Homogeneous and Heterogeneous Equilibria

A homogeneous reaction is one in which every reactant and product is in the same phase, e.g. a gas-phase reaction such as $2HI(g)\rightleftharpoons H_2(g)+I_2(g)$. A heterogeneous reaction involves reactants and products present in more than one phase, e.g. $NH_3(g)+HCl(g)\rightleftharpoons NH_4Cl(s)$. Equilibria set up by each type of reaction are called, respectively, homogeneous and heterogeneous equilibria. In a heterogeneous equilibrium, only the concentrations of gaseous (g) and dissolved (aq) species are included in the equilibrium-constant expression -- a pure solid or pure liquid's concentration (more precisely, its activity) stays constant no matter how much of it is present, so it is absorbed into the value of the constant rather than written explicitly. For example, liquid-vapour equilibrium of ethanol, $C_2H_5OH(l)\rightleftharpoons C_2H_5OH(g)$, gives $K_c=[C_2H_5OH(g)]$ (the constant liquid-phase concentration is folded into $K_c$ itself), and iodine sublimation, $I_2(s)\rightleftharpoons I_2(g)$, gives $K_c=[I_2(g)]$. Problem 12.4 applies this to a heterogeneous decomposition, $2NaHCO_3(s)\rightleftharpoons Na_2CO_3(s)+CO_2(g)+H_2O(g)$: both sodium compounds are pure solids, so $K_c=[CO_2(g)][H_2O(g)]$, with unit $(\text{mol dm}^{-3})^2=\text{mol}^2\,\text{dm}^{-6}$. In general, the unit of $K_c$ depends on the reaction's expression and is found from $\Delta n$ (the difference between the number of moles of species in the numerator and the denominator of the $K_c$ expression): unit of $K_c=(\text{mol dm}^{-3})^{\Delta n}$. Worked examples: for $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$, $\Delta n=2-2=0$, so $K_c$ has NO unit; for $N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)$, $\Delta n=2-4=-2$, so the unit is $(\text{mol dm}^{-3})^{-2}=\text{ …