During the industrial electrolysis of concentrated aqueous sodium chloride (brine solution), what are the principal products liberated at the cathode and anode?
Answer and explanation
Correct answer: A. Cathode: H2 gas; Anode: Cl2 gas
- Step 1: In aqueous , the ions present are , , , and .
- Step 2: At the cathode, (from water) has a higher reduction potential () than (), so water is reduced to liberate hydrogen gas: .
- Step 3: At the anode, although evolution is thermodynamically favored over , oxygen has a very high overpotential on standard electrodes; consequently, chloride oxidation is kinetically favored, liberating chlorine gas: .
- Step 4: The remaining solution contains and ().
Answer
Cathode: H2 gas; Anode: Cl2 gas
Why not the others: 'Cathode: Na metal; Anode: Cl2 gas' occurs during electrolysis of molten (Downs process), not aqueous brine; 'Cathode: H2; Anode: O2' occurs during electrolysis of dilute sulfuric acid or dilute ; 'Cathode: Na; Anode: O2' is physically impossible in aqueous brine.
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