# Potential Energy Diagrams: Exothermic Reactions and Catalysts

This worksheet develops your ability to analyze and interpret potential energy diagrams for exothermic reactions ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23653)) and to explain how catalysts affect activation energy without changing the overall enthalpy change ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)).

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 150 kJ/mol, Products at 80 kJ/mol, Ea = 85 kJ/mol, ΔH = −70 kJ/mol, catalysed path with Ea = 45 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/0a694265-e61b-4031-9a74-a59967f36b35/asset/327)

## Exercises

1. Using the diagram, calculate the enthalpy change (ΔH) for this reaction. Show your work and include the sign and units.

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2. The activation energy for the uncatalyzed pathway is _______ kJ/mol. The activation energy for the catalyzed pathway is _______ kJ/mol. The catalyst lowers the activation energy by _______ kJ/mol.

3. Which statement correctly describes the role of the catalyst in this reaction?

A) The catalyst lowers the energy of the products, making the reaction more exothermic.

B) The catalyst provides an alternate pathway that requires less energy to reach the transition state, without changing ΔH.

C) The catalyst increases the energy released when reactants form products.

D) The catalyst changes the enthalpy change from exothermic to endothermic.

4. A student claims that because the catalyst lowers the activation energy, the products must have lower potential energy than they do in the uncatalyzed reaction. Explain why this claim is incorrect.

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5. The potential energy of the reactants is 150 kJ/mol. Using the diagram and the enthalpy change you calculated in Exercise 1, verify that the potential energy of the products is 80 kJ/mol.

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6. This reaction is classified as exothermic ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23643)). Explain how the diagram supports this classification, and state what would be different if the reaction were endothermic instead.

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## Answer Key

1. ΔH = products − reactants = 80 − 150 = −70 kJ/mol. The negative sign indicates the reaction is exothermic (energy is released).

2. 85 kJ/mol; 45 kJ/mol; 40 kJ/mol.

3. B

4. The catalyst does not change the potential energy of the reactants or products. It only provides an alternate pathway with a lower activation energy. The products have the same potential energy (80 kJ/mol) whether the reaction proceeds through the catalyzed or uncatalyzed route.

5. Figure 1 ΔH = −70 kJ/mol means products are 70 kJ/mol lower in energy than reactants. 150 − 70 = 80 kJ/mol, which matches the product energy shown in the diagram.

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 150 kJ/mol, Products at 80 kJ/mol, Ea = 85 kJ/mol, ΔH = −70 kJ/mol, catalysed path with Ea = 45 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/0a694265-e61b-4031-9a74-a59967f36b35/asset/327)

6. The diagram shows products at lower potential energy than reactants, which means energy is released to the surroundings; this defines an exothermic reaction. In an endothermic reaction, the products would be at higher potential energy than the reactants, and the ΔH would be positive.

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*AILI homework sheet · language en · model claude-haiku-4-5-20251001 · generated 2026-09-17 · id 0a694265-e61b-4031-9a74-a59967f36b35*

### Sources

- node:n1: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)
- node:n2: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23634)
- node:n3: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 2 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23653)
- node:n4: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 2 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)
- node:n5: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 2 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23651)
- node:n6: Chemistry › Chemistry (20, 30) › Chemistry 30 › Unit A: Thermochemical Changes › General Outcome 1 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23643)
- resource:r1: Resources › type#studentsupport, type#teachersupport › SCN3796 (https://goa-cc-uat-aili-app-001.azurewebsites.net/library/resource/YlIUl7k3G0y6R-b8XyPAGw)