Paper 4 Electrochemistry Structured Practice

Five structured questions in the style of IGCSE Chemistry Paper 4, totalling 26 marks. Write your answers in full before checking the mark scheme.

Question 1 (6 marks)

Molten lead(II) bromide is electrolysed using carbon electrodes.

(a) State why lead(II) bromide must be molten for electrolysis to occur. [1]

(b) State the name of the product formed at: (i) the cathode [1] (ii) the anode [1]

(c) Write the half-equation for the reaction at: (i) the cathode [1] (ii) the anode [1]

(d) State whether the reaction at the cathode is oxidation or reduction. Explain your answer. [1]

Mark scheme

(a) When molten, the ions are free to move and carry the electrical charge / current [1]. In the solid, the ions are in fixed positions in the lattice and cannot move.

(b)(i) Lead [1] (b)(ii) Bromine [1]

(c)(i) Pb²⁺ + 2e⁻ → Pb [1] (c)(ii) 2Br⁻ → Br₂ + 2e⁻ [1]

(d) Reduction, because the lead ions gain electrons [1].

Total: 6 marks

Question 2 (5 marks)

Concentrated aqueous sodium chloride solution is electrolysed using inert (carbon) electrodes.

(a) Name the three useful products of this electrolysis. [3]

(b) Explain why sodium is not produced at the cathode during this electrolysis. [2]

Mark scheme

(a) Chlorine (at the anode) [1], hydrogen (at the cathode) [1], and sodium hydroxide (in solution) [1].

(b) Sodium is more reactive than hydrogen / sodium is above hydrogen in the reactivity series [1]. When a solution contains ions of a reactive metal, hydrogen ions (from water) are discharged in preference / the less reactive element is discharged at the cathode [1].

Total: 5 marks

Question 3 (5 marks)

A student electroplates an iron key with copper using the apparatus below: copper anode, iron key as cathode, and copper(II) sulfate solution as the electrolyte.

(a) State why the iron key is connected to the negative terminal. [1]

(b) Write the half-equation for the reaction at the cathode. [1]

(c) State what happens to the copper anode during electroplating. [1]

(d) Explain why the concentration of copper(II) sulfate solution remains approximately constant during the process. [2]

Mark scheme

(a) The key must be the cathode so that copper ions are attracted to it and deposited as copper metal [1].

(b) Cu²⁺ + 2e⁻ → Cu [1]

(c) The copper anode dissolves / decreases in mass / copper atoms are oxidised to Cu²⁺ ions [1].

(d) Copper ions are removed from solution at the cathode (deposited as copper metal) [1], but at the same time the copper anode dissolves to replace them / Cu → Cu²⁺ + 2e⁻, so the concentration stays roughly constant [1].

Total: 5 marks

Question 4 (5 marks)

Dilute sulfuric acid is electrolysed using platinum electrodes. Gases are collected at both electrodes.

(a) Name the gas produced at: (i) the cathode [1] (ii) the anode [1]

(b) State the ratio of volumes of gas collected at the cathode to the anode. [1]

(c) Write the half-equation for the reaction at the anode. [1]

(d) The overall effect of this electrolysis is the decomposition of water. Write the overall equation. [1]

Mark scheme

(a)(i) Hydrogen [1] (a)(ii) Oxygen [1]

(b) 2 : 1 (hydrogen : oxygen) [1]

(c) 4OH⁻ → 2H₂O + O₂ + 4e⁻ [1]

(d) 2H₂O → 2H₂ + O₂ [1]

Total: 5 marks

Question 5 (5 marks)

A hydrogen-oxygen fuel cell is used to generate electricity.

(a) State the overall equation for the reaction in a hydrogen-oxygen fuel cell. [1]

(b) State two advantages of using hydrogen fuel cells compared with burning fossil fuels. [2]

(c) State two disadvantages or challenges of using hydrogen fuel cells. [2]

Mark scheme

(a) 2H₂ + O₂ → 2H₂O [1]

(b) Any two from: the only product is water, so no carbon dioxide is produced / no contribution to climate change [1]; no pollutant gases such as SO₂ or NOₓ / no acid rain [1]; hydrogen is a renewable fuel if produced from water by electrolysis using renewable energy [1].

(c) Any two from: hydrogen is difficult/expensive to store safely (it is highly flammable / needs high pressure or low temperature) [1]; hydrogen is expensive to produce in large quantities [1]; there is limited infrastructure / few hydrogen refuelling stations [1]; the fuel cell itself is expensive due to platinum catalysts [1].

Total: 5 marks