# Potential Energy Diagrams: Exothermic Reactions and Catalysis

This worksheet asks you to analyze and label a potential energy diagram for an exothermic reaction, including reactants, products, enthalpy change and activation energy ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23653)). It also addresses how a catalyst changes the reaction pathway without changing the net energy released ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654), [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23651)).

## Exercises

**1. Labelling the diagram**

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 250 kJ/mol, Products at 150 kJ/mol, Ea = 120 kJ/mol, ΔH = −100 kJ/mol, catalysed path with Ea = 60 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/a07b9ec3-f71d-4c62-b2b9-85e6759fe55f/asset/331)

Using the potential energy diagram described above, answer the following.

a) What is the potential energy of the reactants, in kJ/mol? ______

b) What is the potential energy of the products, in kJ/mol? ______

c) Is this reaction endothermic or exothermic? Justify your answer using the relative positions of reactants and products ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23643)). ______________________________________________

**2. Calculating enthalpy change**

Using the values from the diagram, calculate ΔH for the uncatalysed reaction. Show your work.

ΔH = PE(products) − PE(reactants)

Work: ______________________________________________

ΔH = ______ kJ/mol

State whether this value agrees with the ΔH given in the figure ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636)). ______________________________________________

**3. Activation energy, catalysed and uncatalysed**

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 250 kJ/mol, Products at 150 kJ/mol, Ea = 120 kJ/mol, ΔH = −100 kJ/mol, catalysed path with Ea = 60 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/a07b9ec3-f71d-4c62-b2b9-85e6759fe55f/asset/331)

a) State the activation energy (Ea) for the uncatalysed pathway. ______ kJ/mol

b) State the activation energy (Ea) for the catalysed pathway. ______ kJ/mol

c) Calculate how much lower the activation energy is when the catalyst is used. ______ kJ/mol

d) Define activation energy in your own words ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23651)). ______________________________________________

**4. Effect of the catalyst on ΔH**

A student claims that adding a catalyst makes the reaction release more energy overall. Explain, using the concept from [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654), why this claim is incorrect. Refer to the ΔH value for both the catalysed and uncatalysed pathways in your answer.

______________________________________________

______________________________________________

______________________________________________

**5. Matching terms to definitions**

Match each term on the left to its correct definition on the right by writing the matching letter in the blank.

| Term | Match | Definition |
|---|---|---|
| Activation energy (Ea) | ______ | A) The energy difference between products and reactants |
| Enthalpy change (ΔH) | ______ | B) A substance that provides an alternate pathway with lower Ea |
| Catalyst | ______ | C) The energy barrier that must be overcome for a reaction to occur |
| Exothermic reaction | ______ | D) A reaction in which the products have lower potential energy than the reactants |

**6. Multiple choice**

Which statement correctly describes the effect of a catalyst on the reaction shown in the diagram?

A) The catalyst lowers the potential energy of the products.

B) The catalyst raises the potential energy of the reactants.

C) The catalyst lowers the activation energy but does not change ΔH.

D) The catalyst increases the value of ΔH released by the reaction.

Circle your answer: A B C D

## Answer key

1. a) 250 kJ/mol b) 150 kJ/mol c) Exothermic, because the products have lower potential energy than the reactants ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23643))

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 250 kJ/mol, Products at 150 kJ/mol, Ea = 120 kJ/mol, ΔH = −100 kJ/mol, catalysed path with Ea = 60 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/a07b9ec3-f71d-4c62-b2b9-85e6759fe55f/asset/331)

2. ΔH = 150 − 250 = −100 kJ/mol; this agrees with the ΔH given in the figure ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23636))

3. a) 120 kJ/mol b) 60 kJ/mol c) 60 kJ/mol lower d) Activation energy is the minimum energy that reactant particles must have for a reaction to occur ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23651))

![Figure 1: Potential energy diagram of an exothermic reaction: Reactants at 250 kJ/mol, Products at 150 kJ/mol, Ea = 120 kJ/mol, ΔH = −100 kJ/mol, catalysed path with Ea = 60 kJ/mol](https://goa-cc-uat-aili-app-001.azurewebsites.net/api/generate/a07b9ec3-f71d-4c62-b2b9-85e6759fe55f/asset/331)

4. The claim is incorrect. The catalyst lowers the activation energy from 120 kJ/mol to 60 kJ/mol, giving the reaction an alternate pathway, but ΔH remains −100 kJ/mol for both pathways because the net energy released depends only on the difference between reactant and product potential energy, which the catalyst does not change ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)).

5. Activation energy (Ea) = C; Enthalpy change (ΔH) = A; Catalyst = B; Exothermic reaction = D

6. C

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*AILI homework sheet · language en · model claude-sonnet-5 · generated 2026-09-17 · id a07b9ec3-f71d-4c62-b2b9-85e6759fe55f*

### 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)