Electrolysis and Electrode Reactions

Electrolysis Overview

If an electric current is passed through an electrolyte taken either in solution or in a fused (molten) state, a chemical change occurs. This phenomenon is called electrolysis. Through this process, electrical energy is directly converted into chemical energy.

Electrolysis takes place in an electrolytic cell where two electrodes are immersed in an electrolytic solution. Oxidation occurs at the anode (+) and reduction occurs at the cathode (-), matching the chemical nature found in galvanic cells.

1. Electrolysis of Pure Water

Cathode (Reduction): 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
Anode (Oxidation): 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻

2. Electrolysis of Molten NaCl

In Solution: NaCl —(Heat)→ Na⁺ + Cl⁻
Cathode (Reduction): Na⁺ + e⁻ → Na(s)
Anode (Oxidation): Cl⁻ → 1/2Cl₂(g) + e⁻

3. Electrolysis of Brine Solution (Aqueous NaCl)

Cathode (Reduction): 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
Anode (Oxidation): 2Cl⁻(aq) → Cl₂(g) + 2e⁻

Note: At the cathode, H₂ gas is evolved instead of Na metal deposition. This occurs because water has a much higher (less negative) reduction potential than Na⁺. At the anode, chlorine gas is preferentially evolved over oxygen due to overvoltage kinetic factors.

4. Electrolysis of Aqueous AgNO₃ with Active Silver Electrode

In Solution: AgNO₃ ⇌ Ag⁺ + NO₃⁻
Cathode (Reduction): Ag⁺(aq) + e⁻ → Ag(s)
Anode (Oxidation): Ag(s) → Ag⁺(aq) + e⁻

Note: Since an active silver anode is used, the silver metal of the anode itself dissolves into the solution by losing electrons instead of water undergoing oxidation.

5. Electrolysis of Aqueous AgNO₃ with Inert Pt-Electrode

In Solution: AgNO₃ ⇌ Ag⁺ + NO₃⁻
Cathode (Reduction): Ag⁺(aq) + e⁻ → Ag(s)
Anode (Oxidation): 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻

Note: Because Platinum is inert, it cannot dissolve. Instead, water undergoes oxidation at the anode to produce oxygen gas and acidic H⁺ ions.

6. Electrolysis of Dilute H₂SO₄ with Pt-Electrode

In Solution: H₂SO₄ → 2H⁺ + SO₄²⁻
Cathode (Reduction): 2H⁺(aq) + 2e⁻ → H₂(g)
Anode (Oxidation): 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻

7. Electrolysis of Aqueous CuCl₂ with Pt-Electrode

In Solution: CuCl₂ ⇌ Cu²⁺ + 2Cl⁻
Cathode (Reduction): Cu²⁺(aq) + 2e⁻ → Cu(s)
Anode (Oxidation): 2Cl⁻(aq) → Cl₂(g) + 2e⁻

8. Electrolysis of Aqueous CuSO₄ with Pt-Electrode

In Solution: CuSO₄ ⇌ Cu²⁺ + SO₄²⁻
Cathode (Reduction): Cu²⁺(aq) + 2e⁻ → Cu(s)
Anode (Oxidation): 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