The electrolysis of some other solutions using inert electrodes
4 marks · NormalA student electrolyses aqueous magnesium sulfate using inert electrodes.
State the gas formed at the cathode.
Explain why this product is formed at the cathode, using the reactivity series.
Name the ion that is discharged at the anode.
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) Hydrogen
(b) Magnesium is higher than hydrogen in the reactivity series, so hydrogen ions from water are discharged in preference, producing hydrogen gas. · must show Magnesium reactivity, Hydrogen preferentially discharged
(c) Hydroxide ions (OH⁻) · must show Identifies OH⁻
Common mistakes: Saying magnesium is produced at the cathode
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Marking points
B1 (a) · 1 mark · Gives correct gas at cathode (hydrogen).
M1 (b) · 1 mark · States magnesium is above hydrogen in the reactivity series.
A1 (b) · 1 mark · Concludes that hydrogen ions are discharged instead at the cathode.
B2 (c) · 1 mark · Correct ion at anode: hydroxide (OH⁻).
Worked solution
a) The gas formed at the cathode is hydrogen.
b) Magnesium is above hydrogen in the reactivity series, so magnesium ions are not discharged. Instead, hydrogen ions from water are reduced to hydrogen gas.
c) The ion discharged at the anode is the hydroxide ion (OH⁻).