Chemical changes · GCSE Chemistry
Catalysts
GCSE Chemistry revision on catalysts: alternative pathway, lower activation energy, reaction profiles, and named examples including enzymes and iron in the Haber process.
A catalyst provides an alternative pathway with a lower activation energy. It is not used up. It speeds both directions of a reversible reaction equally and does not change the yield at equilibrium.
The important bits
What you need to know
- 1
A catalyst increases rate by providing an alternative reaction pathway with a lower activation energy, so a larger fraction of collisions are successful at the same temperature.
- 2
The catalyst is not used up in the reaction. It does not appear as a reactant or product in the overall equation, though it may take part in intermediate steps.
- 3
On a reaction profile, the catalyst lowers the hump. The energies of reactants and products stay the same, so ΔH does not change.
- 4
Catalysts do not increase the amount of product from a given amount of limiting reactant; they only help you get that product faster. On an equilibrium graph, they shorten the time to a flat line but the final concentrations stay the same (Higher).
- 5
Enzymes are biological catalysts: protein molecules that catalyse reactions in living cells, usually at around 37 °C, and are specific for one reaction.
- 6
Named examples: MnO₂ for 2H₂O₂ → 2H₂O + O₂; iron in the Haber process; vanadium(V) oxide in the contact process; nickel for hydrogenation of alkenes; platinum/palladium/rhodium in catalytic converters.
- 7
Catalytic converters convert CO, NOₓ and unburnt hydrocarbons into CO₂, N₂ and H₂O. Transition metals are common catalysts (Triple link).
- 8
A poisoned catalyst (impurity adsorbed on the surface) loses activity. That is why industrial gases are purified and why converters fail if the wrong fuel is used.
Quotations worth analysing
Short evidence. Real method.
“A catalyst provides an alternative pathway with a lower activation energy.”
This sentence scores. “It speeds up the reaction” is true but incomplete. “It is used up” is false. “It raises the temperature” is false.
“2H₂O₂(aq) → 2H₂O(l) + O₂(g) MnO₂ catalyst”
Manganese(IV) oxide is still there at the end (often as a black powder you can filter). Oxygen relights a glowing splint. Same products, faster rate.
“N₂ + 3H₂ ⇌ 2NH₃ iron catalyst”
Iron lets the mixture reach equilibrium faster. It does not shift the equilibrium yield. Temperature and pressure do that (Higher / Triple).
Go deeper
Lower Ea, same ΔH — draw it
Sketch reactants, a high hump, products. Label Ea as the rise from reactants to the top of the hump, and ΔH as the difference between products and reactants. A catalyst redraws a lower hump on the same starting and finishing lines. Exothermic profiles still finish lower; endothermic still finish higher. Students draw the catalyst lowering the products as well, which would change ΔH. That is wrong. In collision-theory language: at the same temperature, more particles now have energy greater than the new, lower Ea, so successful collisions are more frequent. The particles are not hotter.
Go deeper
Equilibrium yield is not a rate
If a reaction is reversible, a catalyst speeds the forward and reverse reactions. You get to the same equilibrium position more quickly. Industry loves that because time is money, but if the equilibrium mixture only contains 15% ammonia, the catalyst will not turn it into 90%. Temperature, pressure and concentration change the yield (Le Chatelier, Higher). Mixing those ideas is a classic six-mark failure. For Foundation, it is enough to say the catalyst is not used up and the products are the same, just formed faster. The final mass of product from a given mass of reactant does not increase.
Go deeper
Surface catalysts and enzymes are still the same definition
Many industrial catalysts work on a surface: reactant particles adsorb, bonds weaken, products leave. That is still an alternative pathway with lower Ea. A large surface area (pellets, powders, honeycomb converters) matters for the same reason as marble powder — more successful collisions with the catalytic surface. Enzymes are proteins with a specific active site; they denature if you boil them, which is a Biology crossover. In Chemistry papers, “enzyme = biological catalyst” plus lower activation energy is usually enough. Name one industrial catalyst if the question asks for an example.
See the idea in action
Hydrogen peroxide decomposes slowly at room temperature. Adding MnO₂ produces oxygen rapidly. The overall equation is still 2H₂O₂ → 2H₂O + O₂. MnO₂ is unchanged and can be reused. On the profile, Ea is lower; ΔH is still slightly negative (exothermic). The volume of oxygen from a given amount of H₂O₂ is the same as without a catalyst; it is just collected in less time. Heating would also speed the reaction, but that would not be a catalyst — the mixture’s temperature would be higher.
Exam technique
Turn knowledge into marks
Write “alternative pathway, lower activation energy, not used up”. On profiles, only the hump changes. For reversible reactions, say time to equilibrium falls but the equilibrium yield does not. Name a real catalyst if asked.
Common mistakes
Do not give these marks away
- 01
Writing that a catalyst is used up, or that it increases the temperature or the yield.
- 02
Drawing a catalyst changing the energy of the products on a reaction profile.
- 03
Saying enzymes are not catalysts, or that iron in Haber increases the percentage of ammonia at equilibrium.
How does a catalyst increase the rate of a reaction?
AIt is used up to provide extra energy
BIt provides an alternative pathway with a lower activation energy
CIt increases the concentration of the products
DIt raises the temperature of the surroundings
Show the answer
It provides an alternative pathway with a lower activation energy. More collisions then have enough energy to react, so successful collisions are more frequent. The catalyst is not used up and ΔH stays the same.
Quick questions
If this is the bit you searched
How do catalysts work GCSE Chemistry?
They provide an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful. They are not used up.
Does a catalyst change the products or the yield?
No. You get the same products. For a given limiting reactant you get the same amount of product, just faster. A catalyst does not change ΔH or the equilibrium position.
What is an enzyme?
A biological catalyst, usually a protein, that speeds a specific reaction in a living organism by lowering activation energy.
Name a catalyst used in industry.
Iron in the Haber process, vanadium(V) oxide in the contact process, nickel in hydrogenation, or platinum metals in catalytic converters.