Build Kc expressions and solve simple homogeneous equilibrium calculations.
Method and worked examples
For homogeneous aA+ bB ⇌ cC+ dD, Kc=([A]a[B]b)[C]c[D]d, using equilibrium concentrations. Stoichiometric coefficients become powers, not multipliers. The numerical value depends on the equation as written and on temperature. A large Kc favours products at equilibrium but does not indicate a fast reaction.
Set up initial, change and equilibrium amounts using the balanced ratio. Divide moles by the vessel volume to obtain concentrations before substitution. If the reaction is reversed, its constant is 1/Kc. Simple calculations may give a missing concentration directly; others require an equation for the reaction change. Select solutions that give physically possible non-negative concentrations.
Example 1
For H2+I2 ⇌ 2HI, equilibrium concentrations are 0.20,0.20 and 0.80 mol L−1. Find Kc.
Kc=[HI]([H2][I2])2=0.20×0.200.802=16.
Example 2
Initially [A]=0.60 and [B]=0 for A ⇌ B. At equilibrium [B]=0.20 mol L−1. Find [A] and Kc.
The 1:1 change consumes 0.20 mol L−1A. [A]eq=0.40; Kc=[A][B]=0.400.20=0.50.
Watch out:Kc describes equilibrium composition; rate depends on kinetics and activation barriers.
Equilibrium expressions and quantities | Year 12 Chemistry | Ascent Arcade