Keq

Meaning: At equilibrium the forward and reverse reaction rates are equal, so concentrations stop changing. Keq describes where that balance sits. A large Keq means products dominate at equilibrium; a small Keq means reactants dominate. Keq is linked to free energy: a negative ΔG° corresponds to Keq > 1, and ΔG° = 0 corresponds to Keq = 1.

Equation: For aA + bB ⇌ cC + dD, Keq = [C]^c[D]^d / ([A]^a[B]^b), where brackets are equilibrium molar concentrations (mol/L) and a, b, c, d are stoichiometric coefficients. Keq is unitless in the thermodynamic treatment. Pure solids and pure liquids are left out. The free energy link is ΔG° = −RT ln Keq, where R = 8.314 J/(mol·K), T is in K, and ΔG° is in J/mol.

On the MCAT: You may be asked to compute Keq from equilibrium concentrations, or to compare Q with Keq to predict the shift. If Q < Keq, the reaction moves toward products; if Q > Keq, it moves toward reactants. Passages also ask how Keq changes with temperature: raising T increases Keq for an endothermic reaction and decreases it for an exothermic one. Reversing a reaction gives 1/Keq. Adding reactions multiplies their Keq values.

Common confusion: Changing concentration, pressure, volume, or adding a catalyst does not change Keq. These can shift the position of equilibrium or the speed of reaching it, but only a temperature change alters the value of Keq itself. Also, a large Keq says nothing about how fast equilibrium is reached.