Electroplating

Electroplating is a process where the surface of one metal is coated with a layer of a different metal
The anode is made from the pure metal you want to coat your object with
The cathode is the object to be electroplated
The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode

Electroplating-Tin

A piece of iron being electroplated with tin. The electrolyte is tin(II) chloride, a water-soluble salt of tin

At the anode: Tin atoms lose electrons (is oxidised) to form tin ions in solution
At the cathode: Tin ions gain electrons (is reduced) to form tin atoms which deposit on the strip of iron metal, coating it with a layer of tin

Electroplating is done to make metals more resistant to corrosion or damage
- E.g. chromium and nickel plating
It is also done to improve the appearance of metals,
- E.g. coating cutlery and jewellery with silver

You may be asked to write the half equations for the reaction at each electrode. For the example above, these would be:

At the anode: Sn (s) → Sn²⁺ (aq) + 2e^-

At the cathode: Sn²⁺ (aq) + 2e^- → Sn (s)

Purification of copper

The electrolysis of aqueous CuSO₄ using graphite rods produces oxygen and copper
By changing the electrodes from graphite to pure and impure copper, the products can be changed at each electrode
Electrolysis can be used to purify metals by separating them from their impurities
In the set-up, the impure metal is always the anode, in this case the impure copper
The cathode is a thin sheet of pure copper
The electrolyte used is an aqueous solution of a soluble salt of the pure metal at the anode, e.g. CuSO₄
Copper atoms at the anode lose electrons and go into solution as ions:

Cu ⟶ Cu²⁺ + 2e

The anode thus becomes thinner due to loss of atoms and the impurities fall to the bottom of the cell as sludge
The copper(II) ions are attracted to the cathode where they gain electrons and form now purified copper atoms
The cathode gradually becomes thicker

Cu²⁺ + 2e^- ⟶ Cu

The anode sludge is a highly valuable material and is further refined as it often contains small quantities of precious metals like silver which are found as impurities in the unrefined copper

The cathode increases in mass and the anode decreases in mass

Explain why the electrolyte maintains the blue colour during the electrolysis of aqueous copper sulfate using copper electrodes.

Answer

The concentration of Cu²⁺ stays the same
Cu²⁺ (aq) + 2e^– → Cu (s) occurs at the cathode
Cu (s) → Cu²⁺ (aq) + 2e^– occurs at the anode
Cu²⁺ ions which give the blue colour leave and enter the solution at the same rate