Catalysts — AQA GCSE Combined Science
Test yourself on Catalysts with AQA GCSE practice questions.
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Catalysts explained
A catalyst is a substance added to a reaction that changes its rate without being consumed.
Read the full explanation
It is not a reactant used up in the reaction, so it can be recovered chemically unchanged at the end, even though it takes part in an intermediate step. For example, manganese(IV) oxide speeds up the decomposition of hydrogen peroxide to water and oxygen; filtering it off shows the same mass remains. Catalysts are specific: the catalyst that works for one reaction may not work for another, so different reactions need different catalysts. In living organisms, enzymes are biological catalysts that speed up metabolic reactions such as digestion and respiration at moderate temperatures.
Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy.
Reactions happen when particles collide with at least the activation energy, the minimum energy needed for a successful collision. A catalyst increases the rate by providing a different pathway with a lower activation energy. On an energy profile, the reactants and products stay at the same energy levels, but the peak between them is lower, so a greater proportion of collisions has enough energy to react. For example, adding manganese(IV) oxide to hydrogen peroxide gives a lower-energy route, so oxygen is produced faster. The catalyst is not used up and does not change the overall energy change of the reaction; it only changes the route and therefore the rate.
A reaction profile for a catalysed reaction can be drawn in the following form:
A reaction profile is a graph of energy against reaction progress. For an exothermic reaction, reactants start at a higher energy than products; for an endothermic reaction, products finish higher. The reactants must first reach the activation energy at the peak. A catalyst provides an alternative pathway with a lower activation energy, so the peak is lower, while the energy of reactants and products, and therefore the overall energy change, stays the same. To draw one, plot energy on the y-axis and progress on the x-axis, mark reactant and product levels, draw a curve peaking between them, then draw a second curve with a lower peak for the catalysed route. Label both activation energies from the reactant level to each peak.
Students should be able to identify catalysts in reactions from their effect on the rate of reaction and because they are not included in the chemical equation for the reaction.
A catalyst speeds up a reaction without being used up, so it is not consumed and does not appear in the overall chemical equation. To identify one, compare rates: adding the suspected catalyst increases the rate, while the same amount of an inert substance does not. Then check the equation: a species that appears on both sides, or is written above the arrow, is a catalyst rather than a reactant or product. For example, in the decomposition of hydrogen peroxide, 2H₂O₂ → 2H₂O + O₂, manganese(IV) oxide, MnO₂, speeds up the reaction but is not in the equation; it can be filtered off and reused. Enzymes are biological catalysts that behave in the same way.
Students should be able to explain catalytic action in terms of activation energy.
A catalyst speeds up a reaction without being used up. It works by providing an alternative reaction pathway with a lower activation energy, the minimum energy that colliding particles need to react. On an energy profile, the reactants sit at one level and the products at another; the peak between them is the activation energy. With a catalyst, that peak is lower, so a greater proportion of collisions have enough energy to succeed, increasing the rate. For example, adding a catalyst to the decomposition of hydrogen peroxide lowers the energy barrier, so oxygen is produced faster at the same temperature. The catalyst is not consumed, so it can be recovered chemically unchanged at the end.
Your focus
- Describe what a catalyst does to the rate of a chemical reaction.
- Explain that a catalyst is not used up and can be recovered chemically unchanged.
- Give examples showing that different reactions need different catalysts, including enzymes in biological systems.
Show all 15 objectives
- Explain how a catalyst increases the rate of reaction by providing a different pathway with lower activation energy.
- Interpret an energy profile to compare catalysed and uncatalysed routes.
- State that a catalyst is not used up and does not change the overall energy change of the reaction.
- Draw a labelled reaction profile showing catalysed and uncatalysed pathways for a given reaction.
- Identify the activation energy on a reaction profile as the energy difference between reactants and the peak.
- Explain why a catalyst lowers the peak of a reaction profile without changing the overall energy change.
- Identify a catalyst from rate data showing a faster reaction when it is present.
- Recognise a catalyst in a chemical equation because it is not included as a reactant or product.
- Explain why a catalyst can be recovered unchanged after a reaction.
- Define activation energy as the minimum energy needed for a collision to result in reaction.
- Describe how a catalyst provides an alternative pathway with lower activation energy.
- Explain how a lower activation energy increases the rate by increasing the proportion of successful collisions.
Catalysts exam tips
Marking Points
- A catalyst changes the rate of a chemical reaction, usually increasing it, and is not used up during the reaction.
- The catalyst is not a reactant consumed in the overall reaction; it can be recovered chemically unchanged at the end.
- Catalysts are specific, so different reactions need different catalysts.
- Enzymes are catalysts in biological systems and speed up reactions such as digestion.
- A named example, such as manganese(IV) oxide catalysing hydrogen peroxide decomposition, can support the explanation.
- A catalyst increases the rate of reaction by providing a different pathway.
- The different pathway has a lower activation energy than the uncatalysed route.
- Lower activation energy means a greater proportion of collisions have enough energy to react.
- The catalyst is not used up and does not change the overall energy change of the reaction.
- An energy profile can show the catalysed route with a lower peak between reactants and products.
