Cambridge IGCSE Chemistry · Syllabus 0620 · Chemical Reactions
Catalyst
What is Catalyst?
A substance that increases the rate of a chemical reaction and is chemically unchanged at the end of the reaction. It can be recovered with its mass unchanged, it does not appear in the chemical equation, and it does not change the amount of product that a given amount of reactant can make.
This definition is part of the Chemical Reactions chapter in Cambridge IGCSE Chemistry.
Common mistakes with Catalyst
- 7. “Heating lowers the activation energy.” Supplement 6.2.6 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.” Core 6.2.2 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.” Supplement 6.3.4 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.” Supplement 6.3.4 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
- Drawing the diagram. An answer asked to add a catalysed pathway to a reaction-pathway diagram must keep the reactant level and the product level exactly where they were, and draw a second, lower hump between them. Moving either level changes the energy of the reaction, which a catalyst cannot do; drawing only one lowered hump loses the fact that the uncatalysed route still exists.
- 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.