The electrolysis of sodium chloride solution
3 marks · EasyA student electrolyses sodium chloride solution and is surprised that hydrogen, not sodium, is produced at the cathode.
Name the two positive ions present in the solution that could be discharged at the cathode.
State which of these ions is discharged and forms a gas at the cathode.
Explain why sodium ions are not discharged at the cathode.
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Show answer
(a) Sodium ions (Na⁺) and hydrogen ions (H⁺) · must show Na⁺, H⁺
(b) Hydrogen ions (H⁺) · must show H⁺
(c) Sodium is more reactive than hydrogen so it is harder to discharge sodium ions than hydrogen ions. · must show sodium more reactive, hydrogen less reactive, easier to discharge H⁺
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Marking points
B1 (a) · 1 mark · Mentions both sodium (Na⁺) and hydrogen (H⁺) ions.
B1 (b) · 1 mark · Correctly identifies that hydrogen ions are discharged at the cathode.
B1 (c) · 1 mark · States sodium is more reactive than hydrogen so sodium ions are harder to discharge.
Worked solution
a) The two positive ions in sodium chloride solution are sodium ions (Na⁺) and hydrogen ions (H⁺). b) Hydrogen ions (H⁺) are discharged at the cathode to form hydrogen gas. c) Sodium is more reactive than hydrogen, so it is much harder to discharge sodium ions at the cathode than hydrogen ions.