A-level chemistry definitions

2

2,4-Dinitrophenylhydrazine

Brady's reagent, which gives a yellow or orange precipitate with an aldehyde or a ketone; the melting point of the purified precipitate identifies the carbonyl compound. Test-tube reactions for functional groups, and high-resolution mass spectrometry

A

Absolute zero

The lowest possible temperature, 0 K or −273 °C, at which particles have the minimum possible energy. Gases and the ideal gas equation

Absorbance

A measure of how much light of a chosen colour a solution absorbs; for a dilute solution it is proportional to the concentration of the coloured species. Finding orders from initial rates, half-lives and graphs

Absorption

The removal from the beam of infrared radiation whose frequency matches the vibration frequency of a bond, seen as a dip in transmittance. Infrared spectroscopy

Accuracy

A measurement or result is accurate when it is close to the true value. Measurement, uncertainty and significant figures in chemistry

Acid anhydride

A compound with two acyl groups joined through one oxygen atom, (RCO)₂O. Acylation: acyl chlorides and acid anhydrides

Acid dissociation constant, Ka

The equilibrium constant for the dissociation of a weak acid, Ka = [H⁺][A⁻]/[HA], in mol dm⁻³. Weak acids, Ka and pKa; Kw and strong bases

Acidic buffer

A buffer with a pH below 7, made from a weak acid and a salt of that acid, such as ethanoic acid and sodium ethanoate. Buffer solutions and buffer calculations

Acidic oxide

An oxide that reacts with bases to form a salt and water; in Period 3, a covalent oxide. The Period 3 oxides: structure, bonding and acid–base character

Acidified silver nitrate

Silver nitrate solution to which dilute nitric acid has been added, used to test for halide ions. Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water

Activating group

A group that releases electron density into a benzene ring, so that the ring reacts with electrophiles faster than benzene; OH, NH₂ and alkyl groups direct to positions 2, 4 and 6. Electrophilic substitution: nitration and Friedel–Crafts acylation

Activation energy

The minimum energy that particles must have for a reaction to take place when they collide. Enthalpy change, standard conditions and the definitions

Activation energy, Ea

The minimum energy that colliding particles must have for a reaction to take place. The Arrhenius equation and the rate-determining step

Active site

A place on the surface of a heterogeneous catalyst where reactant molecules are adsorbed and react. Catalysis: heterogeneous, homogeneous and autocatalytic

Actual yield

The mass of pure, dry product actually collected at the end of a preparation. Reacting masses, percentage yield and atom economy

Acyl chloride

A compound containing the functional group –COCl, in which the OH of a carboxylic acid is replaced by a chlorine atom. Acylation: acyl chlorides and acid anhydrides

Acyl group

The RCO– group: a carbonyl carbon bonded to an alkyl or aryl group, left when the OH is removed from a carboxylic acid. Acylation: acyl chlorides and acid anhydrides

Acylation

The substitution of an acyl group, RCO–, for a hydrogen atom on an O or N atom of a nucleophile. Acylation: acyl chlorides and acid anhydrides

Acylium ion

The electrophile in Friedel–Crafts acylation, RCO⁺, with the positive charge on the carbonyl carbon. Electrophilic substitution: nitration and Friedel–Crafts acylation

Addition

A reaction in which two molecules join together to form a single product. Reaction types, curly arrows and how a mechanism is written

Addition polymerisation

The joining of many unsaturated monomer molecules, by opening their double bonds, to form a polymer and no other product. Addition polymers

Addition reaction

A reaction in which two or more molecules join together to form a single product. Reacting masses, percentage yield and atom economy

Adsorption

The formation of weak bonds between reactant molecules and atoms on the surface of a solid. Catalysis: heterogeneous, homogeneous and autocatalytic

Aerial oxidation

Oxidation of a substance by oxygen from the air; the hydroxides of iron(II), manganese(II) and cobalt(II) darken on standing because of it. Reactions of aqua ions with hydroxide, ammonia and carbonate

Alcohol

A compound containing the hydroxyl group, OH, bonded to a saturated carbon atom. Alcohols: hydration, fermentation and classification

Aldehyde

A compound containing the CHO group: a C=O at the end of a carbon chain, with a hydrogen on the carbonyl carbon. Oxidation and elimination of alcohols

Alkali

A base that dissolves in water to release hydroxide ions, OH⁻(aq). The Period 3 elements: reactions with oxygen and water

Alkene

A hydrocarbon containing at least one carbon–carbon double bond; with one double bond and no ring, the general formula is CₙH₂ₙ. Alkenes: structure, electrophilic addition and Markovnikov's rule

Alkoxide ion

The anion RO⁻ formed when an alcohol loses the hydrogen of its OH group, as in sodium ethoxide, CH₃CH₂O⁻Na⁺. Alcohols: hydration, fermentation and classification

Allotrope

One of two or more different structural forms of the same element in the same physical state, such as diamond and graphite. The four crystal structures and their properties

Amide

A compound containing the group –CONH₂, formed when an acyl chloride or anhydride reacts with ammonia. Acylation: acyl chlorides and acid anhydrides

Amide link

The –CONH– group joining monomer residues in a polyamide, formed from a carboxylic acid group and an amine group with loss of water. Condensation polymers and the disposal of polymers

Amine

A compound in which one or more hydrogen atoms of ammonia have been replaced by alkyl groups. Halogenoalkanes: nucleophilic substitution and rates of hydrolysis

Amino acid

A compound with an amine group and a carboxylic acid group; in the α-amino acids that make proteins both are on the same carbon atom. Amino acids, proteins, enzymes and DNA

Amount of substance

The quantity that counts the particles in a sample, measured in moles, symbol n. The mole, the Avogadro constant and molar mass

Amphoteric

Able to act as both an acid and a base, as water does when it donates a proton to ammonia and accepts one from hydrogen chloride. Brønsted–Lowry acids and bases, pH and strong acids

Anhydrous

Describes a substance that contains no water of crystallisation. The mole, the Avogadro constant and molar mass

Anode

The electrode at which oxidation takes place; in a cell delivering a current it is the negative electrode. Commercial cells, rechargeable cells and fuel cells

Anomalous result

A result that does not fit the pattern of the others, such as a titre outside the concordant set, and is left out of the mean. Measurement, uncertainty and significant figures in chemistry

Aqueous

Dissolved in water, shown by the state symbol (aq) in an equation. Formulae, equations and state symbols

Arene

A hydrocarbon containing one or more benzene rings; compounds containing a benzene ring are called aromatic. Benzene: its structure and the evidence for delocalisation

Arrhenius equation

k = Ae^(−Ea/RT), relating the rate constant to the activation energy and the absolute temperature. The Arrhenius equation and the rate-determining step

Atmosphere, atm

A unit of pressure equal to 101.325 kPa, the mean pressure of the air at sea level. Kp: equilibria in the gas phase

Atom economy

The mass of the desired product as a percentage of the total mass of all the reactants, calculated from the balanced equation. Reacting masses, percentage yield and atom economy

Atomic number

The number of protons in the nucleus of an atom, also called the proton number, Z. Protons, neutrons, electrons and isotopes

Atomic radius

Half the distance between the nuclei of two bonded atoms of the same element. Ionisation energies and the evidence for subshells

Autocatalysis

Catalysis of a reaction by one of its own products, so the rate increases as the product builds up. Redox titrations: manganate(VII) and thiosulfate

Avogadro constant

The number of particles in one mole of a substance, 6.022 × 10²³ mol⁻¹, symbol L. The mole, the Avogadro constant and molar mass

Avogadro's law

Equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. Gases and the ideal gas equation

Axial position

One of the two positions in a trigonal bipyramid directly above and below the central atom, at 90° to the equatorial positions. Shapes of molecules and ions

Azo compound

A compound in which two benzene rings are joined by an –N=N– group, made by coupling a diazonium ion with a phenol or an aromatic amine. Amines: preparation, basicity and nucleophilic reactions

B

Back titration

A method in which an excess of a reagent of known amount is added, and the excess left over is then found by titration. Concentration, volumetric solutions and titration calculations

Balanced equation

An equation with the same number of atoms of each element, and the same total charge, on each side. Formulae, equations and state symbols

Base

A proton acceptor: a species that accepts an H⁺ ion using a lone pair. Elimination from halogenoalkanes, and CFCs and the ozone layer

Base peak

The tallest peak in a mass spectrum, given a relative abundance of 100. Test-tube reactions for functional groups, and high-resolution mass spectrometry

Basic buffer

A buffer with a pH above 7, made from a weak base and a salt of that base, such as ammonia and ammonium chloride. Buffer solutions and buffer calculations

Basic oxide

An oxide that reacts with acids to form a salt and water; in Period 3, an ionic oxide containing O²⁻ ions. The Period 3 oxides: structure, bonding and acid–base character

Basicity

The number of protons (H⁺ ions) that one molecule of an acid can donate. Brønsted–Lowry acids and bases, pH and strong acids

Batch process

A process in which a fixed quantity of material is converted in one run, the reactor being emptied and refilled between runs. Group 2: trends down the group and reactions with water

Bidentate ligand

A ligand that forms two coordinate bonds to the same metal ion, such as ethane-1,2-diamine or the ethanedioate ion. Transition metal characteristics and complex formation

Biodegradable

Able to be broken down by microorganisms in the environment into small molecules such as carbon dioxide and water. Addition polymers

Biodiesel

A mixture of methyl esters of long-chain carboxylic acids, made by transesterification of vegetable oils with methanol. Carboxylic acids, esters and biodiesel

Biofuel

A fuel produced from renewable biological material, such as crops. Alcohols: hydration, fermentation and classification

Block

A region of the periodic table in which the highest-energy electron of each element is in the same type of subshell, s, p, d or f. Electron configuration: shells, subshells and orbitals

Boiling temperature

The temperature at which the vapour pressure of a liquid equals atmospheric pressure; a pure liquid distils at a steady temperature matching its data-book value. Preparing, purifying and testing an organic solid and liquid

Bomb calorimeter

A sealed steel vessel in which a sample burns completely in pure oxygen, surrounded by a known mass of water, so that almost no heat is lost. Measuring an enthalpy change: calorimetry

Bond angle

The angle between two bonds that meet at the same atom. Shapes of molecules and ions

Bond dissociation enthalpy

The enthalpy change when one mole of a specified covalent bond is broken, with reactants and products in the gaseous state. Mean bond enthalpies

Bond enthalpy

The enthalpy change when one mole of a given covalent bond is broken, with all species in the gaseous state. Group 7: trends down the group and displacement reactions

Bond length

The distance between the nuclei of two covalently bonded atoms; for the same pair of atoms, the shorter the bond, the stronger it is. Covalent, dative covalent and metallic bonding

Bond vibration

The stretching or bending of a covalent bond at a natural frequency set by the bond strength and the masses of the bonded atoms. Infrared spectroscopy

Bonding pair

A pair of electrons shared between two atoms in a covalent bond. Covalent, dative covalent and metallic bonding

Brønsted–Lowry acid

A species that donates a proton (an H⁺ ion) to another species. Brønsted–Lowry acids and bases, pH and strong acids

Brønsted–Lowry base

A species that accepts a proton (an H⁺ ion) from another species. Brønsted–Lowry acids and bases, pH and strong acids

Buffer

A solution that maintains an approximately constant pH when small amounts of acid or base are added, or when it is diluted. Buffer solutions and buffer calculations

Buffer range

The range of pH, roughly pKa ± 1, over which a buffer resists changes in pH effectively. Buffer solutions and buffer calculations

By-product

A substance formed in a reaction alongside the desired product. Reacting masses, percentage yield and atom economy

C

Cahn–Ingold–Prelog priority rule

On each carbon of the double bond, the group whose first atom has the higher atomic number has the higher priority. Structural isomerism and E/Z isomerism

Calibration

Adjusting a pH meter against buffer solutions of known pH, so that its readings are accurate. Titration curves and choosing an indicator

Calibration curve

A graph of a measured property, such as absorbance, against known concentrations of a substance, used to convert a reading into a concentration. Measuring the rate of a reaction

Calibration graph

A graph of absorbance against known concentrations of the coloured species, used to convert each absorbance reading into a concentration. Finding orders from initial rates, half-lives and graphs

Calorimetry

The measurement of the heat energy transferred in a reaction from the temperature change of a known mass of water or solution. Measuring an enthalpy change: calorimetry

Carbocation

An ion in which a carbon atom has only three bonds and carries a positive charge. Alkenes: structure, electrophilic addition and Markovnikov's rule

Carbon neutral

Describing an activity that has no net annual carbon emissions to the atmosphere. Alcohols: hydration, fermentation and classification

Carboxylate ion

The anion RCOO⁻ formed when a carboxylic acid loses the hydrogen ion from its COOH group. Carboxylic acids, esters and biodiesel

Carboxylic acid

A compound containing the carboxyl group, COOH, at the end of a carbon chain. Oxidation and elimination of alcohols

Carrier gas

The unreactive gas, usually helium or nitrogen, that is the mobile phase in gas chromatography. Chromatography: TLC, column and gas

Catalyst

A substance that increases the rate of a reaction without being changed in chemical composition or amount, by providing an alternative reaction route with a lower activation energy. The Maxwell–Boltzmann distribution, temperature and catalysts

Catalyst poisoning

The loss of catalytic activity when an impurity is strongly adsorbed onto the active sites, blocking them to the reactants. Catalysis: heterogeneous, homogeneous and autocatalytic

Cathode

The electrode at which reduction takes place; in a cell delivering a current it is the positive electrode. Commercial cells, rechargeable cells and fuel cells

Cationic surfactant

A quaternary ammonium salt with a long hydrocarbon chain, whose positive head is attracted to negatively charged surfaces such as fabric and hair. Amines: preparation, basicity and nucleophilic reactions

CFC

A chlorofluorocarbon: a halogenoalkane containing only carbon, chlorine and fluorine. Elimination from halogenoalkanes, and CFCs and the ozone layer

Chain isomers

Structural isomers with the same functional group but a different arrangement of the carbon skeleton. Structural isomerism and E/Z isomerism

Chain reaction

A reaction in which each propagation step regenerates a radical, so that one initiation step leads to many cycles of reaction before termination. Free-radical substitution

Charge density

The charge on an ion relative to its size; a small ion with a large charge has a high charge density. Enthalpies of solution and hydration

Charge/size ratio

The charge on an ion divided by its radius; also called charge density. A small, highly charged ion has a high ratio. Metal-aqua ions and the acidity of their solutions

Chelate effect

The replacement of monodentate ligands by bidentate or multidentate ligands, favoured because the number of particles increases and so the entropy increases. Ligands, the shapes of complexes, isomerism and ligand substitution

Chemical environment

A set of atoms in a molecule that are bonded to identical surroundings, so they absorb at the same chemical shift. Proton and carbon-13 NMR spectroscopy

Chemical shift

The position of an NMR absorption relative to the TMS peak, δ, measured in parts per million (ppm). Proton and carbon-13 NMR spectroscopy

Chiral

Describes a molecule that cannot be superimposed on its mirror image, usually because it contains a chiral centre. Optical isomerism

Chiral centre

A carbon atom bonded to four different atoms or groups of atoms. Optical isomerism

Chlorate(I) ion

The ClO⁻ ion, in which chlorine has an oxidation state of +1; its sodium salt is the active ingredient of bleach. Half-equations and building redox equations

Chlorate(V) ion

The ClO₃⁻ ion, in which chlorine has an oxidation state of +5; formed with chloride when chlorine reacts with hot, concentrated alkali. Half-equations and building redox equations

Chloric(I) acid

HClO, the weak acid formed when chlorine reacts with water, in which chlorine has an oxidation state of +1. Half-equations and building redox equations

Cis–trans isomerism

A special case of E/Z isomerism in which the two carbons of the C=C carry one identical group each, such as H; cis has those groups on the same side, trans on opposite sides. Structural isomerism and E/Z isomerism

Cis–trans isomers

Stereoisomers of a complex in which two identical ligands are adjacent, 90° apart, in the cis isomer and opposite, 180° apart, in the trans isomer. Ligands, the shapes of complexes, isomerism and ligand substitution

Clock reaction

A reaction in which the time taken to form a fixed amount of product is measured, usually by a sudden colour change; 1/t is proportional to the initial rate. Finding orders from initial rates, half-lives and graphs

Closed system

A system that no substance can enter or leave, although energy may be transferred to or from it. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

Coefficient

The number written in front of a formula in an equation, giving how many units of that substance react or form. Formulae, equations and state symbols

Collision frequency

The number of collisions between reactant particles per unit time in a given volume. Collision theory, activation energy and the rate of reaction

Colorimeter

An instrument that measures the absorbance of a solution for light of a chosen colour, selected by a filter. Colour in transition metal complexes and colorimetry

Combustion analysis

Finding the masses of carbon and hydrogen in a compound by burning a weighed sample in excess oxygen and weighing the carbon dioxide and water formed. Empirical and molecular formulae

Common ion effect

The shift in position of an equilibrium involving ions, away from those ions, when a soluble compound supplying one of them is added: ethanoate ions suppress the dissociation of ethanoic acid, and chloride ions lower the solubility of silver chloride. Buffer solutions and buffer calculations

Complementary colours

Two colours of light that together make white light; a substance that absorbs one appears the other. Colour in transition metal complexes and colorimetry

Complete combustion

Burning in a plentiful supply of oxygen, in which the carbon in a fuel is oxidised to carbon dioxide and the hydrogen to water. Alkanes: fractional distillation, cracking and combustion

Complex

A central metal atom or ion surrounded by ligands bonded to it by coordinate bonds. Transition metal characteristics and complex formation

Complex ion

A central metal ion bonded to one or more ligands by coordinate bonds, as in [Ag(NH₃)₂]⁺. Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water

Compound ion

A group of covalently bonded atoms that carries an overall charge, such as SO₄²⁻ or NH₄⁺. Ionic bonding and ionic lattices

Compromise conditions

Conditions chosen to give an acceptable equilibrium yield at an acceptable rate and cost, where the conditions for the highest yield would make the reaction too slow or the plant too expensive. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

Concentration

The amount of solute, in moles, dissolved in each 1 dm³ of solution. Concentration, volumetric solutions and titration calculations

Concordant results

Titres that agree within 0.10 cm³ of each other, from which the mean titre is calculated. Measurement, uncertainty and significant figures in chemistry

Concordant titres

Titres that agree with each other to within 0.10 cm³. Concentration, volumetric solutions and titration calculations

Condensation

A reaction in which two molecules join together with the loss of a small molecule such as water. Reaction types, curly arrows and how a mechanism is written

Condensation polymerisation

The joining of monomers, each with two functional groups, into a polymer with the loss of a small molecule such as water or HCl at each link. Condensation polymers and the disposal of polymers

Condensation reaction

A reaction in which two molecules join together with the loss of a small molecule such as water. Carboxylic acids, esters and biodiesel

Conditions

The temperature, solvent, catalyst and method of heating a reaction needs to give the intended product. Planning a synthetic route

Conjugate acid–base pair

Two species that differ by one proton, such as NH₄⁺ and NH₃; the one with the extra proton is the conjugate acid. Brønsted–Lowry acids and bases, pH and strong acids

Continuous monitoring

Following a single reaction mixture by reading a property that changes, at intervals, through the whole reaction. Measuring the rate of a reaction

Control variable

A variable kept the same in every run so that it cannot affect the result. Measuring the rate of a reaction

Conventional representation

A one-line description of a cell: the electrodes at the ends, single vertical lines for phase boundaries, a double line for the salt bridge, and the oxidised form of each couple next to the salt bridge. Electrode potentials and the standard hydrogen electrode

Coordinate bond

A covalent bond in which both electrons of the shared pair come from the same atom. Transition metal characteristics and complex formation

Coordination number

The number of nearest neighbours of opposite charge that surround an ion in a lattice; six for each ion in sodium chloride. Ionic bonding and ionic lattices

Covalent bond

A shared pair of electrons between two atoms, attracted electrostatically to the nuclei of both. Covalent, dative covalent and metallic bonding

Covalent character

Partial sharing of electrons in an ionic compound, caused by a small, highly charged cation distorting the electron cloud of the anion. The Period 3 oxides: structure, bonding and acid–base character

Cracking

The breaking of C–C bonds in long-chain alkanes to form shorter alkanes and alkenes. Alkanes: fractional distillation, cracking and combustion

Curly arrow

An arrow showing the movement of a pair of electrons, from a lone pair or a bond to where a new bond or lone pair forms. Reaction types, curly arrows and how a mechanism is written

D

d-block element

An element whose last electron, in filling order, enters a d subshell, as from scandium to zinc. Periodicity: trends across Period 3 and the blocks of the table

d-orbital splitting

The separation of the five d orbitals of a metal ion into groups of different energy by the lone pairs of the ligands; the gap between the groups is ΔE. Colour in transition metal complexes and colorimetry

Dash bond

A bond drawn as a hatched wedge or dashed line, showing that it points behind the plane of the paper. Shapes of molecules and ions

Dative covalent bond

A covalent bond in which both electrons of the shared pair come from the same atom; also called a co-ordinate bond. Covalent, dative covalent and metallic bonding

Deactivating group

A group that withdraws electron density from a benzene ring, so that the ring reacts with electrophiles more slowly than benzene; NO₂, COOH and COR direct to position 3. Electrophilic substitution: nitration and Friedel–Crafts acylation

Degree of dissociation

The fraction of the molecules of a substance that have dissociated when equilibrium is reached, written α. Kp: equilibria in the gas phase

Degree of unsaturation

The number of rings plus double bonds in a molecule, found from (2C + 2 + N − H − X) ÷ 2, where a triple bond counts as two. Putting the analysis together: structure from spectra

Dehydration

An elimination reaction in which a molecule of water is removed, as when an alcohol forms an alkene. Oxidation and elimination of alcohols

Delocalisation energy

The difference in energy between benzene and the Kekulé structure, 152 kJ mol⁻¹, by which the delocalised ring is more stable; also called resonance energy. Benzene: its structure and the evidence for delocalisation

Delocalised electrons

Electrons that are not associated with any single atom or bond and are free to move through a structure. Covalent, dative covalent and metallic bonding

