Chem Activity: Potential Energy
Information:
The system is the region that you are focusing on while the surroundings include the
space around the system. When a chemical reaction takes place, the reactants and products are
considered the system, while the container and air or other materials that may come in thermal
contact are considered the surroundings. Heat travels from areas of high heat to low heat.
Endothermic processes occur when heat is absorbed from the surroundings into the system.
Exothermic processes occur when heat is released into the surroundings from the system. All
chemical reactions are accompanied by a change in heat content. Reactions that absorb energy
would be considered endothermic. Reactions that release energy would be considered
exothermic.
Enthalpy (H) is the amount of heat content that a substance has at a given temperature
and pressure. During a chemical reaction, the heat content of the substances changes during the
progression of the reaction. A reaction profile is a graphical representation of what is happening
to the enthalpy (heat) of a reaction. The potential energy (PE) of the reactants and products are
shown on the graph. The activation energy (Ea) is the energy needed to start the reaction. The
transition state (TS) or the activated complex is the brief interval of bond disruption (reactants)
and bond formation (products). Catalysts increase the rate of reaction without being used up in
the reaction. The models used in this activity are an Excel file courtesy of Scott Sinex at Prince
George Community College. Hover on any cell that has a red flag in the corner for some
important tips.
VBCPS 2011
Created by Heather Green and Bridget Mariano
Select the PE Diagram tab from the bottom navigation bar.
Model 1: Potential Energy Diagram
Questions:
1. What is the potential energy of the reactants?
products?
2. What is the potential energy of the transition state or the activated complex?
3. Click on the green “Activation energy” box. Record the value of the activation energy (Ea)
for this reaction.
4. How is the activation energy calculated?
Write the equation.
5. Is the value of the Ea positive or negative?
What are the units?
6. Unclick the green box and click on the brown “Add a catalyst” box. What happens to the
activation energy when a catalyst is added?
7. How does the addition of a catalyst affect the potential energies of the reactants and the
products?
8. Would a catalyst speed up or slow down the rate of reaction?
9. Click on the teal “Activation energy with a catalyst” box (keep brown box checked).
Determine the Ea for the reaction upon the addition of a catalyst. Show your work.
10. Is the value of the Ea positive or negative?
11. Unclick both the brown and teal boxes. Click on the purple “Heat of Rxn” box. Record the
value for the Change of the Enthalpy/Heat of Reaction (H) for this reaction. Include the sign
and the units for the value.
12. How is the heat of reaction (H) calculated?
Write the equation.
13. Is the value of the H positive or negative for this graph?
14. Is energy absorbed or released by this reaction?
15. Is the reaction represented by this graph endothermic or exothermic?
16. Write a general statement concerning the value of the H and the classification of the
reaction as endothermic or exothermic. Sketch and label a sample of this reaction profile on the
last page of this activity.
VBCPS 2011
Created by Heather Green and Bridget Mariano
Uncheck all of the boxes. Use these buttons to manipulate the reaction profile graph.
reactants
activated complex
products
Model 2: Manipulated Potential Energy Diagram
Manipulate the graph so that the potential energy of the reactants is +30 kJ, the potential energy
of the products is +10 kJ and the potential energy of the activation energy is +40 kJ.
Questions:
17. What is the potential energy of the reactants?
products?
18. What is the potential energy of the transition state or the activated complex?
19. Click on the green “Activation energy” box. Record the value of the activation energy (Ea)
for this reaction.
20. Calculate the activation energy (show all work).
21. Is the value of the Ea positive or negative?
What are the units?
22. Unclick the green box and click on the brown “Add a catalyst” box. What happens to the
activation energy when a catalyst is added?
23. How does the addition of a catalyst affect the potential energies of the reactants and the
products?
24. Click on the teal “Activation energy with a catalyst” box (keep the brown box clicked).
Determine the Ea for the reaction upon the addition of a catalyst. Show your work.
25. Is the value of the Ea positive or negative?
26. Unclick both the brown and teal boxes. Click on the purple “Heat of Rxn” box. Record the
value of the Change of the Enthalpy/Heat of Reaction (H) for this reaction. Include the sign
and the units for the value.
27. Calculate the heat of reaction (H) – show all work.
28. Re-click the brown box. Does the H change for the reaction? Does your observation
match your answer to question 23?
29. Is the value of the H positive or negative for this graph?
VBCPS 2011
Created by Heather Green and Bridget Mariano
30. Is energy absorbed or released by this reaction?
31. Is the reaction represented by this graph endothermic or exothermic?
32. Write a general statement concerning the value of the H and the classification of the
reaction as endothermic or exothermic. Sketch and label a sample of this reaction profile on the
last page of this activity.
Extension activity
Complete the assigned worksheets on reaction profiles.
VBCPS 2011
Created by Heather Green and Bridget Mariano