Overview
What you’ll learn
- Explain what an exothermic and an endothermic reaction does to the temperature of the surroundings.
- Read a reaction pathway diagram to identify the reactants, products, activation energy and overall energy change.
- Describe bond breaking as endothermic and bond forming as exothermic (Supplement).
- Define activation energy and explain why colliding particles need it (Supplement).
- Calculate the enthalpy change of a reaction from bond energies (Supplement).
Syllabus checklist (0620)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 5.1State that an exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases.
- 5.1State that an endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases.
- 5.1Interpret reaction pathway diagrams that show the reactants, the products and the overall energy change.
Supplement — Extended candidates only
- 5.1Explain that the thermal energy transferred during a reaction comes from the breaking and forming of bonds.
- 5.1Define activation energy, Eₐ, as the minimum energy that colliding particles need in order to react.
- 5.1Interpret reaction pathway diagrams that also show the activation energy.
- 5.1State that breaking a bond is endothermic and forming a bond is exothermic.
- 5.1Calculate the enthalpy change of a reaction from the bond energies of the bonds broken and the bonds formed.
In 30 seconds
- 1An exothermic reaction transfers thermal energy to the surroundings, so their temperature rises.
- 2An endothermic reaction takes in thermal energy from the surroundings, so their temperature falls.
- 3On a reaction pathway diagram, the products of an exothermic reaction sit lower in energy than the reactants.
- 4Energy is taken in when bonds break and released when new bonds form (Supplement).
- 5Activation energy (Eₐ) is the minimum energy that colliding particles must have before they can react (Supplement).
- 6Enthalpy change = energy taken in to break bonds − energy released when new bonds form (Supplement).
Notes · 1 of 4
Exothermic and endothermic reactions
Every reaction either gives thermal energy to the surroundings or takes it in.
When substances react, energy is transferred between the reaction mixture and everything around it. If the transfer is out of the mixture, the surroundings get warmer and the reaction is exothermic. If the transfer is into the mixture, the surroundings get cooler and the reaction is endothermic. The temperature you measure with a thermometer is the temperature of the *surroundings*, so it is the simplest way to tell the two apart.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Direction of energy transfer | From the reaction to the surroundings | From the surroundings to the reaction |
| Temperature of the surroundings | Rises | Falls |
| Sign of the overall energy change | Negative | Positive |
| Everyday example | Burning a fuel | A cold pack for an injury |
Exothermic
Combustion
Burning a fuel such as methane in oxygen gives out thermal energy, so the surroundings warm up.
Exothermic
Neutralisation
An acid reacting with an alkali releases energy, so the mixture becomes warmer.
Endothermic
Thermal decomposition
Breaking calcium carbonate down by heating takes in energy from the surroundings.
Endothermic
Photosynthesis
Plants absorb energy from sunlight in order to build glucose from carbon dioxide and water.
Notes · 2 of 4
Reaction pathway diagrams
A diagram of energy against the progress of a reaction shows the reactants, the products and the energy change.
A reaction pathway diagram plots the energy of the reacting particles as a reaction proceeds. The curve begins at the energy level of the reactants and finishes at the level of the products. Between them the curve rises to a peak, because the particles must first be pulled apart before new bonds can be made. The difference between the two flat levels is the overall energy change of the reaction.
- If the products finish below the reactants, energy has been lost to the surroundings, so the reaction is exothermic.
- If the products finish above the reactants, energy has been gained from the surroundings, so the reaction is endothermic.
- The larger the gap between the two levels, the greater the energy change.
- The height of the peak above the reactants is the activation energy, which is a separate quantity from the overall energy change (Supplement).
Supplement: the same curve shows the activation energy. Drawing a horizontal line from the reactant level across to the peak makes the vertical distance between them easy to see, and that distance is Eₐ.
Notes · 3 of 4
Activation energy (Supplement)
Colliding particles need a minimum energy before they can react.
Supplement: particles only react when they collide with enough energy to overcome the repulsion between them and to start breaking the existing bonds. The activation energy, Eₐ, is the minimum energy that colliding particles must have for a reaction to take place. On a reaction pathway diagram it is the vertical distance from the reactant level up to the top of the curve.
- A high activation energy means very few collisions carry enough energy, so the reaction is slow to get going.
- Raising the temperature gives the particles more energy on average, so a greater fraction of collisions is successful.
- A catalyst lowers the activation energy by providing a different route for the reaction, so it speeds the reaction up (met again in Topic 6).
Notes · 4 of 4
Bond energies and energy calculations (Supplement)
The overall energy change comes from the energy needed to break bonds minus the energy released when new bonds form.
Supplement: a reaction swaps the bonds in the reactants for the bonds in the products. Breaking a bond always takes in energy, and forming a bond always releases energy. If more energy is released than is taken in, the reaction is exothermic; if more is taken in than is released, it is endothermic. A bond energy is the energy needed to break one mole of a particular bond, measured in kJ per mol.
| Bond | Bond energy | Bond | Bond energy |
|---|---|---|---|
| H–H | 436 | H–Cl | 432 |
| Cl–Cl | 243 | O=O | 498 |
| H–O | 464 | C–H | 413 |
- 1Add up the bonds broken
Total the bond energies of every bond in the reactants. This energy is taken in.