Delocalised model of benzene

A planar ring of six carbon atoms joined by σ bonds, in which a p orbital on each carbon overlaps with those on both neighbours, giving a ring of six delocalised electrons above and below the plane. Benzene: its structure and the evidence for delocalisation

Dependent variable

The variable that is measured in each run to see the effect of the change. Measuring the rate of a reaction

Deprotonation

The removal of a proton from a species; here, from a water ligand bonded to a metal ion, leaving a hydroxide ligand. Reactions of aqua ions with hydroxide, ammonia and carbonate

Derivative

A solid made from an unknown compound by a reaction of its functional group, whose melting point is compared with data to identify the compound. Aldehydes and ketones: nucleophilic addition and the tests that tell them apart

Desorption

The breaking of the bonds between product molecules and a catalyst surface, which releases the products. Catalysis: heterogeneous, homogeneous and autocatalytic

Deuterated solvent

A solvent such as CDCl₃ in which ¹H atoms are replaced by deuterium, so that it gives no peaks in a proton NMR spectrum. Proton and carbon-13 NMR spectroscopy

Diatomic molecule

A molecule made of two atoms, as in the elements hydrogen, nitrogen, oxygen and the halogens. Formulae, equations and state symbols

Diazonium ion

An ion containing the –N₂⁺ group bonded to a benzene ring, such as C₆H₅N₂⁺, made from an aromatic amine and nitrous acid below 10 °C. Amines: preparation, basicity and nucleophilic reactions

Dimer

A molecule formed when two identical molecules join, as two AlCl₃ molecules join to form Al₂Cl₆. The Period 3 elements: reactions with oxygen and water

Diol

A compound with two OH groups; cold dilute acidified manganate(VII) adds one to each carbon of a C=C bond. Alkenes: structure, electrophilic addition and Markovnikov's rule

Dipole moment

A measure of the size of the overall separation of charge in a molecule; zero when the bond dipoles cancel. Electronegativity, polarity and intermolecular forces

Displacement reaction

A reaction in which a more reactive element takes the place of a less reactive one in a compound or in solution. Group 7: trends down the group and displacement reactions

Displayed formula

A formula showing the relative positioning of all the atoms in a molecule and every bond between them. Empirical and molecular formulae

Disproportionation

A redox reaction in which the same element in one species is simultaneously oxidised and reduced. Half-equations and building redox equations

Distillation

Separating a liquid from a mixture by boiling it and condensing the vapour in a separate vessel. Apparatus, techniques and safety: the twelve required practicals in outline

Double bond

Two shared pairs of electrons between the same two atoms, shorter and stronger than a single bond between them. Covalent, dative covalent and metallic bonding

Drying agent

An anhydrous salt, such as magnesium sulfate or calcium chloride, that removes water from an organic liquid by absorbing it as water of crystallisation. Preparing, purifying and testing an organic solid and liquid

Dynamic equilibrium

The state of a reversible reaction in a closed system in which the forward and reverse reactions proceed at equal rates, so the concentrations of reactants and products remain constant. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

D₂O exchange

Shaking a sample with D₂O replaces the hydrogen of O–H and N–H groups with deuterium, so their proton NMR peak disappears. Putting the analysis together: structure from spectra

E

E/Z isomerism

A form of stereoisomerism caused by restricted rotation about a C=C double bond, where each carbon of the double bond carries two different groups. Structural isomerism and E/Z isomerism

E/Z isomers

Stereoisomers that exist because rotation about a C=C bond is restricted and each carbon of the double bond carries two different groups. Elimination from halogenoalkanes, and CFCs and the ozone layer

Effervescence

Bubbles of gas produced in a liquid by a reaction; here carbon dioxide, which turns limewater milky. Reactions of aqua ions with hydroxide, ammonia and carbonate

Elastic collision

A collision in which the total kinetic energy of the colliding particles is unchanged. Gases and the ideal gas equation

Electrochemical series

A list of half-equations written as reductions, in order of their standard electrode potentials. The electrochemical series and predicting feasibility

Electrode potential

The potential difference between an electrode and the solution it is in; it can be measured only against a second electrode. Electrode potentials and the standard hydrogen electrode

Electrolysis

The decomposition of an ionic compound, molten or in solution, by an electric current from an external supply. Commercial cells, rechargeable cells and fuel cells

Electrolyte

The ionic solution, melt or solid between the electrodes of a cell, through which ions move to complete the circuit. Commercial cells, rechargeable cells and fuel cells

Electron

A particle outside the nucleus with a relative mass of 1/1840 and a relative charge of −1. Protons, neutrons, electrons and isotopes

Electron configuration

The arrangement of the electrons of an atom or ion in shells and subshells, written for example as 1s² 2s² 2p⁶. Electron configuration: shells, subshells and orbitals

Electron density map

A contour map, calculated from the X-ray diffraction pattern of a crystal, showing how the density of electrons varies through the structure. Ionic bonding and ionic lattices

Electron impact ionisation

Ionisation in which high-energy electrons from an electron gun knock an electron off each gaseous particle, forming a 1+ ion. Time-of-flight mass spectrometry and relative atomic mass

Electron-pair repulsion theory

The theory that the electron pairs round a central atom repel each other and move as far apart as possible, which fixes the shape of the molecule. Shapes of molecules and ions

Electronegativity

The power of an atom to attract the pair of electrons in a covalent bond towards itself. Electronegativity, polarity and intermolecular forces

Electrophile

An electron pair acceptor, which is attracted to a region of high electron density. Reaction types, curly arrows and how a mechanism is written

Electrophilic addition

A reaction in which the double bond of an alkene attacks an electrophile, and two groups add across the double bond to give one saturated product. Alkenes: structure, electrophilic addition and Markovnikov's rule

Electrophilic substitution

A reaction in which an electrophile replaces an atom, usually hydrogen, on an electron-rich compound such as benzene. Electrophilic substitution: nitration and Friedel–Crafts acylation

Electrophoresis

The separation of charged species, such as amino acids in a buffer of fixed pH, by their movement towards the oppositely charged electrode when a potential difference is applied. Amino acids, proteins, enzymes and DNA

Electrospray ionisation

Ionisation in which a solution of the sample is sprayed from a needle at a high positive voltage, so that each molecule gains a proton, forming XH⁺. Time-of-flight mass spectrometry and relative atomic mass

Elimination

A reaction in which a small molecule is removed from a larger one, leaving a double bond. Reaction types, curly arrows and how a mechanism is written

Eluent

The solvent run continuously through a chromatography column as the mobile phase, carrying the components down it. Chromatography: TLC, column and gas

EMF

The potential difference between the two electrodes of a cell when no current flows. Electrode potentials and the standard hydrogen electrode

Empirical formula

The simplest whole-number ratio of atoms of each element in a compound. Empirical and molecular formulae

Empirical formula mass

The sum of the relative atomic masses of the atoms in the empirical formula. Empirical and molecular formulae

Enantiomers

Stereoisomers that are non-superimposable mirror images of each other. Optical isomerism

End point

The point in a titration at which the indicator first changes colour permanently. Concentration, volumetric solutions and titration calculations

Endothermic

Describes a reaction in which heat energy is taken in from the surroundings, so ΔH is positive. Enthalpy change, standard conditions and the definitions

Enthalpy change, ΔH

The heat energy change measured under conditions of constant pressure. Enthalpy change, standard conditions and the definitions

Enthalpy cycle

A diagram showing two or more routes between the same reactants and products, with the enthalpy change for each step, used to calculate an unknown enthalpy change. Hess's law and enthalpy cycles

Enthalpy of atomisation

The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state. Mean bond enthalpies

Enthalpy of formation

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. Lattice enthalpy and Born–Haber cycles

Enthalpy of formation of an element

Zero for an element in its standard state, because forming the element from itself involves no change. Hess's law and enthalpy cycles

Enthalpy of fusion

The enthalpy change when one mole of a solid is converted into a liquid at its melting point. The four crystal structures and their properties

Enthalpy of hydration

The enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions. Enthalpies of solution and hydration

Enthalpy of hydrogenation

The enthalpy change when one mole of an unsaturated compound reacts completely with hydrogen to give a saturated compound. Benzene: its structure and the evidence for delocalisation

Enthalpy of solution

The enthalpy change when one mole of a solute dissolves in enough solvent that further dilution causes no further heat change. Hess's law and enthalpy cycles

Enthalpy of vaporisation

The enthalpy change when one mole of a liquid is converted into a gas at its boiling point. The four crystal structures and their properties

Enthalpy profile

A diagram of enthalpy against the progress of a reaction, showing the reactants, the transition state at the peak and the products, from which Ea and ΔH are read. Collision theory, activation energy and the rate of reaction

Entropy

A measure of the disorder of a system: the number of ways the particles and their energy can be arranged. Entropy and Gibbs free-energy change

Entropy change of a reaction

The sum of the standard entropies of the products minus the sum of the standard entropies of the reactants, each multiplied by its number of moles in the equation. Entropy and Gibbs free-energy change

Entropy change of the surroundings

The entropy change of everything outside the reacting system, equal to −ΔH/T; it is positive for an exothermic reaction. Entropy and Gibbs free-energy change

Equatorial position

One of the three positions in a trigonal bipyramid that lie in a triangle round the central atom, at 120° to each other. Shapes of molecules and ions

Equilibrium concentration

The concentration of a substance, in mol dm⁻³, in a mixture that has reached equilibrium; it is what each term in Kc stands for. The equilibrium constant Kc

Equilibrium constant, Kc

The concentrations of the products at equilibrium, each raised to the power of its coefficient and multiplied together, divided by the same expression for the reactants, at a stated temperature. The equilibrium constant Kc

Equilibrium yield

The proportion of the reactants converted into products once the reaction mixture has reached equilibrium. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

Equivalence point

The point at which the reactants have been added in exactly the amounts the equation requires. Concentration, volumetric solutions and titration calculations

Equivalent atoms

Atoms that are interchanged by the symmetry of the molecule, so they share an environment and give a single NMR peak. Putting the analysis together: structure from spectra

Ester link

The –COO– group joining monomer residues in a polyester, formed from a carboxylic acid group and an alcohol group with loss of water. Condensation polymers and the disposal of polymers

Esterification

The reaction of a carboxylic acid with an alcohol, catalysed by a strong acid, to form an ester and water. Carboxylic acids, esters and biodiesel

Excess (of a reagent)

More of a reagent than is needed to react with all of the other reactant present. Reactions of aqua ions with hydroxide, ammonia and carbonate

Excited state

A configuration in which one or more electrons occupy a higher-energy orbital than in the ground state. Electron configuration: shells, subshells and orbitals

Exothermic

Describes a reaction in which heat energy is transferred to the surroundings, so ΔH is negative. Enthalpy change, standard conditions and the definitions

External calibration

Finding a concentration by measuring standards of known concentration, plotting the response against concentration, and reading the unknown from the graph. Putting the analysis together: structure from spectra

Extrapolated temperature rise

The temperature change read at the time of mixing from a cooling line extended backwards, which corrects for heat lost to the surroundings during the reaction. Measuring an enthalpy change: calorimetry

E°cell

The EMF of a cell under standard conditions, equal to E° of the couple being reduced minus E° of the couple being oxidised; a positive value means the reaction is feasible. Variable oxidation states: vanadium, chromium and manganate(VII) titrations

