On a reaction profile, measure the forward activation energy from the reactants to the peak. Measure the reverse activation energy from the products to the peak. A catalyst provides a pathway with a lower activation barrier while the energies of the reactants and products stay the same.
The 2027 SEC G3 Chemistry syllabus (K324) covers chemical energetics and the effect of catalysts on reaction rates. This example uses made-up energy levels to show how three different energy differences come from the same diagram.
- Identify the starting level for the direction you are asked about.
- Subtract that starting level from the pathway’s peak.
- Find the forward enthalpy change from the endpoints.
Worked example | Compare an uncatalysed peak with a lower peak
In a simplified profile, reactants are at 35 kJ/mol and products at 10 kJ/mol on the same energy scale. The uncatalysed peak is at 95 kJ/mol. The forward activation energy is 95 − 35 = 60 kJ/mol. The reverse activation energy is 95 − 10 = 85 kJ/mol.
The simplified catalysed path peaks at 65 kJ/mol. Its forward activation energy is 65 − 35 = 30 kJ/mol. Its reverse activation energy is 65 − 10 = 55 kJ/mol.
The forward enthalpy change is 10 − 35 = −25 kJ/mol for both pathways. The negative value means the forward reaction is exothermic. For the reverse reaction, the enthalpy change is +25 kJ/mol.
Use the endpoints as a check
The reverse barrier is larger than the forward barrier here because the products are lower in energy than the reactants. The barriers differ by 25 kJ/mol on each of the two simplified paths.
If you get an enthalpy change of −30 kJ/mol by subtracting the two peak heights, check what that difference means. In this example, it is the amount by which the activation barrier falls.
Try a forward endothermic profile
Reactants are at 20, products at 45 and the peak at 80 kJ/mol. The forward activation energy is 60 kJ/mol, the reverse activation energy is 35 kJ/mol and the forward enthalpy change is +25 kJ/mol.
A study partner can point to a pair of levels and ask what the gap means. Say which direction the reaction is going before identifying the starting level.
Frequently asked questions
Can a catalyst turn an exothermic reaction into an endothermic one?
For the same reactants and products under the same conditions, the endpoint energy difference stays the same.
Is the peak value itself the activation energy?
Use the difference between the peak and the starting level for the stated direction.
Official information
SEAB’s 2027 SEC G3 Chemistry syllabus (K324), Chemical Energetics and Rate of Reactions. Checked 10 November 2026.