Equilibrium constants and concentration expressions
≈ 50 minEquilibrium constants and concentration expressions
The equilibrium constant Kc expresses the ratio of product and reactant concentrations at equilibrium, each raised to their stoichiometric coefficients. For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d/[A]^a[B]^b for species included in the expression. Kc has a fixed value at a fixed temperature; changing concentration may shift the system temporarily but does not change Kc unless temperature changes. A large Kc indicates products are favoured at equilibrium, while a small Kc indicates reactants are favoured. Do not substitute initial concentrations when the question asks for equilibrium values.
Work it through
For A + B ⇌ 2C, if [A] = 0.20, [B] = 0.50 and [C] = 0.40 mol dm⁻³ at equilibrium, Kc = (0.40)²/(0.20 × 0.50) = 0.16/0.10 = 1.6. The square on C comes from the coefficient 2. Units are often treated according to the course convention; concentrate on correct equilibrium expression and values.
Mastery target
Write an equilibrium expression from a balanced equation, calculate Kc from equilibrium data and interpret whether products or reactants are favoured.
For A + B ⇌ 2C, which Kc expression is correct?
Name the key chemistry term from Equilibrium constants and concentration expressions that best fits the explanation and visual model.
For A + B ⇌ 2C, [A] = 0.20, [B] = 0.50 and [C] = 0.40 mol dm⁻³ at equilibrium. Calculate Kc.
Which statement corrects a common misunderstanding in Equilibrium constants and concentration expressions?