Topic 6.2 Notes – Energy Diagrams

1. What an Energy Diagram Shows

An energy diagram is a graph of energy versus reaction progress.

• y-axis → potential energy (often enthalpy, $H$)
• x-axis → reaction progress (the pathway from reactants to products, not time)

It visually shows:

• Potential energy of reactants ($PE_r$)
• Potential energy of products ($PE_p$)
• Activation energy ($E_a$)
• Enthalpy change ($\Delta H$)
• The activated complex (transition state)

The whole point is this: the diagram tells you how hard it is to start the process and whether the system ends up at a higher or lower energy than it began.

2. Key Parts of an Energy Diagram

Here’s what each piece means and how to recognize it. Use the exothermic diagram below as your reference as you read.

Exothermic energy diagram

Potential Energy of Reactants ($PE_r$)

• Energy level at the start.
• Shown as the left plateau labeled A + B.
• Comes from bond energies and intermolecular forces in the reactants.

Potential Energy of Products ($PE_p$)

• Energy level at the end.
• Shown as the right plateau labeled C + D.
• Comparing $PE_p$ to $PE_r$ tells you if the reaction is exothermic or endothermic.

Activation Energy ($E_a$)

The minimum energy needed to start the reaction.

$$E_a = PE_{\text{activated complex}} - PE_r$$

• Measured from the reactant energy level up to the peak marked as the transition step.
• Represents energy needed to break initial bonds.
• Even exothermic reactions have a positive $E_a$. This trips people up constantly.

Activated Complex (Transition State)

• The highest point on the curve.
• Labeled as the transition step in the diagram.
• Extremely unstable arrangement of atoms.
• Bonds are partially broken and partially formed.
• Exists for an instant.

Enthalpy Change ($\Delta H$)

Overall energy change of the reaction:

$$\Delta H = PE_p - PE_r$$

• $\Delta H < 0$ → exothermic
• $\Delta H > 0$ → endothermic

In the diagram, this is the vertical difference between the reactant plateau and the product plateau. Notice this depends only on start vs. end energy, not on how tall the peak is.

3. Exothermic vs Endothermic Reactions

The relative heights of reactants and products tell the story.

Energy-wise:

• Exothermic → forming new bonds releases more energy than breaking old bonds requires.
• Endothermic → breaking bonds requires more energy than forming new bonds releases.

On tests, you’re often given a diagram with numbers on the y-axis and asked to calculate $E_a$ and $\Delta H$. Always subtract carefully and watch the sign.

4. Energy Diagrams for Phase Changes

Energy diagrams also describe physical processes, not just chemical reactions.

Endothermic Phase Changes ($\Delta H > 0$)

• Melting $(s \rightarrow l)$
• Vaporization $(l \rightarrow g)$
• Sublimation $(s \rightarrow g)$

Energy is absorbed to overcome intermolecular forces.

Exothermic Phase Changes ($\Delta H < 0$)

• Freezing $(l \rightarrow s)$
• Condensation $(g \rightarrow l)$
• Deposition $(g \rightarrow s)$

Energy is released as intermolecular forces form.

Important detail for exams:

• During a phase change, temperature stays constant.
• The energy involved is latent heat.
• On an energy diagram, endothermic phase changes end higher; exothermic ones end lower.

5. How to Read or Construct an Energy Diagram

When you’re given one:

1. Read $PE_r$ and $PE_p$ from the y-axis.
2. Calculate $\Delta H = PE_p - PE_r$.
3. Identify reaction type from the sign.
4. Calculate $E_a = PE_{\text{peak}} - PE_r$.
5. Label the peak as the activated complex.

If you’re asked to draw one:

• Axes labeled correctly.
• One smooth curve.
• Clear peak.
• Reactants and products at correct relative heights.
• Arrows labeled $E_a$ and $\Delta H$.

Common mistake I see on FRQs: students label the x-axis as time. It must say reaction progress.

Also remember:

• $E_a$ relates to rate (kinetics).
• $\Delta H$ relates to energy change (thermodynamics).

They are not the same idea.