Catalysts — AQA A-Level Chemistry
Test yourself on Catalysts with AQA A-Level practice questions.
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Catalysts explained
A catalyst speeds a reaction up and is present in the same chemical form and the same amount at the end as at the start.
Read the full explanation
That does not mean it is a spectator: it usually reacts, forming an intermediate, and is then regenerated in a later step, which is why the amount is unchanged rather than merely unaffected. Heterogeneous catalysts are in a different phase from the reactants, typically a solid surface with gases passing over it. Homogeneous catalysts share the phase. An autocatalyst is a product of the reaction that then catalyses it, so the rate rises as the reaction proceeds, as with the manganese(II) ions produced when manganate(VII) oxidises ethanedioate ions. A catalyst changes the rate only, never ∆H or the equilibrium yield.
Catalysts work by providing an alternative reaction route of lower activation energy.
A catalyst does not push the existing reaction harder; it opens a different one. The catalysed route passes through different intermediates and has a lower activation energy, so at the same temperature a much larger proportion of colliding particles now has enough energy to react, and the frequency of successful collisions rises. In heterogeneous catalysis the reactants adsorb onto active sites, which weakens their bonds and holds them in a favourable orientation before the products desorb. In homogeneous catalysis an intermediate forms and then breaks down to release the catalyst. The manganate(VII) and ethanedioate reaction shows why the route matters: two negative ions repel each other, giving a high Eₐ, whereas the positive manganese ions of the catalysed path attract the ethanedioate ions.
Students should be able to: use a Maxwell–Boltzmann distribution to help explain how a catalyst increases the rate of a reaction involving a gas.
The catalyst question and the temperature question look alike but need opposite diagrams. Adding a catalyst does not change the temperature, so the distribution curve is not redrawn at all: a single curve is used and a second, lower activation energy is marked to the left of the original. The shaded area beyond that new barrier is far larger than the area beyond the original one, meaning many more molecules now have energy greater than or equal to the catalysed activation energy. The proportion of successful collisions rises and so does the rate, even though the mean energy of the molecules and the collision frequency are both unaltered. Marking two barriers on one unchanged curve is what the mark scheme is looking for.
Your focus
- Decide whether a named species in a described reaction is acting as a catalyst, a reactant or an intermediate, justifying the choice from whether it is present at the start, at the end, or only part way through.
- Classify a given catalysed reaction as heterogeneous or homogeneous and justify the classification.
- Explain how a catalyst can take part in a reaction and still be present unchanged at the end.
Show all 12 objectives
- Identify autocatalysis from a concentration against time curve that steepens partway through the reaction, or from a rate against time curve that rises to a maximum before falling.
- Explain how a catalyst increases rate by reference to an alternative route with a lower activation energy.
- Sketch an enthalpy profile showing catalysed and uncatalysed routes with the same reactants, products and ∆H.
- Describe the steps of heterogeneous catalysis, from adsorption at active sites to desorption of products.
- Explain why a reaction between two ions of like charge has a high activation energy and how a catalyst of opposite charge overcomes it.
- Sketch one Maxwell–Boltzmann distribution marked with both catalysed and uncatalysed activation energies.
- Explain a catalytic rate increase using the change in the area beyond the activation energy on an unchanged curve.
- Distinguish the diagram required for a catalyst from that required for a temperature rise, and justify the difference.
- Deduce what happens to the mean molecular energy and the collision frequency when a catalyst is added.
Catalysts exam tips
Marking Points
- one mark for stating that a catalyst increases the rate of reaction
- one mark for adding that it is chemically unchanged, and unchanged in amount, at the end
- one mark for defining heterogeneous as being in a different phase or state from the reactants
- one mark for defining an autocatalyst as a product of the reaction which catalyses that reaction
- one mark for stating that the catalyst provides an alternative route or mechanism
- one mark for that route having a lower activation energy
- one mark for concluding that a greater proportion of collisions is successful at the same temperature
- one mark for describing the catalysed route in a named example, such as adsorption onto active sites or formation of an intermediate
- one mark for explaining a high uncatalysed Eₐ by repulsion between ions of like charge, where relevant
- one mark for a single correctly shaped distribution curve with both axes labelled
- one mark for two activation energies marked on the energy axis, the catalysed one lower
- one mark for stating that more molecules have energy greater than or equal to the catalysed activation energy
- one mark for concluding that the proportion, and hence the frequency, of successful collisions increases so the rate rises
Examiner Tips
- 💡Give both halves of the definition, rate increased and chemically unchanged in composition and amount, for a two mark answer.
- 💡For heterogeneous, say different phase or state from the reactants; different substance is not the point.
- 💡If a reaction speeds up partway through with no change in conditions, suspect autocatalysis.
- 💡Use the exact phrase alternative route of lower activation energy; both halves are credited.
- 💡On a profile diagram draw a second, lower hump between the same reactant and product levels, and label both barriers.
- 💡For an ionic example, explain the high uncatalysed barrier in terms of repulsion between ions of the same charge.
- 💡Ask yourself first whether the temperature changed; if it did not, use one curve only.
- 💡Label the two barriers Eₐ and Eₐ(catalysed) and shade both tails so the comparison is visible.
- 💡Finish with a sentence naming the proportion of successful collisions, which is the explanation mark.
Common Mistakes
- saying a catalyst takes no part in the reaction, when it takes part and is regenerated
- defining an autocatalyst as a reactant that catalyses the reaction, when it must be a product
- claiming a catalyst increases the yield of product rather than the rate at which the yield is reached
- saying a catalyst is not used up but omitting that it is chemically unchanged in composition
- describing a heterogeneous catalyst as being in a different phase from the products only
- saying the catalyst lowers the activation energy of the reaction, rather than providing a route with a lower one
- claiming the catalyst increases the collision frequency or the energy of the particles
- stating that the catalyst makes the reaction more exothermic, when ∆H is unaffected
- drawing a catalysed profile with a different product energy level
- attributing the effect to the catalyst being a reactant that is consumed
- drawing a second curve for the catalysed reaction, as though the temperature had changed
- shifting the distribution to the right instead of moving the activation energy to the left
- saying the catalyst gives the molecules more energy
- marking the catalysed Eₐ to the right of the original one
- explaining the effect as an increase in collision frequency