AQA AS Chemistry

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ASChemistryAQARevision Notes1. Physical Chemistry1.8 Chemical Equilibria, Le Chatelier's Principle & Kc1.8.3 The Equilibrium Constant, Kc

1.8.3 The Equilibrium Constant, Kc

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The Equilibrium Constant, Kc

Equilibrium expression & constant

The equilibrium expression links the equilibrium constant, K_c, to the concentrations of reactants and products at equilibrium taking the stoichiometry of the equation into account
So, for a given reaction:

aA + bB ⇌ cC + dD

the K_c is defined as follows:

Equilibria Equilibrium Expression, downloadable AS & A Level Chemistry revision notes

Equilibrium expression linking the equilibrium concentration of reactants and products at equilibrium

Solids are ignored in equilibrium expressions
The K_c of a reaction is specific and only changes if the temperature of the reaction changes

Worked example

Deducing equilibrium expressions

Deduce the equilibrium expression for the following reactions:

Ag⁺(aq) + Fe²⁺(aq) ⇌ Ag (s) + Fe³⁺(aq)
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

Answer 1:

- K_c = $\frac{[{Fe}^{3 +} (aq)]}{[{Fe}^{2 +} (aq)] [{Ag}^{+} (aq)]}$
- [Ag (s)] is not included in the equilibrium expression as it is a solid

Answer 2:

- K_c = $\frac{{[{NH}_{^{3}} (g)]}^{2}}{[N_{2} (g)] {[H_{2} (g)]}^{3}}$

Answer 3:

- K_c = $\frac{{[{SO}_{3} (g)]}^{2}}{{[{SO}_{2} (g)]}^{2} [O_{2} (g)]}$

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