What is Electrolysis?
Electrolysis is the non-spontaneous decomposition of a compound using electrical energy. At the anode (positive), oxidation occurs; at the cathode (negative), reduction occurs — driven entirely by the external power source.
Electrolysis splits a compound into elements or ions using an electric current. An external power source (not the cell) drives the non-spontaneous reaction, making products that wouldn't form naturally.
- 1↓Power source connectedExternal voltage E > |E°cell| (non-spontaneous)
- 2↓Anode (positive)2H₂O → O₂ + 4H⁺ + 4e⁻ (oxidation)
- 3↓Cathode (negative)4H⁺ + 4e⁻ → 2H₂ (reduction)
- 4Products formO₂ gas (anode) + H₂ gas (cathode) + aqueous H⁺
Step-by-step worked examples
Electrolyse molten NaCl. Write anode and cathode reactions.
Anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻ Cathode (reduction): Na⁺ + e⁻ → Na Overall: 2NaCl → 2Na + Cl₂
Calculate charge (in Coulombs) needed to deposit 1 mol of Cu from CuSO₄ by electrolysis.
Cu²⁺ + 2e⁻ → Cu (2 electrons per Cu) Q = n × F = 2 mol e⁻ × 96485 C/mol = 192970 C
If 5 A of current flows for 1 hour, how many moles of electrons transferred?
Q = I × t = 5 A × 3600 s = 18000 C n(e⁻) = Q / F = 18000 / 96485 = 0.187 mol
Flashcards
Quick quiz
Q1.In the electrolysis of water, which gas forms at the anode?
Q2.To deposit 2 mol of Cu (Cu²⁺ + 2e⁻ → Cu), how many moles of e⁻?
Q3.If I = 10 A for 1 hour, what is the charge Q in Coulombs?
Q4.Electrolysis is spontaneous or non-spontaneous?
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Common mistakes
Electrolysis is spontaneous like a galvanic cell. — Correct: Electrolysis is non-spontaneous — it requires external energy.
The cathode is positive. — Correct: The cathode is negative (connected to −ve terminal of power source).
Fewer electrons needed for higher charged ions. — Correct: Higher charge means MORE electrons per ion (e.g., Al³⁺ needs 3 e⁻).
Electrolysis only makes new compounds. — Correct: It splits compounds into elements (e.g., H₂O → H₂ + O₂).
FAQ
What is the definition of electrolysis?
The process of splitting a compound using an external electrical current to drive a non-spontaneous redox reaction.
What are the products of water electrolysis?
H₂ gas at the cathode and O₂ gas at the anode in a 2:1 ratio.
How many moles of electrons are needed per mole of metal deposited?
It equals the charge of the ion: Cu²⁺ needs 2 mol e⁻, Al³⁺ needs 3 mol e⁻.
What is Faraday's constant in electrolysis?
96485 C/mol — the charge of one mole of electrons.