Electrolysis
7 marks · HardElectrolysis of sodium chloride solution and copper(II) sulfate solution can be represented by balanced half-equations.
Write a balanced half-equation for the reaction at the cathode during electrolysis of sodium chloride solution.
Write a balanced half-equation for the reaction at the anode during electrolysis of sodium chloride solution.
Write a balanced half-equation for the reaction at the cathode during electrolysis of copper(II) sulfate solution using inert electrodes.
Write a balanced half-equation for the reaction at the anode during electrolysis of copper(II) sulfate solution using inert electrodes.
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.
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(a) 2H2O(l) + 2e- → H2(g) + 2OH-(aq) OR 2H+(aq) + 2e- → H2(g) · must show Either accepted half-equation
(b) 2Cl-(aq) → Cl2(g) + 2e- · must show Balanced electrons
(c) Cu2+(aq) + 2e- → Cu(s) · must show Balanced charge and electrons
(d) 4OH-(aq) → 2H2O(l) + O2(g) + 4e- OR 2H2O(l) → O2(g) + 4H+(aq) + 4e- · must show Either accepted half-equation (OH- or H2O version, with balanced electrons)
Also accept: State symbols both present and omitted
Common mistakes: Incorrect balancing of electrons; Writing molecules instead of ions where inappropriate; Missing state symbols
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct species present (H2O/H+ as reactant, H2/OH- as products)
A1 (a) · 1 mark · Balanced equation and electrons for cathode (NaCl solution)
B1 (b) · 1 mark · Correct half-equation for anode (2Cl− → Cl2 + 2e−), balanced
M2 (c) · 1 mark · Correct species (Cu2+, e–, Cu)
A2 (c) · 1 mark · Fully balanced half-equation at cathode for CuSO4
M3 (d) · 1 mark · Correct species for anode (OH– or H2O as reactant, O2 and electrons as products)
A3 (d) · 1 mark · Fully balanced half-equation at anode for CuSO4 (either accepted version)
Worked solution
(a) One accepted half-equation for the cathode (NaCl solution) is:
Alternatively: 2H⁺(aq) + 2e⁻ → H₂(g)
(b) The half-equation at the anode:
(c) The cathode half-equation for CuSO₄ solution:
(d) The anode half-equation for CuSO₄ solution (either is accepted):
or: