Equilibrium Constant — Core Principles

Core Principles

The equilibrium constant ($K$) is a quantitative measure of the extent of a reversible chemical reaction at equilibrium. For a general reaction $aA + bB \rightleftharpoons cC + dD$, $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ (using molar concentrations) and $K_p = \frac{(P_C)^c(P_D)^d}{(P_A)^a(P_B)^b}$ (using partial pressures for gases).

The value of $K$ is constant for a given reaction at a specific temperature and indicates the relative amounts of products and reactants at equilibrium. A large $K$ means products are favored, while a small $K$ means reactants are favored.

Only temperature changes the value of $K$. Solids and pure liquids are excluded from the $K$ expression because their concentrations are constant. The relationship $K_p = K_c(RT)^{Delta n_g}$ connects the two constants, where $Delta n_g$ is the change in moles of gaseous species.

The reaction quotient ($Q$) is calculated similarly to $K$ but for non-equilibrium conditions, allowing prediction of reaction direction.

Important Differences

vs Reaction Quotient (Q)

Aspect	This Topic	Reaction Quotient (Q)
Definition	Equilibrium Constant (K): Ratio of product to reactant concentrations/pressures at *equilibrium*.	Reaction Quotient (Q): Ratio of product to reactant concentrations/pressures at *any given time*.
Value	K is a constant for a given reaction at a specific temperature.	Q's value changes as the reaction proceeds towards equilibrium.
Purpose	Indicates the extent of a reaction at equilibrium and the relative amounts of products/reactants.	Used to predict the direction a reaction will shift to reach equilibrium by comparing it to K.
Condition	Applicable only when the system is at chemical equilibrium.	Applicable at any point during the reaction, whether at equilibrium or not.

The equilibrium constant (K) is a fixed value for a specific reaction at a given temperature, representing the ratio of products to reactants when the system has reached a state of dynamic balance. It quantifies the extent of the reaction. In contrast, the reaction quotient (Q) is calculated using the same mathematical expression as K but applies to concentrations or partial pressures at any arbitrary point in time, not necessarily at equilibrium. By comparing Q with K, one can predict the direction in which a reaction will proceed to achieve equilibrium.