Copper sulphate solution

Next, electrolyse aqueous copper sulphate with carbon electrodes.

Activity 12.2.3

You will need: copper sulphate solution, carbon rods, connecting wires, a 9 V battery

Connect the electrodes to the battery as in figure 12.2.6, then dip both in the copper sulphate solution. Observe and report.

The observation

Gas bubbles come off the positive terminal (anode).
Copper is deposited on the negative terminal (cathode).
The blue colour of the solution slowly fades.

The solution mainly holds the Cu²⁺ and SO₄^2- ions formed as copper sulphate ionises, plus a very small amount of H⁺ and OH^- from the slight dissociation of water (figure 12.2.7).

At the cathode

Cu²⁺ and H⁺ move to the negative electrode. Copper is below hydrogen in the activity series, so Cu²⁺ is reduced and copper deposits on the cathode. As Cu²⁺ — the cause of the blue colour — leaves the solution, the blue fades.

At the anode

SO₄^2- and OH^- ions are attracted; the OH^- ions are the more likely to be oxidised, so oxygen bubbles off.

Cu²⁺ (aq) + 2e → Cu (s)

4OH^- (aq) → O₂ (g) + 2H₂O (l) + 4e

(1) cathode; (2) anode

Writing the water reactions another way

Because water has so few H⁺ ions, the cathode reaction is sometimes written with water itself instead of 2H⁺ + 2e → H₂. Likewise the anode reaction is sometimes written with water instead of OH^-.

2H₂O (l) + 2e → 2OH^- (aq) + H₂ (g)

2H₂O (l) → O₂ (g) + 4H⁺ (aq) + 4e

Sulphate stays put

Sulphate ions are attracted to the anode but are not oxidised; they stay in the solution. It is the hydroxide ions (or water) that give oxygen.

Copper on the cathode, oxygen at the anode

Electrolysing copper sulphate with carbon electrodes plates copper on the cathode, gives oxygen at the anode, and fades the blue colour.