The electrolysis of sodium chloride solution

2 marks · Hard

Explain why sodium ions are not discharged at the cathode during the electrolysis of sodium chloride solution, whereas hydrogen is.

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Hydrogen is less reactive than sodium, so hydrogen ions are more easily reduced (gain electrons) than sodium ions. Therefore, hydrogen is discharged in preference to sodium at the cathode.

Must show: Hydrogen is less reactive than sodium, Hydrogen ions are more easily reduced than sodium ions, Thus, hydrogen is discharged

Also accept: Sodium is too reactive and remains as ions in solution; hydrogen ions are reduced first.

Common mistakes: Stating sodium is discharged; Ignoring the reactivity series

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Marking points

M1 · 1 mark · States or implies that hydrogen is less reactive than sodium

A1 · 1 mark · Correctly links easier reduction/discharge of hydrogen ions, so hydrogen is formed at cathode

Worked solution

Hydrogen ions are discharged at the cathode because hydrogen is less reactive than sodium. More reactive metals, such as sodium, have a greater tendency to remain as ions in solution and are not easily reduced. Hydrogen ions are reduced (gain electrons) more readily, so hydrogen gas is produced in preference to sodium.