6.3: Heterogeneous Equilibria

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A heterogeneous reaction is one where one or more states within the reaction differ (the Greek word "heteros" means "different"). For example, the formation of an aqueous solution of lead(II) iodide creates a heterogeneous mixture dealing with molecules in both the solid and aqueous states:

\[PbI_{2 (s)} \rightleftharpoons Pb^{+2}_{(aq)} + 2I^-_{(aq)}\]

The decomposition of sodium hydrogen carbonate (baking soda) at high elevations is another example of a heterogeneous mixture, this reaction deals with molecules in both the solid and gaseous states:

\[ 2NaHCO_{3 (s)} \rightleftharpoons Na_2CO_{3 (s)} + H_2O_{ (g)} + CO_{2 (g)} \]

\[ C_{(s)} + O_{2 (g)} \rightleftharpoons CO_{2 (g)} \]

Once again, this difference is emphasized so that students remember that equilibrium constant calculations are different from heterogeneous mixtures.