The equilibrium constant for a gas reaction

We consider 6rst of all a reaction in a perfect mixture.f For each component of such a mixture, as in equation (3 19), 

The right-hand side of this equation is a function only of temperature. Therefore the quantity called the equilibrium constant of the reaction, is also a function only of temperature. In brief the above theorem shows that there exists a certain function of the equilibrium partial pressures which has a constant value, at a given temperature, whatever are the individual values of these partial pressures. 

It may be noted that the equilibrium constant, as conventionally defined, contains in the numerator the partial pressures of those substances which are on the right-hand side of the chemical equation, as usuaUy written, and in the denominator the partial pressures of those substances which are on the left-hand side of this equation. Notice also that equilibrium constarUs are always dimensionless and this is so even in the case of reactions such as A+B C where the number of molecules changes. This is because of (3-1) the standard pressure has subsequently been taken as unity and has therefore not been included explicitly in the equilibrium constant in fact each in Kp is really a ratio pjp.  

The significance of Kp may perhaps be seen more clearly when it is brought to mind that there is an infinite number of sets of chemical potentials and partial pressures which satisfy the condition of reaction equilibrium at a given temperature Let andp be members of one such set and let and be members of another set. For the 

This shows, perhaps more clearly than in (4 11), that the product of any set of equilibrium partial pressures, each to the power of the appropriate stoichiometric coefficient, is a constant at any particular temperature. 

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