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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.”
Over voltages are of three types:
Ohmic Over Voltage \(({ \eta }_{ O })\):
In some electrolysis processes, a film of oxide of some other insoluble material is formed on the surface of cathode. This oxide layer increase the resistance of the cell and opposes the applied potential across the electrodes. If I is the current density in mA/cm and R is the resistance, then ohmic voltage is given by, $$ { \eta }_{ O } = IR $$
Concentration Over Voltage \(({ \eta }_{ C })\):
Consider a AgNO3 solution and Ag electrode is immersed in it. During the electrolysis deposition of Ag from solution will take place, then the concentration of Ag+ ions in the electrolyte in the vicinity of electrodes (near the electrode) will decrease. Let It is C(vicinity) and the bulk concentration is C(bulk). Now decomposition potential (Ed) depends on C(vicinity) and Theoretical reversible potential (Er) depends on C(bulk).
As $$ { C }_{ vicinity } \neq { C }_{ bulk } $$ $$ \therefore { E }_{ d } \neq { E }_{ r } $$ and the difference between two is known as concentration over voltage \(({ \eta }_{ C })\).$$ { \eta }_{ C } = { E }_{ d } - { E }_{ r } $$Activation Over Voltage \(({ \eta }_{ A })\):
For any chemical reaction to take place there is a minimum amount of energy needs to be supplied. Without this supplied energy the reaction will not start. This minimum energy to initiate a reaction is Activation Energy.
In case of electrolysis, several chemical reactions take place simultaneously, e.g. conversion of H+ ions and OH- ions into H2 and O2 gases in electrolysis of 1N H2SO4 and this energy is supplied form the applied potential. So the increase in applied voltage or over voltage due to this minimum energy or activation energy is termed as Activation Over Voltage \(({ \eta }_{ A })\)
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