Electrolysis

5 marks · Easy

This question is about balancing half-equations for the electrolysis of ionic compounds.

(a)

Write the balanced ionic half-equation (including state symbols) for the reaction at the anode during the electrolysis of molten zinc(II) chloride.

[2 marks]

(b)

State the product formed at the anode and the product formed at the cathode during the electrolysis of concentrated aqueous sodium chloride solution.

[2 marks]

(c)

Explain why less chlorine gas is collected at the anode than expected, even if the reaction has gone to completion.

[1 mark]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

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(a) 2Cl^-(l) → Cl2(g) + 2e^- · must show correct balancing, charges balanced, state symbols present

(b) Anode: chlorine gas (Cl2); cathode: hydrogen gas (H2).

Show mark scheme and worked solution

Marking points

A1 (a) · 1 mark · Atoms and state symbols correct in half-equation.

A2 (a) · 1 mark · Charges correctly balanced; 2 electrons shown on right.

A3 (b) · 1 mark · Correct anode product: chlorine gas (Cl2). Accept correct formula or name.

A4 (b) · 1 mark · Correct cathode product: hydrogen gas (H2). Accept correct formula or name.

A5 (c) · 1 mark · Explains that less chlorine is collected because it is soluble in water.

Worked solution

a) The anode reaction in molten zinc(II) chloride is the oxidation of chloride ions.

Atoms and charges are balanced, and correct state symbols are used.

b) For concentrated aqueous sodium chloride:

At the anode, chlorine gas (Cl2) is produced.

At the cathode, hydrogen gas (H2) is produced.

c) Less chlorine gas is collected than expected because chlorine is soluble in water, so some dissolves rather than being released as a gas.