An energy profile diagram shows two separate quantities. Activation energy is the climb from the reactants to the peak, and the energy change is the difference between the reactants and the products.
This lesson is part of SPM Chemistry rate of reaction. The same diagram is used again in reading energy-level diagrams in thermochemistry.
What are the two measurements?
Read the vertical axis, which is energy, and find three levels: the reactants, the peak and the products.
- Activation energy, Ea: peak level − reactants level.
- Energy change, ΔH: products level − reactants level.
A negative ΔH means the reaction is exothermic. A positive ΔH means it is endothermic.
Worked example: an original energy profile
Suppose the reactants are at 50 kJ, the peak is at 130 kJ and the products are at 20 kJ.
Ea = 130 − 50 = 80 kJ.
ΔH = 20 − 50 = −30 kJ, so the reaction is exothermic. The products sit below the reactants, which means energy is released to the surroundings.
What changes when a catalyst is added?
Suppose the catalysed pathway has its peak at 100 kJ, with the same reactants and products.
New Ea = 100 − 50 = 50 kJ. The activation energy falls by 30 kJ.
ΔH = 20 − 50 = −30 kJ. It has not changed, because the reactants and products are at the same levels.
| Quantity | Without catalyst | With catalyst |
|---|---|---|
| Activation energy | 80 kJ | 50 kJ |
| Energy change ΔH | −30 kJ | −30 kJ |
| Rate | Lower | Higher |
The lower peak means a larger fraction of collisions reach the activation energy, which is why the rate is higher. You can link this to the explanations in collision theory.
The mistake that costs marks
The common slip is measuring activation energy from the bottom axis, or all the way to the products. Start the measuring arrow at the reactants and end it at the peak.
A second slip is saying a catalyst “makes the reaction more exothermic”. The energy released is set by the reactants and products, which a catalyst does not alter.
Check yourself
An endothermic reaction has reactants at 20 kJ, a peak at 95 kJ and products at 60 kJ. Find Ea and ΔH. A catalyst lowers the peak to 75 kJ. Find the new Ea and ΔH.
Answer
Ea = 95 − 20 = 75 kJ.
ΔH = 60 − 20 = +40 kJ, so the reaction is endothermic because the products are higher.
With the catalyst, Ea = 75 − 20 = 55 kJ, and ΔH is still +40 kJ.
What to study next
Test your reading of diagrams and graphs together with the rate of reaction practice set. For the energy-change side, continue with reading energy-level diagrams.
If you want a teacher to check your own diagrams and labels, see online one-to-one Chemistry tuition.