catalyst

Meaning: A catalyst opens a different mechanism with a lower activation energy (Ea). At a given temperature, more collisions then have enough energy to react. The catalyst may take part in intermediate steps, but it is regenerated, so it does not appear in the net equation. Because it lowers Ea for the forward and reverse reactions by the same amount, equilibrium is reached sooner but is not shifted. A homogeneous catalyst is in the same phase as the reactants; a heterogeneous catalyst is in a different phase, such as a solid metal surface. Enzymes are biological catalysts.

Equation: k = A e^(−Ea/RT), where k is the rate constant (units depend on reaction order), A is the frequency factor (same units as k), Ea is activation energy (J/mol), R is 8.314 J/(mol·K), and T is temperature (K). A lower Ea gives a larger k.

On the MCAT: You may read an energy diagram where the catalyzed curve has a lower peak but the same reactant and product levels. Expect to predict that rate rises while ΔH, ΔG, Keq, and equilibrium yield stay the same. Enzyme passages use the same logic.

Common confusion: A catalyst does not make a reaction more favorable or raise the equilibrium yield. ΔG depends only on the reactants and products, which are unchanged. A catalyst cannot make a nonspontaneous reaction spontaneous.