Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- An energy diagram shows the energy of the system at each stage of a process. The surroundings are not drawn.
- Vertical axis: energy, usually in kJ mol⁻¹ (per mole of the process as written). Horizontal axis: progress from start to finish, with no scale and no unit.
- Each stage is a horizontal level; the arrow runs from the start level to the end level and is labelled ΔH.
- Zero can go anywhere. Only differences between levels mean anything.
- Diagrams work for reactions, phase changes and dissolving. A hump (activation energy) can be added, but it does not affect ΔH.
Key relationships
| Idea | Rule |
|---|---|
| Energy change | ΔH = E(final) − E(start) |
| Exothermic | final level lower, arrow down, ΔH < 0, surroundings warm |
| Endothermic | final level higher, arrow up, ΔH > 0, surroundings cool |
| Reverse process | same two levels, arrow reversed, ΔH changes sign |
| Phase levels for one substance | gas highest, then liquid, then solid |
| Dissolving step diagram | step 1 up, step 2 up, step 3 down; overall = sum of the three |
Assumptions behind a diagram
- All levels refer to the same conditions (same temperature and pressure), so the differences are meaningful.
- ΔH is per mole of the equation as written; a different equation would need different levels.
- Step values for dissolving are a model: the real process does not happen in three separate steps.
Mistakes to avoid
- Unlabelled or unit-less axis. Write “Energy (kJ mol⁻¹)”.
- Uneven ticks or levels placed by eye instead of by the scale.
- Arrow drawn from high to low whatever the process. It goes from start to end.
- Comparing arrow lengths on different axes. Read the numbers.
- Changing ΔH when a catalyst is added. Only the hump moves.
- “Temperature” on the vertical axis. It is energy.
Quick self-check
- Reactants at 20 kJ mol⁻¹, products at 85 kJ mol⁻¹. ΔH? Endo or exo? (+65 kJ mol⁻¹, endothermic.)
- A reaction has ΔH = −184 kJ mol⁻¹. What is ΔH for the reverse, and which way does its arrow point? (+184 kJ mol⁻¹; up.)
- On one diagram for a pure substance, which phase sits lowest? (The solid.)
Next: practice questions.