F

Faraday constant

The charge on one mole of electrons, 96 500 C mol⁻¹. The electrochemical series and predicting feasibility

Feasible

Able to take place without a continuous input of energy from outside, which is so when ΔG is zero or negative. Entropy and Gibbs free-energy change

Feedstock recycling

Heating waste polymer to crack it into small molecules that are used to make new polymers or fuels. Addition polymers

Fehling's solution

An alkaline solution of a copper(II) complex that is reduced by an aldehyde to a brick-red precipitate of copper(I) oxide. Test-tube reactions for functional groups, and high-resolution mass spectrometry

Fermentation

The conversion of glucose into ethanol and carbon dioxide by enzymes in yeast, in the absence of air. Alcohols: hydration, fermentation and classification

Filtrate

The liquid that passes through the filter paper during filtration. Apparatus, techniques and safety: the twelve required practicals in outline

Filtration under reduced pressure

Filtration through a Büchner funnel with a pump lowering the pressure in the flask below, which is faster and leaves a drier solid. Preparing, purifying and testing an organic solid and liquid

Fingerprint region

The region of an infrared spectrum below 1500 cm⁻¹, whose pattern of absorptions is unique to a compound. Infrared spectroscopy

First electron affinity

The enthalpy change when one mole of gaseous atoms each gain one electron to form one mole of gaseous 1− ions. Lattice enthalpy and Born–Haber cycles

First ionisation energy

The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions. Ionisation energies and the evidence for subshells

First order

A reactant is first order when the rate is directly proportional to its concentration: doubling the concentration doubles the rate. Rate equations, orders of reaction and the rate constant

Fishhook arrow

A curly arrow with half a head, showing the movement of a single electron. Free-radical substitution

Flame test

A test in which a compound is heated in a flame and the colour of the light emitted identifies the metal ion present. Group 2 compounds: solubility, uses and the sulfate test

Flue-gas desulfurisation

The removal of sulfur dioxide from the waste gases of a power station by reaction with a base such as calcium oxide or calcium carbonate. Group 2 compounds: solubility, uses and the sulfate test

Formula unit

The group of ions given by the formula of an ionic compound, in the ratio they occur in the lattice; for CaCl₂, one Ca²⁺ and two Cl⁻. Ionic bonding and ionic lattices

Fraction

A mixture of hydrocarbons with similar chain lengths, and so with boiling points in a narrow range. Alkanes: fractional distillation, cracking and combustion

Fractional distillation

Separation of a mixture into fractions by boiling point, in a column with a temperature gradient that is hottest at the bottom. Alkanes: fractional distillation, cracking and combustion

Fragmentation

The breaking of a molecular ion into a positive ion and a neutral radical, of which only the ion is detected. Test-tube reactions for functional groups, and high-resolution mass spectrometry

Free-radical substitution

A reaction in which an atom in a molecule is replaced by another, by way of a chain of steps involving radicals. Free-radical substitution

Friedel–Crafts acylation

The electrophilic substitution of an acyl group, RCO, onto a benzene ring, using an acyl chloride and an aluminium chloride catalyst, giving a ketone. Electrophilic substitution: nitration and Friedel–Crafts acylation

Friedel–Crafts alkylation

The electrophilic substitution of an alkyl group onto a benzene ring, using a halogenoalkane and an aluminium chloride catalyst. Electrophilic substitution: nitration and Friedel–Crafts acylation

Fuel cell

A cell that generates an electric current from a fuel and oxygen supplied continuously from outside, and does not need to be electrically recharged. Commercial cells, rechargeable cells and fuel cells

Fullerene

A molecular allotrope of carbon in which each atom is bonded to three others in a closed cage or tube, such as C₆₀. The four crystal structures and their properties

Functional group

An atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions. Naming and representing organic molecules

Functional group isomers

Structural isomers with the same molecular formula but different functional groups. Structural isomerism and E/Z isomerism

G

Gas constant

The constant R in the ideal gas equation, equal to 8.31 J K⁻¹ mol⁻¹. Gases and the ideal gas equation

General formula

An algebraic formula that can describe any member of a homologous series, such as CₙH₂ₙ₊₂ for the alkanes. Naming and representing organic molecules

Giant covalent structure

A lattice in which every atom is joined to its neighbours by covalent bonds extending throughout the whole structure. Periodicity: trends across Period 3 and the blocks of the table

Giant ionic lattice

A regular three-dimensional arrangement of oppositely charged ions, repeated throughout the crystal and held by electrostatic attraction in all directions. Ionic bonding and ionic lattices

Giant structure

A structure in which the bonding extends throughout the whole crystal, with no separate molecules, as in ionic, metallic and macromolecular crystals. The four crystal structures and their properties

Gibbs free-energy change

The quantity ΔG = ΔH − TΔS, which combines the enthalpy change, the entropy change and the temperature; a reaction is feasible when it is zero or negative. Entropy and Gibbs free-energy change

Global warming

The rise in the mean temperature of the Earth's surface and lower atmosphere caused by increasing concentrations of greenhouse gases. Infrared spectroscopy

Gravimetric analysis

Finding the amount of a substance by converting it into a precipitate of known formula, which is filtered, dried and weighed. Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water

Greenhouse gas

A gas whose molecules absorb infrared radiation emitted by the Earth's surface and re-emit it, warming the lower atmosphere. Alkanes: fractional distillation, cracking and combustion

Grignard reagent

An organomagnesium halide, RMgX, made from a halogenoalkane and magnesium in dry ether, whose carbon bonded to magnesium acts as a nucleophile and forms a new C–C bond. Planning a synthetic route

Ground state

The lowest-energy electron configuration of an atom or ion, with each electron in the lowest available orbital. Electron configuration: shells, subshells and orbitals

H

Half-cell

An element in contact with a solution of its ions, or an inert electrode in a solution of an oxidised and a reduced form, at which a redox equilibrium is set up. Electrode potentials and the standard hydrogen electrode

Half-equation

An equation for the oxidation or the reduction half of a redox reaction on its own, with electrons shown to balance the charge. Formulae, equations and state symbols

Half-life, t½

The time taken for the concentration of a reactant to fall to half of its starting value. Rate equations, orders of reaction and the rate constant

Half-neutralisation point

The point in the titration of a weak acid at which half the acid has been neutralised, so [HA] = [A⁻] and pH = pKa. Titration curves and choosing an indicator

Halogen carrier

A compound such as AlCl₃ that accepts a lone pair from a halogen atom and so generates a positive electrophile. Electrophilic substitution: nitration and Friedel–Crafts acylation

Halogenoalkane

A compound in which one or more hydrogen atoms of an alkane have been replaced by halogen atoms. Halogenoalkanes: nucleophilic substitution and rates of hydrolysis

Halogenoarene

A compound in which a halogen atom is bonded directly to a carbon atom of a benzene ring, such as chlorobenzene. Benzene: its structure and the evidence for delocalisation

Hazard

A substance, object or activity with the potential to cause harm. Apparatus, techniques and safety: the twelve required practicals in outline

Heating to constant mass

Repeating the cycle of heating, cooling and weighing until two successive masses agree, which shows the change is complete. The mole, the Avogadro constant and molar mass

Heating under reflux

Heating a reaction mixture with a vertical condenser fitted, so that vapours condense and return to the flask and no volatile substance is lost. Apparatus, techniques and safety: the twelve required practicals in outline

Henderson–Hasselbalch equation

pH = pKa + log([A⁻]/[HA]), the Ka expression for a weak acid rearranged in logarithmic form. Buffer solutions and buffer calculations

Hess's law

The enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. Hess's law and enthalpy cycles

Heterogeneous catalyst

A catalyst in a different phase from the reactants, such as solid iron in the Haber process. The Maxwell–Boltzmann distribution, temperature and catalysts

Heterogeneous equilibrium

An equilibrium in which the reactants and products are in more than one phase, such as a solid with a gas. The equilibrium constant Kc

Heterolytic fission

The breaking of a covalent bond in which one atom takes both electrons from the shared pair, forming a positive and a negative ion. Reaction types, curly arrows and how a mechanism is written

HFC

A hydrofluorocarbon: a compound of hydrogen, fluorine and carbon with no chlorine, used to replace CFCs. Elimination from halogenoalkanes, and CFCs and the ozone layer

Homogeneous catalyst

A catalyst in the same phase as the reactants, such as iodide ions dissolved with hydrogen peroxide. The Maxwell–Boltzmann distribution, temperature and catalysts

Homogeneous equilibrium

An equilibrium in which all the reactants and products are in the same phase. The equilibrium constant Kc

Homologous series

A series of organic compounds with the same functional group and general formula, in which each member differs from the next by CH₂. Naming and representing organic molecules

Homolytic fission

The breaking of a covalent bond so that each atom takes one electron from the shared pair, forming two radicals. Mean bond enthalpies

HPLC

High-performance liquid chromatography, in which a liquid mobile phase is pumped at high pressure through a column packed with very small particles of stationary phase. Chromatography: TLC, column and gas

Hund's rule

Electrons fill the orbitals of a subshell singly, with parallel spins, before any orbital takes a pair. Electron configuration: shells, subshells and orbitals

Hybridisation

The mixing of an atom's s and p orbitals to give a set of identical hybrid orbitals, sp³, sp² or sp, that point along its σ bonds. Covalent, dative covalent and metallic bonding

Hydrated

Describes a crystalline compound that contains water of crystallisation. The mole, the Avogadro constant and molar mass

Hydrated ion

An ion in aqueous solution surrounded by water molecules held to it by ion–dipole attractions. Enthalpies of solution and hydration

Hydration

The addition of water across a carbon–carbon double bond, as when ethene and steam form ethanol. Alcohols: hydration, fermentation and classification

Hydride ion

H⁻, a hydrogen atom with an extra electron, which acts as a nucleophile through its lone pair; NaBH₄ is its source. Aldehydes and ketones: nucleophilic addition and the tests that tell them apart

Hydrocarbon

A compound that contains carbon and hydrogen only. Empirical and molecular formulae

Hydrogen bond

An attraction between a lone pair on an N, O or F atom and a δ+ hydrogen atom covalently bonded to N, O or F in another molecule. Electronegativity, polarity and intermolecular forces

Hydrogen halide

A compound of hydrogen and a halogen, HX, which forms misty white fumes in moist air. Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water

Hydrolysis

A reaction in which a bond in a compound is broken by reaction with water. The Period 3 oxides: structure, bonding and acid–base character

Hydrolysis (of a metal-aqua ion)

The reaction of a metal-aqua ion with water in which a coordinated water molecule loses a proton to a solvent water molecule, forming H₃O⁺. Metal-aqua ions and the acidity of their solutions

Hydroxynitrile

A compound containing a hydroxyl group and a nitrile group, formed by nucleophilic addition of HCN to an aldehyde or ketone. Aldehydes and ketones: nucleophilic addition and the tests that tell them apart

I

ICE table

A table of the initial amount, the change in amount and the equilibrium amount of each species, with the changes in the ratio of the coefficients in the equation. The equilibrium constant Kc

Ideal gas

A gas whose molecules have negligible volume and no intermolecular forces, so that it obeys pV = nRT exactly. Gases and the ideal gas equation

