The Equilibrium Constant, Kc
Equilibrium expression & constant
- The equilibrium constant expression is an expression that links the equilibrium constant, K, 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 corresponding equilibrium constant expression is written as:
-
- Where:
- [A] and [B] = equilibrium reactant concentrations (mol dm-3)
- [C] and [D] = equilibrium product concentrations (mol dm-3)
- a, b, c and d = number of moles of corresponding reactants and products
- Where:
- Solids are ignored in equilibrium constant expressions
- The equilibrium constant, K, of a reaction is specific to a given equation
Worked example
Deducing equilibrium expressions
Deduce the equilibrium constant 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:
- [Ag (s)] is not included in the equilibrium constant expression as it is a solid
Answer 2:
Answer 3: