The electrolysis of sodium chloride solution

4 marks · Normal

Here are two possible half-equations for a reaction occurring at the cathode during electrolysis of sodium chloride solution:

(a)

Name all ions present in the solution that could be reduced at the cathode.

[1 mark]

(b)

Which of the two half-equations shows the production of hydrogen gas from water? Explain your answer.

[2 marks]

(c)

Write a word equation for the cathode reaction using water as the starting material.

[1 mark]

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(a) Na+(aq), H+(aq)

(b) The second equation shows hydrogen gas production from water, because H2O is used as a reactant; hydrogen ions in the first come from water splitting, but the second makes it clear OH- is also produced.

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Marking points

B1 (a) · 1 mark · States both Na+ and H+ ions as possible at cathode.

B2 (b) · 1 mark · Identifies the second half-equation as representing hydrogen gas production from water.

B3 (b) · 1 mark · Explains H2O is a reactant and OH- is formed, making it water as the source of H2.

B4 (c) · 1 mark · Correctly writes a word equation: Water + electrons → hydrogen gas + hydroxide ions.

Worked solution

a) The ions that could move to the cathode are Na+ and H+.

b) The second half-equation shows hydrogen gas produced from water, because water is used as a reactant and hydroxide ions are produced as well.

c) Water + electrons → hydrogen gas + hydroxide ions.