chemical equilibrium

Meaning: A reversible reaction reaches equilibrium when the forward and reverse rates are equal. Concentrations then stay constant, but both reactions keep running. Organic examples include Fischer esterification, hemiacetal and acetal formation, keto-enol tautomerism, and the interconversion of sugar anomers. The position of equilibrium reflects the relative stability of products and reactants. Le Chatelier's principle predicts the response: adding a reactant or removing a product shifts the reaction toward products.

Equation: Keq = [products] / [reactants], each concentration raised to its stoichiometric coefficient; ΔG° = −RT ln Keq, where Keq is unitless, concentrations are in mol/L, ΔG° is in J/mol, R = 8.314 J/(mol·K), and T is in K.

On the MCAT: Passages often show an acid-catalyzed esterification or acetal formation and ask how to raise the yield. Use excess alcohol or remove water. Other questions ask which tautomer is favored (the keto form for simple ketones) or whether a catalyst or a temperature change alters the outcome. A negative ΔG° means Keq > 1 and products are favored.

Common confusion: Equilibrium does not mean equal concentrations of reactants and products, and the reaction does not stop. A catalyst, such as an acid, speeds the approach to equilibrium in both directions but does not change Keq or the final composition. For a given reaction, only a change in temperature changes the value of Keq. Adding or removing a species shifts the position, but Keq stays the same.