For aA + bB ⇌ cC + dD: Kc = [C]^c [D]^d / [A]^a [B]^b
At equilibrium, the rates of the forward and reverse reactions are equal, and concentrations remain constant. The equilibrium constant Kc = [products]^coefficients / [reactants]^coefficients. Large Kc (>>1) means products are favored; small Kc (<<1) means reactants are favored. Kc is related to Gibbs energy: ΔG° = −RT ln(K). For gas-phase reactions, Kp uses partial pressures: Kp = Kc(RT)^Δn, where Δn = moles of gaseous products − moles of gaseous reactants. Le Chatelier's principle predicts how equilibrium shifts when conditions change: adding reactants shifts forward, increasing temperature shifts toward the endothermic direction, and pressure changes affect reactions with Δn ≠ 0. Kc depends only on temperature, not on initial concentrations or catalysts.