Make a reaction run uphill.
An electrolytic cell uses a power supply to drive a non-spontaneous reaction. Reduction still happens at the cathode, but this cathode is negative; the anode is positive. Molten NaCl yields sodium and chlorine. In aqueous NaCl, water can compete: hydrogen forms at the cathode instead of sodium.
Products depend on the species present, concentration and electrode material. Concentrated aqueous NaCl with inert electrodes typically gives H₂ and Cl₂. Dilute chloride solutions can favour oxygen at the anode. CuSO₄ with inert electrodes gives copper at the cathode and oxygen at the anode; a copper anode dissolves instead.
Charge q = It = n()F, with F ≈ 96485 C mol⁻¹. Use the balanced half-equation to convert electron moles to product moles. The object to be electroplated is the cathode.
Build a copper coating
Ideal CuSO₄ electroplating with a copper anode and 100% current efficiency. F = 96485 C mol⁻¹; M(Cu) = 63.546 g mol⁻¹. Thickness is exaggerated.
Explore: Identify the species discharged at each electrode and link them to oxidation or reduction.
A 2.00 A current plates Cu for 965 s. Use F = 96500 C mol⁻¹ and M(Cu) = 63.5 g mol⁻¹. Calculate deposited mass.
- q = 2.00 × 965 = 1930 C.
- n() = = 0.0200 mol; n(Cu) = 0.0100 mol.
- m = 0.0100 × 63.5 = 0.635 g.
Answer: 0.635 g
Assumed knowledge
I in amperes, t in seconds; these calculations assume 100% current efficiency.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Predict products for molten and aqueous electrolytes.
- Identify electrode roles in electroplating.
- Calculate charge and product mass.