Interpret catalysed and uncatalysed pathways
Construct and compare energy profiles for catalysed and uncatalysed reactions.
Section 8 · Reaction kinetics
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 should remain unchanged between catalysed and uncatalysed profiles?
What you will learn
- Construct a reaction pathway diagram with and without catalyst
- Compare activation energies
- Show that ΔH is unchanged by the catalyst
Keep start and finish energy levels fixed
On a reaction pathway diagram, plot energy vertically and reaction progress horizontally. The uncatalysed route has a higher peak; the catalysed route has a lower peak because it follows a different mechanism with lower activation energy.
Both routes begin at the same reactant level and finish at the same product level. Therefore the overall ΔH is unchanged. The catalyst changes the barrier, not the difference in energy between reactants and products.
Read each activation energy from the reactants
For each route, measure Eₐ vertically upward from the same reactant level to that route's peak. The catalysed Eₐ is smaller. A diagram is qualitative unless energy values are provided; do not infer a numerical Eₐ from an unscaled sketch.
An effective catalyst can change the mechanism, so the route may have intermediate steps. The essential comparison is that the catalysed pathway has a lower highest energy barrier and retains the same overall reactant and product levels.
Pause and explain
How do you show an effective catalyst on a pathway diagram?
Worked example
Describe how to add a catalysed route to an uncatalysed energy profile.
Show the worked solution
- Keep the same reactant and product levels.
- Draw the catalysed route with a lower peak than the uncatalysed route.
- Label each forward Eₐ from the reactant level and show ΔH unchanged.
Answer The catalysed pathway has a lower peak and lower Eₐ, while the reactant/product levels and ΔH stay the same.
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Test this topic →From guided practice to a new situation
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.
Sketch in words catalysed and uncatalysed pathways for an exothermic reaction.
Give me a hint
Keep the same lower product level and compare each peak with the common reactant level.
Compare my reasoning
- Draw energy against reaction progress.
- Set products below reactants for the exothermic ΔH.
- Draw a higher uncatalysed peak and a lower catalysed peak; the start/end levels stay fixed.
Both pathways have the same reactant and product levels, with the catalysed peak lower and its Eₐ smaller.
Look for these in your work
- Axes identified
- Exothermic start/end levels correct
- Catalysed barrier lower
An uncatalysed profile has reactants at 10 kJ mol⁻¹ and a peak at 85 kJ mol⁻¹. A catalysed route peaks at 52 kJ mol⁻¹; products are at −8 kJ mol⁻¹. Find both forward Eₐ values and ΔH.
Give me a hint
Use the common reactant level for each Eₐ and product minus reactants for ΔH.
Compare my reasoning
- Uncatalysed Eₐ = 85 − 10 = 75 kJ mol⁻¹.
- Catalysed Eₐ = 52 − 10 = 42 kJ mol⁻¹.
- ΔH = −8 − 10 = −18 kJ mol⁻¹ for both pathways.
Uncatalysed Eₐ = 75 kJ mol⁻¹; catalysed Eₐ = 42 kJ mol⁻¹; ΔH = −18 kJ mol⁻¹ for both.
Look for these in your work
- Each barrier measured from reactants
- Both barriers compared
- Same ΔH calculated
A diagram shows the catalysed path with lower Eₐ but a different final product level. Explain why this does not represent catalysis of the same overall reaction.
Give me a hint
Compare the reaction's start and finish, not just its peak.
Compare my reasoning
- A catalyst offers a different mechanism for the same overall transformation.
- Different product energy indicates different products or an incorrect diagram.
- For the same reaction, start/end levels and ΔH must match.
The diagram changes the overall reaction as well as the mechanism. A valid catalysed profile keeps the same reactants, products and ΔH while lowering Eₐ.
Look for these in your work
- Mechanism separated from overall reaction
- Product-level error identified
- Correct invariant stated
Common mistakes
- Changing the product energy to show a catalyst
- Measuring Eₐ from zero
- Drawing the catalysed peak higher
- Treating the reaction-progress axis as time
What can you explain now?
What should remain unchanged between catalysed and uncatalysed profiles?
Compare with the explanation
Reactant and product energy levels
The overall reaction has the same start and finish energies.
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Make it stick
Which profile features show lower Eₐ without changing the overall reaction?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.