The electrolysis of sodium chloride solution
4 marks · NormalIn the electrolysis of sodium chloride solution, hydrogen gas forms at the cathode.
State the ions present in sodium chloride solution that move to the cathode.
Write a balanced half-equation to show the reaction occurring at the cathode. (You may use either half-equation involving H+ or H2O.)
Explain, in one sentence, what happens to the electrons at the cathode during this reaction.
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) Na+(aq) and H+(aq)
(b) 2H+(aq) + 2e- → H2(g) or 2H2O(l) + 2e- → H2(g) + 2OH-(aq)
(c) Electrons are gained by hydrogen ions (reduction) to form hydrogen gas.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies both Na+ and H+ (no penalty for state symbols) as moving to the cathode.
B2 (b) · 2 marks · Writes a correct balanced half-equation: either 2H+(aq) + 2e- → H2(g) or 2H2O(l) + 2e- → H2(g) + 2OH-(aq). (1 mark for correct reactants/products, 1 mark for balance/physical states).
B3 (c) · 1 mark · States electrons are gained at the cathode by hydrogen ions (or hydrogen atoms), or that reduction occurs.
Worked solution
a) The ions are sodium ions, Na+, and hydrogen ions, H+, from water.
OR
c) The electrons at the cathode are gained by hydrogen ions, producing hydrogen gas (reduction).