The Equilibrium Constant, Kc (AQA A-Level Chemistry): Revision Notes

1.8.3 The Equilibrium Constant, Kc

Introduction to $K_c$

Expression of $K_c$

For a general reaction:

$$aA + bB \rightleftharpoons cC + dD$$

The equilibrium constant, $K_c$, is expressed as:

$$K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$$

Where:

• [X] represents the concentration of species $X$ in mol $dm^3$.
• The exponents $a$, $b$, $c$ and $d$ correspond to the stoichiometric coefficients from the balanced equation.

Interpretation of $K_c$

• If $K_c > 1:$ The concentration of products is greater than that of the reactants at equilibrium, meaning the equilibrium lies to the right (favouring products).
• If $K_c < 1$: The concentration of reactants is greater than that of the products, meaning the equilibrium lies to the left (favouring reactants).

Homogeneous vs. Heterogeneous Equilibria

• Homogeneous reactions involve reactants and products all in the same phase (e.g., all gases or all solutions).
• Heterogeneous reactions involve reactants and products in different phases. In such cases, only the concentrations of gaseous or aqueous species are included in the $K_c$ expression; solids and pure liquids are omitted.

Units of $K_c$

The units of $K_c$ depend on the reaction's stoicheiometry.

For a reaction where the number of moles of reactants equals the number of moles of products, $K_c$ will be unitless. Otherwise, the units of $K_c$ will be derived based on the number of moles on each side of the equation, generally in terms of mol $dm^3$.