Ideal gas equation

pV = nRT, relating the pressure, volume, amount and kelvin temperature of an ideal gas. Gases and the ideal gas equation

Immiscible

Describes two liquids that do not mix and form two separate layers. Preparing, purifying and testing an organic solid and liquid

In excess

Present in more than the amount needed to react with the limiting reagent, so that some is left at the end. Reacting masses, percentage yield and atom economy

Incomplete combustion

Combustion in a limited supply of oxygen, giving carbon monoxide or carbon as well as or instead of carbon dioxide, and releasing less energy. Measuring an enthalpy change: calorimetry

Independent variable

The variable that is deliberately changed from one run to the next. Measuring the rate of a reaction

Indicator

A weak acid whose undissociated form and conjugate base have different colours, used to show the end point of a titration. Titration curves and choosing an indicator

Indirect determination

Finding an enthalpy change that cannot be measured directly by combining, in a Hess cycle, enthalpy changes that can be measured. Hess's law and enthalpy cycles

Induced dipole

A temporary separation of charge in a molecule caused by a nearby region of charge; the π electrons of an alkene repel the Br–Br bonding pair, making the nearer bromine δ+. Alkenes: structure, electrophilic addition and Markovnikov's rule

Induced dipole–dipole force

An attraction between an instantaneous dipole in one molecule and the dipole it induces in a neighbouring molecule; it acts between all molecules. Electronegativity, polarity and intermolecular forces

Inductive effect

The release of electron density along a σ bond by an alkyl group, which stabilises a positive charge on the carbon it is joined to. Alkenes: structure, electrophilic addition and Markovnikov's rule

Inert electrode

An electrode, usually platinum, that conducts electrons to and from a redox equilibrium in the solution without reacting or setting up an equilibrium of its own; used when neither species of the couple is a metal. Electrode potentials and the standard hydrogen electrode

Infrared spectroscopy

A technique that identifies the bonds in a compound from the wavenumbers of infrared radiation that they absorb. Infrared spectroscopy

Initial rate

The rate of reaction at the start, when t = 0, found from the gradient of the tangent to a concentration–time graph at t = 0. Collision theory, activation energy and the rate of reaction

Initiation

The step in which radicals are first formed, here by homolytic fission of Cl₂ in ultraviolet light. Free-radical substitution

Integration ratio

The ratio of the areas under the peaks in a proton NMR spectrum, equal to the ratio of the numbers of hydrogens in each environment. Proton and carbon-13 NMR spectroscopy

Intermediate

A species formed in one step of a mechanism and used up in a later step, so it does not appear in the overall equation. The Arrhenius equation and the rate-determining step

Intermolecular force

An attraction between separate molecules, much weaker than the covalent bonds within them. The four crystal structures and their properties

Inversion of configuration

The reversal of the arrangement of groups around a chiral centre when a nucleophile attacks from the side opposite the leaving group, as in an SN2 reaction. Optical isomerism

Ion

An atom or group of atoms that carries an overall charge because it has lost or gained electrons. Protons, neutrons, electrons and isotopes

Ionic bonding

The strong electrostatic attraction between oppositely charged ions, acting in all directions through a lattice. Ionic bonding and ionic lattices

Ionic equation

An equation showing only the ions and other species that take part in a reaction, with spectator ions left out. Formulae, equations and state symbols

Ionic product of water, Kw

The product of the concentrations of hydrogen ions and hydroxide ions in water or an aqueous solution, Kw = [H⁺][OH⁻]. Weak acids, Ka and pKa; Kw and strong bases

Ionic radius

The radius of an ion in a crystal, found from the distance between the centres of neighbouring ions. Ionic bonding and ionic lattices

Ion–dipole attraction

The electrostatic attraction between an ion and the oppositely charged end of a polar molecule such as water. Enthalpies of solution and hydration

Isoelectric point

The pH at which an amino acid exists as the zwitterion and has no overall charge. Amino acids, proteins, enzymes and DNA

Isoelectronic

Having the same number of electrons, as Na⁺, Ne and F⁻ do. Protons, neutrons, electrons and isotopes

Isolation method

Finding the order with respect to one reactant by putting every other reactant in large excess, so that only the isolated reactant's concentration changes during the run. Finding orders from initial rates, half-lives and graphs

Isomers

Compounds with the same molecular formula but a different arrangement of atoms. Structural isomerism and E/Z isomerism

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons. Protons, neutrons, electrons and isotopes

K

Kekulé structure

A model of benzene as a ring of six carbon atoms joined by alternating single and double bonds, cyclohexa-1,3,5-triene. Benzene: its structure and the evidence for delocalisation

Kelvin temperature

Temperature measured from absolute zero, in K, found by adding 273 to the temperature in °C. Gases and the ideal gas equation

Ketone

A compound containing a C=O group bonded to two other carbon atoms. Oxidation and elimination of alcohols

Kinetically stable

Describes a mixture that does not react at a measurable rate because its activation energy is high, even though ΔH for its reaction is negative. Enthalpy change, standard conditions and the definitions

Kp

The equilibrium constant for a reaction between gases, written with the partial pressure of each gas at equilibrium in place of its concentration. Kp: equilibria in the gas phase

L

Lattice enthalpy of dissociation

The enthalpy change when one mole of a solid ionic compound is separated into its gaseous ions under standard conditions. Lattice enthalpy and Born–Haber cycles

Lattice enthalpy of formation

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions. Lattice enthalpy and Born–Haber cycles

Le Chatelier's principle

If a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts in the direction that opposes the change. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

Leaving group

An atom or group that departs with the bonding pair of electrons when its bond breaks, such as Cl⁻ from an acyl chloride. Acylation: acyl chlorides and acid anhydrides

Ligand

A molecule or ion that forms a coordinate bond with a transition metal ion by donating a pair of electrons. Transition metal characteristics and complex formation

Ligand substitution

A reaction in which one or more ligands in a complex are replaced by different ligands. Ligands, the shapes of complexes, isomerism and ligand substitution

Limiting reagent

The reactant that is completely used up first, and so determines the maximum amount of product. Reacting masses, percentage yield and atom economy

Lone pair

A pair of outer-shell electrons on one atom that is not used in bonding. Covalent, dative covalent and metallic bonding

M

M+1 peak

A small peak one m/z unit above the molecular ion peak, from molecules that contain one carbon-13 atom. Time-of-flight mass spectrometry and relative atomic mass

M+2 peak

A peak two m/z units above the molecular ion peak, from molecules that contain chlorine-37 or bromine-81 in place of the lighter isotope. Time-of-flight mass spectrometry and relative atomic mass

m/z

The mass of an ion divided by its charge, equal to the relative mass of the ion when its charge is +1. Time-of-flight mass spectrometry and relative atomic mass

Macromolecular crystal

A giant structure of atoms joined by covalent bonds in a network that extends throughout the crystal; also called giant covalent. The four crystal structures and their properties

Markovnikov's rule

When a hydrogen halide adds to an unsymmetrical alkene, the hydrogen bonds to the carbon of the double bond that already carries more hydrogen atoms. Alkenes: structure, electrophilic addition and Markovnikov's rule

Mass concentration

The mass of solute dissolved in each 1 dm³ of solution, in g dm⁻³. Concentration, volumetric solutions and titration calculations

Mass number

The total number of protons and neutrons in the nucleus of an atom, A. Protons, neutrons, electrons and isotopes

Maxwell–Boltzmann distribution

The distribution of the energies of the molecules in a gas at a given temperature, shown as the number of molecules with each energy. The Maxwell–Boltzmann distribution, temperature and catalysts

Mean bond enthalpy

The enthalpy needed to break one mole of a given type of covalent bond in the gaseous state, averaged over a range of different compounds. Mean bond enthalpies

Mean energy

The average energy of the molecules, which lies to the right of the most probable energy because the distribution is not symmetrical. The Maxwell–Boltzmann distribution, temperature and catalysts

Mean rate

The total change in the amount or concentration of a substance divided by the time interval over which it changed. Collision theory, activation energy and the rate of reaction

Melting point of a pure solid

The single temperature, reached sharply, at which a pure solid turns to liquid; impurities lower it and widen the range. Preparing, purifying and testing an organic solid and liquid

Metal hydride

A compound of a metal with hydrogen, such as NaH or CaH₂, containing H⁻ ions, in which hydrogen has an oxidation state of −1. Oxidation states and the rules for assigning them

Metal-aqua ion

A complex ion in which water molecules are the ligands, each bonded to the central metal ion by a coordinate bond from a lone pair on its oxygen atom. Metal-aqua ions and the acidity of their solutions

Metallic bonding

The electrostatic attraction between a lattice of positive metal ions and the delocalised electrons around them. Covalent, dative covalent and metallic bonding

Mobile phase

The liquid or gas that moves through or over the stationary phase and carries the components with it. Chromatography: TLC, column and gas

Molar gas volume

The volume occupied by one mole of any gas at a stated temperature and pressure, 24.0 dm³ mol⁻¹ at room temperature and pressure. Gases and the ideal gas equation

Molar mass

The mass of one mole of a substance, in g mol⁻¹, numerically equal to its relative formula mass. The mole, the Avogadro constant and molar mass

Mole

The amount of substance that contains 6.022 × 10²³ particles, the same number as there are atoms in 12 g of carbon-12. The mole, the Avogadro constant and molar mass

Mole fraction

The amount in moles of one gas in a mixture divided by the total amount in moles of all the gases. Kp: equilibria in the gas phase

Molecular crystal

A regular arrangement of simple molecules held in place by intermolecular forces, with covalent bonds only within each molecule. The four crystal structures and their properties

Molecular formula

The actual number of atoms of each element in one molecule of a compound. Empirical and molecular formulae

Molecular ion

The ion formed when a whole molecule loses one electron, M⁺, whose m/z gives the relative molecular mass. Time-of-flight mass spectrometry and relative atomic mass

Monodentate ligand

A ligand that forms one coordinate bond to a metal ion, such as water, ammonia or the chloride ion. Transition metal characteristics and complex formation

Monomer

A small molecule that joins with many others of the same kind to form a polymer. Addition polymers

Monoprotic acid

An acid in which each molecule can donate one proton, such as HCl or CH₃COOH. Weak acids, Ka and pKa; Kw and strong bases

Most probable energy

The energy at the peak of the Maxwell–Boltzmann distribution, the energy possessed by the greatest number of molecules. The Maxwell–Boltzmann distribution, temperature and catalysts

Multidentate ligand

A ligand that forms more than two coordinate bonds to the same metal ion, such as EDTA⁴⁻, which forms six. Transition metal characteristics and complex formation

N

n + 1 rule

A proton NMR peak is split into n + 1 lines, where n is the number of hydrogens on the adjacent carbon atoms. Putting the analysis together: structure from spectra

N-substituted amide

An amide in which one or both hydrogens on the nitrogen are replaced by alkyl groups, formed from a primary or secondary amine. Acylation: acyl chlorides and acid anhydrides

Nernst equation

E = E° + (0.059/z) log₁₀([oxidised form] / [reduced form]) at 298 K, giving the electrode potential of a half-cell at concentrations that are not standard. The electrochemical series and predicting feasibility

