Example 1
For aqueous CuSO4 with inert electrodes, assume Cu deposition and water oxidation. Write both half-equations.
Cathode: Cu2++2e−→ Cu. Anode: 2H2O→O2+4H++4e−. The cathode is negative and the anode positive.
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Year 12 · SACE Stage 2
Aqueous and molten electrolysis products
Select electrode products from supplied electrolyte and electrode conditions.
Method and worked examples
Aqueous electrolysis can involve water as well as dissolved ions. Use the given discharge conditions rather than assuming every metal cation becomes metal. With inert electrodes and the supplied conditions, aqueous CuSO4 deposits Cu and evolves O2, while concentrated aqueous NaCl produces H2 and Cl2. Molten NaCl contains no water and can produce Na metal.
Reduction occurs at the negative cathode and oxidation at the positive anode. Cu2++2e−→ Cu is cathodic reduction. Water oxidation can be written 2H2O→O2+4H++4e−. Electrode material also matters: a soluble copper anode can oxidise to Cu2+ instead of evolving oxygen. Balance both atoms and charge for each stated system.
Example 1
Example 2
Compare sodium-chloride electrolysis products using the supplied conditions: concentrated aqueous with inert electrodes versus molten with inert electrodes.
Concentrated aqueous NaCl gives H2 at the cathode because water is reduced, and Cl2 at the anode under the supplied conditions. Molten NaCl gives Na at the cathode and Cl2 at the anode because no water competes.
Watch out: Water may be reduced preferentially; use the metal reactivity, medium and supplied discharge conditions.