Tuesday, February 15, 2011


Electrolysis of water:

Water may be electrolyzed in the apparatus shown below. Pure water is however a very poor conductor of electricity, and one has to add dilute sulphuric acid in order to have a significant current flow.

Apparatus for the electrolysis of dilute sulphuric acid

The electrodes consist of platinum foil. The electrolyte is dilute sulphuric acid. Hydrogen gas is evolved at the cathode, and oxygen at the anode.

The ratio, by volume, of hydrogen to oxygen, is exactly 2:1.

Remember that electron flow in the circuit is opposite to the conventional current flow. Thus, while the conventional current flows from the positive pole through the electrolyte and ends up at the negative pole, electrons flow from the negative pole in the reverse direction. The positive pole of a battery accepta electrons from the electrolyte by means of the anode of the electrolytic cell. The reaction at the cathode (tube A) is the reduction of protons:

Reduction of hydrogen ion

Oxidation takes place at the anode (tube B). There are two anions competing to give up their electrons, namely sulphate (SO42-), and hydroxide (OH-) from the ionization of water. The oxidation of OH- according to the reaction

Oxidation of hydroxide ion

has a standard electrode potential of -0.40V, compared to the oxidation of sulphate (-0.17V), and consequently, OH- will be oxidized preferentially. The overall reaction is therefore

Electrolysis of water: overall reaction

and the electrons are reurned to the battery, thus completing the circuit.


  1. which of the substances is weak or strong electrolyte; water, sulfuric acid and why?

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