Catalysis, equilibrium effect, promoter

Catalysis

A catalyst is a substance that changes the rate of a chemical reaction without being consumed in the reaction (its mass and chemical nature are the same at the end). The phenomenon is called catalysis. Example: MnO₂ speeds up the decomposition of KClO₃ to O₂.

How a catalyst alters the rate

A catalyst provides a new reaction path with a lower activation energy (Ea). More reacting molecules then have energy equal to or greater than the lowered threshold at the same temperature, so the number of effective collisions rises and the reaction becomes faster (rate constant k = A e^(−Ea/RT) increases). The catalyst is regenerated at the end.

Activation energy on the path R → P

  uncatalysed : ████████████  (high Ea)
  catalysed   : ████          (low Ea)

  Catalyst lowers the barrier; R and P levels unchanged.

The energies of reactants (R) and products (P) are unchanged, so ΔH is the same; only the barrier is lowered.

Effect on equilibrium

A catalyst does not change the position of equilibrium or the equilibrium constant. It speeds up the forward and the backward reactions by the same factor (it lowers the activation energy of both by the same amount), so the equilibrium is reached faster but the equilibrium composition is unchanged. For example, in N₂ + 3H₂ ⇌ 2NH₃, Fe makes equilibrium arrive quickly but does not increase the final percentage of NH₃.

Promoter

Promoters (activators) are substances that, though not themselves catalysts, increase the activity of a catalyst when added in a small quantity. They may increase the active surface or change the crystal structure (creating more defects/active sites), or they may improve the adsorption of the reactants.

• In Haber's process, Mo (or Al₂O₃ + K₂O) is a promoter for the iron catalyst.
• In the water-gas shift reaction, Cr₂O₃ promotes Fe₂O₃.
• Cu promotes Ni in some hydrogenation reactions.