Electrolysis

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FreeScienceLessons · AQA ChemistryElectrolysis

Cognito · AQA ChemistryElectrolysis of solutions

Key facts

Key definitions

Electrolysis — using electricity to break down an ionic compound.
Electrolyte — the ionic liquid (molten or dissolved) that conducts electricity.
Cathode (–) — negative electrode. Cations (+) move here. Reduction (gain electrons).
Anode (+) — positive electrode. Anions (–) move here. Oxidation (lose electrons).
MNEMONIC: OILRIG — Oxidation Is Loss, Reduction Is Gain (of electrons).
OA CA — Oxidation at Anode, Cathode is where reduction hAppens.

Products of electrolysis

Molten ionic compounds: always get the two elements. E.g. molten NaCl → Na at cathode, Cl₂ at anode.
Aqueous solutions: water also present, so competition occurs.
At cathode: metal deposited if it is below hydrogen in reactivity series; otherwise H₂ gas.
At anode: Cl₂ if high concentration of Cl⁻ ions; otherwise O₂.
Dilute aqueous NaCl → H₂ at cathode, O₂ at anode.
Concentrated aqueous NaCl (brine) → H₂ at cathode, Cl₂ at anode, NaOH in solution.

Half equations — how to write them

At cathode (reduction): Cu²⁺ + 2e⁻ → Cu or 2H⁺ + 2e⁻ → H₂
At anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻ or 4OH⁻ → 2H₂O + O₂ + 4e⁻
Check: charges must balance on both sides.
Electrolysis of brine half equations: Cathode: 2H⁺ + 2e⁻ → H₂. Anode: 2Cl⁻ → Cl₂ + 2e⁻
Uses of electrolysis products of brine: H₂ → fuel/margarine production. Cl₂ → disinfectants/PVC. NaOH → soap/paper.

Exam questions — 4 questions · 12 marks

1 markCathode productAQA 8462 P1 style

Copper sulfate solution is electrolysed using copper electrodes. What is produced at the cathode?

A Oxygen gas

B Copper metal

C Sulfur dioxide

D Hydrogen gas

1 markElectrolysis of brineAQA 8462 P1 style

What are the three useful products of the electrolysis of concentrated sodium chloride solution (brine)?

A Sodium metal, chlorine gas, water

B Hydrogen gas, chlorine gas, sodium hydroxide solution

C Hydrogen gas, oxygen gas, sodium chloride

D Sodium metal, oxygen gas, hydrochloric acid

4 marksHalf equations for brineAQA 8462 P1 style

Write the half equations for the reactions at the cathode and anode during the electrolysis of concentrated brine. State what is produced at each electrode. (4 marks)

Hint: OILRIG: cathode = reduction = gain electrons. Anode = oxidation = lose electrons. H⁺ ions at cathode, Cl⁻ ions at anode (high concentration).

+40 XP

4 marksExplain electrode productsAQA 8462 P1 style

Dilute sodium chloride solution is electrolysed. Explain why the products at each electrode are different from those obtained from concentrated sodium chloride solution. (4 marks)

Hint: The key difference is concentration of Cl⁻ ions vs OH⁻ ions at the anode. In dilute solution, OH⁻ dominates → O₂. In concentrated, Cl⁻ dominates → Cl₂.

+40 XP

Key definitions

Products of electrolysis

Half equations — how to write them

Module complete! 🎉