Questions on covalent, dative covalent and metallic bonding

Question 1

Compare the bonding in solid sodium with the bonding in solid sodium chloride, and use the comparison to explain the differences in their electrical conductivity and in how they respond to a hammer blow.

Accepted Answer

Indicative content

both are giant lattices containing Na⁺ ions, held by electrostatic attraction acting in all directions through the whole structure
in sodium the Na⁺ ions are attracted to a sea of delocalised electrons, one released by each atom, which belong to no single ion
in sodium chloride each Na⁺ ion is attracted to the oppositely charged Cl⁻ ions around it, and there are no delocalised electrons at all
sodium conducts as a solid because its delocalised electrons move through the lattice and carry charge when a potential difference is applied
solid sodium chloride does not conduct because its ions are fixed in place; it conducts only when molten or dissolved, with ions carrying the charge
sodium bends because layers of ions slide while the electrons move with them, but sodium chloride shatters because a shifted layer brings like charges together

Levels

Level 3 (5–6 marks) both structures are compared point for point: the delocalised electrons of sodium against the Na⁺ and Cl⁻ ions of the salt, conductivity in the solid explained for both, and the sliding of metal layers against the shattering of the ionic lattice; every point names both substances
Level 2 (3–4 marks) metallic and ionic bonding are described and the conductivity difference explained; the hammer blow is answered without the layer argument, or some points speak of one substance only
Level 1 (1–2 marks) sodium is said to have delocalised electrons or sodium chloride to be ionic; solid sodium chloride is often said to conduct, or both solids to bend under the hammer

Question 2

Explain why magnesium has a higher melting point than sodium.

Accepted Answer

B1 magnesium forms Mg²⁺ ions, sodium forms Na⁺ ions B1 magnesium releases two delocalised electrons per atom, sodium one B1 Mg²⁺ is smaller than Na⁺ B1 so the attraction between the positive ions and the delocalised electrons is stronger in magnesium, and more energy is needed to overcome it

Question 3

Explain why metals conduct electricity and why they are malleable.

Accepted Answer

B1 a metal has delocalised electrons that are not attached to any one atom B1 the delocalised electrons move through the lattice when a potential difference is applied, carrying charge B1 metals are malleable because layers of positive ions can slide over one another B1 the delocalised electrons move with the ions, so the bonding is not broken and like charges are not forced together

Question 4

Explain, using hybridisation, why the bond angles round each carbon atom in ethene, C₂H₄, are about 120° and why the molecule is planar.

Accepted Answer

B1 each carbon mixes its 2s orbital with two 2p orbitals to form three sp² hybrid orbitals B1 the three sp² orbitals lie in a plane at 120° and form σ bonds to two hydrogen atoms and the other carbon B1 the unhybridised 2p orbital on each carbon lies at right angles to that plane, and the two overlap sideways to form a π bond B1 sideways overlap needs the two p orbitals to be parallel, so both CH₂ groups are held in the same plane

Question 5

Describe the dot-and-cross diagram for ammonia, NH₃, showing outer electrons only.

Accepted Answer

B1 three shared pairs between nitrogen and the three hydrogen atoms, each pair one dot and one cross B1 one lone pair on nitrogen, both electrons in nitrogen's symbol B1 nitrogen has eight electrons in its outer shell and each hydrogen has two

Question 6

Describe the dot-and-cross diagram for a nitrogen molecule, N₂.

Accepted Answer

B1 three shared pairs between the two nitrogen atoms, a triple bond, six electrons in the overlap B1 three of the shared electrons are dots and three are crosses, one from each atom in every pair B1 each nitrogen atom also has one lone pair, giving each atom eight outer electrons

Question 7

Explain how an ammonium ion forms from an ammonia molecule and a hydrogen ion, and why its four N–H bonds are identical.

Accepted Answer

B1 the lone pair on the nitrogen of NH₃ is donated to the H⁺, which has no electrons B1 this forms a dative covalent bond, both electrons of the shared pair coming from nitrogen B1 once formed, a dative bond is a shared pair like any other, so all four N–H bonds have the same length and strength

Question 8

Explain how two AlCl₃ molecules join to form an Al₂Cl₆ molecule.

Accepted Answer

B1 aluminium in AlCl₃ has only six outer electrons, so it can accept an electron pair B1 a chlorine atom in each AlCl₃ molecule has lone pairs B1 a lone pair on a chlorine of each molecule forms a dative bond to the aluminium of the other, so two dative bonds join the molecules and each aluminium has eight outer electrons

Question 9

Suggest how the bonding in a carbon monoxide molecule, CO, allows both atoms to have eight outer electrons, given that the molecule contains a triple bond.

Accepted Answer

B1 two of the shared pairs are ordinary covalent bonds, each with one electron from carbon and one from oxygen B1 the third shared pair is a dative bond, both electrons coming from a lone pair on oxygen, drawn as O→C B1 carbon then has six shared electrons plus one lone pair, and oxygen six shared electrons plus one lone pair, eight each

Question 10

Suggest why bronze, an alloy of copper with tin, is harder than pure copper.

Accepted Answer

B1 tin atoms are a different size from copper atoms B1 they disrupt the regular layers of ions in the metal lattice B1 so the layers cannot slide over each other as easily when a force is applied

Question 11

Suggest why phosphorus forms both PCl₃ and PCl₅, but nitrogen forms NCl₃ and no NCl₅.

Accepted Answer

B1 nitrogen is in Period 2, so its outer shell can hold at most eight electrons B1 phosphorus is in Period 3, so its outer shell has room for more than eight electrons B1 in PCl₅ phosphorus shares all five outer electrons, giving it ten; nitrogen cannot share all five

Question 12

Calculate the number of delocalised electrons in 1.00 g of magnesium. Ar of Mg = 24.3; L = 6.022 × 10²³ mol⁻¹.

Accepted Answer

M1 amount of Mg = 1.00 ÷ 24.3 = 0.04115 mol M1 each atom releases two electrons, so electrons = 2 × 0.04115 × 6.022 × 10²³ A1 4.96 × 10²² delocalised electrons, to three significant figures

Question 13

Calculate the energy needed to break every N≡N bond in 2.40 dm³ of nitrogen gas at room temperature and pressure. N≡N bond enthalpy = 945 kJ mol⁻¹; molar gas volume = 24.0 dm³ mol⁻¹.

Accepted Answer

M1 amount of N₂ = 2.40 ÷ 24.0 = 0.100 mol M1 one N≡N bond per molecule, so energy = 0.100 × 945 A1 94.5 kJ, to three significant figures

Question 14

Evaluate the claim that the dative bond in an ammonium ion is weaker than its other three N–H bonds and can be identified in the ion.

Accepted Answer

B1 the claim is wrong: the dative bond differs only in where both electrons came from when it formed B1 once formed it is a shared pair attracted to both nuclei, like the other three bonds B1 all four N–H bonds have the same length and strength and the ion is a regular tetrahedron, so no bond can be identified

Question 15

Calculate an estimate of the energy needed to break only the π bonds in 5.60 g of nitrogen gas, N₂, taking the σ part of the triple bond as equal to an N–N single bond. Bond enthalpies: N–N 163 kJ mol⁻¹, N≡N 945 kJ mol⁻¹; Mr of N₂ = 28.0.

Accepted Answer

M1 N≡N is one σ and two π bonds, so the two π bonds together are worth 945 − 163 = 782 kJ mol⁻¹ M1 amount of nitrogen = 5.60 ÷ 28.0 = 0.200 mol, so energy = 0.200 × 782; error carried forward from the π value A1 156 kJ, to three significant figures; reject 78.2 kJ, which counts only one of the two π bonds

Question 16

Suggest why a hot wire placed in hydrogen iodide produces violet fumes of iodine, but the same wire has no effect on hydrogen chloride. Bond enthalpies: H–Cl 432 kJ mol⁻¹, H–I 298 kJ mol⁻¹.

Accepted Answer

B1 an iodine atom is larger than a chlorine atom, so the H–I bond is longer B1 the shared pair is further from the iodine nucleus, so the H–I bond is weaker, 298 against 432 kJ mol⁻¹ B1 the hot wire supplies enough energy to break H–I bonds, giving hydrogen and iodine, but not enough to break H–Cl bonds

Question 17

State what is meant by a covalent bond.

Accepted Answer

B1 a shared pair of electrons between two atoms B1 the pair is attracted electrostatically to the nuclei of both atoms

Question 18

State what is meant by a dative covalent bond, and state how one is shown in a displayed formula.

Accepted Answer

B1 a shared pair of electrons in which both electrons come from the same atom B1 it is drawn as an arrow from the atom that donates the pair to the atom that accepts it

Question 19

Give the number of bonding pairs and the number of lone pairs round the central atom in H₂O and in CO₂.

Accepted Answer

B1 H₂O: two bonding pairs and two lone pairs on oxygen B1 CO₂: four bonding pairs, as two double bonds, and no lone pairs on carbon

Question 20

Explain why nitrogen gas, N₂, is very unreactive.

Accepted Answer

B1 the two atoms are joined by a triple bond, three shared pairs of electrons B1 the triple bond is very strong, 945 kJ mol⁻¹, so a large amount of energy is needed to break it before N₂ can react

Question 21

Give the number of σ bonds and the number of π bonds in a molecule of hydrogen cyanide, HCN.

Accepted Answer

B1 two σ bonds: the C–H bond and one of the three bonds in C≡N B1 two π bonds, both in the C≡N triple bond