The electrolysis of sodium chloride solution
5 marks · HardA student sets up an electrolytic cell using sodium chloride solution and collects the gases produced. They notice that the solution near the cathode becomes increasingly alkaline as the reaction proceeds.
Name the ion that increases in concentration around the cathode.
Write a balanced chemical equation, with state symbols, for the process at the cathode that explains this change.
Explain why the solution near the cathode becomes alkaline as electrolysis continues. Relate your answer to the ions produced and removed at the electrode.
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.
Show answer
(a) Hydroxide ion (OH⁻) · must show OH⁻
(b) 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq) · must show Balanced equation, State symbols
(c) Hydrogen ions from water are reduced and removed as hydrogen gas at the cathode. This leaves behind hydroxide ions, so their concentration increases, making the solution around the cathode alkaline. · must show Hydrogen ions are removed/discharged, Hydroxide ions remain and increase, Alkaline condition explained
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly names 'hydroxide ion' (OH⁻) as the ion increasing near the cathode
M1 (b) · 1 mark · Correct equation written for cathode process (accepts either valid cathode half-equation)
A1 (b) · 1 mark · All state symbols are correctly given for every species in the equation
M2 (c) · 1 mark · Explains that H⁺/H₂O ions are discharged/removed at the cathode
A2 (c) · 1 mark · Explains hydroxide ions remain/increase and cause alkalinity
Worked solution
(a) The ion increasing is the hydroxide ion, OH⁻.
(b) The half-equation with state symbols is:
(c) As the electrolysis proceeds, hydrogen ions (or water molecules) are discharged at the cathode and converted to hydrogen gas, removing them from the solution. Each time this happens, hydroxide ions remain in solution, so their concentration increases. Higher OH⁻ ion concentration makes the solution alkaline around the cathode.