Homogeneous and heterogeneous catalysts — common mistakes

When asked to explain a catalytic mechanism with equations, always write at least two steps. The first step should show the catalyst being consumed, and the second step must show it being regenerated. Make sure that if you add your two (or more) steps together, you get the overall equation for the reaction, with the catalyst cancelling out on both sides.

A catalyst increases the rate of both the forward and the reverse reactions by the same factor. It lowers the activation energy for both directions equally. Therefore, the ratio of the rates (which determines the equilibrium constant, $K_c$) remains unchanged, and the final proportions of reactants and products at equilibrium are the same. Equilibrium is simply reached much faster.

Autocatalysis is a specific type of homogeneous catalysis where one of the reaction products acts as the catalyst for the reaction. A classic example is the reaction between manganate(VII) ions and ethanedioate ions: 2MnO₄⁻(aq) + 16H⁺(aq) + 5C₂O₄²⁻(aq) → 2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g). The Mn²⁺(aq) ions produced catalyse the reaction, which is why the reaction starts slowly and then speeds up as more product is formed.

Enzymes are biological catalysts. They are large protein molecules that are typically soluble in the aqueous environment of a cell, just like their substrates (reactants). In this sense, they behave as homogeneous catalysts. However, their mechanism involves the substrate binding to a specific 'active site' on the large enzyme molecule, which is very similar to the surface action of a heterogeneous catalyst. For A-Level purposes, they are best considered a special class of highly specific catalysts, but if forced to choose, their solubility makes them analogous to homogeneous catalysts.