- 2Add up the bonds formed
Total the bond energies of every bond in the products. This energy is released.
- 3Subtract
Overall energy change = energy taken in − energy released. A negative answer means the reaction is exothermic.
Revise
Key terms
The words examiners expect you to use precisely.
- Exothermic reaction
- A reaction that transfers thermal energy to the surroundings, so the temperature of the surroundings rises.
- Endothermic reaction
- A reaction that takes in thermal energy from the surroundings, so the temperature of the surroundings falls.
- Reaction pathway diagram
- A graph of energy against the progress of a reaction, showing the reactants, the products and the overall energy change.
- Activation energy (Eₐ)
- The minimum energy that colliding particles must have for a reaction to take place.
- Bond energy
- The energy needed to break one mole of a particular covalent bond, in kJ per mol; the same energy is released when that bond forms.
- Bond breaking
- An endothermic process that takes in energy to separate two bonded atoms.
Show 3 more key termsShow fewer key terms
- Bond forming
- An exothermic process that releases energy as a new bond is made between two atoms.
- Enthalpy change
- The overall energy change of a reaction; negative for an exothermic reaction and positive for an endothermic one.
- Combustion
- A rapid exothermic reaction of a fuel with oxygen that transfers thermal energy to the surroundings.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Saying that bond breaking releases energy.
Separating two bonded atoms needs an input of energy; the energy is only released again when the atoms form new bonds.
Instead: Bond breaking is endothermic and bond forming is exothermic.
Mistake: Reading a reaction pathway diagram from right to left.
The energy change is found by comparing where the curve finishes with where it started, so direction matters.
Instead: Read left to right: reactants at the start, products at the end.
Show 8 more mistakesShow fewer mistakes
Mistake: Adding the energy released by bond forming to the energy taken in by bond breaking.
The two transfers act in opposite directions, so the overall change is the difference between them, not their total.
Instead: Overall energy change = energy taken in to break bonds − energy released when new bonds form.
Mistake: Treating activation energy and the overall energy change as the same quantity.
They are two different gaps on the same diagram and they answer different questions.
Instead: Eₐ goes from the reactants up to the peak; the overall change goes from the reactants across to the products.
Mistake: Claiming that the surroundings get hotter because energy is "created" inside the reaction.
Energy is transferred, not created; the total amount stays the same.
Instead: An exothermic reaction transfers thermal energy out of the mixture into the surroundings, which is why they warm up.
Mistake: Forgetting to use the coefficients when totalling bonds formed.
Every molecule in the balanced equation contributes its own bonds, so 2HCl means two H–Cl bonds, not one.
Instead: Multiply each bond energy by the number of that bond shown in the balanced equation.
Mistake: Writing that an exothermic reaction “produces” or “creates” energy.
Energy cannot be created; it is converted from stored chemical energy into thermal energy and passed on.
Instead: An exothermic reaction transfers thermal energy from the reaction mixture to the surroundings.
Mistake: Claiming a catalyst changes the overall energy change of a reaction.
The energy levels of the reactants and products depend on the substances, not on the route taken between them.
Instead: A catalyst lowers the activation energy but leaves the overall energy change exactly the same.
Mistake: Reading the height of the peak as the overall energy change.
The peak height measures the barrier from the reactants, not the difference between reactants and products.
Instead: The overall energy change is the vertical gap between the reactant level and the product level.
Mistake: Quoting an enthalpy change without a sign or a direction.
Without the sign there is no way to tell whether energy was released or absorbed.
Instead: A negative change means energy is released (exothermic); a positive change means energy is absorbed (endothermic).
Check yourself
Quick quiz
18 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Describe the effect of an exothermic reaction on the surroundings
In an exothermic reaction, what happens to the temperature of the surroundings?
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Practise
Topical questions
10 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 5 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Topic 5: Chemical energetics.
Mark scheme
PROTopic 5 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
Experimental skills
Want a full paper? Try the Paper 2-style multiple choice, Paper 4-style structured or Paper 6-style practical sets.
Questions
Frequently asked questions
What is the difference between an exothermic and an endothermic reaction?
An exothermic reaction transfers thermal energy to the surroundings, so their temperature rises. An endothermic reaction takes thermal energy in from the surroundings, so their temperature falls. In both cases energy is transferred, never created or destroyed.
Why is bond breaking endothermic but bond forming exothermic?
Energy has to be supplied to pull two bonded atoms apart, so breaking a bond takes energy in. When the same bond forms again, the atoms move to a lower, more stable energy, and that energy is released to the surroundings.
How do I calculate the energy change of a reaction from bond energies?
Add up the bond energies of all the bonds broken in the reactants, then add up the bond energies of all the bonds formed in the products. Subtract the second total from the first. A negative answer means the reaction is exothermic; a positive answer means it is endothermic.
Is activation energy needed for IGCSE Chemistry?
The idea that reactions need a minimum energy is Supplement material, examined on the Extended papers, along with reading Eₐ from a reaction pathway diagram and calculating energy changes from bond energies. Everything about exothermic and endothermic reactions and simple pathway diagrams is Core.
Up next
Topic 6: Chemical reactions