dynamic equilibrium

Meaning: In a reversible reaction in a closed system, the forward rate falls as reactants are used up and the reverse rate rises as products build. Eventually the two rates are equal. From then on, each reactant and product is consumed and formed at the same rate, so macroscopic properties such as concentration, pressure and color stay constant. The same idea applies to phase changes (liquid ⇌ vapor at equal evaporation and condensation rates) and to dissolving (solid ⇌ dissolved ions in a saturated solution).

Equation: rate(forward) = rate(reverse), so kf[A] = kr[B] for A ⇌ B, and Keq = kf/kr = [B]/[A]. Here kf and kr are the forward and reverse rate constants (units depend on reaction order, s⁻¹ for first order), and the brackets are concentrations in mol/L. At equilibrium ΔG = 0.

On the MCAT: Passages show a concentration-versus-time graph that levels off, and ask what happens after a stress (Le Châtelier): adding reactant or removing product shifts the reaction forward, and the system reaches a new equilibrium. Isotope-labeling setups show the label appearing in both reactants and products, which proves reactions continue. A catalyst speeds both directions equally and changes the time to reach equilibrium, not Keq.

Common confusion: Students think the reaction has stopped or that reactant and product concentrations are equal. Equal rates, not equal concentrations, define it. Keq can be far above or below 1, and the constant concentrations hide ongoing molecular change.