An energy profile is a graph of the energy of the chemicals as a reaction goes from reactants to products. The products’ level compared with the reactants’ level tells you whether the reaction is exothermic or endothermic, and the gap between them gives the energy change, ΔH.
It is the second lesson in energy changes and bonds, and it gives you the picture behind the words from distinguishing energy released from energy absorbed.
What does each part of the diagram mean?
The vertical axis shows energy. The horizontal axis shows the progress of the reaction, from left (reactants) to right (products). It is not a time axis and it has no scale in seconds.
- Left flat line: the energy of the reactants.
- Right flat line: the energy of the products.
- Peak in the middle: the highest energy along the route. The climb from the reactants up to the peak is the activation energy, covered in the next lesson.
- Vertical arrow between the two flat lines: the overall energy change, ΔH.
How do you read it, step by step?
- Label the reactant and product levels first, before anything else.
- Compare them. Products lower means exothermic; products higher means endothermic.
- Write ΔH as products minus reactants if the levels have numbers.
- Check the sign against step 2. Lower products must give a negative answer.
- Add units, usually kJ/mol.
Worked example
The values below are invented for practice. A fictional reaction has reactants at 100 kJ/mol, a peak at 250 kJ/mol and products at 40 kJ/mol.
| Part of the diagram | Energy (kJ/mol) |
|---|---|
| Reactants | 100 |
| Peak | 250 |
| Products | 40 |
Step 1, compare levels: the products (40) are lower than the reactants (100), so the reaction is exothermic.
Step 2, calculate ΔH: products minus reactants = 40 − 100 = −60 kJ/mol.
Step 3, check: the sign is negative, which matches exothermic.
Step 4, the climb to the peak: 250 − 100 = 150 kJ/mol. That is the activation energy, not the energy change.
Now reverse the picture. A second fictional reaction has reactants at 50, a peak at 210 and products at 130. The products are higher, so it is endothermic, and ΔH = 130 − 50 = +80 kJ/mol.
Check: the first answer is negative for lower products, the second is positive for higher products. Both agree with the direction of the arrow.
The mistake to watch for
The typical slip is to use the peak as if it were the product level.
Mistaken answer: “ΔH = 250 − 100 = +150 kJ/mol, so the reaction is endothermic.”
The student subtracted the peak from the reactant level, which measures the energy barrier, not the overall change.
The correction is to use the two flat lines only. The products are at 40, so ΔH = 40 − 100 = −60 kJ/mol and the reaction is exothermic. A quick habit helps: put a finger on the left flat line, slide it to the right flat line, and ask whether you moved down or up.
Check yourself
1. Reactants are at 200 kJ/mol, the peak is at 320 kJ/mol and products are at 150 kJ/mol. Is the reaction exothermic or endothermic, and what is ΔH?
Show answer
Products (150) are lower than reactants (200), so it is exothermic. ΔH = 150 − 200 = −50 kJ/mol.
2. A profile has products higher than reactants. Which way does the ΔH arrow point, and what sign does ΔH have?
Show answer
The arrow points up from the reactant level to the product level, and ΔH is positive. The reaction is endothermic.
3. A student says a taller peak means a more exothermic reaction. Is that correct?
Show answer
No. The peak shows the energy barrier. Whether a reaction is exothermic depends on whether the products are lower than the reactants.
Where this leads next
Next, use the climb to the peak in explaining activation energy on a diagram. When you want mixed questions, try the energy changes and bonds practice set.
Labelling and reading diagrams under exam time is a skill that improves with spoken feedback. A teacher in online one-to-one Chemistry tuition can ask you to justify each arrow and show you where your reading drifts.