Define activation energy on a reaction pathway
Identify the activation-energy barrier and distinguish it from the enthalpy change on an energy profile.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Rate data and diagrams are illustrative unless a question supplies measurements. A diagram is qualitative unless labelled values are provided. Written practice is self-checked
Before you begin
- Rearrange simple ratios and use consistent units for time, volume, mass and concentration.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does activation energy represent?
What you will learn
- Define activation energy
- Read a reaction pathway diagram
- Distinguish activation energy from enthalpy change
Activation energy is the minimum energy required for an effective collision
Activation energy, Eₐ, is the minimum energy required for a collision to be effective. On an energy profile, the forward activation energy is the energy difference from the reactant level to the peak of the pathway.
The peak represents the high-energy transition region in this simplified profile. Eₐ is an energy difference, not the absolute height of the peak above zero unless the reactant level is zero on the supplied scale.
Do not confuse Eₐ with ΔH
The enthalpy change ΔH is the difference between product and reactant energy levels. Activation energy is the barrier from reactants up to the peak. They answer different questions: ΔH compares start and finish; Eₐ describes the collision barrier.
For the example profile, reactants are at 50 kJ mol⁻¹, products at 20 kJ mol⁻¹ and the peak at 110 kJ mol⁻¹. Therefore Eₐ = 110 − 50 = 60 kJ mol⁻¹ and ΔH = 20 − 50 = −30 kJ mol⁻¹.
Pause and explain
On a forward reaction pathway, from which level is Eₐ measured?
Worked example
A pathway diagram labels reactants at 35 kJ mol⁻¹ and the peak at 92 kJ mol⁻¹. Calculate the forward activation energy.
Show the worked solution
- Measure upward from the reactant level, not from zero.
- Eₐ = peak energy − reactant energy.
- Eₐ = 92 − 35 kJ mol⁻¹.
Answer Eₐ = 57 kJ mol⁻¹.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A profile has reactants at 20 kJ mol⁻¹ and a peak at 76 kJ mol⁻¹. Find the forward Eₐ.
Give me a hint
Subtract the reactant energy from the peak energy.
Compare my reasoning
- Eₐ = 76 − 20.
- The difference is 56.
- Attach kJ mol⁻¹.
Eₐ = 56 kJ mol⁻¹.
Look for these in your work
- Correct reference level
- Subtraction correct
- Units stated
Reactants are at 40 kJ mol⁻¹, products at 12 kJ mol⁻¹ and the peak at 100 kJ mol⁻¹. Find forward Eₐ and ΔH.
Give me a hint
Use separate energy differences for the barrier and start-to-finish change.
Compare my reasoning
- Eₐ = 100 − 40 = 60 kJ mol⁻¹.
- ΔH = 12 − 40 = −28 kJ mol⁻¹.
- The negative ΔH means products are lower in energy in this supplied profile.
Eₐ = 60 kJ mol⁻¹; ΔH = −28 kJ mol⁻¹.
Look for these in your work
- Barrier calculated from reactants
- Products minus reactants used for ΔH
- Signs and units checked
A student reads a 90 kJ mol⁻¹ peak as Eₐ even though the reactants sit at 25 kJ mol⁻¹. Correct the answer.
Give me a hint
Activation energy is a difference from the reactant energy level.
Compare my reasoning
- The peak is at 90 kJ mol⁻¹.
- Reactants are already at 25 kJ mol⁻¹.
- Eₐ = 90 − 25 = 65 kJ mol⁻¹.
The forward activation energy is 65 kJ mol⁻¹, not 90 kJ mol⁻¹.
Look for these in your work
- Starting level identified
- Difference calculated
- Absolute peak height corrected
Common mistakes
- Subtracting product energy from the peak for forward Eₐ
- Calling ΔH the activation energy
- Using the peak height above zero instead of the rise from reactants
- Giving an energy difference without units
What can you explain now?
What does activation energy represent?
Compare with the explanation
Minimum energy required for a collision to be effective
It is the energy barrier particles must overcome in the collision model.
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Make it stick
How do you read forward activation energy and ΔH from the same energy profile?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.