Equilibrium Constant Calculations
Heterogeneous Equilibria Examples
Heterogeneous equilibria involve reactants and products in different phases. The concentration of pure solids and liquids are essentially constant and therefore are omitted from the equilibrium expression.
Key Idea: Only gaseous and aqueous species appear in the equilibrium constant expression, $ K $ .
Examples:
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Dissolution of a slightly soluble salt:
Consider the dissolution of silver chloride ( $ AgCl $ ) in water:
$ AgCl(s) \rightleftharpoons Ag^+(aq) + Cl^-(aq) $
The equilibrium constant expression is: $ K_{sp} = [Ag^+]][Cl^-]] $ ( $ K_{sp} $ is the solubility product constant). Note that solid $ AgCl $ is omitted. Solubility Product Constant
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Decomposition of a solid:
Consider the decomposition of calcium carbonate ( $ CaCO_3 $ ):
$ CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g) $
The equilibrium constant expression is: $ K_p = P_{CO_2} $ where $ P_{CO_2} $ is the partial pressure of $ CO_2 $ . Both $ CaCO_3(s) $ and $ CaO(s) $ are omitted.
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Reactions involving gases and solids:
Consider the reaction between carbon monoxide and iron(III) oxide:
$ 3CO(g) + Fe_2O_3(s) \rightleftharpoons 2Fe(s) + 3CO_2(g) $
The equilibrium constant expression is: $ K = \frac{P_{CO_2}^3}{P_{CO}^3} $
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Reactions with water:
The equilibrium between gaseous carbon dioxide and aqueous carbonic acid: $$ CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq) $$ The equilibrium expression would be: $ K = \frac{[H_2CO_3}{P_{CO_2}} $ . Liquid water is omitted. Acid-Base Equilibria
General Considerations:
- The equilibrium constant, $ K $ , is temperature dependent. A change in temperature will shift the equilibrium. Le Chateliers Principle
- Adding more solid or liquid reactant or product will not affect the equilibrium position, since their concentrations are constant.
- Changes in pressure only affect the equilibrium if gases are involved. Increasing pressure will favor the side with fewer gas molecules. Gas Laws