Electrolysis

6 marks · Normal

A student carries out the electrolysis of molten calcium chloride (CaCl2) in a laboratory using inert carbon electrodes.

(a)

State what is meant by 'discharging an ion' during electrolysis.

[2 marks]

(b)

Identify the ions in molten calcium chloride and their movement during electrolysis.

[1 mark]

(c)

Write the half-equation for the discharge of chloride ions at the anode.

[1 mark]

(d)

Explain, using electron transfer, what happens to a calcium ion during electrolysis.

[2 marks]

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.

Show answer

(a) 'Discharging an ion' means the ion loses its charge at the electrode, by either gaining or losing electrons to become a neutral atom or molecule. · must show Losing charge, Gaining/losing electrons, Form neutral atom/molecule

(b) Calcium ions (Ca^2+) move to the cathode; chloride ions (Cl^-) move to the anode. · must show Ca^2+ to cathode, Cl^- to anode

(d) A calcium ion (Ca^2+) gains two electrons at the cathode to become a neutral calcium atom. This is reduction. · must show Ca^2+ gains electrons, becomes calcium atom, called reduction

Common mistakes: Describing discharge only as forming atoms without mention of charge loss; Confusing anode and cathode; Not mentioning movement of ions

Show mark scheme and worked solution

Marking points

M1 (a) · 1 mark · States that discharging an ion means it loses its charge at the electrode.

C1 (a) · 1 mark · Mentions process involves gain or loss of electrons, becoming a neutral atom or molecule.

B1 (b) · 1 mark · Identifies Ca^2+ moves to cathode and Cl^- to anode.

A1 (c) · 1 mark · Correctly writes balanced half-equation: 2Cl^- → Cl2 + 2e^-.

M1 (d) · 1 mark · States that Ca^2+ gains two electrons at the cathode.

C1 (d) · 1 mark · Explains this process is reduction, forming a calcium atom.

Worked solution

a) Discharging an ion means the ion loses its charge at the electrode, either by gaining or losing electrons and becoming a neutral atom or molecule.

b) The ions present are calcium ions (Ca^2+) and chloride ions (Cl^-). Ca^2+ move to the cathode and Cl^- to the anode.

c) The half-equation for chloride ions at the anode is:

d) At the cathode, each Ca^2+ gains two electrons to become a neutral calcium atom (Ca^2+ + 2e^- → Ca). This is reduction because the ion gains electrons.