Neutral

Describes a solution in which the concentration of hydrogen ions equals the concentration of hydroxide ions. Weak acids, Ka and pKa; Kw and strong bases

Neutralisation

The reaction of an acid with a base to form a salt and water. The Period 3 oxides: structure, bonding and acid–base character

Neutron

A particle in the nucleus with a relative mass of 1 and no charge. Protons, neutrons, electrons and isotopes

Nitrile

A compound containing the C≡N group, named from the whole carbon chain including the carbon of the C≡N. Halogenoalkanes: nucleophilic substitution and rates of hydrolysis

Nitronium ion

The electrophile in nitration, NO₂⁺, formed when concentrated sulfuric acid protonates nitric acid, which then loses water. Electrophilic substitution: nitration and Friedel–Crafts acylation

Non-rechargeable cell

A cell in which the reaction cannot be reversed, so the cell is discarded when its reactants are used up. Commercial cells, rechargeable cells and fuel cells

nth ionisation energy

The energy required to remove one electron from each ion in one mole of gaseous (n − 1)+ ions to form one mole of gaseous n+ ions. Ionisation energies and the evidence for subshells

Nuclear charge

The total positive charge of the nucleus, equal to the number of protons in it. Ionisation energies and the evidence for subshells

Nuclear spin

The property of some nuclei, including every nucleus with an odd number of nucleons such as ¹H or ¹³C, that lets them align with or against an applied magnetic field. Proton and carbon-13 NMR spectroscopy

Nucleon

A proton or a neutron: either of the two particles found in the nucleus. Protons, neutrons, electrons and isotopes

Nucleophile

An electron pair donor, which is attracted to an electron-deficient atom such as a δ+ carbon. Reaction types, curly arrows and how a mechanism is written

Nucleophilic addition

A reaction in which a nucleophile attacks the δ+ carbon of a C=O bond and two molecules join to give one product. Aldehydes and ketones: nucleophilic addition and the tests that tell them apart

Nucleophilic addition–elimination

A mechanism in which a nucleophile adds to the carbonyl carbon of an acyl compound and a small molecule such as HCl is then eliminated, reforming the C=O bond. Acylation: acyl chlorides and acid anhydrides

Nucleophilic substitution

A reaction in which a nucleophile attacks an electron-deficient carbon and replaces an atom or group attached to it. Halogenoalkanes: nucleophilic substitution and rates of hydrolysis

Nucleotide

The monomer of DNA, made of a phosphate group, the sugar 2-deoxyribose and one of four bases. Amino acids, proteins, enzymes and DNA

O

Octahedral complex

A complex with six coordinate bonds pointing to the corners of an octahedron, with bond angles of 90°. Ligands, the shapes of complexes, isomerism and ligand substitution

Open system

A system that substances can enter or leave, so a reversible reaction in it cannot reach equilibrium. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

Optical isomerism

Stereoisomerism that occurs when a molecule contains a chiral centre, giving two enantiomers. Optical isomerism

Optical isomers

Stereoisomers that are non-superimposable mirror images of each other. Ligands, the shapes of complexes, isomerism and ligand substitution

Optically active

Able to rotate the plane of plane-polarised light; the two enantiomers rotate it by equal amounts in opposite directions. Optical isomerism

Orbital

A region of space around the nucleus where there is a high probability of finding an electron; it holds up to two electrons, with opposite spins. Electron configuration: shells, subshells and orbitals

Order with respect to a reactant

The power to which the concentration of that reactant is raised in the rate equation. Rate equations, orders of reaction and the rate constant

Overall order

The sum of the powers of all the concentration terms in the rate equation. Rate equations, orders of reaction and the rate constant

Overall redox equation

The equation formed by adding an oxidation half-equation and a reduction half-equation with equal numbers of electrons, so that the electrons cancel. Half-equations and building redox equations

Overall yield

The yield of a synthesis of several steps, found by multiplying together the fractional yields of the steps. Reacting masses, percentage yield and atom economy

Oxidation

The loss of electrons by a species, which raises the oxidation state of one of its elements. Oxidation states and the rules for assigning them

Oxidation number

Another name for oxidation state: the same number, found by the same rules. Oxidation states and the rules for assigning them

Oxidation state

The charge an atom would have if all its bonds were ionic, with the bonding electrons assigned to the more electronegative atom. Oxidation states and the rules for assigning them

Oxide

A compound containing oxygen and one other element only. The Period 3 elements: reactions with oxygen and water

Oxidising agent

An electron acceptor: a species that oxidises another and is itself reduced. Oxidation states and the rules for assigning them

Oxoacid

An acid in which the acidic hydrogen atoms are bonded to oxygen atoms, as in H₂SO₄ and H₃PO₄. The Period 3 oxides: structure, bonding and acid–base character

Ozone layer

The region of the stratosphere where ozone is concentrated, which absorbs most of the ultraviolet radiation from the Sun. Elimination from halogenoalkanes, and CFCs and the ozone layer

P

p-block element

An element whose last electron, in filling order, enters a p subshell, as in groups 3 to 0. Periodicity: trends across Period 3 and the blocks of the table

Parallax error

An error in reading a scale caused by viewing it from an angle instead of with the eye level with the mark or meniscus being read. Measurement, uncertainty and significant figures in chemistry

Partial pressure

The pressure that one gas in a mixture would exert if it alone occupied the whole volume; its mole fraction multiplied by the total pressure. Kp: equilibria in the gas phase

Partition coefficient, Kpc

The ratio of the concentrations of a solute in two immiscible solvents at equilibrium, at a stated temperature. The equilibrium constant Kc

Parts per million

The mass of solute in one million units of mass of solution, or for a gas the volume in one million units of volume of air. Concentration, volumetric solutions and titration calculations

Pauli principle

No orbital holds more than two electrons, and two electrons in the same orbital must have opposite spins. Electron configuration: shells, subshells and orbitals

Peptide link

The –CONH– group that joins two amino acids, formed by a condensation reaction that eliminates water. Amino acids, proteins, enzymes and DNA

Percentage atom economy

The mass of the desired product as a percentage of the total mass of all the reactants in the balanced equation. Reaction types, curly arrows and how a mechanism is written

Percentage by mass

The mass of an element in a compound as a percentage of the mass of the compound, equal to its mass in grams in 100 g. Empirical and molecular formulae

Percentage purity

The mass of the pure substance in a sample, as found by analysis, divided by the mass of the sample, multiplied by 100. Redox titrations: manganate(VII) and thiosulfate

Percentage uncertainty

The absolute uncertainty of a measurement divided by the measured value, multiplied by 100. Measurement, uncertainty and significant figures in chemistry

Percentage yield

The actual mass of product obtained as a percentage of the theoretical maximum mass. Reacting masses, percentage yield and atom economy

Perfect ionic model

A model of an ionic lattice in which the ions are perfect spheres with evenly spread charge and the only interactions are electrostatic attractions and repulsions. Lattice enthalpy and Born–Haber cycles

Periodicity

A repeating pattern of physical or chemical properties across the periods of the periodic table. Periodicity: trends across Period 3 and the blocks of the table

Permanent dipole

A separation of charge within a molecule, caused by polar bonds, that is present at all times. Electronegativity, polarity and intermolecular forces

Permanent dipole–dipole force

An attraction between the δ+ end of one polar molecule and the δ− end of another. Electronegativity, polarity and intermolecular forces

Peroxide

A compound containing an O–O single bond, such as H₂O₂ or Na₂O₂, in which each oxygen has an oxidation state of −1. Oxidation states and the rules for assigning them

pH

The negative logarithm to base 10 of the hydrogen ion concentration in mol dm⁻³, pH = −log[H⁺]. Brønsted–Lowry acids and bases, pH and strong acids

pH curve

A graph of the pH of the solution against the volume of acid or base added during a titration. Titration curves and choosing an indicator

Phase

A part of a system with the same composition and properties throughout: a solid, a liquid, a gas or a solution. The equilibrium constant Kc

Phenol

A compound with an OH group bonded directly to a benzene ring; also the name of the simplest, C₆H₅OH. Benzene: its structure and the evidence for delocalisation

Phenoxide ion

The ion C₆H₅O⁻ formed when phenol loses a proton, its negative charge delocalised into the ring. Benzene: its structure and the evidence for delocalisation

Phenyl group

The C₆H₅– group, a benzene ring with one hydrogen atom removed, attached to the rest of a molecule. Benzene: its structure and the evidence for delocalisation

Photochemical smog

A haze formed when nitrogen oxides and unburnt hydrocarbons react in sunlight, containing ground-level ozone and PAN. Alkanes: fractional distillation, cracking and combustion

Photodegradable

Able to be broken down by light, usually ultraviolet absorbed by groups such as C=O built into the polymer chain. Addition polymers

Pi (π) bond

A covalent bond formed by sideways overlap of two parallel p orbitals, with electron density above and below the line between the nuclei. Covalent, dative covalent and metallic bonding

pKa

The negative logarithm to base 10 of the acid dissociation constant, pKa = −log Ka; the smaller the pKa, the stronger the acid. Weak acids, Ka and pKa; Kw and strong bases

pKIn

The negative logarithm of an indicator's dissociation constant; the pH at which [HIn] = [In⁻] and the colour change is half complete. Titration curves and choosing an indicator

pKw

The negative logarithm to base 10 of the ionic product of water, pKw = −log Kw, equal to 14.00 at 298 K. Weak acids, Ka and pKa; Kw and strong bases

Plane-polarised light

Light in which the vibrations occur in one plane only. Optical isomerism

Plasticiser

A small molecule added to a polymer that gets between the chains, pushes them apart and weakens the forces between them, so the polymer becomes flexible. Addition polymers

Polar bond

A covalent bond between atoms of different electronegativity, in which the bonding pair is shared unequally and the atoms carry partial charges δ+ and δ−. Electronegativity, polarity and intermolecular forces

Polar molecule

A molecule with an overall permanent dipole, because its bond dipoles do not cancel. Electronegativity, polarity and intermolecular forces

Polarisability

How easily the electron cloud of an anion is distorted; it is greater for a larger anion with a larger charge. Lattice enthalpy and Born–Haber cycles

Polarisation

The distortion of the electron cloud of an anion by a neighbouring cation, giving the bond some covalent character. Lattice enthalpy and Born–Haber cycles

Polarising power

The ability of a cation to distort the electron cloud of an anion; it is greater for a smaller cation with a larger charge, a higher charge density. Lattice enthalpy and Born–Haber cycles

Polyamide

A condensation polymer in which the monomer residues are joined by amide links, –CONH–. Condensation polymers and the disposal of polymers

Polyatomic ion

An ion made of two or more atoms covalently bonded together and carrying an overall charge, such as SO₄²⁻ or NH₄⁺. Formulae, equations and state symbols

Polyester

A condensation polymer in which the monomer residues are joined by ester links, –COO–. Condensation polymers and the disposal of polymers

Polymer

A long-chain molecule made by joining many monomer molecules together. Addition polymers

Position isomers

Structural isomers with the same carbon skeleton and functional group, the functional group being attached to a different carbon atom. Structural isomerism and E/Z isomerism

Position of equilibrium

The relative proportions of reactants and products in a mixture at equilibrium. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

