JAMB - Chemistry (2025 - No. 19)

In the electrolysis of concentrated sodium chloride solution (brine) using neutral (inert) electrodes (like graphite or carbon), several ions are present: Na\(^+\), Cl\(^−\), H\(^+\), and OH\(^−\) (from the dissociation of NaCl and water, H\(_2\)O).

At the anode (positive electrode), negative ions (anions) are attracted. Both Cl\(^−\) and OH\(^-\) ions migrate to the anode, eliminating options A and B.

In the case of concentrated brine, Cl− ions are preferentially discharged over OH\(^−\) ions due to their high concentration (one of the factors considered for selective preferential discharge of ions in solutions).

The reaction at the anode is the oxidation of chloride ions to form chlorine gas (Cl\(_2\)): 2Cl\(^−\)(aq) → Cl\(_2\)(g) + 2e\(^−\)