equilibrium

Meaning: Chemical equilibrium is the state of a reversible reaction in which forward and reverse rates are equal, so concentrations stay constant. It is dynamic: molecules keep reacting in both directions, and ΔG = 0. Le Châtelier's principle says a disturbance (adding or removing a species, changing gas volume or pressure, changing temperature) shifts the system in the direction that partly offsets it. A catalyst speeds both directions equally and does not change the position. In physics, mechanical equilibrium means net force and net torque are zero.

Equation: Keq = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ) for aA + bB ⇌ cC + dD, where brackets are equilibrium concentrations (mol/L), lowercase letters are coefficients, and Keq is dimensionless; pure solids and liquids are omitted.

On the MCAT: Compare the reaction quotient Q (same form, current concentrations) with Keq: Q < Keq shifts forward, Q > Keq shifts reverse. Expect shift-direction questions, ICE-table calculations, and concentration-versus-time graphs that level off.

Common confusion: Equilibrium means equal rates, not equal concentrations, and the reaction has not stopped. Of the disturbances, temperature changes the value of Keq; concentration or volume changes shift the position but Keq stays the same.