A-LevelChemistryElectrochemistry9701/42 · May/June 2018 · Q310 Marks

Complete the table by predicting the identity of the substance liberated at each electrode during electrolysis with inert electrodes. electrolyte | substance liberated at the anode | substance liberated at the cathode ---|---|--- NaOH(aq) | | dilute CuCl2(aq) | | concentrated MgCl2(aq) | |

(a)Complete the table by predicting the identity of the substance liberated at each electrode during electrolysis with inert electrodes.[3]
(b)(i)The electrolysis of molten ZnBr₂ is a redox process. Identify the ion that is oxidised and the ion that is reduced. Use ionic half-equations to explain your answer.[3]
(b)(ii)Describe one visual observation that would be made during this electrolysis.[1]
(c)Dilute sulfuric acid is electrolysed for 50.0 minutes using inert electrodes and a current of 1.20 A. A different gas is collected above each electrode. The volumes of the two gases are measured under room conditions. Calculate the maximum volume of gas that could be collected at the cathode.[3]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[1] NaOH (aq): anode oxygen / O2, cathode hydrogen / H2
[1] dilute CuCl2 (aq): anode oxygen / O2, cathode copper / Cu
[1] conc MgCl2 (aq): anode chlorine / Cl2, cathode hydrogen / H2
[1] 2Br⁻ → Br2 + 2e⁻ or 2Br⁻ – 2e⁻ → Br2
[1] Zn²⁺ + 2e⁻ → Zn
[1] Zinc/Zn²⁺ reduced and Br⁻/bromide oxidised
[1] liquid / molten metal or orange-brown / reddish brown vapour / gas (at anode) or amount of melt / electrolyte decreases
[1] 50 × 60 × 1.2 or 3600 C (calculation of number of Coulombs)
[1] 3600 / 96 500 or 0.0373 F (calculation of number of Faradays)
[1] 0.0373 F / 2 or 0.01865 / 0.0187 mol H2 (use of stoichiometry)
[1] 0.01865 × 24 000 = 448–449 (Use of 24 000 & answer to 3sf)

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