equilibrium point

Meaning: The equilibrium point is the state of a reversible reaction in which forward and reverse rates are equal, so reactant and product concentrations stop changing at a given temperature. It is dynamic: both reactions continue. The same state can be reached from reactants, products, or a mixture, and ΔG = 0 there. A catalyst speeds the approach but does not move the position. (In physics, it means a point where net force is zero.)

Equation: K = Q at equilibrium, where Q = [C]^c[D]^d / ([A]^a[B]^b) for aA + bB ⇌ cC + dD; brackets are concentrations (mol/L), exponents are coefficients, and K and Q are treated as unitless.

On the MCAT: Expect concentration-versus-time graphs that level off, or comparisons of Q with K: if Q < K the net reaction goes forward, and if Q > K it goes in reverse. Le Châtelier questions ask which way the system shifts after a change in concentration, pressure, or volume. Only a temperature change alters K.

Common confusion: Students think equilibrium means equal concentrations or that the reaction has stopped. Concentrations are equal only if K happens to be near 1; what defines equilibrium is equal rates and Q = K. It is also not the equivalence point of a titration.