Heat Evolution within Electrolytic Cells

As has been already mentioned the theoretical decomposition voltage of water equals 1.23 V while its practical value amounts to minimum 1.7 V and the actual operating voltage to 1.9 V. It would be erroneous to think that all the energy corresponding to the difference between the actual and the theoretical voltage across the electrolytic cell is converted into heat which warms up the electrolyte. 

In the theoretical section of this book we learnt that the change in free energy enoountered in a reversible reaction proceeding at a constant temperature can be expressed by the difference between the corresponding change in enthalpy AH and latent heat Qiev which must be released by the system to the sur- 

This holds good for galvanic and electrolytic cells. In the case of electrolysis of water at 25 °C the following values have to be considered  

If we first use the equation (VII-4) which expresses the relation between the change in free energy and the theoretical decomposition voltage (ER)t, we obtain  

This value is in complete agreement with the result obtained on the basis of electrode potentials. 

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