Explain catalyst types and lower-energy mechanisms
Define catalysis and distinguish homogeneous from heterogeneous catalysts by phase and mechanism.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Rate data and diagrams are illustrative unless a question supplies measurements. A diagram is qualitative unless labelled values are provided. Written practice is self-checked
Before you begin
- Rearrange simple ratios and use consistent units for time, volume, mass and concentration.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the key phase test for homogeneous catalysis?
What you will learn
- Explain the term catalyst
- Distinguish homogeneous and heterogeneous catalysis
- Explain that a catalyst provides a different mechanism with lower activation energy
Catalysis provides another reaction route
A catalyst increases reaction rate by providing a different mechanism with a lower activation energy. The lower barrier allows more particles to make effective collisions at the same temperature.
In the overall reaction, the catalyst is regenerated rather than permanently consumed. Its presence changes the route and rate, not the reactant and product identities in the overall equation.
The phases identify the catalyst type
A homogeneous catalyst is in the same physical phase as the reactants. A heterogeneous catalyst is in a different phase; for example, a solid catalyst can provide a surface on which gaseous or dissolved reactants react.
These labels describe phase relationships. They do not specify one universal mechanism or mean the catalyst has no interactions with reactants. Adsorption, intermediate steps and regeneration depend on the particular catalysed reaction.
Pause and explain
How does an effective catalyst increase rate?
Worked example
A gas reaction is accelerated by adding a solid catalyst. Classify the catalysis and explain the rate effect.
Show the worked solution
- The reactants are gases and the catalyst is a solid.
- They are in different phases, so the catalysis is heterogeneous.
- The catalyst provides a different route with lower Eₐ.
Answer This is heterogeneous catalysis. The solid catalyst provides a different mechanism with lower activation energy, increasing the reaction rate.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A dissolved acid catalyst and liquid reactants are all in one aqueous phase. Classify the catalysis.
Give me a hint
Compare the catalyst phase with the reactant phase.
Compare my reasoning
- The catalyst is dissolved in the same liquid phase.
- The reactants are also in that phase.
- Therefore the catalysis is homogeneous.
Homogeneous catalysis, because the catalyst and reactants share the same phase.
Look for these in your work
- Phases identified
- Same-phase comparison made
- Classification justified
A solid catalyst speeds a reaction between gaseous reactants. State the catalyst type and the pathway change.
Give me a hint
Compare the solid with the gas phase, then state the energy-barrier effect.
Compare my reasoning
- The catalyst and reactants are in different phases.
- This is heterogeneous catalysis.
- An alternative mechanism has a lower activation energy.
Heterogeneous catalysis; the solid provides a different mechanism with a lower activation energy.
Look for these in your work
- Different phases recognised
- Type named
- Lower-Eₐ route stated
A student says, ‘The catalyst gives the reactant particles more energy, so they cross the same barrier.’ Correct this explanation.
Give me a hint
The particles' temperature distribution need not change when a catalyst is added.
Compare my reasoning
- The catalyst does not have to raise the mixture's temperature.
- It creates a different reaction mechanism with a lower Eₐ.
- At the same temperature, more particles can overcome the lower barrier.
The catalyst provides a different, lower-activation-energy route; it need not increase particle energies or temperature.
Look for these in your work
- Reject energy transfer claim
- Alternative mechanism stated
- Lower barrier linked to rate
Common mistakes
- Defining homogeneous as a catalyst that dissolves every reactant
- Calling a catalyst a reactant that is used up overall
- Saying a catalyst raises particle energy
- Treating a lower activation energy as a change in ΔH
What can you explain now?
What is the key phase test for homogeneous catalysis?
Compare with the explanation
Catalyst and reactants are in the same phase
Homogeneous describes a shared physical phase.
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Make it stick
What makes catalysis homogeneous or heterogeneous, and what changes in the reaction pathway?
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