Copper-plating an iron spoon

Activity 12.2.5 carries out the plan of the last page.

Activity 12.2.5

You will need: an iron spoon, a copper plate, connecting wires, copper sulphate solution, a 9 V battery

Connect the copper plate and the iron spoon to the electric cell with wires, and dip them both at once in the copper sulphate solution. Record your observations.

Figure to be added

Figure 12.2.10 — a copper plate (anode) and an iron spoon (cathode) in copper sulphate solution, connected to a battery.

At the anode (positive electrode)

SO₄^2- and OH^- ions are attracted to the anode, and of these OH^- is the more likely to be oxidised. So you might expect 4OH^- → O₂ + 2H₂O + 4e at the anode — but it does not happen. Because the anode is a metal, it is easier for its own atoms to be oxidised to ions. The copper anode slowly dissolves.

Cu (s) → Cu²⁺ (aq) + 2e

the anode reaction

At the cathode (negative electrode)

The solution contains Cu²⁺ ions and a few H⁺ ions from the dissociation of water. Of these, the one more likely to be reduced is the less active Cu²⁺. So copper is plated onto the cathode — the iron spoon.

Cu²⁺ (aq) + 2e → Cu (s)

the cathode reaction

Copper moves from plate to spoon

Put the two half reactions side by side. For every copper atom that leaves the anode as Cu²⁺, one Cu²⁺ ion becomes a copper atom on the spoon. In effect copper is carried from the plate, through the solution, onto the spoon, and the blue colour of the solution does not fade — unlike the electrolysis with carbon electrodes, where nothing replaced the Cu²⁺. Refining copper works the same way: impure copper is made the anode, and pure copper builds up on the cathode while the impurities are left behind.

The anode feeds the cathode

In copper plating the copper anode dissolves as Cu²⁺ and the same amount of copper is deposited on the object at the cathode.