The Equilibrium Law, The Equilibrium Constant, K
Specification Reference R2.3.2
Quick Notes:
- The equilibrium constant (K) gives the ratio of concentrations of products to reactants at equilibrium.
- For a homogeneous reaction (all species in the same phase), the equilibrium expression includes concentration terms for all species.
- The expression is derived from the balanced chemical equation.
- K is constant at a fixed temperature.
- For a general reaction:
K =![IB Chemistry equilibrium constant expression showing K = [C]^c [D]^d / [A]^a [B]^b.](images/kcexpression.png)
Full Notes:
What Is the Equilibrium Law?
The equilibrium law relates the concentrations of reactants and products in a reversible reaction at equilibrium.
It allows us to calculate the equilibrium constant (K).
For a general homogeneous reaction:
The equilibrium expression is:
- Square brackets [ ] represent concentration in mol dm⁻³.
- Each term is raised to the power of its stoichiometric coefficient from the balanced equation.
Matt’s exam tip
Solids aren’t ever included in K expressions and if water is a solvent as well as a reactant or product, it also isn’t included.
Homogeneous Reactions
A reaction is homogeneous if all reactants and products are in the same phase (usually gaseous or aqueous).
Example Gaseous reaction
N2(g) + 3H2(g) ⇌ 2NH3(g) all species are (g)
K = [NH3]2 / [N2][H2]3
Example Aqueous reaction
Fe3+(aq) + SCN−(aq) ⇌ [FeSCN]2+(aq) all species are (aq)
K = [[FeSCN]2+] / [Fe3+][SCN−]
Important Notes
- K is temperature dependent: changing temperature will change the value of K.
- K can have different units depending on the reaction.
Summary
- The equilibrium law expresses the ratio of concentrations of products to reactants at equilibrium.
- K is written from the balanced equation with concentrations raised to their stoichiometric powers.
- Only aqueous and gaseous species are included in K expressions.
- K varies with temperature and may have different units for different reactions.