Metallic bonding and electrolysis – IGCSE Chemistry exam questions with mark scheme

1Grade 6–7Triple only4 marks

(a) Describe the structure and bonding in a metal. [2]

(b) Explain why metals are malleable. [2]

(a) (giant) lattice of positive ions in a sea of delocalised electrons (allow metal ions / cations for positive ions; allow 'positive ions surrounded by delocalised electrons' – 'lattice' not required; reject atoms / protons / nuclei for positive ions)(1)
(a) (strong) electrostatic attraction between the positive ions and the delocalised electrons (allow strong forces of attraction between the ions and the electrons; reject attraction between ions / between nuclei and electrons in atoms / covalent or ionic bonds)(1)
(b) (regular) layers of (positive) ions (allow atoms)(1)
(b) (layers) can slide over each other (when a force is applied) (ignore extra correct detail, e.g. delocalised electrons still hold the ions together so the bonding is not broken)(1)
Answer: (a) lattice of positive ions in delocalised electrons, held by electrostatic attraction (b) layers of ions slide
Examiner tip: Write 'positive ions', not 'atoms' or 'protons', in the description of metallic bonding.

2Grade 4–5Triple only4 marks

The diagram shows the apparatus used to electrolyse molten lead(II) bromide, PbBr₂. Both electrodes are made of carbon.

(a) Name the product formed at the positive electrode and state what you would observe there. [2]

(b) Write the half-equation for the reaction at the negative electrode. [2]

(a) bromine (accept Br₂; reject bromide)(1)
(a) brown / red-brown / orange-brown gas or vapour (accept fumes; allow orange)(1)
(b) $\text{Pb}^{2+} + 2\text{e}^- \rightarrow \text{Pb}$: formulae of all species correct (ignore state symbols)(1)
(b) correctly balanced (two electrons) (accept $\text{Pb}^{2+} \rightarrow \text{Pb} - 2\text{e}^-$)(1)
Answer: (a) bromine; brown vapour (b) Pb²⁺ + 2e⁻ → Pb
Examiner tip: Positive ions go to the negative electrode — remember 'PANIC': Positive Anode, Negative Is Cathode.

3Grade 6–7Triple only5 marks

Concentrated sodium chloride solution is electrolysed using inert electrodes.

(a) Name the products formed at the anode and at the cathode. [2]

(b) Explain why sodium is not formed at the cathode. [2]

(a) anode: chlorine (accept Cl₂)(1)
(a) cathode: hydrogen (accept H₂)(1)
(b) solution contains H⁺ ions (from water) as well as Na⁺ ions(1)
(b) sodium is more reactive than hydrogen, so hydrogen (ions) are discharged instead / H⁺ ions gain electrons more easily than Na⁺ ions(1)
(c) $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-$ (accept $2\text{Cl}^- - 2\text{e}^- \rightarrow \text{Cl}_2$; ignore state symbols)(1)
Answer: (a) Cl₂ at anode, H₂ at cathode (b) H⁺ from water discharged as Na more reactive (c) 2Cl⁻ → Cl₂ + 2e⁻
Examiner tip: Always state where the H⁺ ions come from (water) — examiners want both ions named in the comparison.

4Grade 8–9Triple only5 marks

Dilute sulfuric acid is electrolysed using inert electrodes. Hydrogen forms at the cathode and oxygen forms at the anode.

(a) Write the half-equation for the formation of oxygen at the anode. [2]

(b) The volume of hydrogen collected is twice the volume of oxygen collected. Use the half-equations to explain this. [2]

(a) $4\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^-$: all species correct (accept $\text{H}_2\text{O} \rightarrow \text{O}_2 + \text{H}^+ + \text{e}^-$ species; ignore state symbols)(1)
(a) correctly balanced (accept $2\text{H}_2\text{O} \rightarrow \text{O}_2 + 4\text{H}^+ + 4\text{e}^-$ for both marks; accept $4\text{OH}^- - 4\text{e}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O}$)(1)
(b) 4 electrons are needed to make one O₂ molecule but only 2 electrons for one H₂ molecule (accept $2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}_2$ quoted)(1)
(b) same number of electrons flow at each electrode, so 2 mol H₂ form for every 1 mol O₂ (equal moles of gas occupy equal volumes)(1)
(c) increases (because water is removed)(1)
Answer: (a) 4OH⁻ → O₂ + 2H₂O + 4e⁻ (b) 4e⁻ per O₂ but 2e⁻ per H₂, same charge passes (c) increases
Examiner tip: Link the 2 : 1 ratio to electrons — just saying 'water is H₂O' scores no marks.

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