5.11 Catalysis

What a catalyst does

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy, without being consumed overall. It's regenerated at the end of the mechanism, so it doesn't appear in the overall balanced equation. Because more particles have enough energy to react, the rate constant k increases, raising the reaction rate at a given temperature.

Homogeneous vs. heterogeneous catalysts

A homogeneous catalyst exists in the same phase as the reactants (often dissolved in solution) and typically reacts to form an intermediate, which later reacts further to regenerate the catalyst. A heterogeneous catalyst exists in a different phase (commonly a solid) and works by adsorption — reactant molecules bind to its surface, weakening bonds and lowering the activation energy for the reaction.

Catalysts and energy diagrams

On a potential energy diagram, a catalyzed pathway shows a lower activation energy peak (or an added intermediate dip) compared to the uncatalyzed path, but the energies of reactants and products (ΔH) stay the same. Catalysts change the mechanism and rate, not the overall thermodynamics of the reaction.

Key terms

Catalyst

A substance that increases reaction rate by lowering activation energy without being consumed overall.

Homogeneous catalyst

A catalyst in the same phase as the reactants; typically forms an intermediate before regenerating.

Heterogeneous catalyst

A catalyst in a different phase than reactants (often solid); works via surface adsorption of reactants.

Adsorption

The binding of reactant molecules to a catalyst's surface, which weakens bonds and lowers activation energy.

Effect of catalyst on ΔH

None — a catalyst changes the reaction pathway/rate but does not change the overall enthalpy change of the reaction.

Try an AP-style question

How does a catalyst speed up a reaction?

1. It provides a pathway with a lower activation energy
2. It raises the temperature of the mixture
3. It shifts the equilibrium toward products
4. It makes the reaction's ΔH more negative