Alternative discussion of the galvanic cell

In the leist section the cell was discussed under conditions where it is in process of performing external work reversibly and the equations were based on the relation — dC = — dw ax. f 2 4. An alternative method is to consider the cell on open circuity not performing work. This method has the advantage of showing that the cell obeys the condition of reaction equilibrium  

Consider the same cell as in the last section and let dn be any variation in the number of moles of hydrogen chloride. Because of the stoichiometry of the recustion  

When the cell is held at constant temperature and pressure, but performs no work the condition of equilibrium is a minimum of O. Using equation (2 61) we have for the change in U in the above process 

Let it be supposed that the mercury is connected to a copper terminal I. Since electrons are able to pass freely between the mercury and the copper they have the same chemical potential in each. Similarly, let it be supposed that the platinum is connected to a second copper terminal II. Then the last equation may be written 

Equation (4 63) is the same as (4 66) of the last section. This important equation has therefore been obtained by a method which is in harmony with the theory of reaction equilibrium of the present chapter. 

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