Characteristics of Equilibrium Constant

How $K_c$ and $K_p$ behave under different conditions and stoichiometric changes.

By chemca Team • Updated Jan 2026

The Equilibrium Constant ($K$) is a fundamental value that defines the state of a reversible reaction at equilibrium. While it is constant for a given reaction at a fixed temperature, its value depends on specific factors and follows strict mathematical rules when the reaction equation is manipulated.

1. Dependence on Temperature

Temperature is the ONLY factor that changes the numerical value of the Equilibrium Constant.

Endothermic Reactions ($\Delta H > 0$): As Temperature increases, $K$ increases (Product formation favored).
Exothermic Reactions ($\Delta H < 0$): As Temperature increases, $K$ decreases (Reactant formation favored).

Van't Hoff Equation:

$$ \ln \left( \frac{K_2}{K_1} \right) = \frac{\Delta H^\circ}{R} \left( \frac{1}{T_1} - \frac{1}{T_2} \right) $$

2. Dependence on Stoichiometry

If the chemical equation is modified, the expression and value of $K$ change accordingly.

A. Reversing the Reaction

If a reaction is reversed, the new constant is the reciprocal of the original.

$A \rightleftharpoons B \quad (K)$
$B \rightleftharpoons A \quad (K' = 1/K)$

B. Multiplying by a Coefficient

If a reaction is multiplied by a factor $n$, the new constant is the original raised to the power of $n$.

$A \rightleftharpoons B \quad (K)$
$nA \rightleftharpoons nB \quad (K' = K^n)$

C. Adding Reactions

If multiple reactions are added to give a net reaction, their equilibrium constants are multiplied.

Rxn 1: $A \rightleftharpoons B \quad (K_1)$
Rxn 2: $B \rightleftharpoons C \quad (K_2)$
Net: $A \rightleftharpoons C \quad (K_{net} = K_1 \times K_2)$

3. Factors That Do NOT Affect K

Initial Concentrations/Pressures: $K$ is independent of the starting amounts of reactants or products.
Presence of a Catalyst: A catalyst speeds up both the forward and reverse reactions equally. It helps reach equilibrium faster but does not change the value of $K$ or the composition of the equilibrium mixture.
Total Pressure/Volume: While changing pressure may shift the equilibrium position (Le Chatelier's Principle) to maintain the constant, the numerical value of $K$ itself remains unchanged (at constant T).
Inert Gas Addition: Does not change $K$.

4. General Properties

Applicable only when the reaction has attained equilibrium.
Expressions for $K$ do not include pure solids or pure liquids (their active mass is taken as unity).
The value of $K$ relates to the standard Gibbs Free Energy change: $\Delta G^\circ = -RT \ln K$.

Knowledge Check

Test your understanding of Equilibrium Constants

Search This Blog

Monday