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D3.6 · Interpret simple reaction potential-energy diagrams

Learn to interpret simple reaction potential-energy diagrams through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Energy Changes and Rates of Reaction

SCH4U study topic D3.6: interpret the energy levels and changes shown in a simple reaction diagram

A reaction may release energy to its surroundings or take in energy from them. A potential-energy diagram represents these changes by showing how the energy of the reacting system changes from reactants to products. The diagram is a model: it helps you compare energy levels and interpret the energy barrier that must be crossed. Begin by reading the axes and locating the reactant and product levels. Then compare their energies and measure the vertical gaps shown.

What you will learn

  • Identify the reactants, products, and activated complex on a simple potential-energy diagram.
  • Determine the activation energy for the forward or reverse reaction from the diagram.
  • Use the relative energy levels of reactants and products to decide whether a reaction is exothermic or endothermic.
  • Explain how a catalyst changes a potential-energy diagram without changing the energy difference between reactants and products.

1. Read the axes and the energy profile

A potential-energy diagram is a graph of the potential energy of a reacting system as a reaction proceeds. Potential energy is stored energy associated with the arrangement of particles. The horizontal axis usually represents reaction progress. It shows the change from reactants to products, but it is not usually a measure of time.
The vertical axis represents potential energy. Its units are often kilojoules per mole, written as kJ mol−1\mathrm{kJ\,mol^{-1}}. The graph’s curve begins at the reactant energy level, rises to a high point, and then falls to the product energy level. Read the energy values from the vertical scale before drawing conclusions.
The reactants are the starting substances in the reaction. The products are the substances formed. The high point on a simple profile represents the activated complex: a short-lived, high-energy arrangement of particles as bonds are breaking and forming. You do not need to identify its exact structure to interpret the diagram.
  • The vertical axis shows potential energy; the horizontal axis shows reaction progress.
  • The curve’s high point is the activated complex.
  • The reactant and product levels, not the shape of the curve alone, determine the overall energy change.

2. Find activation energy and reaction energy change

Activation energy is the energy needed to raise the reacting system from its starting level to the activated complex. It is the vertical energy difference from the reactants to the top of the curve for the forward reaction. A larger forward activation energy means a taller energy barrier on that diagram.
For the reverse reaction, the products are the starting point. Its activation energy is the vertical difference from the product level up to the same high point. The forward and reverse activation energies can therefore be different.
The reaction energy change, often written as ΔH\Delta H, is the difference between product energy and reactant energy. The sign tells you whether the products are higher or lower in energy. If the products are lower, energy is released by the reacting system and the reaction is exothermic. If the products are higher, energy is taken in by the reacting system and the reaction is endothermic.
For an exothermic reaction, ΔH\Delta H is negative. For an endothermic reaction, ΔH\Delta H is positive. These signs describe the change from reactants to products. Do not use the height of the peak to decide whether a reaction is exothermic or endothermic; compare the two endpoint levels.
Ea,forward=Epeak−Ereactants;Ea,reverse=Epeak−Eproducts;ΔH=Eproducts−EreactantsE_{\mathrm{a,forward}}=E_{\mathrm{peak}}-E_{\mathrm{reactants}};\quad E_{\mathrm{a,reverse}}=E_{\mathrm{peak}}-E_{\mathrm{products}};\quad \Delta H=E_{\mathrm{products}}-E_{\mathrm{reactants}}
  • Forward activation energy is the gap from the reactant level to the peak.
  • Reverse activation energy is the gap from the product level to the peak.
  • Reaction energy change is product energy minus reactant energy.
  • Products lower than reactants indicate an exothermic reaction; products higher than reactants indicate an endothermic reaction.

3. Connect the diagram to particles and catalysts

At the particle level, reactant particles must reach an arrangement with enough energy for the reaction to proceed. The activated complex represents that high-energy arrangement. The activation energy is the energy gap between the starting particles and this arrangement on the diagram.
A catalyst provides a different reaction pathway with a lower activation-energy peak. A pathway is the route represented by the profile as the reaction changes from reactants to products. On a diagram comparing catalysed and uncatalysed pathways, both start at the same reactant level and end at the same product level. The catalysed peak is lower.
Because the reactant and product levels stay the same, a catalyst does not change ΔH\Delta H. It lowers the activation-energy barrier for both forward and reverse reactions. When interpreting a diagram, keep these two comparisons separate: endpoint levels show the reaction energy change, while the peak relative to an endpoint shows activation energy.
  • The peak represents an energy barrier, not the overall energy change.
  • A catalyst lowers the peak on the reaction pathway.
  • A catalyst does not change the reactant level, product level, or ΔH\Delta H.

4. A reliable method for interpreting a diagram

First, check the axis labels and units. Confirm which side represents reactants and which side represents products. Then read the energy of each endpoint and the peak from the vertical scale.
Next, identify what the question asks. For forward activation energy, subtract the reactant energy from the peak energy. For reverse activation energy, subtract the product energy from the peak energy. For the reaction energy change, subtract reactant energy from product energy.
Keep units with the values during the calculation. If all energies are given in kJ mol−1\mathrm{kJ\,mol^{-1}}, the differences are also in kJ mol−1\mathrm{kJ\,mol^{-1}}. A negative reaction energy change is meaningful; it indicates that products have less energy than reactants. Activation energies are reported as positive energy gaps.
Finally, use the calculated sign or comparison to state what the diagram means. Name the reaction as exothermic or endothermic, or identify which direction has the larger activation energy. Do not claim that the diagram gives a reaction time or an exact molecular structure.
  • Read the graph before calculating.
  • Subtract the starting level from the peak for activation energy.
  • Subtract reactant energy from product energy for ΔH\Delta H.
  • Include units and interpret the sign in words.

