# Potential Energy Diagrams and Thermochemical Change

This worksheet asks you to interpret and calculate values from a potential energy diagram of an exothermic reaction, both with and without a catalyst, in line with 30-A2.1k, 30-A2.3k and 30-A2.4k. It also reviews the classification of reactions as exothermic or endothermic (30-A1.10k) and the application of Q = mcΔt to heat transfer (30-A1.1k).

## Exercises

**1. Reading 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/8214d446-425e-41dc-9ea5-f001b66df96b/asset/631)

Using the labelled potential energy diagram, record the following values.

Potential energy of the reactants: ______________ kJ/mol

Potential energy of the products: ______________ kJ/mol

Activation energy of the uncatalyzed reaction (Ea): ______________ kJ/mol

Enthalpy change of the reaction (ΔH): ______________ kJ/mol

**2. Calculating enthalpy change**

Using the reactant and product energy values from Exercise 1, show the calculation for ΔH and confirm that the reaction is exothermic.

ΔH = potential energy of products − potential energy of reactants

Work: ____________________________________________

ΔH = ______________ kJ/mol

Is the reaction exothermic or endothermic? Explain how the sign of ΔH supports your answer.

______________________________________________________________

______________________________________________________________

**3. Effect of a catalyst on activation energy**

![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/8214d446-425e-41dc-9ea5-f001b66df96b/asset/631)

The diagram shows a second reaction pathway with a lower activation energy.

a) State the activation energy of the catalyzed pathway. ______________ kJ/mol

b) Calculate how much lower the catalyzed Ea is compared to the uncatalyzed Ea.

Work: ____________________________________________

Difference: ______________ kJ/mol

c) Does the catalyst change the value of ΔH? Explain your answer in terms of the reactant and product energy levels.

______________________________________________________________

______________________________________________________________

**4. Multiple choice**

Circle the best answer.

A catalyst increases the rate of a reaction by:

A) increasing the potential energy of the products
B) decreasing the potential energy of the reactants
C) providing an alternate reaction pathway with a lower activation energy
D) increasing the value of ΔH

**5. Matching definitions**

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

| Term | Answer | Definition |
|---|---|---|
| 1. Activation energy | _____ | A) The energy barrier that must be overcome for a chemical reaction to occur |
| 2. Enthalpy change (ΔH) | _____ | B) A substance that increases reaction rate without affecting the net energy change of the reaction |
| 3. Catalyst | _____ | C) The difference in potential energy between products and reactants |

**6. Applying Q = mcΔt**

A student runs an exothermic reaction in a calorimeter containing 200.0 g of water. The water temperature rises from 21.5°C to 38.2°C. The specific heat capacity of water is 4.18 J/(g·°C).

a) Calculate the heat absorbed by the water.

Work: ____________________________________________

Q = ______________ J

b) Convert your answer to kilojoules.

Q = ______________ kJ

c) Classify the reaction that took place in the calorimeter as exothermic or endothermic, and justify your classification using the direction of heat flow.

______________________________________________________________

______________________________________________________________

## Answer key

1. Reactants = 250 kJ/mol; Products = 150 kJ/mol; Ea (uncatalyzed) = 120 kJ/mol; ΔH = −100 kJ/mol.

![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/8214d446-425e-41dc-9ea5-f001b66df96b/asset/631)

2. ΔH = 150 − 250 = −100 kJ/mol; exothermic, because ΔH is negative, meaning the products have less potential energy than the reactants and energy is released to the surroundings.
3. a) Ea (catalyzed) = 60 kJ/mol. b) 120 − 60 = 60 kJ/mol lower. c) No, ΔH stays at −100 kJ/mol because the catalyst only lowers the activation energy of the pathway; it does not change the potential energy of the reactants or products.

![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/8214d446-425e-41dc-9ea5-f001b66df96b/asset/631)

4. C
5. 1-A, 2-C, 3-B
6. a) Q = mcΔt = 200.0 g × 4.18 J/(g·°C) × (38.2 − 21.5)°C = 200.0 × 4.18 × 16.7 = 13,961.2 J. b) Q ≈ 14.0 kJ. c) Exothermic, because heat flows from the reaction into the water, raising its temperature.

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*AILI homework sheet · language en · model claude-sonnet-5 · generated 2026-09-17 · id 8214d446-425e-41dc-9ea5-f001b66df96b*

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