The electrolysis of a concentrated solution of sodium chloride, also called brine, produces hydrogen gas at the cathode, chlorine gas at the anode, and sodium hydroxide solution as a by‑product.
The electrolytic cell used in this electrolysis is shown in Figure 16.3. The cathode used in this cell is made of platinum while the anode must be made of graphite to avoid the reaction with chlorine gas.
Ions present in solution:
Na⁺ and H₂O migrate towards cathode. Since water molecules and hydrogen ions have a greater tendency to get reduced as compared to sodium ions (Na⁺), they pick up electrons from the cathode to form hydrogen gas.
At the Cathode
2H⁺ + 2e⁻ → H₂
2H₂O + 2e⁻ → H₂ + 2OH⁻
The ionic equilibrium of water is, thus, disturbed due to the discharge of H⁺ and more water molecules are ionized to restore it.
Did You Know? Spontaneous reactions occur naturally without any energy input, while non‑spontaneous reactions require a continuous input of energy (like electrolysis).
The solution around the cathode becomes alkaline due to the formation of hydroxide ions (OH⁻).
At the Anode
2Cl⁻ → Cl₂ + 2e⁻
The solution gradually gets alkaline due to the presence of OH⁻. The volumes of hydrogen and chlorine gases obtained are equal.
KEYNOTE: Brine → H₂ (cathode) + Cl₂ (anode) + NaOH (by‑product).
TRICK: At cathode: H⁺ or H₂O reduced → H₂. At anode: Cl⁻ oxidized → Cl₂.
MEMORIZE: “Platinum cathode, Graphite anode – because Cl₂ attacks metals but not graphite.”
Order of discharge: H₂O / H⁺ > Na⁺ (so H₂ forms).
NaOH remains in solution.