In an exothermic reaction profile, products sit lower than reactants; in an endothermic one, products sit higher. Free notes and 11 questions answered.
Updated 25 Mar 2026 · Revise Right Team
Drawing and Understanding Reaction Profiles is part of Exothermic and endothermic reactions in GCSE AQA Higher Chemistry.
4 min
Reading time
11
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
What this lesson covers
In an exothermic reaction profile, products sit lower than reactants; in an endothermic one, products sit higher. Free notes and 11 questions answered.
Draw reaction profiles for exothermic and endothermic reactions showing reactants, products, and activation energy
Calculate overall energy changes from reaction profiles using relative energy levels
Explain the significance of activation energy in determining reaction rates
Compare energy changes in exothermic and endothermic reactions using diagrams
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Use this lesson to stay inside Exothermic and endothermic reactions while connecting the detail back to the wider Topic 5: Energy changes topic.
Topic 5: Energy changes
Exothermic and endothermic reactions
GCSE AQA Higher Chemistry
Revision notes
Read the core explanations from Drawing and Understanding Reaction Profiles before testing yourself from memory.
1. What Are Reaction Profiles?
Picture a rollercoaster track - it starts at one height, climbs to a peak, then drops to a different end height. Chemical reactions follow the same pattern with energy instead of height.
Every reaction profile has four key features you must be able to identify and draw. The horizontal axis shows 'progress of reaction' (time), while the vertical axis shows 'energy' in kJ/mol.
2. The Four Essential Components
Every reaction profile contains exactly four components that examiners expect you to identify and label correctly. Missing any of these costs marks.
Reactants: Starting energy level on the left side. This is where your reaction begins - the energy of the substances before they react.
Products: Final energy level on the right side. This shows the energy of the substances after the reaction is complete.
Overall Energy Change: The difference between products and reactants. If products are lower than reactants, energy is released (exothermic). If products are higher, energy is absorbed (endothermic).
3. Drawing Exothermic Reaction Profiles
Exothermic reactions release energy to the surroundings. Think of burning fuel - it gives out heat and light. On a reaction profile, this means products end up at a lower energy level than reactants.
Worked example: Drawing an Exothermic Reaction Profile
Draw a reaction profile for the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O (releases 890 kJ/mol)
Vertical axis: 'Energy (kJ/mol)'
Horizontal axis: 'Progress of reaction' — Always start with properly labelled axes
Draw horizontal line on left side, label 'CH₄ + 2O₂' — This is your starting energy level
Draw horizontal line BELOW reactants, label 'CO₂ + 2H₂O' — Products lower = energy released
Draw curved line from reactants up to peak, then down to products — The peak represents the energy barrier
Arrow from reactants to peak: 'Activation energy'
Arrow from reactants to products: 'Overall energy change = -890 kJ/mol' — Negative value shows energy is released
Answer: Complete exothermic profile with products 890 kJ/mol below reactants
Key feature: The curved line connecting reactants to products must go up first (activation energy barrier) then down to the lower product level. Never draw straight lines - reactions don't work that way.
4. Drawing Endothermic Reaction Profiles
Endothermic reactions absorb energy from the surroundings. Think of photosynthesis - plants absorb light energy to make glucose. Products end up at a higher energy level than reactants.
Worked example: Drawing an Endothermic Reaction Profile
Draw a reaction profile for thermal decomposition: CaCO₃ → CaO + CO₂ (requires 178 kJ/mol)
Vertical axis: 'Energy (kJ/mol)'
Horizontal axis: 'Progress of reaction' — Same axes as exothermic reactions
Draw horizontal line on left side, label 'CaCO₃' — Starting energy level for calcium carbonate
Draw horizontal line ABOVE reactants, label 'CaO + CO₂' — Products higher = energy absorbed
Draw curved line from reactants up to peak, then down to products — Peak is still above both reactants and products
Arrow from reactants to peak: 'Activation energy'
Arrow from reactants to products: 'Overall energy change = +178 kJ/mol' — Positive value shows energy is absorbed
Answer: Complete endothermic profile with products 178 kJ/mol above reactants
5. Reading and Interpreting Reaction Profiles
Exam questions often give you a reaction profile and ask you to extract information. Here's what examiners are looking for and the common traps students fall into.
