Equilibrium Constant
Definition and meaning of Equilibrium Constant in chemistry.
Equilibrium constant (K) is the numerical value of the ratio of product activities to reactant activities, each raised to its stoichiometric coefficient, at chemical equilibrium and a given temperature. It quantifies the extent to which a reversible reaction proceeds toward products before forward and reverse rates become equal.
In more detail
For a general reaction aA + bB ⇌ cC + dD, K = [C]^c[D]^d / [A]^a[B]^b (using concentrations, Kc, or partial pressures, Kp). A large K (K >> 1) indicates the equilibrium mixture favors products, while a small K (K << 1) favors reactants. K depends only on temperature, not on initial concentrations, pressure, or the presence of a catalyst, and it is related to the standard Gibbs free energy change by ΔG° = -RT ln K.
Key facts
| Symbol | K (Kc, Kp, Ka, Kb, Ksp, etc.) |
|---|---|
| Formula | K = [products]^coeff / [reactants]^coeff |
| Depends on | Temperature only |
| Field | Physical Chemistry |
For the Haber process, N2(g) + 3H2(g) ⇌ 2NH3(g), Kc = [NH3]²/([N2][H2]³). At 25°C this Kc is very large, showing the equilibrium strongly favors ammonia, but Kc decreases sharply as temperature rises, reflecting the reaction's exothermic nature.
Frequently asked questions
Does a catalyst change the equilibrium constant?
No. A catalyst speeds up both the forward and reverse reactions equally, helping equilibrium be reached faster, but it does not change the value of K.
What does it mean if K equals 1?
K = 1 means that at equilibrium, the products and reactants have comparable activities, so neither side is strongly favored.