Catalysts

Specification Reference 3.2.2 (c)–(e)

Quick Notes

A catalyst increases the rate of a reaction without being used up.
Catalysts provide an alternative reaction pathway with a lower activation energy (E_a).
Types of catalysis:
- Homogeneous: catalyst and reactants in same phase.
- Heterogeneous: catalyst in a different phase (e.g. solid catalyst with gaseous reactants).
Common techniques to measure reaction rates:
- Gas volume (e.g. using a gas syringe)
- Mass loss (e.g. CO₂ escape in open container)
- Colour change or pH shift (e.g. clock reactions)
- Time measurements

Full Notes

What is a Catalyst?

A catalyst is a substance that speeds up a reaction without being chemically changed or used up.

It works by providing an alternative reaction pathway with a lower activation energy (E_a).

We can show how catalysts lower the activation energy using a reaction profile diagram:

The catalysed pathway has a lower peak (activation energy).
The enthalpy change (ΔH) is the same with or without the catalyst.

Types of Catalysis

Note - types of catalyst, along with specific examples are outlined later in the course here.

There are two main types of catalyst: homogeneous and heterogeneous.

Heterogeneous Catalysis

Heterogeneous catalysts are in a different phase than the reactants.

The reaction occurs on the catalyst’s surface at active sites.

Homogeneous Catalysis

Homogeneous catalysts are in the same phase as the reactants.

The reaction proceeds via an intermediate species. Transition metals are effective due to variable oxidation states.

Benefits of Using Catalysts

Lower activation energy allows reactions to occur at lower temperatures and pressures.
Reduces fuel consumption, lowering CO₂ emissions.
Catalysts are not used up and can be reused, increasing efficiency.
Economically beneficial, reducing operational costs.
Supports greener chemistry and environmentally friendly industrial processes.

Experimental Methods for Measuring Reaction Rates

Different methods can be used to measure rate depending on the type of reaction and substances involved. The key idea is to measure how the concentration of a reactant or product changes over time.

Mass loss: Used when a gaseous product is formed. Measure decrease in mass over time using a balance.
Gas volume: Collect gas produced using a gas syringe or inverted burette/measuring cylinder filled with water. Record volume at regular intervals.
Timing: Measure how long it takes for a visible change (e.g. colour change, precipitate formation).
Initial rate method: Start reactions with varying concentrations; measure time to a visible endpoint.

Summary

Catalysts speed up reactions without being used up.
They provide an alternative pathway with lower activation energy.
Homogeneous and heterogeneous catalysis differ by phase relationship.
Benefits include efficiency, cost reduction, and sustainability.
Rates can be measured via mass loss, gas volume, timing, or initial rate methods.