equilibrium constant

Meaning: Keq describes where a reversible reaction settles when forward and reverse rates are equal. It is the product concentrations over the reactant concentrations, each raised to its coefficient. Pure solids and pure liquids are left out. A Keq much greater than 1 means products dominate at equilibrium; much less than 1 means reactants dominate.

Equation: For aA + bB ⇌ cC + dD, Keq = [C]^c [D]^d / ([A]^a [B]^b), where brackets are equilibrium concentrations in mol/L (use partial pressures in atm or bar for Kp). Also ΔG° = −RT ln Keq, with ΔG° in J/mol, R = 8.314 J/(mol·K) and T in K.

On the MCAT: Expect an ICE table calculation, or a comparison of the reaction quotient Q with Keq: if Q < Keq the reaction proceeds forward, and if Q > Keq it proceeds in reverse. Adding reactant or changing pressure shifts the position but leaves Keq unchanged. Temperature does change Keq: it rises with temperature for an endothermic reaction and falls for an exothermic one. Reversing a reaction gives 1/Keq, and adding two reactions multiplies their K values.

Common confusion: A large Keq does not mean a fast reaction. Keq is thermodynamic, set by ΔG°, while rate depends on activation energy. A catalyst speeds the forward and reverse reactions and leaves Keq unchanged.