Explain how a catalyst increases rate by providing an alternative pathway with lower activation energy.

How catalysts lower activation energy, the difference between homogeneous and heterogeneous catalysts, enzymes, and the effect on the energy profile and Maxwell-Boltzmann distribution.

Generated by Claude Opus 4.76 min answerUpdated 2026-05-30

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What this dot point is asking

You must explain the mechanism by which a catalyst works and describe its effect on the energy profile and on the spread of particle energies.

What a catalyst does

A catalyst increases the rate of a chemical reaction without being permanently used up. It takes part in the reaction, often forming an intermediate, but is regenerated by the end so the same catalyst molecule acts again and again. Because it is not consumed, only a small amount is needed.

The lower activation energy pathway

The key idea is that a catalyst lowers the activation energy. On an energy profile diagram, the catalysed pathway shows a lower peak (energy barrier) than the uncatalysed pathway, while the energy of the reactants and products stays the same. Because the barrier is lower, a greater proportion of colliding particles have enough energy to react.

On a Maxwell-Boltzmann distribution, lowering the activation energy moves the $E_a$ line to the left, so the shaded area to its right (the fraction of particles that can react) is much larger. The temperature is unchanged, so the curve itself does not move.

Types of catalyst

Homogeneous catalysts are in the same physical state as the reactants, typically all in solution. They usually work by forming an intermediate species.

Heterogeneous catalysts are in a different state, commonly a solid catalyst with gaseous or liquid reactants. They work by adsorbing reactant molecules onto active sites on their surface, which weakens bonds and holds molecules in a favourable orientation. Industrial examples include iron in the Haber process and platinum or palladium in catalytic converters.

Enzymes are biological catalysts, large protein molecules that catalyse reactions in living things with great specificity. Each enzyme has an active site shaped to fit a particular substrate.

example

Reading a catalysed energy profile

A reaction has an uncatalysed activation energy of $120\ \text{kJ mol}^{-1}$ and an enthalpy change of $-40\ \text{kJ mol}^{-1}$ . A catalyst lowers the activation energy to $75\ \text{kJ mol}^{-1}$ . State the activation energy of the reverse reaction with the catalyst present.

Step 1: For the forward catalysed reaction, $E_a = 75\ \text{kJ mol}^{-1}$ .

Step 2: The reverse activation energy is the forward barrier minus the enthalpy change: $E_{a,\text{reverse}} = E_{a,\text{forward}} - \Delta H = 75 - (-40) = 115\ \text{kJ mol}^{-1}$ .

Step 3: So with the catalyst the reverse reaction must climb a $115\ \text{kJ mol}^{-1}$ barrier, compared with $160\ \text{kJ mol}^{-1}$ uncatalysed.

In the exam, always state that the catalyst provides an alternative pathway with lower activation energy, and stress that it is regenerated and does not alter the enthalpy change or equilibrium position.

Exam-style practice questions

Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2023 TASC3 marksThe reaction 4NH3(g) + 5O2(g) -> 4NO(g) + 6H2O(g) usually takes place in the presence of a catalyst such as platinum or rhodium. Using a molecular-energy distribution diagram and collision theory, explain how a catalyst speeds up a reaction.

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A catalyst provides an alternative reaction pathway with a lower activation energy (Ea). On the Maxwell-Boltzmann distribution this is shown by drawing the catalysed Ea further to the left of the uncatalysed Ea.

Because the catalysed Ea is lower, a larger fraction of the molecules (a greater area under the curve to the right of Ea) now have enough energy to react on collision.

So at the same temperature a greater proportion of collisions are successful per unit time, which increases the reaction rate. The catalyst itself is not consumed. (3 marks: alternative pathway, lower Ea, larger fraction of successful collisions.)

2022 TASC2 marksIndustrially, a vanadium-based catalyst is used to increase the rate of the conversion of SO2 to SO3. Outline how a catalyst changes the rate of a chemical reaction.

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A catalyst speeds up a reaction by providing an alternative reaction pathway (mechanism) that has a lower activation energy than the uncatalysed reaction.

Because the activation energy is lower, a greater proportion of the colliding molecules have enough energy to react, so the number of successful collisions per unit time increases and the rate rises. The catalyst is regenerated and so is not used up overall. It does not change the enthalpy change or the position of equilibrium. (2 marks.)

2021 TASC2 marksStatement found online: UV light acts as a catalyst to speed up the decomposition of hydrogen peroxide into water and oxygen. Justify that this statement is incorrect, referring to what is meant by a catalyst in your answer.

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A catalyst is a substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy, and that is not consumed overall (it is regenerated).

UV light is a form of energy, not a substance, so it cannot be a catalyst. UV speeds up the decomposition by supplying energy to the reactant molecules (photochemical activation), helping them reach the activation energy, rather than by lowering the activation energy. It therefore does not fit the definition of a catalyst, so the statement is incorrect. (2 marks: definition of catalyst, and UV is energy not a substance.)

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