- Axes are labelled correctly: energy on the y-axis and reaction progress or extent of reaction on the x-axis.
- Reactant and product energy levels are marked and joined by a curve with a single maximum representing the transition state.
- The catalysed curve has a lower peak than the uncatalysed curve, showing a lower activation energy.
- The reactant and product energy levels are identical on both curves, so the overall energy change is unchanged by the catalyst.
- Activation energy is shown as the vertical difference from the reactant energy level to the peak of each curve.
- For an exothermic reaction the product level is below the reactant level; for an endothermic reaction it is above.
- A catalyst increases the rate of a reaction while remaining chemically unchanged at the end.
- A catalyst is not used up, so it can often be recovered and reused.
- A catalyst does not appear in the overall chemical equation because it is not a reactant or a product.
- A species written above the reaction arrow, or shown on both sides of an equation, indicates a catalyst.
- Comparing the rate with and without the suspected substance provides evidence that it is a catalyst.
- Enzymes are biological catalysts and can be identified by the same rate and equation evidence.
- A catalyst increases the rate of a reaction without being used up or chemically changed at the end.
- Activation energy is the minimum energy that particles must have when they collide in order to react.
- A catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed route.
- On an energy profile, the catalysed pathway has a lower peak between reactants and products than the uncatalysed pathway.
- Because the energy barrier is lower, a greater proportion of collisions possess at least the activation energy, so more successful collisions occur per unit time.
- The overall energy change between reactants and products is unchanged by a catalyst; only the activation energy is lowered.
Examiner Tips
- 💡State clearly that the catalyst is not used up and can be recovered chemically unchanged.
- 💡Use a named example, such as manganese(IV) oxide for hydrogen peroxide, to show the catalyst is not consumed.
- 💡When asked about enzymes, link them to biological systems and to changing the rate of metabolic reactions.
- 💡Use the phrase lower activation energy and link it to a greater proportion of successful collisions.
- 💡Sketch or interpret an energy profile showing the catalysed peak lower than the uncatalysed peak.
- 💡State that the catalyst is not used up and does not alter the overall energy change.
- 💡Sketch both curves on the same axes so the unchanged reactant and product levels are obvious to the examiner.
- 💡Label the axes, the two peaks and both activation energies; unlabelled curves lose clarity even when the shape is right.
- 💡State explicitly that the catalyst lowers the activation energy by providing an alternative pathway, not that it changes the energy of reactants or products.
- 💡Quote the rate evidence and the equation evidence together; one alone is usually insufficient for full credit.
- 💡When a species appears above the arrow, name it as the catalyst and explain that it is not included in the equation.
- 💡Use the phrase 'not used up' or 'chemically unchanged' rather than 'not involved', which is inaccurate.
- 💡Sketch or annotate an energy profile with two curves and label the lower peak as the catalysed route with lower activation energy.
- 💡Use the phrase 'alternative pathway with lower activation energy' rather than 'lowers the activation energy of the reaction' alone.
- 💡Link the lower barrier to a greater proportion of successful collisions to explain the increased rate, not just to a lower number.
Common Mistakes
- Saying the catalyst is used up: correct this by stating it is recovered chemically unchanged at the end, even though it may take part in an intermediate step.
- Thinking one catalyst works for every reaction: correct this by stating catalysts are specific, so different reactions need different catalysts.
- Confusing enzymes with reactants or with energy: correct this by describing enzymes as biological catalysts that change rate without being consumed.
- Saying the catalyst lowers the energy of the reactants or products: correct this by stating it lowers the activation energy of the pathway, not the energy levels of reactants or products.
- Saying the catalyst gives particles more energy: correct this by stating it provides a different pathway with a lower activation energy.
- Saying the catalyst changes the overall energy change: correct this by stating the energy change between reactants and products is unchanged.
- Drawing the catalysed curve with lower reactant or product levels: the error is changing the overall energy change; the correction is to keep both levels the same and lower only the peak.
- Drawing the catalysed curve with a higher peak: the error is reversing the effect of a catalyst; the correction is that a catalyst lowers the activation energy, so the peak must be lower.
- Labelling activation energy from the product level to the peak: the error is measuring from the wrong baseline; the correction is to measure from the reactant energy level to the peak.
- Assuming any substance that speeds up a reaction is a catalyst: the error is ignoring whether it is consumed; the correction is to check that it is unchanged and absent from the overall equation.
- Treating a catalyst as a reactant because it is added at the start: the error is confusing addition with consumption; the correction is that a catalyst is not used up and is not written as a reactant.
- Claiming a catalyst increases the yield of a reaction: the error is confusing rate with equilibrium position; the correction is that a catalyst speeds up the forward and reverse reactions equally and does not change the yield.
- Saying a catalyst lowers the energy of the reactants or products: it does not change their energies; it lowers the activation energy peak of the pathway.
- Stating that a catalyst increases the energy of collisions or makes particles collide more often: it lowers the energy barrier, allowing more existing collisions to be successful.
- Claiming a catalyst is used up or becomes part of the product: it is recovered chemically unchanged, which is why only a small amount is needed.