Catalysts and Enzymes
Explain how catalysts lower activation energy, distinguish elements from compound catalysts, and understand enzymes as biological catalysts.
On this page
A catalyst changes the pathway by which a reaction happens. Unlike raising temperature, adding a catalyst does not give reactant particles extra kinetic energy. Revise Collision Theory if activation energy is unfamiliar.
What a catalyst does
A catalyst increases the rate of a reaction and is chemically unchanged at the end. It can take part in intermediate steps, but is regenerated rather than used up in the overall reaction. A catalyst works for a particular reaction; one substance does not catalyse every reaction.
The catalysed pathway has a lower activation energy, Eₐ. At the same temperature, a larger fraction of collisions has enough energy to react, so effective collisions occur more frequently. The explanation concerns the fraction of successful collisions; it does not require an increase in total collision frequency.
Swipe or scroll sideways to inspect the complete overview.
The diagram shows two possible pathways for the same overall reaction. A catalyst changes neither the reactant and product energy levels nor their difference, Δ H. The lower barrier means faster reaction, not more energy released.
Elements and compounds can be catalysts
| Catalyst | Example reaction | What to distinguish |
|---|---|---|
| Iron | Manufacture of ammonia in the Haber process | Iron is an element, not a compound. |
| Vanadium(V) oxide, V₂O₅ | Conversion of sulfur dioxide to sulfur trioxide in sulfuric acid manufacture | This catalyst is a compound. |
| Manganese(IV) oxide, MnO₂ | Decomposition of hydrogen peroxide to water and oxygen | This is another compound catalyst. |
For hydrogen peroxide decomposition, the catalyst is not a reactant in the overall equation:
2H₂O₂(aq) → 2H₂O(l) + O₂(g)
Use the Haber-process lesson and sulfuric-acid lesson for the industrial processes. Here the key point is that catalysts can be elements or compounds.
Enzymes are biological catalysts
Enzymes catalyse reactions in living organisms. For example, the enzyme catalase speeds up the same decomposition of hydrogen peroxide shown above. It helps break down hydrogen peroxide in cells; enzymes can also be used in industrial processes.
Like other catalysts, an enzyme provides a pathway with a lower activation energy and is not used up in the overall reaction. Enzymes work on particular reactants. Many are proteins whose working shape can be damaged by sufficiently high temperature: this is denaturation, and catalytic activity can fall. Do not assume that heating an enzyme-catalysed reaction always makes it faster. You do not need a detailed enzyme mechanism for this Chemistry topic.
The Royal Society of Chemistry’s catalase experiment explains the biological example and its comparison with manganese(IV) oxide.
Rate and final amount are different
For the same chosen reaction, starting amounts and conditions, a catalyst can give more product at a fixed early time because the reaction proceeds faster. It does not increase the theoretical amount available when the limiting reactant has been fully used up. In a reversible reaction at fixed conditions, it helps the mixture reach equilibrium sooner without changing the equilibrium composition.
Keep temperature, reactant amounts and the measurement method fixed when comparing a catalysed and uncatalysed reaction. Distinguish “after 30 s” from “after completion”. See Reversible Reactions for the equilibrium case.
Worked examples
Modelled example 1
Compare two pathways
Problem
An illustrative energy model gives reactants at 20 kJ mol⁻¹, products at −10 kJ mol⁻¹, an uncatalysed peak at 70 kJ mol⁻¹ and a catalysed peak at 40 kJ mol⁻¹. Calculate both activation energies and the enthalpy change.
Study the worked solution
Measure each barrier from the reactants
Method
Subtract the reactant energy from each peak.Reason
Activation energy is the upward energy difference to the barrier, not the absolute peak value.
Working
Uncatalysed: 70-20 = 50 kJ mol⁻¹. Catalysed: 40-20 = 20 kJ mol⁻¹.
Use the unchanged endpoints
Method
Subtract reactant energy from product energy.Reason
The same reactants and products give the same enthalpy change for either pathway.
Working
Δ H = -10-20 = -30 kJ mol⁻¹. Both pathways are exothermic.
Guided practice 2
Catalyst (what changes, what doesn’t)
Problem
Distinguish pathway from particle energy
Hints
Hint 1: change the barrier
Hint 2: compare with temperature
View solution step by step
Change the reaction pathway
Method
Provide an alternative pathway with lower activation energy.Reason
A lower threshold lets a larger proportion of the existing collisions have enough energy.Working
Lower Eₐ → larger effective-collision proportion.Reject extra particle energy
Method
Keep particle kinetic energies unchanged at the same temperature.Reason
The catalyst changes the pathway, not the temperature.Working
It does not necessarily raise total collision frequency or give particles energy.Keep the energy difference stable
Method
Leave reactant level, product level and Δ H unchanged.Reason
Only the activation-energy peak is lowered; the reaction endpoints are identical.Working
More frequent effective collisions per unit time, so rate increases without changing Δ H.
Practise independently
Mind stretcher 1: Product at a fixed time or at completion?Extension
Two illustrative trials decompose identical hydrogen peroxide portions at the same temperature. One uses a catalyst. At 30 s, the catalysed trial has collected 36 cm³ of oxygen and the uncatalysed trial 12 cm³. At completion, both have collected 60 cm³.
Question: Compare their average oxygen-production rates over the first 30 s. Explain why “the catalyst makes three times as much oxygen” is misleading.
Show Answer
The average rates are 36/30 = 1.2 cm³ s⁻¹ and 12/30 = 0.40 cm³ s⁻¹. The catalysed trial has a three-times-greater average rate over that interval and three times the collected volume at 30 s. It does not give three times the final amount: both finish at 60 cm³ because the same starting amount can decompose. The catalyst changes the pathway and rate, not the amount available from the reactant.
Mind stretcher 2: Two catalysts, one overall reactionExtension
Question: Catalase and manganese(IV) oxide both speed up hydrogen peroxide decomposition. What makes catalase an enzyme? Does seeing oxygen bubbles alone prove that the substance added is a catalyst?
Show Answer
Catalase is a biological catalyst. Manganese(IV) oxide is an inorganic compound catalyst. Oxygen bubbles show gas production, but a substance used up as a reactant could also make gas. Evidence for catalysis must distinguish an increased rate from an added reactant and show that the catalyst is regenerated overall; bubbles alone establish neither point.
Keep these distinctions clear
- Temperature changes reactant particle energies; a catalyst changes the pathway barrier.
- A catalyst is regenerated overall, even if it participates in intermediate steps.
- A lower activation energy does not change Δ H for the same overall reaction.
- More product at an early time does not mean more product at completion.
- Iron is an element; vanadium(V) oxide and manganese(IV) oxide are compounds.
Use the topic check to practise collision explanations, catalysts and rate-data interpretation.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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