**Writing Equilibrium Equations for Heterogeneous Equilibria **

Write the equilibrium equation for each of the following reactions:

(a) CO_2(g) + C(s) \rightleftharpoons 2 CO(g)

(b) \mathrm{Hg}(l)+\mathrm{Hg}^{2+}(a q) \rightleftharpoons \mathrm{Hg}_2^{2+}(a q)

**STRATEGY **

Write the usual equilibrium-constant expressions but omit the pure solid carbon in part (a) and the pure liquid mercury in part (b) because the ratio of their concentrations to their concentrations in the standard state is equal to 1.

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(a) K_{\mathrm{c}}=\frac{[\mathrm{CO}]^2}{\left[\mathrm{CO}_2\right]}

Alternatively, because CO and CO_2 are gases, the equilibrium equation can be written using partial pressures:

K_{\mathrm{p}}=\frac{\left(P_{\mathrm{CO}}\right)^2}{P_{\mathrm{CO}_2}}The relationship between K_p and K_c is K_p = K_c(RT)^{Δn} = K_c(RT), because Δn = 2 – 1 = 1.

(b) K_c=\frac{\left[\mathrm{Hg}_2^{2+}\right]}{\left[\mathrm{Hg}^{2+}\right]}

In this case, it’s not appropriate to write an expression for K_p because none of the reactants or products is a gas.

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