Read reaction pathway diagrams
Interpret reactant and product enthalpy levels, reaction enthalpy and activation energy on an energy profile.
Section 5 · Chemical energetics
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Report signs, states, units and significant figures consistently. Standard conditions use 298 K and 101 kPa in this syllabus. Written practice is self-checked
Before you begin
- Rearrange a formula, balance a chemical equation and use temperature differences.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is ΔH calculated from reactant and product levels?
What you will learn
- Construct and interpret a reaction pathway diagram
- Identify ΔH and activation energy from relative energy levels
- Distinguish exothermic and endothermic profiles
A pathway diagram tracks enthalpy through a reaction
Plot enthalpy vertically and reaction progress horizontally. Reactants and products occupy energy levels; the curve rises to a peak representing the activated complex along the pathway.
The reaction enthalpy is ΔH = H(products) − H(reactants). Products below reactants give negative ΔH and an exothermic reaction; products above reactants give positive ΔH and an endothermic reaction.
Activation energy is measured from a starting level to the peak
The forward activation energy is the energy difference from the reactants to the top of the pathway. The reverse activation energy is measured from the products to that same peak.
Activation energy is not the same as ΔH. A catalyst can change the pathway and lower activation energy, but the reactant and product enthalpy levels, and therefore ΔH, remain the same for the same reaction conditions.
Pause and explain
From which level is forward activation energy measured?
Worked example
Products are 65 kJ mol⁻¹ below reactants and the pathway peak is 120 kJ mol⁻¹ above reactants. Find ΔH and the forward activation energy.
Show the worked solution
- ΔH = H(products) − H(reactants) = −65 kJ mol⁻¹.
- Forward activation energy is measured from reactants to the peak.
- Ea(forward) = 120 kJ mol⁻¹.
Answer ΔH = −65 kJ mol⁻¹; forward Ea = 120 kJ mol⁻¹.
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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.
An endothermic reaction has products 42 kJ mol⁻¹ above the reactants and a peak 95 kJ mol⁻¹ above the reactants. State ΔH and forward Ea.
Give me a hint
Take products minus reactants for ΔH; take the peak minus reactants for Ea.
Compare my reasoning
- ΔH = +42 kJ mol⁻¹.
- Ea(forward) = 95 kJ mol⁻¹.
- Keep the two vertical differences separate.
ΔH = +42 kJ mol⁻¹ and forward Ea = 95 kJ mol⁻¹.
Look for these in your work
- Positive ΔH correct
- Forward barrier measured from reactants
- Quantities distinguished
A reaction has ΔH = −75 kJ mol⁻¹ and forward activation energy 110 kJ mol⁻¹. Find reverse activation energy.
Give me a hint
The reverse barrier starts at products, which lie 75 kJ mol⁻¹ below reactants.
Compare my reasoning
- Set reactants at 0 and products at −75 kJ mol⁻¹.
- The peak is at +110 kJ mol⁻¹.
- Reverse Ea = 110 − (−75).
Reverse activation energy = 185 kJ mol⁻¹.
Look for these in your work
- Product level set using ΔH
- Peak positioned from forward Ea
- Reverse difference calculated
A catalyst lowers the pathway peak by 30 kJ mol⁻¹ without changing reactants or products. State what changes and what stays the same.
Give me a hint
Compare the peak-relative-to-starting levels with the unchanged endpoints.
Compare my reasoning
- The activation energy is lower.
- Reactant and product enthalpy levels are unchanged.
- Therefore ΔH is unchanged.
The activation energy decreases by 30 kJ mol⁻¹, while ΔH stays the same because the reactant and product levels are unchanged.
Look for these in your work
- Activation energy change identified
- Endpoints retained
- ΔH unchanged explained
Common mistakes
- Measuring activation energy from the product level for the forward reaction
- Reading ΔH as the peak height
- Reversing the sign of products minus reactants
- Claiming a catalyst changes ΔH
What can you explain now?
How is ΔH calculated from reactant and product levels?
Compare with the explanation
H(products) − H(reactants)
The change is final minus initial enthalpy.
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Make it stick
How do the starting level, product level and pathway peak determine ΔH and activation energy?
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.