Draw the essential components of this electrolytic cell and identify the products that form at each electrode.

Product formed at positive electrode (anode):

Product formed at negative electrode (cathode):

State the half-equations for the oxidation and reduction processes and deduce the overall cell reaction, including state symbols.

Oxidation half-equation:

Reduction half-equation:

Overall cell reaction:

Explain why solid sodium chloride does not conduct electricity.

Cell showing:

container, liquid, electrodes and power supply;

No labels are required, but do not award mark if incorrect labels are used (e.g. sodium chloride solution). A line must be drawn on the container to indicate the presence of a liquid. If power supply is a battery then do not penalize electrodes incorrectly assigned as + or –.

Positive electrode (anode):

chlorine (gas) / \({\text{C}}{{\text{l}}_2}{\text{(g)}}\)

and

Negative electrode (cathode):

sodium (liquid) / Na(l);

Ignore state symbols in (i) but do not award mark for Cl.

Oxidation half-equation:

\({\text{2C}}{{\text{l}}^ - } \to {\text{C}}{{\text{l}}_2} + {\text{2}}{{\text{e}}^ - }/{\text{C}}{{\text{l}}^ - } \to \frac{1}{2}{\text{C}}{{\text{l}}_2}{{\text{e}}^ - }\)

and

Reduction half-equation:

\({\text{N}}{{\text{a}}^ + } + {{\text{e}}^ - } \to {\text{Na}}/{\text{2N}}{{\text{a}}^ + } + {\text{2}}{{\text{e}}^ - } \to {\text{2Na}}\);

Allow e instead of \({e^ - }\).

Overall cell reaction:

\({\text{2NaCl(l)}} \to {\text{2Na(l)}} + {\text{C}}{{\text{l}}_2}{\text{(g)}}/{\text{NaCl(l)}} \to {\text{Na(l)}} + \frac{1}{2}{\text{C}}{{\text{l}}_2}{\text{(g)}}\);

Award [1] for oxidation and reduction half-equations.

Award [1] for overall cell reaction, including correct state symbols.

Accept Na⁺(l) + Cl^–(l) instead of NaCl(l) as a reactant.

Penalize equilibrium arrows once only.

ions not free to move when solid / ions in rigid lattice / OWTTE;

In part (a)(i) Some students mixed up electrolytic cells with voltaic cells and salt bridges were often seen. Others mixed up the products at the cathode and anode. For the anode, Cl was sometimes given instead of \({\text{C}}{{\text{l}}_{\text{2}}}\) meaning that the mark was not awarded. Also occasionally the electrolyte was incorrectly given as an aqueous solution.

In part (ii) The most common mistake here involved the incorrect set of state symbols. Very few candidates realised that sodium would be a liquid. Also there were equilibrium arrows incorrectly used in the redox equations.

In part (b) many candidates did not refer to ions in their answer and instead referred to the lack of delocalised electrons.