The electrolysis of molten compounds

5 marks · Hard

Molten iron(III) bromide ( ) is electrolysed using inert electrodes.

(a)

State the formulae of the ions present in molten .

[1 mark]

(b)

Identify the product formed at the cathode.

[1 mark]

(c)

Write the half-equation for the reaction at the cathode.

[1 mark]

(d)

Write the half-equation for the reaction at the anode.

[1 mark]

(e)

Explain why only the ions in are discharged during the electrolysis of its molten form.

[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) ,

(b) Iron (Fe)

(c)

(d)

(e) Because only and ions are present when is molten; there is no water or other ions.

Common mistakes: Wrong charges on ions; Confusing oxidation/reduction; Incorrect electron numbers

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Marking points

B1 (a) · 1 mark · States correct formulae of ions: , .

B1 (b) · 1 mark · Identifies iron/Fe as cathode product.

M1 (c) · 1 mark · Writes correct half-equation for : .

M1 (d) · 1 mark · Writes correct half-equation for : .

B1 (e) · 1 mark · Explains only and ions are present since it is molten; no other ions.

Worked solution

a) Ions present: and .

b) At the cathode, iron is formed ().

c) Half-equation for cathode:

d) Half-equation for anode:

e) Only and ions are present because iron(III) bromide is molten; there are no other ions to be discharged.