# Potential Energy Diagrams: Exothermic Reactions and Catalysts

You will analyze and interpret potential energy diagrams for chemical reactions, identifying activation energy, enthalpy change, and the effect of catalysts on reaction pathways ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23653), [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)). This sheet develops your ability to explain energy changes in chemical reactions and to recognize how catalysts function without altering the net energy released ([Topic](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23650), [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23652)).

![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/6379b3de-a6f2-4c30-9be3-1c64f80f85f7/asset/805)

## Exercises

1. **Multiple choice.** For the uncatalyzed reaction shown in the diagram, what is the activation energy?

A) 45 kJ/mol
B) 70 kJ/mol
C) 80 kJ/mol
D) 85 kJ/mol

2. **Short answer.** Using the diagram, calculate the enthalpy change (ΔH) for this reaction. Show your work.

ΔH = __________________ kJ/mol

Work:

_______________________________________________________________

3. **Matching.** Match each term on the left with its value from the diagram on the right.

| Term | Value |
|---|---|
| Potential energy of reactants | A) 45 kJ/mol |
| Potential energy of products | B) 80 kJ/mol |
| Activation energy (catalyzed) | C) 150 kJ/mol |
| Activation energy (uncatalyzed) | D) 85 kJ/mol |

4. **Short answer.** Explain why the catalyzed pathway has a lower activation energy than the uncatalyzed pathway. Does the catalyst change the overall energy released by the reaction?

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

5. **Calculation.** A chemist wants to compare the two reaction pathways. How much lower is the activation energy for the catalyzed reaction than for the uncatalyzed reaction?

Difference = __________________ kJ/mol

6. **Analysis.** Figure 1 This reaction is exothermic. Which of the following statements correctly explains why?

![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/6379b3de-a6f2-4c30-9be3-1c64f80f85f7/asset/805)

A) The activation energy is greater than 70 kJ/mol.
B) The potential energy of the products is lower than the potential energy of the reactants.
C) The catalyst provides an alternate pathway with lower activation energy.
D) The potential energy of the reactants is 150 kJ/mol.

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## Answer Key

1. D) 85 kJ/mol

2. ΔH = −70 kJ/mol. Work: ΔH = PE(products) − PE(reactants) = 80 − 150 = −70 kJ/mol.

3. Potential energy of reactants: C) 150 kJ/mol | Potential energy of products: B) 80 kJ/mol | Activation energy (catalyzed): A) 45 kJ/mol | Activation energy (uncatalyzed): D) 85 kJ/mol

4. The catalyst provides an alternate reaction pathway that requires less energy to initiate the reaction. The catalyst does not change the overall energy released; ΔH remains −70 kJ/mol for both pathways.

5. 85 − 45 = 40 kJ/mol

6. B) The potential energy of the products is lower than the potential energy of the reactants.

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*AILI homework sheet · language en · model claude-haiku-4-5-20251001 · generated 2026-09-17 · id 6379b3de-a6f2-4c30-9be3-1c64f80f85f7*

### Sources

- node:n1: Unit A: Thermochemical Changes (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23632)
- node:n2: General Outcome 2 (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23650)
- node:n3: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23651)
- node:n4: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23652)
- node:n5: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23653)
- node:n6: Specific Outcomes for Knowledge (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)
- node:n7: Specific Outcomes for Skills (https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23660)