The over voltage can be defined as “The difference between the potential of the electrode when gas evolution is actually observed and the theoretical reversible potential of the involved galvanic cell.”
Following are the factors which affect the over voltage:
1) Current Density: It is found that the over voltage depends on the current density. As the current density increases over voltage increases.
2) The surface area of the Electrodes: As effective surface area of the electrodes increases, current density decreases, So over voltage decreases
3) Nature of the surface of the electrode: On smooth and polished surface, the over voltage is greater than on the rough and non polished surface for e.g. The Hydrogen over voltage on rough and non-polished surface is 0.005 volts, while that on smooth surface is 0.09 volts for same solution.
4) Pressure: It is practically observed that at higher pressure, over voltage slightly decreases and at low pressure it increase rapidly.
5) Temperature: As the over voltage is slow process of discharge of H+ ions. If the temperature is high, the process is fast and so the over voltage decreases. It is found that over voltage decreases by 2mv for 1°C rise of temperature.
6) pH of the Solution: In strongly acidic or alkaline solution, there is large concentration of H+ ions and OH- ions in the vicinity of the electrode, due to the large concentration, the deviation occurs.
Application of over voltage:
1) We can separate two isotopes of hydrogen i.e. 1H1 and 2H2 which are present in dil. H2SO4. When the electrolysis of it is carried out using platinum electrodes. As the over voltage of 1H1 is low on platinum and that of 2H2 is high on platinum. So 1H1 will be discharged first leaving 2H2 in the solution. Thus with the help of over voltage we can separate two isotopes of Hydrogen.
2) As many metals have low over potential than hydrogen and so can be deposited on the electrodes instead of Hydrogen when the electrolysis is carried out. Thus electroplating of the metal can be done easily.
3) In the lead accumulators: When the recharging of the lead accumulator is carried out, the lead is deposited on the lead electrode and not Hydrogen. Because the over voltage of Hydrogen on the surface of lead is higher than the Pb+2 ions on the lead electrode, so the deposition of lead from PbSO4 take place and not H2 gas from aqueous solution of H2SO4.
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