Properties of the Equilibrium Constant

Chemical Equilibrium

Chemical equilibrium is achieved in a reaction when the rate of the forward reaction is equal to the rate of the reverse reaction.
At equilibrium, the concentrations of reactants and products remain constant over time, but they aren’t necessarily equal.

Equilibrium Constant (K)

The equilibrium constant (K) is a measure of the ratio of product concentrations to reactant concentrations when a chemical reaction reaches equilibrium.
This constant is determined using the expression: K = [C]^c [D]^d / [A]^a [B]^b. Here, [A], [B], [C] and [D] represent molar concentrations of reactants (A and B) and products (C and D); while a, b, c, and d are their respective coefficients in the balanced chemical equation.
K is calculated at a specific temperature and remains constant unless the temperature changes.

Interpreting Equilibrium Constant

If K > 1, the reaction favours formation of products at equilibrium. This means the equilibrium lies to the right.
If K < 1, the reaction favours formation of reactants at equilibrium, i.e., the equilibrium lies to the left.
If K = 1, the amounts of products and reactants at equilibrium are approximately equal.

Relating K to ΔG

The equilibrium constant is related to the standard free-energy change (ΔG°) according to the following equation: ΔG° = -RT lnK. Here, R refers to the gas constant and T stands for temperature in Kelvin.
A large positive ΔG° (ΔG° » 0) means equilibrium constant K is very small (K « 1), showing reaction favours the reactants. Conversely, a large negative ΔG° (ΔG° « 0) corresponds to large K (K » 1), indicating reaction favours the products.
At equilibrium, ΔG° = 0.

Effect of Changing Conditions on K

Changes in concentration or pressure do not change the value of K. They may shift the position of equilibrium but K will remain the same.
Only changes in temperature can alter the value of the equilibrium constant K. If the forward reaction is exothermic (releases heat), increasing temperature will cause K to decrease. If the forward reaction is endothermic (absorbs heat), increasing temperature will cause K to increase.

Significance of Equilibrium Constant

Knowledge of the equilibrium constant helps scientists predict the direction of the reaction in given conditions, allowing us to understand, predict and control chemical reactions in laboratories and industrial processes.