Gas-Phase Equilibria

The concentrations of the species in the law of mass action are expressed in moles per liter (M). If,
however, the reaction is in the gas phase (if the reactants and products are gases), the more
common approach is to formulate the law of mass action in terms of the partial pressures of the
species. For the reaction

aA  (g) +   bB  (g) →   cC  (g) +   dD  (g),

the law of mass action is usually written in the form:

where PC is the partial pressure of gas C, et cetera. All partial pressures should be in units of
atmospheres. For the reaction mentioned at the beginning of this chapter and the one in the
example problem above, then, one can write the following mass action expressions:

The numerical value of Kp would in general be different from that of Kc, the equilibrium constant
written with molarities. What is unchanged, however, is the essence of the law of mass action: At
equilibrium, a certain relationship will always prevail among the amount of reactants and products,
no matter how you choose to report these amounts.