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CatalystsAQA A-Level Chemistry: Revision notes

Section 1

What a catalyst is

A catalyst is a substance that increases the rate of a chemical reaction without being changed in chemical composition or amount. It is not used up, so it does not appear in the overall equation, although it is often written above the arrow, for example MnO₂ in 2H₂O₂ → 2H₂O + O₂.

A catalyst may take part in the reaction mechanism, but it is regenerated by the end. It can be physically altered (a solid catalyst may become powdery), so the definition refers to chemical composition and amount.

Key termscatalyst
Common mistake

A catalyst does not change the yield at equilibrium, the enthalpy change of reaction or the products. It only changes how fast the reaction happens.

Section 2

How catalysts work: an alternative route

Catalysts work by providing an alternative reaction route (pathway) that has a lower activation energy. The activation energy is the minimum energy colliding particles need for a reaction to occur.

On a reaction profile the catalysed route has a lower peak. The reactants and products are at the same energy levels, so ΔH is unchanged. For example, if Ea falls from 120 to 70 kJ mol⁻¹, the reduction is 50 kJ mol⁻¹ but ΔH stays the same.

Key termsactivation energyalternative route

Section 3

Maxwell–Boltzmann distribution

In a gas, molecules have a range of energies. The Maxwell–Boltzmann distribution plots the number of molecules against energy.

  • The curve starts at the origin (no molecules have zero energy)
  • It has a peak, then a tail that never touches the energy axis
  • The area under the curve equals the total number of molecules
  • Only molecules with energy ≥ Ea in the tail can react on collision
Key termsMaxwell–Boltzmann distributionEa marker

Section 4

Using the distribution to explain a catalyst

To explain why a catalyst increases the rate of a gas-phase reaction:

  1. The catalyst provides an alternative route with a lower Ea
  2. The temperature is unchanged, so the curve does not change shape or move
  3. The Ea marker moves left, so a greater proportion (area) of molecules have energy ≥ Ea
  4. There are more frequent successful collisions, so the rate increases
Key termsproportion of molecules
Common mistake

Do not move the curve to the right for a catalyst. That is what raising the temperature does (and the peak also falls). With a catalyst only the activation energy moves.

Section 5

Worked example and exam approach

Worked example. A catalyst lowers Ea from 120 to 70 kJ mol⁻¹. State the effect on ΔH and the rate.

  • ΔH is unchanged (reactants and products are the same)
  • More molecules have energy ≥ 70 kJ mol⁻¹ than ≥ 120 kJ mol⁻¹, so the rate increases.

In 6-mark answers, link: alternative route, lower Ea, same curve, greater proportion above Ea, more frequent successful collisions, higher rate.

Key termssuccessful collision
Exam tip

Use the phrase 'greater proportion of molecules with energy greater than or equal to the activation energy', not 'more energetic molecules'.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Catalysts

  1. In a school laboratory a student adds a spatula of black manganese(IV) oxide powder to 25 cm³ of hydrogen peroxide solution at room temperature. Oxygen is evolved rapidly, whereas without the powder it forms extremely slowly. At the end of the experiment the student filters off the powder, washes and dries it, and finds that its mass is unchanged and that it is still black manganese(IV) oxide.
    Write the balanced equation for the decomposition of hydrogen peroxide and explain why manganese(IV) oxide does not appear in it.2 marks
  2. The gas-phase reaction between two reactants is exothermic, with an enthalpy change of −45 kJ mol⁻¹. Without a catalyst the activation energy is 120 kJ mol⁻¹. When a solid catalyst is used, the reaction follows an alternative route with an activation energy of 70 kJ mol⁻¹. The temperature is the same in both cases.
    Explain why the reaction is faster when the catalyst is used, even though the temperature is unchanged.2 marks
  3. In a car's catalytic converter, the exhaust gases nitrogen monoxide and carbon monoxide react at about 600 K: 2NO(g) + 2CO(g) → N₂(g) + 2CO₂(g). Without a catalyst the reaction is extremely slow, but platinum and rhodium on a ceramic support allow it to occur in the short time the gases spend in the converter.
    Describe the shape of the Maxwell–Boltzmann distribution of molecular energies for the exhaust gases at 600 K, and state what the area under the curve represents.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).