Change of K with Pressure

We have seen that unlike T and T°, the pressure of the standard state (P°) may or may not be the same as the system pressure (P). Therefore differentiating with respect to system pressure may not be the same as differentiating with respect to standard state pressure. Differentiating In K with respect to system pressure, we have 

if all standard states involved in the reaction have been defined as being at a constant pressure P, normally 1 bar, these states are unaffected by changes in system pressure P, and 

Thus in this case the equilibrium constant K is independent of P. 

On the other hand, if even one of the reaction constituents has a standard state with a variable pressure, normally P° = P, then equation (13.27) is not true, and integration of equation (13.28) requires a knowledge of how ArG° varies with pressure. We will simplify the following discussion by assuming that reaction constituents having the same physical state (solid, liquid, gas, solute) will have the same kind of standard 

The above equations are for reactions involving aqueous species only. For reactions involving minerals and aqueous species  

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