Electrolytic processes

These are processes in which chemicals (electrolytes) are made to conduct electricity by passing a direct current through them by means of cells or batteries connected to rods known as electrodes.

The pole(electrode) through which current from the battery enters the electrolyte is known as anode. The anode is a positive electrode so it is connected to the negative side of the battery (Electrochemical cell). Oxidation takes place at this electrode.

The pole(electrode) through which current leave the electrolyte is called cathode. It is a negative electrode so it is connected to the positive side of the battery. Reduction takes place at this electrode so that metals get deposited to increase the mass of the cathode.

Electrolyte is a liquid or molten compound which decomposes into ions when current is passed through it. Examples are sulphuric acid(H2SO4), copper sulphate (CuSO4), sodium chloride (NaCl) etc.

Mechanism of electrolysis

When the battery is switched on the ions(anions and cations) in the electrolyte become mobile and migrate to the electrodes. Positive ions(cations) migrate to the negative electrode(cathode) while negative ions migrate to the positive electrode(anode). 

If there is more than one negative or positive ion in an electrolyte the ion which gets discharged by the solution is determined by three factors arranged below in their order of priority:

Nature of electrodes

Concentration of ions

Relative position in activity series

Nature of electrode: Some electrodes have affinity for ions. Mercury cathode for example has a great affinity for sodium ion in an electrolyte solution to form a mixture of sodium and Mercury known as sodium amalgam (Na/Hg). Whenever this effect is not desired inert electrodes are used. Examples of common inert electrodes are platinum and graphite electrodes,,, they do not have affinity for ions in the electrolyte solution.

Concentration of ions: If there are two ions of the same charge in a solution the one with highest concentration gets discharged. For instance in molten sodium chloride and lead chlorise there are two anions (Cl- and OH-) as current is passed, the chloride ions leave the solution and migrate to the anode since they have higher concentration,,, at the anode they undergo oxidation to form chlorine gas. 2Cl- = Cl2 + 2e-

Relative position in activity series:  If there two identically charged ions in a solution the least active one gets discharged and leaves the solution. In AgNO3 solution for instance, there are two positive ions Ag+ and H+,,, the Ag+ ion being less active than H+ leaves the electrolyte solution and gets discharged,, it migrates to the cathode where it is reduced to the metal.

Note: The most powerful and prior of these factor is the nature of the electrode. The nature of the electrode used should be considered first then the concentration of the ions then the relative position in activity series.


Author: Peter A.

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