Factors Affecting the Selective Discharge – Types of Electrode

Factors Affecting the Selective Discharge – Types of Electrode

Type of Electrode

  1. There are 2 types of electrode:
    1. Inert electrode
      Inert electrodes do not react with electrolyte and do not take part in the chemical reaction. (Example: Carbon, platinum)
    2. Active electrode
      The active electrode will react with the electrolyte and take part in the reaction. (Example: Copper, Silver, Mercury)

Example: Electrolysis of Copper(II) Sulphate with Carbon/Copper Electrodes

Using Carbon Electrode
Using Copper Electrode

Ions presence at
Anode:

OH–, SO42-

Cathode:

H+, Cu2+

Ions presence at
Anode:

OH–, SO42-

Cathode:

H+, Cu2+

Observation at
Anode:
Colourless gas is produced. When a glowing wooden splinter is inserted into the test tube that contains the gas, the wooden splinter is rekindled.

Cathode
The electrode become thicker.

Colour of the Solution:
Become paler

Observation at
Anode:
The electrode becomes thinner.

Cathode
The electrode becomes thicker.

Colour of the Solution:
Remain unchanged

Half Equation of the Reaction at
Anode:

4OH– → 2H2O + O2 + 4e

Cathode:

Cu2+ + 2e → Cu

Half Equation of the Reaction at
Anode:

Cu → Cu2+ + 2e

Cathode:

Cu2+ + 2e → Cu

Note:
For the electrolysis of copper(II) sulphate solution by using copper electrodes, both the hydroxide and sulphate ions are not selected to be discharged, Instead, the copper electrode ionises to form copper(II) ions. The electrode has taken part in the reaction!