Heat Capacity of a System in Chemical Equilibrium

We now return to the case of isomerization discussed in the preceding subsection. Again we have N molecules in an ideal-gas phase, and each molecule has an internal PF 

The summation in (2.4.74) is over all states of a single molecule (excluding the momentum PF). The PF of the system is 

Here each molecule has a kinetic energy of IkTand an average internal energy of where the sum is over all internal states of a single molecule. 

Now we choose to view our system as a two-component system. We follow the same grouping of all states into two groups as in section 2.4.4 and rewrite (2.4.76) as 

The first term on the rhs of (2.4.79) is as in (2.4.77) due to the translational degrees of freedom. The significance of the other two terms is as follows Suppose that transition between states belonging to the two groups is forbidden, say by introducing an inhibitor 

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