Copper Sulfate Electrolysis: Active versus Inert Electrodes
Two students electrolyse separate 0.50 mol/dm³ copper(II) sulfate solutions using a current of 0.50 A for 20.0 minutes. Student A uses two clean copper electrodes. Student B uses two graphite electrodes. Both start with the same blue solution. After electrolysis, Student A reports that the anode lost mass, the cathode gained mass, and the blue colour was unchanged. Student B reports a pink-brown deposit on the cathode, colourless bubbles at the anode that relight a glowing splint, and a pale blue solution that has become noticeably paler. Student B claims their experiment 'proves' that copper is not discharged at the anode, while Student A claims their experiment 'proves' that copper ions are not discharged at the cathode. (1 faraday = 96 500 C/mol; Aᵣ Cu = 63.5)
1. Calculate the charge passed and the expected mass change at the cathode.
Read the worked answer
Q = I × t = 0.50 A × (20.0 × 60 s) = 0.50 × 1200 = 600 C. n(e⁻) = Q / F = 600 / 96 500 = 6.22 × 10⁻³ mol. The cathode reaction is Cu²⁺ + 2e⁻ → Cu, so n(Cu) = 6.22 × 10⁻³ / 2 = 3.11 × 10⁻³ mol. m(Cu) = n × Aᵣ = 3.11 × 10⁻³ × 63.5 = 0.197 g ≈ 0.20 g. So the cathode should gain about 0.20 g.
2. Explain the observations at each electrode for Student B and write the electrode equations.
Read the worked answer
At the graphite cathode, copper(II) ions are reduced: Cu²⁺ + 2e⁻ → Cu. This gives the pink-brown copper deposit, and removal of Cu²⁺ from solution makes the blue colour fade. At the graphite anode, copper metal is not available to dissolve, so water (or hydroxide ions) is oxidised: 4OH⁻ → O₂ + 2H₂O + 4e⁻, or equivalently 2H₂O → O₂ + 4H⁺ + 4e⁻. The colourless gas is oxygen, which relights a glowing splint. The solution becomes more acidic as H⁺ ions are produced.
3. Evaluate the two students' claims.
Read the worked answer
Both claims are wrong. In Student A's cell, the copper anode dissolves: Cu → Cu²⁺ + 2e⁻, so copper is oxidised at the anode, not 'not discharged'. At the cathode, Cu²⁺ + 2e⁻ → Cu deposits copper. Because copper ions leave the anode at the same rate as they are removed at the cathode, the concentration of Cu²⁺ stays approximately constant and the blue colour does not change. In Student B's cell, the cathode does discharge copper ions, producing the pink-brown deposit. The difference between the two cells is the electrode material: an active copper anode can dissolve, while an inert graphite anode forces water or hydroxide ions to be oxidised.