8.3 Catalysts
🎯 Learning outcomes
After completing this lesson, you should be able to:
- explain and use the terms catalyst and catalysis
- explain that, in the presence of a catalyst, a reaction has a different mechanism — one of lower activation energy
- explain this catalytic effect in terms of the Boltzmann distribution
- construct and interpret a reaction pathway diagram, for a reaction in the presence and absence of an effective catalyst
- distinguish between homogeneous and heterogeneous catalysts
1⚗️What is a catalyst?
A catalyst is a substance that increases the rate of a reaction but remains chemically unchanged at the end of the reaction.
Catalysis is the increase in the rate of a chemical reaction brought about by the addition of a substance which is not itself used up in the reaction.
A catalyst may take part in the reaction mechanism — even forming temporary intermediates along the way — but by the end of the reaction it is fully regenerated, unchanged in mass and chemical identity.
Catalysts are everywhere in industry and biology: iron is used to catalyse the Haber process for making ammonia, platinum-based catalysts clean up exhaust gases in catalytic converters, and enzymes act as highly efficient biological catalysts inside living cells.
2🛤️How catalysts work: an alternative pathway
Catalysts increase the rate of a reaction because they allow it to proceed by a different reaction pathway (mechanism) — one that has a lower activation energy than the uncatalysed route.
Because the alternative pathway has a lower activation energy, EA(cat), a much greater proportion of collisions between reacting particles now have enough energy to be effective — so the rate of reaction increases, at the same temperature.
A catalyst does not increase the energy of the reacting particles, and it does not change the overall enthalpy change, ΔH, of the reaction. It only opens up a lower-energy route from reactants to products — the products, and the overall energy released or absorbed, stay exactly the same.
3📈The Boltzmann distribution and catalysts
At a fixed temperature, the Boltzmann distribution of molecular kinetic energies is completely determined by the temperature — so a catalyst, added at constant temperature, does not change the shape of the curve at all.
What a catalyst changes is where the activation energy line sits on that same, unchanged curve. Since EA(cat) is lower than EA, it cuts the curve further to the left — enclosing a much larger shaded area, and therefore a much larger proportion of molecules that already have enough energy to react.
Don’t confuse this with the effect of temperature. Raising the temperature reshapes the whole curve (peak shifts and flattens). A catalyst, at constant temperature, leaves the curve exactly as it was — it only moves EA itself, to a lower value.
4⛰️Reading a reaction pathway diagram
A reaction pathway diagram (enthalpy profile) can show the catalysed and uncatalysed routes for the same reaction side by side, so you can compare them directly.
When constructing or interpreting a diagram like this, check for three things:
- the reactants and products energy levels are the same for both curves — a catalyst cannot change ΔH
- the catalysed curve has a lower peak than the uncatalysed curve — this height difference represents the drop from EA to EA(cat)
- both curves still start and finish at the same two points — the catalyst only changes the pathway in between, not the overall energy change
5🔬Homogeneous vs heterogeneous catalysts
A homogeneous catalyst is in the same phase as the reactants — for example, a catalyst dissolved in the same aqueous solution as the reactants.
A heterogeneous catalyst is in a different phase to the reactants — most commonly a solid catalyst acting on liquid or gaseous reactants.
| Type | Phase relationship | Typical example |
|---|---|---|
| Homogeneous | same phase as reactants | dilute H₂SO₄ catalysing esterification of an alcohol and carboxylic acid (all liquid) |
| Heterogeneous | different phase to reactants | iron catalyst (solid) in the Haber process with N₂ and H₂ (gases) |
Heterogeneous catalysts are especially convenient industrially because the solid catalyst is easy to separate from liquid or gaseous products at the end of the process — for example, the solid platinum/palladium/rhodium catalyst in a car’s catalytic converter stays fixed in place as exhaust gases pass over it.
📌 Exam tips & quick summary
✅ Do remember
- catalyst = increases rate, chemically unchanged at the end of the reaction
- catalysts work by providing an alternative pathway with a lower activation energy
- at constant temperature, a catalyst does not change the Boltzmann curve — it only moves the EA line to a lower value, so more molecules already qualify
- ΔH is identical for the catalysed and uncatalysed pathway — only the peak height (EA) differs
- homogeneous = same phase as reactants; heterogeneous = different phase
🚫 Do not confuse
- a catalyst lowering EA is not the same as it changing ΔH — thermodynamics (ΔH) and kinetics (rate, EA) are separate ideas
- catalyst effect on the Boltzmann curve ≠ temperature effect — temperature reshapes the whole curve; a catalyst only shifts the EA line
- “unchanged at the end” does not mean a catalyst can’t participate in intermediate steps of the mechanism
🔁 Quick recap
- Catalyst: speeds up a reaction, unchanged at the end
- Mechanism: alternative pathway, lower EA(cat) < EA, same ΔH
- Boltzmann view: same curve, lower EA line → bigger reacting fraction
- Homogeneous: same phase · Heterogeneous: different phase