An energy-level diagram has two measurements. The activation energy, Ea, goes from the reactants up to the peak, and the heat of reaction, ΔH, goes from the reactants to the products.
This lesson is part of the thermochemistry chapter. The next lesson, explaining sign conventions and sources of error, uses ΔH in calculations.
What do the labels on the diagram mean?
The vertical axis is energy. The reactants sit on the left level and the products on the right level, joined by a curve that rises to a peak.
- Ea = energy at the peak − energy of the reactants.
- ΔH = energy of the products − energy of the reactants.
Both are measured from the reactants’ level, but Ea ends at the peak and ΔH ends at the products.
Worked example: an exothermic reaction
In a diagram the reactants are at 100 kJ, the peak is at 150 kJ, and the products are at 40 kJ. These values are invented for practice.
- Ea = 150 − 100 = +50 kJ mol⁻¹.
- ΔH = 40 − 100 = −60 kJ mol⁻¹.
The products are lower than the reactants, so the reaction is exothermic. The surroundings would become warmer because 60 kJ per mole is released as heat.
A catalyst is added and the peak drops to 120 kJ. Now Ea = 120 − 100 = 20 kJ mol⁻¹. The reactants and products stay at 100 and 40, so ΔH remains −60 kJ mol⁻¹.
Worked example: an endothermic reaction
In a second diagram the reactants are at 30 kJ, the peak is at 95 kJ, and the products are at 70 kJ.
- Ea = 95 − 30 = +65 kJ mol⁻¹.
- ΔH = 70 − 30 = +40 kJ mol⁻¹.
The products are higher than the reactants, so the reaction is endothermic. Energy is absorbed, and the surroundings would cool.
The mistake that costs marks
The common slip is to measure ΔH from the peak to the products, which gives the wrong size and sign.
| Quantity | Wrong measurement | Right measurement |
|---|---|---|
| Ea | reactants to products | reactants to peak |
| ΔH (exothermic) | peak to products, shown as positive | reactants to products, negative |
| Effect of a catalyst | changes ΔH | lowers Ea only |
A quick check: ΔH is negative when the right-hand level is lower, whatever the height of the peak.
Check yourself
A diagram shows reactants at 80 kJ, a peak at 130 kJ and products at 35 kJ. Find Ea and ΔH, and state whether the reaction is exothermic or endothermic.
Answer
Ea = 130 − 80 = 50 kJ mol⁻¹.
ΔH = 35 − 80 = −45 kJ mol⁻¹.
The products are lower than the reactants, so the reaction is exothermic.
What to study next
Continue with explaining sign conventions and sources of error to see how temperature data gives ΔH. The mole and stoichiometry steps tool helps when you need the number of moles for the calculation.
If diagram questions still feel like guessing, see online one-to-one Chemistry tuition.