To find the overall energy change, measure the vertical distance between reactants and products. If products are lower, the value is negative (exothermic). If products are higher, the value is positive (endothermic).
Worked example: Calculating Energy Changes from a Profile
A reaction profile shows reactants at 150 kJ/mol and products at 80 kJ/mol. Calculate the overall energy change.
Reactants = 150 kJ/mol
Products = 80 kJ/mol — Read values from the y-axis
Overall energy change = Products - Reactants — This formula works for both reaction types
Overall energy change = 80 - 150 = -70 kJ/mol — Negative result indicates exothermic
The reaction releases 70 kJ/mol of energy — Negative = releases energy (exothermic)
Answer: -70 kJ/mol (exothermic reaction)
6. Exam Technique and Common Mistakes
Reaction profile questions are mark-heavy in exams. Here's how to maximise your score and avoid the traps that catch most students.
The biggest mistake: Students often confuse activation energy with overall energy change. Activation energy is always from reactants to the peak. Overall energy change is from reactants to products.
Mark-winning phrases: Use 'energy is released to the surroundings' for exothermic and 'energy is absorbed from the surroundings' for endothermic. This precise language gets you the communication marks.
Exam precision note: In Drawing and Understanding Reaction Profiles - Reaction profiles - Exothermic and endothermic reactions, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
A reaction profile does not plot energy against time. The horizontal axis shows progress of reaction, which is a way of moving from reactants to products over an energy barrier. The curve shape matters because it shows an energy barrier at the top. Activation energy is measured from the reactants level up to that peak, while the overall energy change is measured between the reactants and products levels. Students often confuse the peak itself with activation energy, so diagrams should always show the difference as an arrow or gap.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-25.
What does an exothermic reaction profile look like?
In an exothermic reaction profile, the products are at a lower energy level than the reactants, because energy is released to the surroundings. For methane combustion, the products are 890 kJ/mol lower than the reactants, so the overall energy change is -890 kJ/mol.
What does an endothermic reaction profile look like?
In an endothermic reaction profile, the products are at a higher energy level than the reactants, because energy is absorbed from the surroundings. For the thermal decomposition of calcium carbonate, the products are 178 kJ/mol higher, so the overall energy change is +178 kJ/mol.
How do you draw a reaction profile?
To draw a reaction profile, label the vertical axis energy and the horizontal axis progress of reaction, draw the reactant and product energy levels, then join them with a curve that rises to a peak. Add arrows for the activation energy (reactants to peak) and the overall energy change (reactants to products).
How do you calculate the energy change from a reaction profile?
Overall energy change = energy of products - energy of reactants, read from the vertical axis of the reaction profile. For reactants at 150 kJ/mol and products at 80 kJ/mol, the change is 80 - 150 = -70 kJ/mol, so the reaction is exothermic.
What is the difference between activation energy and overall energy change?
On a reaction profile, activation energy is measured from the reactants level up to the peak, while the overall energy change is measured from the reactants level to the products level. Confusing the two is the biggest mistake students make.
What are the axes on a reaction profile?
A reaction profile has energy, in kJ/mol, on the vertical axis and progress of reaction on the horizontal axis. Reactants are drawn on the left, products on the right, joined by a curved line over an energy barrier.
What does the peak of a reaction profile show?
The peak of a reaction profile shows the energy barrier the reaction must get over to happen. Activation energy is the gap from the reactants level up to that peak, not the peak itself, so it should be shown as an arrow.
Why is a reaction profile curved?
A reaction profile is curved because the line must go up first, over the activation energy barrier, then down to the product level. It should never be drawn as a straight line, because reactions don't work that way.
What is the energy change for the thermal decomposition of calcium carbonate?
The thermal decomposition of calcium carbonate, CaCO₃ → CaO + CO₂, has an overall energy change of +178 kJ/mol. It is endothermic, so on a reaction profile the products are drawn 178 kJ/mol higher than the reactants.
What are the parts of a reaction profile?
A reaction profile has four key components: the reactants energy level, the products energy level, the activation energy, and the overall energy change. Each must be labelled, with arrows for the activation energy and the overall energy change.
Does a reaction profile show energy against time?
A reaction profile does not plot energy against time. The horizontal axis shows progress of reaction, a way of moving from reactants to products over an energy barrier, while the vertical axis shows energy.