Heterogeneous Equilibria

Heterogeneous Equilibria

A heterogeneous equilibrium is a system in which reactants and products are found in two or more phases. The phases may be any combination of solid, liquid, or gas phases, and solutions. When dealing with these equilibria, remember that solids and pure liquids do not appear in equilibrium constant expressions (the activities of pure solids, pure liquids, and solvents are 1).

Some heterogeneous equilibria involve chemical changes; for example:

\({\text{PbCl}}_{2}\left(s\right)⇌{\text{Pb}}^{2+}\left(aq\right)+2{\text{Cl}}^{\text{−}}\left(aq\right)\phantom{\rule{5em}{0ex}}{K}_{c}=\left[{\text{Pb}}^{2+}\right]{\left[{\text{Cl}}^{\text{−}}\right]}^{2}\)

\(\text{CaO}\left(s\right)+{\text{CO}}_{2}\left(g\right)⇌{\text{CaCO}}_{3}\left(s\right)\phantom{\rule{5em}{0ex}}{K}_{c}=\phantom{\rule{0.2em}{0ex}}\cfrac{1}{\left[\text{CO}_{2}\right]}\)

\(\text{C}\left(s\right)+2\text{S}\left(g\right)⇌{\text{CS}}_{2}\left(g\right)\phantom{\rule{5em}{0ex}}{K}_{c}=\phantom{\rule{0.2em}{0ex}}\cfrac{\left[\text{CS}_{2}\right]}{\left[\text{S}\right]^{2}}\)

Other heterogeneous equilibria involve phase changes, for example, the evaporation of liquid bromine, as shown in the following equation:

\({\text{Br}}_{2}\left(l\right)⇌{\text{Br}}_{2}\left(g\right)\phantom{\rule{5em}{0ex}}{K}_{c}=\left[{\text{Br}}_{2}\right]\)

We can write equations for reaction quotients of heterogeneous equilibria that involve gases, using partial pressures instead of concentrations. Two examples are:

\(\text{CaO}\left(s\right)+{\text{CO}}_{2}\left(g\right)⇌{\text{CaCO}}_{3}\left(s\right)\phantom{\rule{5em}{0ex}}{K}_{P}=\phantom{\rule{0.2em}{0ex}}\cfrac{1}{{P}_{{\text{CO}}_{2}}}\)

\(\text{C}\left(s\right)+2\text{S}\left(g\right)⇌{\text{CS}}_{2}\left(g\right)\phantom{\rule{5em}{0ex}}{K}_{P}=\phantom{\rule{0.2em}{0ex}}\cfrac{{P}_{{\text{CS}}_{2}}}{{\left({P}_{\text{S}}\right)}^{2}}\)