Homogeneous and heterogeneous catalysts — common mistakes

Examiners frequently ask for the equations of catalysed pathways. For homogeneous catalysis (e.g., Fe²⁺), show how the catalyst is used and then regenerated in two separate steps. For heterogeneous catalysis (e.g., V₂O₅), show the oxidation/reduction of the catalyst. Also, remember that a catalyst lowers $E_a$ but has NO effect on the enthalpy change of reaction, $\Delta H$, or the position of equilibrium, $K_c$. It only affects the rate at which equilibrium is reached.

Although catalysts are chemically regenerated in each cycle, they degrade physically and chemically over time. Heterogeneous catalysts can be deactivated by poisoning (where impurities block active sites) or sintering (where high temperatures cause particles to fuse, reducing surface area). This loss of activity eventually makes it more economical to replace the catalyst.

It offers a different set of elementary steps for the reaction to proceed. For a homogeneous catalyst, this involves forming a new intermediate species. For a heterogeneous catalyst, it involves binding reactants to a surface, which weakens their internal bonds and holds them in a favourable orientation for reaction. Both these new pathways have lower activation energies than the uncatalysed route.

Autocatalysis is a specific type of 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, where the Mn²⁺(aq) product catalyses its own formation. Unlike a typical catalysed reaction which is fastest at the start, an autocatalysed reaction starts slowly and then accelerates as the concentration of the catalytic product increases.