Positive inductive effect

The release of electron density by an alkyl group towards the atom it is bonded to. Amines: preparation, basicity and nucleophilic reactions

Pre-exponential factor, A

The constant in the Arrhenius equation, with the same units as k, related to the frequency of collisions with the correct orientation. The Arrhenius equation and the rate-determining step

Precipitate

An insoluble solid formed when two solutions are mixed and react. Formulae, equations and state symbols

Precise relative molecular mass

The relative molecular mass calculated from the precise masses of the most abundant isotopes, given to four decimal places. Test-tube reactions for functional groups, and high-resolution mass spectrometry

Precision

Results are precise when repeated measurements are close to one another, with little spread about the mean. Measurement, uncertainty and significant figures in chemistry

Primary alcohol

An alcohol in which the carbon bonded to the OH group is bonded to one other carbon atom (or none, in methanol). Alcohols: hydration, fermentation and classification

Primary amine

An amine in which the nitrogen atom is bonded to one carbon group, RNH₂. Amines: preparation, basicity and nucleophilic reactions

Primary halogenoalkane

A halogenoalkane in which the carbon bonded to the halogen is bonded to only one other carbon atom. Halogenoalkanes: nucleophilic substitution and rates of hydrolysis

Primary standard

A pure, stable, non-hygroscopic solid that can be weighed accurately to make a standard solution directly. Concentration, volumetric solutions and titration calculations

Primary structure

The sequence of amino acids in a polypeptide chain. Amino acids, proteins, enzymes and DNA

Principal quantum number

The number n of the shell an electron occupies, 1, 2, 3 and so on outwards from the nucleus. Electron configuration: shells, subshells and orbitals

Propagation

A step in which a radical reacts with a molecule to give a product and another radical, so the number of radicals is unchanged. Free-radical substitution

Proton

A particle in the nucleus with a relative mass of 1 and a relative charge of +1. Protons, neutrons, electrons and isotopes

Q

Quaternary ammonium salt

An ionic compound containing a nitrogen atom bonded to four carbon groups and carrying a positive charge, R₄N⁺, with a negative ion. Amines: preparation, basicity and nucleophilic reactions

Quenching

Slowing a reaction almost to a stop, by cooling, dilution or removing a reactant or catalyst, so that the composition of a sample does not change while it is analysed. The equilibrium constant Kc

R

Racemic mixture

A mixture containing equal amounts of two enantiomers, which is optically inactive because their rotations cancel. Optical isomerism

Radical

A species with an unpaired electron, such as a hydrogen or chlorine atom formed by homolytic fission. Mean bond enthalpies

Random error

An error that makes repeated readings scatter unpredictably on both sides of the true value; its effect is reduced by repeating the measurement and taking a mean. Measurement, uncertainty and significant figures in chemistry

Rate constant, k

The constant of proportionality in the rate equation; it is fixed at a given temperature and increases as the temperature rises. Rate equations, orders of reaction and the rate constant

Rate equation

An expression, found by experiment, of the form rate = k[A]ᵐ[B]ⁿ, showing how the rate depends on the concentration of each substance. Rate equations, orders of reaction and the rate constant

Rate of reaction

The change in the concentration of a reactant or a product per unit time, measured in mol dm⁻³ s⁻¹. Collision theory, activation energy and the rate of reaction

Rate-determining step

The slowest step in a reaction mechanism, which limits the overall rate of the reaction. The Arrhenius equation and the rate-determining step

Reaction mechanism

The sequence of steps by which a reaction takes place, which add up to the overall equation. The Arrhenius equation and the rate-determining step

Reaction quotient, Q

The ratio with the same form as Kc but using the concentrations at any moment; it equals Kc only when the mixture is at equilibrium. The equilibrium constant Kc

Reaction quotient, Qp

The Kp expression evaluated with the partial pressures at any moment; it equals Kp only when the mixture is at equilibrium. Kp: equilibria in the gas phase

Reagent

The substance added to bring about a reaction, named in full or by a correct formula. Planning a synthetic route

Rechargeable cell

A cell in which the reaction can be reversed by applying an external voltage greater than its EMF, regenerating the reactants. Commercial cells, rechargeable cells and fuel cells

Recrystallisation

Purifying a solid by dissolving it in the minimum volume of hot solvent and letting the solution cool, so that the product crystallises and soluble impurities stay in solution. Apparatus, techniques and safety: the twelve required practicals in outline

Redox reaction

A reaction in which one species is oxidised and another is reduced, with electrons transferred from one to the other. Oxidation states and the rules for assigning them

Redox titration

A titration in which the reaction between the solution in the burette and the solution in the flask is a redox reaction. Redox titrations: manganate(VII) and thiosulfate

Reducing ability

How readily a species donates electrons; for the halide ions it increases from F⁻ to I⁻. Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water

Reducing agent

An electron donor: a species that reduces another and is itself oxidised. Oxidation states and the rules for assigning them

Reduction

The gain of electrons by a species, which lowers the oxidation state of one of its elements. Oxidation states and the rules for assigning them

Reflux

Continuous boiling and condensing of a reaction mixture, a vertical condenser returning the vapour to the flask, so it can be heated for a long time without losing volatile substances. Oxidation and elimination of alcohols

Reforming

The conversion of straight-chain alkanes into branched alkanes and cyclic hydrocarbons, over a platinum catalyst, to give fuels that burn more efficiently. Alkanes: fractional distillation, cracking and combustion

Region of electron density

A lone pair, a single bond, a double bond or a triple bond, each counted once when deciding the shape round a central atom. Shapes of molecules and ions

Relative atomic mass

The weighted mean mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12, Ar. Time-of-flight mass spectrometry and relative atomic mass

Relative formula mass

The weighted mean mass of a formula unit of a compound compared with one twelfth of the mass of an atom of carbon-12, used for ionic compounds. The mole, the Avogadro constant and molar mass

Relative isotopic mass

The mass of an atom of an isotope relative to one twelfth of the mass of an atom of carbon-12. Time-of-flight mass spectrometry and relative atomic mass

Relative molecular mass

The weighted mean mass of a molecule relative to one twelfth of the mass of an atom of carbon-12, Mr. Time-of-flight mass spectrometry and relative atomic mass

Repeating unit

The smallest group of atoms in a polymer chain that, repeated, gives the whole chain; for an addition polymer, the two carbons of the monomer's double bond with their side groups. Addition polymers

Resolution

The smallest change in the quantity being measured that gives a change in the reading on the instrument. Measurement, uncertainty and significant figures in chemistry

Retention time

The time taken for a component to pass from the injection point to the detector in gas chromatography. Chromatography: TLC, column and gas

Retrosynthesis

Planning a synthesis by working backwards from the target, asking at each stage which compound could give it in a single step. Planning a synthetic route

Reversible reaction

A reaction that can proceed in both the forward and the reverse direction under the same conditions. Dynamic equilibrium, Le Chatelier's principle and industrial compromise

Rf value

The distance moved by a component divided by the distance moved by the solvent front, both measured from the baseline. Chromatography: TLC, column and gas

Risk

The likelihood that harm from a hazard will occur in the way an activity is carried out, together with how severe that harm would be. Apparatus, techniques and safety: the twelve required practicals in outline

Room temperature and pressure

About 293 K and 101 kPa, the conditions at which the molar gas volume is taken as 24.0 dm³ mol⁻¹. Gases and the ideal gas equation

S

s-block element

An element whose last electron, in filling order, enters an s subshell, as in groups 1 and 2. Periodicity: trends across Period 3 and the blocks of the table

Salt

A compound formed when the hydrogen ions of an acid are replaced by metal ions or ammonium ions. Formulae, equations and state symbols

Salt bridge

A connection between two half-cells, usually filter paper soaked in saturated potassium nitrate solution, that allows ions to move between them to complete the circuit without the solutions mixing. Electrode potentials and the standard hydrogen electrode

Saponification

The alkaline hydrolysis of a fat or oil, giving glycerol and the salts of the fatty acids, which are soap. Carboxylic acids, esters and biodiesel

Saturated

Containing only single bonds between carbon atoms. Naming and representing organic molecules

Second electron affinity

The enthalpy change when one mole of gaseous 1− ions each gain one electron to form one mole of gaseous 2− ions. Lattice enthalpy and Born–Haber cycles

Second ionisation energy

The energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions. Ionisation energies and the evidence for subshells

Second order

A reactant is second order when the rate is proportional to the square of its concentration: doubling the concentration multiplies the rate by four. Rate equations, orders of reaction and the rate constant

Secondary alcohol

An alcohol in which the carbon bonded to the OH group is bonded to two other carbon atoms. Alcohols: hydration, fermentation and classification

Secondary amine

An amine in which the nitrogen atom is bonded to two carbon groups, R₂NH. Amines: preparation, basicity and nucleophilic reactions

Secondary structure

The coiling of a polypeptide chain into an α-helix or its folding into a β-pleated sheet, held by hydrogen bonds between peptide links. Amino acids, proteins, enzymes and DNA

Self-indicating

Describes a titration in which a reagent's own colour change shows the end point, so no separate indicator is added. Redox titrations: manganate(VII) and thiosulfate

Self-indicating titration

A titration in which a reactant changes colour at the end point, so no separate indicator is needed. Variable oxidation states: vanadium, chromium and manganate(VII) titrations

Shell

A main energy level in an atom, numbered n = 1, 2, 3 outwards from the nucleus, holding up to 2n² electrons. Electron configuration: shells, subshells and orbitals

Shielding

The repulsion of an outer electron by the electrons in inner shells, which reduces the attraction of the nucleus it feels. Ionisation energies and the evidence for subshells

Sigma (σ) bond

A covalent bond formed by end-on overlap of two orbitals, with the shared pair lying directly between the two nuclei. Covalent, dative covalent and metallic bonding

Skeletal formula

A simplified formula showing the carbon skeleton as lines, with hydrogen atoms on carbon removed and functional groups shown. Naming and representing organic molecules

SN1

Nucleophilic substitution in two steps, in which the carbon–halogen bond breaks heterolytically in the slow first step to give a carbocation, so only the halogenoalkane is in the rate equation. The Arrhenius equation and the rate-determining step

SN2

Nucleophilic substitution in one step, in which the nucleophile and the halogenoalkane are both in the rate-determining step, so the reaction is second order. The Arrhenius equation and the rate-determining step

Sodium chlorate(I)

NaClO, the bleaching and disinfecting compound formed when chlorine reacts with cold dilute sodium hydroxide. Halide ions, the silver nitrate test, concentrated sulfuric acid, and chlorine in water

Solubility

The amount of a solute that dissolves in a stated volume of solvent to give a saturated solution at a stated temperature, here in mol dm⁻³. Buffer solutions and buffer calculations

Solubility product, Ksp

The product of the concentrations of the ions in a saturated solution of a sparingly soluble salt, each raised to the power of its coefficient in the equation. Buffer solutions and buffer calculations

Solvation

The surrounding of a dissolved ion or molecule by solvent molecules attracted to it; hydration is solvation by water. Enthalpies of solution and hydration

Solvent front

The furthest point reached by the solvent on a chromatogram, marked as soon as the plate is removed from the tank. Chromatography: TLC, column and gas

