When using the formula ΔG° = –RT ln K to calculate K, K will be Kc for reactions in solution and concentrations would be expressed in molarity units. But if you have a gas phase reaction, K from this formula will be Kp and the concentrations of the substances are expressed in standard pressure units such as atmospheres. The equilibrium constant obtained from ΔG° = –RT ln K is called the "thermodynamic" equilibrium constant. In the thermodynamic equilibrium constant expression for a reaction involving gases, gases are always given as partial pressures; for solutions, always molarity. If the equilibrium is heterogeneous and has some substances in solution and others in the gas phase, both units, molarity and atmospheres, are used. It is confusing, but that is the convention!  |
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