# Potential Energy Diagrams: Exothermic Reactions and Catalysts This worksheet asks you to analyze and label a potential energy diagram for an exothermic reaction, calculate enthalpy and activation energy values, and explain how a catalyst changes the reaction pathway without changing the net energy change ([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), [Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)). You will also connect the diagram to bond breaking, bond forming, and changes in potential and kinetic energy ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23652)). ## Exercises **1. Label 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/4d369ab0-2770-4643-9190-509d47a2beab/asset/333) Using the potential energy diagram described above, fill in the missing values on the lines below. Potential energy of reactants: ______ kJ/mol Potential energy of products: ______ kJ/mol Activation energy (uncatalyzed forward reaction), Ea: ______ kJ/mol Enthalpy change, ΔH: ______ kJ/mol Activation energy (catalyzed forward reaction): ______ kJ/mol **2. Calculations** Using the values from the diagram, show your work for each calculation. a) Calculate the potential energy of the transition state (activated complex) for the uncatalyzed pathway. ______________________________________________ b) Calculate the activation energy for the reverse reaction (products → reactants) along the uncatalyzed pathway. ______________________________________________ c) By how many kJ/mol does the catalyst lower the activation energy of the forward reaction? ______________________________________________ **3. Multiple choice** Circle the best answer. Adding a catalyst to this reaction changes: A) ΔH only B) Ea only C) both ΔH and Ea D) neither ΔH nor Ea **4. Matching** Match each term on the left to its correct description on the right by writing the matching letter in the blank. | Term | Blank | Description | |---|---|---| | 1. Activation energy | ______ | A) The energy barrier that must be overcome for a reaction to occur | | 2. Catalyst | ______ | B) The difference in potential energy between products and reactants | | 3. Enthalpy change (ΔH) | ______ | C) A substance that provides an alternate pathway with lower Ea | | 4. Exothermic reaction | ______ | D) A reaction in which the products have lower potential energy than the reactants | **5. True or False** Write True or False on each line. If a statement is false, correct it. a) In this reaction, breaking bonds in the reactants releases more energy than forming bonds in the products absorbs. ______________________________________________ b) The catalyzed pathway has a lower activation energy but the same ΔH as the uncatalyzed pathway. ______________________________________________ c) A catalyst increases the amount of energy released by the reaction. ______________________________________________ **6. Short answer** ![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/4d369ab0-2770-4643-9190-509d47a2beab/asset/333) Explain, using the values from the diagram, why the catalyzed pathway allows the reaction to proceed faster while the enthalpy change of the reaction stays at −70 kJ/mol. Refer to the activation energy of both pathways in your answer ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)). ______________________________________________ ______________________________________________ ______________________________________________ ## Answer key 1. Reactants: 150 kJ/mol; Products: 80 kJ/mol; Ea (uncatalyzed): 85 kJ/mol; ΔH: −70 kJ/mol; Ea (catalyzed): 45 kJ/mol. ![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/4d369ab0-2770-4643-9190-509d47a2beab/asset/333) 2. a) 150 kJ/mol + 85 kJ/mol = 235 kJ/mol; b) 235 kJ/mol − 80 kJ/mol = 155 kJ/mol; c) 85 kJ/mol − 45 kJ/mol = 40 kJ/mol. 3. B) Ea only, since a catalyst provides an alternate pathway with a lower activation energy but does not change the net energy of the reaction ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)). 4. 1, A, 2, C, 3, B, 4, D ([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)). 5. a) True, since the reaction is exothermic (ΔH = −70 kJ/mol), meaning energy released in forming product bonds exceeds energy absorbed in breaking reactant bonds ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23652)). b) True, the catalyst lowers Ea from 85 kJ/mol to 45 kJ/mol but ΔH remains −70 kJ/mol ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)). c) False; a catalyst does not change the amount of energy released, only the rate at which the reaction reaches that energy change ([Outcome](https://goa-cc-uat-aili-app-001.azurewebsites.net/explore/node/23654)). 6. The uncatalyzed pathway requires 85 kJ/mol of activation energy, while the catalyzed pathway requires only 45 kJ/mol, so more reactant particles have enough energy to reach the lower energy barrier, increasing reaction rate. The reactants (150 kJ/mol) and products (80 kJ/mol) are unchanged, so ΔH remains −70 kJ/mol on both pathways ([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/23653)). ![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/4d369ab0-2770-4643-9190-509d47a2beab/asset/333) --- *AILI homework sheet · language en · model claude-sonnet-5 · generated 2026-09-17 · id 4d369ab0-2770-4643-9190-509d47a2beab* ### 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)