Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Catalyst
What is Catalyst?
A substance that increases the rate of a chemical reaction by providing an alternative reaction pathway of lower activation energy, and that is chemically unchanged at the end of the reaction. A catalyst does not alter the enthalpy change of the reaction and does not increase the yield of product at equilibrium.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Catalyst in context
Chemical reactions are changes in which new substances form as bonds break and re-form between particles, and this chapter answers three questions about them. Rate asks how fast a reaction happens, and is governed by collision theory: particles must collide with combined energy at least equal to the activation energy, \(E_\mathrm{a}\), and factors such as concentration, pressure, surface area, temperature and catalysts all work by changing how often successful collisions occur. Equilibrium asks how far a reversible reaction goes before the forward and reverse rates become equal. Redox asks which species lost electrons and which gained them, tracked through oxidation numbers.
A reversible reaction, shown with the symbol \(\rightleftharpoons\), can run in both directions at once, and in a closed system it reaches dynamic equilibrium when the forward and reverse rates are equal, so the concentrations of reactants and products stop changing. Changing temperature, pressure or concentration shifts the position of equilibrium, but a catalyst never does — it speeds up both directions equally, reaching the same equilibrium sooner rather than reaching a different one. The Haber and Contact processes each pick operating conditions that trade off rate, yield, safety and cost rather than chasing the highest possible yield alone. In redox terms, oxidation is loss of electrons and reduction is gain of electrons, so an oxidising agent is itself reduced and a reducing agent is itself oxidised.
Common mistakes with Catalyst
- 7. “Heating lowers the activation energy.” Why temptingHeating and catalysis both make more collisions succeed. Repair\(E_\mathrm{a}\) is a fixed property of the reaction pathway. Heating raises the particles towards the barrier; only a catalyst lowers the barrier, by offering a different pathway. TransferOn an energy profile, which of the two changes would you be able to see? The catalyst; heating changes nothing on the diagram.
- 9. “The catalyst is used up, so you have to keep adding it.” Why temptingIt clearly takes part, so it feels like a reactant. RepairIt is regenerated. Filter it off at the end, dry it and weigh it: the mass is unchanged. That is the experimental evidence, and it is worth quoting. TransferWhy does a catalyst not appear in the equation? It is neither consumed nor produced, so it cancels from both sides.
- 10. “A catalyst increases the yield.” Why temptingMore product appears sooner, which looks like more product. RepairIt lowers the activation energy of forward and reverse reactions equally, so both rates rise by the same factor. Equilibrium arrives sooner at exactly the same composition. TransferExplain why iron is used in the Haber process. To reach equilibrium fast enough at \(450\,{}^\circ\mathrm{C}\) — not to raise the yield.
- 16. “Heat acts as a catalyst.” Why temptingBoth speed reactions up. RepairA catalyst lowers \(E_\mathrm{a}\) and leaves the position of equilibrium alone. Heat leaves \(E_\mathrm{a}\) alone and does move the position. They are opposites in both respects. TransferWhich of the two changes the yield? Heat.
Examiner tips on Catalyst
- The pressure is the discriminating fact. Both processes run at \(450\,{}^\circ\mathrm{C}\), and both use a catalyst, so the temperature and the idea of a catalyst do not tell the two apart. The pressures differ by a factor of a hundred: \(200\) atmospheres for Haber, \(2\) atmospheres for Contact. Both numbers contain the digits “200”, so check the unit every time before you write one of them down.
Questions students ask about Catalyst
What is the difference between rate of reaction and position of equilibrium?
Rate measures how fast a reaction happens; position of equilibrium measures how far it goes. A catalyst is the clearest proof they are separate: it raises the rate by lowering the activation energy for both directions, but it never moves the position of equilibrium, because it speeds up the forward and reverse reactions by the same factor.