Electrolysis of Aqueous Solutions

Electrolysis splits an ionic substance using electricity. In aqueous solutions, water is present, so water can also supply ions. This means the products may come from the solute or from water.

Charge movement

• Cathode (negative): attracts cations (+). Reduction happens (gain of electrons).
• Anode (positive): attracts anions (−). Oxidation happens (loss of electrons).
• Electrons move through the wires; ions move in the solution.

Simple predicting rules

• At the cathode: Either a metal forms or hydrogen gas forms. Metals less reactive than hydrogen (e.g. copper, silver) are deposited. For more reactive metals (e.g. sodium, potassium, magnesium), hydrogen is produced from water: $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$.
• At the anode (inert electrode): Halide ions give halogen if concentrated (Cl⁻ → chlorine, Br⁻ → bromine, I⁻ → iodine). If no halide or solution is dilute, oxygen forms from water: $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$.

Key examples and observations

1) Dilute sulfuric acid (H2SO4) with inert electrodes

• Cathode: Hydrogen gas. $2H^+ + 2e^- \rightarrow H_2$. Bubbles; “pop” with a lighted splint.
• Anode: Oxygen gas. $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$. Bubbles; relights a glowing splint.
• Gas volumes near 2:1 (H2:O2).

2) Concentrated aqueous sodium chloride (brine), inert electrodes

• Cathode: Hydrogen gas from water: $2H_2O + 2e^- \rightarrow H_2 + 2OH^-$.
• Anode: Chlorine gas: $2Cl^- \rightarrow Cl_2 + 2e^-$. Greenish gas, bleaching effect; solution becomes alkaline (OH⁻ left in solution).

3) Aqueous copper(II) sulfate (CuSO4)

• Inert electrodes: Cathode deposits copper: $Cu^{2+} + 2e^- \rightarrow Cu$ (pink-brown solid). Anode gives oxygen: $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$. Blue solution may get paler.
• Copper electrodes: Cathode gains copper (electroplating). Anode dissolves: $Cu \rightarrow Cu^{2+} + 2e^-$. Blue color stays roughly constant.