Answer parts (a)–(c) in a writing booklet - HSC - SSCE Chemistry - Question 32 - 2010 - Paper 1

The electrolysis of molten sodium chloride involves the melting of solid NaCl to produce a conductive liquid, allowing the following half reactions:

• At the cathode (reduction): $\text{Na}^+ + e^- \rightarrow \text{Na (s)}$

• At the anode (oxidation): $\text{2Cl}^- \rightarrow \text{Cl}_2 (g) + 2e^-$

The overall reaction for molten NaCl is:

$\text{2NaCl (l) \rightarrow 2Na (s) + Cl}_2\text{(g)}$

In contrast, the electrolysis of aqueous sodium chloride involves water competing with chloride ions. The half reactions are:

• At the cathode: \text{2H}_2\text{O (l) + 2e^- \rightarrow H}_2\text{(g) + 2OH}^-

• At the anode: $\text{2Cl}^- \rightarrow \text{Cl}_2 (g) + 2e^-$

The overall reaction for aqueous NaCl is:

$\text{2NaCl (aq) + 2H}_2\text{O (l) \rightarrow Cl}_2\text{(g) + H}_2\text{(g) + 2NaOH (aq)}$

The key differences are that:

• In molten NaCl, sodium is produced as a metal, while in aqueous NaCl, sodium hydroxide is produced.

At time B, a new equilibrium position is established due to changes in the concentrations of the reactants and products. This can occur due to factors such as:

1. Shift in Concentrations: If additional reactants or products are added or removed from the system, the equilibrium position adjusts to counteract this change, according to Le Châtelier's Principle.
2. Temperature Changes: If the temperature of the system changes, it can favor either the forward or reverse reaction, thus altering the concentration of species and moving the equilibrium position.
3. Pressure Changes: In gaseous systems, changing pressure can shift the equilibrium position if there is a difference in the number of moles of gases on either side of the reaction.

In this case, it’s important to observe the concentration changes from time A to time B to fully understand the shift in equilibrium.