Write the equilibrium constant expressions for these...

Write the equilibrium constant expressions for these reactions: (a) $3 \mathrm{O}_2(\mathrm{~g}) \rightleftharpoons 2 \mathrm{O}_3(\mathrm{~g})$ (b) $\mathrm{H}_2 \mathrm{O}(\mathrm{g}) \rightleftharpoons \mathrm{H}_2 \mathrm{O}(\mathrm{l})$ (c) $\mathrm{MgCO}_3(\mathrm{~s}) \rightleftharpoons \mathrm{MgO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$ (d) $2 \mathrm{Bi}^{3+}(a q)+3 \mathrm{H}_2 \mathrm{~S}(a q) \rightleftharpoons \mathrm{Bi}_2 \mathrm{~S}_3(s)+6 \mathrm{H}^{+}(a q)$

Write the equilibrium constant expressions for these reactions:
(a) $3 \mathrm{O}_2(\mathrm{~g}) \rightleftharpoons 2 \mathrm{O}_3(\mathrm{~g})$
(b) $\mathrm{H}_2 \mathrm{O}(\mathrm{g}) \rightleftharpoons \mathrm{H}_2 \mathrm{O}(\mathrm{l})$
(c) $\mathrm{MgCO}_3(\mathrm{~s}) \rightleftharpoons \mathrm{MgO}(\mathrm{s})+\mathrm{CO}_2(\mathrm{~g})$
(d) $2 \mathrm{Bi}^{3+}(a q)+3 \mathrm{H}_2 \mathrm{~S}(a q) \rightleftharpoons \mathrm{Bi}_2 \mathrm{~S}_3(s)+6 \mathrm{H}^{+}(a q)$

Key Concepts

Homogeneous vs. Heterogeneous Equilibria

Reactions can involve species in the same phase (homogeneous) or different phases (heterogeneous). In equilibrium constant expressions, the concentrations or pressures of pure solids and pure liquids are omitted because their activities are constant (typically taken as one), focusing only on the species in the gas phase or in solution.

Stoichiometric Coefficients

These coefficients from the balanced chemical equation dictate the exponents used in the equilibrium constant expression. They ensure that the expression reflects the proportionate change in reactants and products, as dictated by the reaction's stoichiometry.

Equilibrium Constant Expression

This represents the ratio of the activities (or effective concentrations or pressures) of the products to those of the reactants at equilibrium. The expression is derived from the law of mass action, where each species' concentration or partial pressure is raised to the power of its stoichiometric coefficient in the balanced chemical reaction.

Chemical Activity

In equilibrium expressions, the concept of activity is used to account for the effective concentration of a species. In solutions, this often approximates to the molar concentration, and in gases, to partial pressures. However, the exact activity depends on factors like intermolecular interactions, and for pure solids and liquids, the activity is defined as one.

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