5.2.6Additional lessons
Energy Profiles With and Without Catalysts — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 71 of 98
Part 1 of 6Comparing Pathways
A catalyst changes the route, not the starting or end points.
Uncatalysed path
- Shown as a single high peak between reactants and products, which sets the activation energy of the normal reaction.
Catalysed path
- Shown as a lower peak (or several lower peaks) on the same graph, because the catalyst offers another route.
Same levels
- Reactants and products are at the same enthalpy for both pathways, so ΔH is unchanged.
Both directions
- The catalyst lowers the activation energy by the same amount for both the forward and the reverse reaction.
Part 2 of 6Drawing the Diagram
1
Draw axes with enthalpy vertical and reaction progress horizontal, and mark the reactant and product levels.
2
Draw one smooth curve with a high peak and label it as the uncatalysed reaction.
3
Draw a second curve with a lower peak between the same two levels and label it as catalysed.
4
Mark Eₐ for each pathway with vertical arrows from the reactants and mark ΔH between the levels.
Part 3 of 6How Catalysts Work
How Catalysts Work
Surface action
- In heterogeneous catalysis, reactants adsorb onto a solid surface where bonds weaken and reaction is easier.
Regeneration
- The catalyst is released unchanged at the end, so a small amount can convert large amounts of reactant.
No change in yield
- Because ΔH and the energy levels stay the same, a catalyst does not alter the amount of product at the end.
Selectivity
- Some catalysts speed up only one reaction, which helps in producing a desired product.
Part 4 of 6Uses of Catalysts
Uses of Catalysts
Industry
- Iron is used in the Haber process and vanadium(V) oxide in the contact process, allowing lower temperatures to be used.
Cars
- Platinum and rhodium in catalytic converters convert CO and NOₓ into harmless gases.
Biology
- Enzymes are biological catalysts that allow reactions to proceed at body temperature.
Benefit
- Lower temperatures save energy and reduce costs and emissions.
Part 5 of 6Worked Example
Question
- An uncatalysed reaction has Eₐ = 80 kJ mol⁻¹ and ΔH = −30 kJ mol⁻¹.
- A catalyst lowers the forward Eₐ to 50 kJ mol⁻¹.
- State ΔH with the catalyst and the reverse activation energies without and with the catalyst.
- A catalyst does not change the reactant and product levels, so ΔH is still −30 kJ mol⁻¹.
- Reverse Eₐ without catalyst = 80 − (−30) = 110 kJ mol⁻¹.
- Reverse Eₐ with catalyst = 50 + 30 = 80 kJ mol⁻¹.
- Both Eₐ values fall by 30 kJ mol⁻¹.
Answer
- ΔH stays −30 kJ mol⁻¹;
- reverse Eₐ is 110 kJ mol⁻¹ without and 80 kJ mol⁻¹ with the catalyst.
Part 6 of 6Exam Checklist & Tips
5.2.6 – Key Points for the Exam
- Catalyst A substance which speeds up a reaction by providing an alternative pathway with lower activation energy.
- Alternative pathway A different reaction route with a lower energy barrier.
- Heterogeneous catalyst A catalyst in a different phase from the reactants, such as a solid metal for gases.
- Enzyme A biological catalyst, usually a protein.
Common Mistakes
- Drawing different product levels for the catalysed reaction.
- Saying that a catalyst changes ΔH.
- Claiming that the catalyst lowers only the forward activation energy.
- Saying that a catalyst is consumed in the reaction.
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