# Potential Energy Diagrams: Exothermic Reactions and Catalysts

Energy diagrams show the path a reaction takes from reactants to products and reveal the energy barrier that must be overcome ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23651), [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23653)). Catalysts lower this barrier without changing the overall energy released, allowing reactions to proceed faster ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)).

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 0 kJ/mol, Products at −80 kJ/mol, Ea = 150 kJ/mol, ΔH = −80 kJ/mol, catalysed path with Ea = 90 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/2dbd6de0-ad98-4aeb-a102-747c2d031be2/asset/329)

## Exercises

1. **Multiple choice.** An exothermic reaction has reactants at 0 kJ/mol and products at −80 kJ/mol. What is ΔH for this reaction?

 A) −80 kJ/mol
 B) +80 kJ/mol
 C) 150 kJ/mol
 D) 90 kJ/mol

2. **Short answer.** Using the diagram, identify the activation energy (Ea) of the uncatalysed reaction. Write your answer with units.

 ____________

3. **Matching.** Match each term on the left to its definition on the right.

 | Term | Definition |
 |---|---|
 | Activation energy | The energy difference between reactants and products |
 | Enthalpy change (ΔH) | The minimum energy needed for reactants to form products |
 | Catalyst | A substance that speeds up a reaction by lowering the activation energy |

4. **Short answer.** The diagram shows two pathways: one uncatalysed (Ea = 150 kJ/mol) and one catalysed (Ea = 90 kJ/mol). By how many kilojoules does the catalyst lower the activation energy?

 ____________

5. **Analysis.** A student claims that adding a catalyst to this reaction will make the products have more energy than they do in the uncatalysed reaction. Is this claim correct? Explain your reasoning using the diagram.

 ____________________________________________________________

 ____________________________________________________________

6. **Calculation.** In the uncatalysed pathway, reactants must gain 150 kJ/mol to reach the transition state. In the catalysed pathway, reactants must gain 90 kJ/mol to reach the transition state. Express the ratio of the catalysed activation energy to the uncatalysed activation energy as a fraction in lowest terms.

 ____________

## Answer Key

1. A) −80 kJ/mol

2. 150 kJ/mol

3. Activation energy → The minimum energy needed for reactants to form products; Enthalpy change (ΔH) → The energy difference between reactants and products; Catalyst → A substance that speeds up a reaction by lowering the activation energy

4. 60 kJ/mol (150 − 90 = 60)

5. No, the claim is incorrect. Figure 1 shows that both pathways end at the same product energy level (−80 kJ/mol). The catalyst provides an alternate pathway with lower activation energy but does not change the final energy of the products ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)).

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 0 kJ/mol, Products at −80 kJ/mol, Ea = 150 kJ/mol, ΔH = −80 kJ/mol, catalysed path with Ea = 90 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/2dbd6de0-ad98-4aeb-a102-747c2d031be2/asset/329)

6. 3/5 (90 ÷ 150 = 90/150 = 3/5)

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*AILI homework sheet · language en · model claude-haiku-4-5-20251001 · generated 2026-09-17 · id 2dbd6de0-ad98-4aeb-a102-747c2d031be2*

### 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)