2021 O Level Chem P1 Q13 — Electrolysis of Aqueous Solutions Solution
Question
A concentrated aqueous solution containing each of the following ions, Cl⁻, SO₄²⁻, Na⁺ and Cu²⁺, is electrolysed using platinum electrodes. Which row shows the ions discharged first at the two electrodes?
Correct answer: A
Video Solution
Explanation
Answer: A. At the negative electrode (cathode) cations are discharged, and the less reactive metal ion is preferred. Cu²⁺ is below Na⁺ in the reactivity series, so copper is deposited: Cu²⁺ + 2e⁻ → Cu (sodium ions stay in solution).
At the positive electrode (anode) anions and OH⁻ compete. In a concentrated halide solution the chloride ion is discharged in preference to OH⁻: 2Cl⁻ → Cl₂ + 2e⁻. Sulfate ions are not discharged under these conditions.
So the ions discharged first are Cl⁻ at the positive electrode and Cu²⁺ at the negative electrode.
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