Sparingly soluble

Dissolving only to a very small extent, so that most of the solid remains undissolved, as magnesium hydroxide does in water. Group 2 compounds: solubility, uses and the sulfate test

Specific heat capacity, c

The energy needed to raise the temperature of 1 g of a substance by 1 K. Measuring an enthalpy change: calorimetry

Spectator ion

An ion present in the reaction mixture that is unchanged by the reaction and appears identically on both sides of the equation. Formulae, equations and state symbols

Spin-pair repulsion

The repulsion between two electrons in the same orbital, which makes one of them easier to remove than an unpaired electron. Ionisation energies and the evidence for subshells

Spin-spin splitting

The splitting of an NMR peak into lines by the hydrogens on adjacent carbon atoms, n neighbouring hydrogens giving n + 1 lines. Proton and carbon-13 NMR spectroscopy

Square planar complex

A complex with four coordinate bonds pointing to the corners of a square round the metal, with bond angles of 90°. Ligands, the shapes of complexes, isomerism and ligand substitution

Stability constant

The equilibrium constant for the formation of a complex ion from the aqua ion in aqueous solution; the larger the value, the more stable the complex. Ligands, the shapes of complexes, isomerism and ligand substitution

Standard cell potential

The EMF of a cell in which both half-cells are under standard conditions, E°cell = E°right − E°left. Electrode potentials and the standard hydrogen electrode

Standard conditions

A pressure of 100 kPa and a stated temperature, usually 298 K, with any solutions at 1.00 mol dm⁻³. Enthalpy change, standard conditions and the definitions

Standard electrode potential

The EMF of a cell made from the half-cell and the standard hydrogen electrode, measured under standard conditions. Electrode potentials and the standard hydrogen electrode

Standard enthalpy of combustion, ΔcH°

The enthalpy change when one mole of a substance is burned completely in oxygen, with all reactants and products in their standard states under standard conditions. Enthalpy change, standard conditions and the definitions

Standard enthalpy of formation, ΔfH°

The enthalpy change when one mole of a compound is formed from its constituent elements, with all reactants and products in their standard states under standard conditions. Enthalpy change, standard conditions and the definitions

Standard enthalpy of neutralisation, ΔneutH°

The enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce one mole of water. Enthalpy change, standard conditions and the definitions

Standard enthalpy of reaction, ΔrH°

The enthalpy change when the amounts of reactants shown in the equation react under standard conditions, with all substances in their standard states. Enthalpy change, standard conditions and the definitions

Standard entropy

The entropy of one mole of a substance in its standard state at 100 kPa and a stated temperature, usually 298 K. Entropy and Gibbs free-energy change

Standard hydrogen electrode

A half-cell in which hydrogen gas at 100 kPa is bubbled over a platinum electrode in a solution of H⁺ ions at 1.00 mol dm⁻³ at 298 K; its electrode potential is defined as 0.00 V. Electrode potentials and the standard hydrogen electrode

Standard solution

A solution whose concentration is known accurately, made by dissolving a known mass of solute and making it up to a known volume. Apparatus, techniques and safety: the twelve required practicals in outline

Standard state

The physical state of a substance at 100 kPa and 298 K, such as H₂O(l), O₂(g) and C(graphite). Enthalpy change, standard conditions and the definitions

State symbol

A symbol after a formula giving its physical state: (s) solid, (l) liquid, (g) gas or (aq) dissolved in water. Formulae, equations and state symbols

Stationary phase

The phase that does not move, on or in which the components of a mixture are held, such as silica on a TLC plate. Chromatography: TLC, column and gas

Steam distillation

Distilling an organic liquid that is immiscible with water together with steam, so that it distils below 100 °C, well below its own boiling point. Preparing, purifying and testing an organic solid and liquid

Stereoisomers

Compounds with the same structural formula but a different arrangement of their atoms in space. Ligands, the shapes of complexes, isomerism and ligand substitution

Stereospecific active site

An active site that can bond to only one enantiomeric form of a substrate or drug, because the site itself is chiral. Amino acids, proteins, enzymes and DNA

Strong acid

An acid that is completely dissociated into ions in aqueous solution. Measuring an enthalpy change: calorimetry

Strong base

A base that dissociates completely into ions in aqueous solution, such as sodium hydroxide. Weak acids, Ka and pKa; Kw and strong bases

Structural formula

The minimal detail that shows the arrangement of atoms in a molecule, group by group, such as CH₃CH₂OH. Empirical and molecular formulae

Structural isomers

Compounds with the same molecular formula but a different structural formula. Empirical and molecular formulae

Sublimation

The change of state from solid directly to gas, without passing through the liquid. The four crystal structures and their properties

Subshell

A division of a shell made of orbitals of equal energy, labelled s, p, d or f. Electron configuration: shells, subshells and orbitals

Substitution

A reaction in which an atom or group of atoms in a molecule is replaced by a different atom or group. Reaction types, curly arrows and how a mechanism is written

Successful collision

A collision between particles that have at least the activation energy and are in the correct orientation, so that it leads to a reaction. Collision theory, activation energy and the rate of reaction

Successive ionisation energies

The energies needed to remove the electrons of an atom one at a time, first, second, third and so on. Ionisation energies and the evidence for subshells

Support medium

An unreactive material with a large surface area over which a thin layer of catalyst is spread, to increase the surface area for each gram of catalyst. Catalysis: heterogeneous, homogeneous and autocatalytic

Surface tension

The energy needed to increase the surface area of a liquid, which arises because molecules at the surface are pulled inwards by the molecules below and beside them. Electronegativity, polarity and intermolecular forces

Synthetic route

A sequence of reactions that converts a starting material into a target compound, with the reagents and conditions for each step. Planning a synthetic route

Systematic error

An error that shifts every reading from the true value in the same direction by a similar amount; repeating the measurement neither reveals it nor reduces it. Measurement, uncertainty and significant figures in chemistry

T

Tangent

A straight line that touches a curve at one point without crossing it there; its gradient is the rate at that moment. Collision theory, activation energy and the rate of reaction

Termination

A step in which two radicals combine to form a molecule, removing both radicals from the chain. Free-radical substitution

Tertiary alcohol

An alcohol in which the carbon bonded to the OH group is bonded to three other carbon atoms. Alcohols: hydration, fermentation and classification

Tertiary amine

An amine in which the nitrogen atom is bonded to three carbon groups, R₃N. Amines: preparation, basicity and nucleophilic reactions

Tertiary structure

The folding of the whole polypeptide chain into a three-dimensional shape, held by hydrogen bonds, ionic bonds and disulfide bridges between side chains. Amino acids, proteins, enzymes and DNA

Tetrahedral complex

A complex with four coordinate bonds pointing to the corners of a tetrahedron, with bond angles of 109.5°. Ligands, the shapes of complexes, isomerism and ligand substitution

Theoretical yield

The mass of product that would form if all of the limiting reagent reacted as the equation shows. Reacting masses, percentage yield and atom economy

Thermal decomposition

The breaking down of a compound into simpler substances by heating it. Hess's law and enthalpy cycles

Thermal stability

The resistance of a compound to decomposition when it is heated. Group 7: trends down the group and displacement reactions

Thermodynamically stable

Describes a substance or mixture for which the possible reaction has a positive ΔG, so the reaction is not feasible under the conditions stated. Entropy and Gibbs free-energy change

Titre

The volume of solution added from the burette to reach the end point. Concentration, volumetric solutions and titration calculations

TMS

Tetramethylsilane, Si(CH₃)₄, the reference compound whose single peak is defined as δ = 0. Proton and carbon-13 NMR spectroscopy

Tollens' reagent

An aqueous solution of [Ag(NH₃)₂]⁺ that is reduced to silver by an aldehyde, giving a silver mirror, but does not react with a ketone. Test-tube reactions for functional groups, and high-resolution mass spectrometry

Total entropy change

The sum of the entropy changes of the system and of the surroundings; a change takes place naturally when it is positive. Entropy and Gibbs free-energy change

Total pressure

The pressure of a gas mixture, equal to the sum of the partial pressures of all the gases in it. Kp: equilibria in the gas phase

Transesterification

The exchange of the alcohol part of an ester for another alcohol, as when methanol replaces glycerol in making biodiesel. Carboxylic acids, esters and biodiesel

Transition metal

An element that forms at least one stable ion with an incomplete d subshell. Transition metal characteristics and complex formation

Transition state

The highest-energy arrangement of atoms on the route from reactants to products, in which bonds are partly broken and partly formed. Collision theory, activation energy and the rate of reaction

Transmittance

The percentage of the incident infrared radiation at a given wavenumber that passes through the sample. Infrared spectroscopy

Transmitted light

The light that passes through a sample without being absorbed; its colour is the colour the sample appears. Colour in transition metal complexes and colorimetry

Tribasic acid

An acid of which each molecule can donate three protons, such as phosphoric(V) acid, H₃PO₄. Brønsted–Lowry acids and bases, pH and strong acids

Triglyceride

An ester formed from propane-1,2,3-triol (glycerol) and three fatty acid molecules. Carboxylic acids, esters and biodiesel

Triiodomethane test

Warming with iodine and aqueous sodium hydroxide; a pale yellow precipitate of CHI₃ shows a CH₃CO group or a CH₃CH(OH) group. Test-tube reactions for functional groups, and high-resolution mass spectrometry

U

Uncertainty

The interval within which the true value of a measurement can be expected to lie, written as ± a value in the unit of the measurement. Measurement, uncertainty and significant figures in chemistry

Unsaturated

Containing at least one carbon–carbon double or triple bond. Naming and representing organic molecules

V

Van der Waals forces

Weak attractions between molecules caused by temporary dipoles, which induce dipoles in neighbouring molecules. Periodicity: trends across Period 3 and the blocks of the table

Vertical section

The steep part of a pH curve around the equivalence point, where a very small volume added changes the pH by several units. Titration curves and choosing an indicator

W

Water of crystallisation

Water molecules held in a fixed ratio within the crystal lattice of a hydrated salt. Formulae, equations and state symbols

Wavenumber

The reciprocal of the wavelength, 1/λ, measured in cm⁻¹; it is proportional to the frequency of the radiation. Infrared spectroscopy

Weak acid

An acid that is only partially dissociated into ions in aqueous solution. Measuring an enthalpy change: calorimetry

Weak base

A base that is only partially protonated in aqueous solution, such as ammonia, so [OH⁻] is much less than the concentration of the base. Weak acids, Ka and pKa; Kw and strong bases

Wedge bond

A bond drawn as a solid wedge, showing that it points out of the plane of the paper towards the viewer. Shapes of molecules and ions

Weighing by difference

Finding the mass of solid transferred by weighing its container before and after the transfer and subtracting. Apparatus, techniques and safety: the twelve required practicals in outline

Z

Zero order

A reactant is zero order when changing its concentration has no effect on the rate; it does not appear in the rate equation. Rate equations, orders of reaction and the rate constant

Zwitterion

An ion with both a positive and a negative charge and no overall charge, as in ⁺H₃NCH(R)COO⁻. Amino acids, proteins, enzymes and DNA

Π

π bond

A bond formed by the sideways overlap of p orbitals, with its electron density above and below the line between the two nuclei. Alkenes: structure, electrophilic addition and Markovnikov's rule