Chemistry › Definitions › Acids and bases

Acids and bases definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

33 terms

Brønsted–Lowry acids and bases, pH and strong acids

Brønsted–Lowry acid: A species that donates a proton (an H⁺ ion) to another species.
Brønsted–Lowry base: A species that accepts a proton (an H⁺ ion) from another species.
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.
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.
Strong acid: An acid that dissociates completely into ions in aqueous solution.
Weak acid: An acid that only partially dissociates into ions in aqueous solution.
pH: The negative logarithm to base 10 of the hydrogen ion concentration in mol dm⁻³, pH = −log[H⁺].
Basicity: The number of protons (H⁺ ions) that one molecule of an acid can donate.
Tribasic acid: An acid of which each molecule can donate three protons, such as phosphoric(V) acid, H₃PO₄.

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⁻].
Neutral: Describes a solution in which the concentration of hydrogen ions equals the concentration of hydroxide ions.
Strong base: A base that dissociates completely into ions in aqueous solution, such as sodium hydroxide.
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.
Acid dissociation constant, Ka: The equilibrium constant for the dissociation of a weak acid, Ka = [H⁺][A⁻]/[HA], in mol dm⁻³.
pKa: The negative logarithm to base 10 of the acid dissociation constant, pKa = −log Ka; the smaller the pKa, the stronger the acid.
Monoprotic acid: An acid in which each molecule can donate one proton, such as HCl or CH₃COOH.
pKw: The negative logarithm to base 10 of the ionic product of water, pKw = −log Kw, equal to 14.00 at 298 K.

pH curve: A graph of the pH of the solution against the volume of acid or base added during a titration.
Equivalence point: The point in a titration at which the amount of base added exactly reacts with the amount of acid present, according to the balanced equation.
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.
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.
Indicator: A weak acid whose undissociated form and conjugate base have different colours, used to show the end point of a titration.
End point: The point in a titration at which the indicator changes colour; with a suitable indicator it coincides with the equivalence point.
pKIn: The negative logarithm of an indicator's dissociation constant; the pH at which [HIn] = [In⁻] and the colour change is half complete.
Calibration: Adjusting a pH meter against buffer solutions of known pH, so that its readings are accurate.

Buffer: A solution that maintains an approximately constant pH when small amounts of acid or base are added, or when it is diluted.
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.
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.
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.
Henderson–Hasselbalch equation: pH = pKa + log([A⁻]/[HA]), the Ka expression for a weak acid rearranged in logarithmic form.
Buffer range: The range of pH, roughly pKa ± 1, over which a buffer resists changes in pH effectively.
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.
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.