Worked example

Compare forward and reverse energy changes

A simple potential-energy diagram has reactants at 35 kJ mol−135\ \mathrm{kJ\,mol^{-1}}, a peak at 92 kJ mol−192\ \mathrm{kJ\,mol^{-1}}, and products at 12 kJ mol−112\ \mathrm{kJ\,mol^{-1}}. Find the forward activation energy, reverse activation energy, and reaction energy change. State whether the reaction is exothermic or endothermic.
  1. Identify the levels
    The reactant and product levels are the starting and ending energies. The peak is the activated complex. The problem gives all three values in the same units, so their differences will also be in kilojoules per mole.
  2. Calculate forward activation energy
    For the forward direction, the system starts at the reactant level and must reach the peak. Subtract the reactant energy from the peak energy.
    Ea,forward=92−35=57 kJ mol−1E_{\mathrm{a,forward}}=92-35=57\ \mathrm{kJ\,mol^{-1}}
  3. Calculate reverse activation energy
    For the reverse direction, the products are the starting point. Subtract the product energy from the same peak energy.
    Ea,reverse=92−12=80 kJ mol−1E_{\mathrm{a,reverse}}=92-12=80\ \mathrm{kJ\,mol^{-1}}
  4. Calculate and interpret the reaction energy change
    Subtract the reactant energy from the product energy. The negative result means that the products are lower in energy than the reactants, so the reaction is exothermic.
    ΔH=12−35=−23 kJ mol−1\Delta H=12-35=-23\ \mathrm{kJ\,mol^{-1}}
Answer: The forward activation energy is 57 kJ mol−157\ \mathrm{kJ\,mol^{-1}}. The reverse activation energy is 80 kJ mol−180\ \mathrm{kJ\,mol^{-1}}. The reaction energy change is −23 kJ mol−1-23\ \mathrm{kJ\,mol^{-1}}, so the reaction is exothermic.
Check: The forward and reverse activation energies differ by 23 kJ mol−123\ \mathrm{kJ\,mol^{-1}}, matching the energy difference between the reactants and products.

Common mistakes and how to avoid them

Using the peak height alone to label a reaction exothermic or endothermic.
Correction: Compare product energy with reactant energy. The peak is used to find activation energy.
Finding activation energy by subtracting the peak from the starting level.
Correction: Subtract the starting energy level from the peak. The resulting activation-energy gap is positive.
Using the forward activation energy as the reverse activation energy.
Correction: For the reverse direction, use the product level as the starting point and compare it with the peak.
Saying that a catalyst changes the reaction energy change.
Correction: A catalyst lowers the activation-energy peak but leaves the reactant and product energy levels unchanged.

Lesson summary

  • A potential-energy diagram shows energy against reaction progress.
  • The curve’s peak represents the activated complex.
  • Activation energy is the energy gap from a starting level to the peak.
  • The reaction energy change is product energy minus reactant energy.
  • A negative ΔH\Delta H indicates an exothermic reaction; a positive ΔH\Delta H indicates an endothermic reaction.
  • A catalyst lowers the activation-energy barrier without changing ΔH\Delta H.

Check your understanding

Question 1

On a diagram, reactants are at 18 kJ mol−118\ \mathrm{kJ\,mol^{-1}} and products are at 46 kJ mol−146\ \mathrm{kJ\,mol^{-1}}. What is the reaction energy change?
  1. −28 kJ mol−1-28\ \mathrm{kJ\,mol^{-1}}; exothermic
  2. +28 kJ mol−1+28\ \mathrm{kJ\,mol^{-1}}; endothermic
  3. +64 kJ mol−1+64\ \mathrm{kJ\,mol^{-1}}; endothermic
  4. correctIndex: 1
Show answer and explanation
+28 kJ mol−1+28\ \mathrm{kJ\,mol^{-1}}; endothermic
Subtract reactant energy from product energy: 46−18=+28 kJ mol−146-18=+28\ \mathrm{kJ\,mol^{-1}}. Products are higher in energy, so the reaction is endothermic.

Question 2

A reactant level is 24 kJ mol−124\ \mathrm{kJ\,mol^{-1}} and the peak is 71 kJ mol−171\ \mathrm{kJ\,mol^{-1}}. What is the forward activation energy?
  1. 47 kJ mol−147\ \mathrm{kJ\,mol^{-1}}
  2. 95 kJ mol−195\ \mathrm{kJ\,mol^{-1}}
  3. −47 kJ mol−1-47\ \mathrm{kJ\,mol^{-1}}
  4. correctIndex: 0
Show answer and explanation
47 kJ mol−147\ \mathrm{kJ\,mol^{-1}}
Subtract the reactant level from the peak: 71−24=47 kJ mol−171-24=47\ \mathrm{kJ\,mol^{-1}}.

Question 3

A catalyst is added to a reaction. Which change should appear on a simple potential-energy diagram?
  1. The product energy level moves while the reactant level stays fixed.
  2. The peak is lower, while the reactant and product levels stay the same.
  3. The reactant and product levels both move to the peak.
  4. correctIndex: 1
Show answer and explanation
The peak is lower, while the reactant and product levels stay the same.
A catalyst lowers the activation-energy barrier. It does not change the energy levels of the reactants or products.

Key terms

Potential energy
Stored energy associated with the arrangement of particles.
Activated complex
The short-lived, high-energy arrangement of particles represented by the peak of a simple reaction profile.
Activation energy
The energy difference from the starting level of a reaction direction to the activated complex.
Exothermic reaction
A reaction in which the products have less energy than the reactants; its reaction energy change is negative.
Endothermic reaction
A reaction in which the products have more energy than the reactants; its reaction energy change is positive.
Catalyst
A substance that provides a reaction pathway with a lower activation-energy barrier without changing the reactant and product energy levels.

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