The Equilibrium Constant for a Heterogeneous Reaction

If we have both vapor and condensed phases present, we simply treat the vapor species and the condensed species as we did before. We must always remember that the mole fractions in these expressions, however, refer to the mole fraction in a given phase, not the total mole fraction. 

Calcium carbonate can dissociate according to the following reaction  

Consider a closed system with pure CaCOs in vacuum at 1000 K. What is the equilibrium pressure of the system Assume that the two solid phases are completely immiscible. At 1000 K, the following Gibbs energies of formation are reported  

SOLUTION Applying the definition of the equilibrium constant given by Equation (9.16)  

We must treat each of the three pure phases distinctly. If we assume that the pressure wUl be low enough that we have an ideal gas at equilibrium, we can rewrite the equilibrium constant as  

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