Exothermic and endothermic reactions
Energy transfer to and from the surroundings, energy level diagrams, activation energy and bond breaking/making.
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 5.1.1Describe exothermic and endothermic reactions in terms of temperature change and energy transfer.
- 5.1.2Draw and interpret energy level (reaction pathway) diagrams including activation energy and ΔH.
- 5.1.3Explain overall energy change in terms of bond breaking and bond making.Supplement
6 minute read
Energy changes in reactions
The two types
Exothermic reactions transfer energy to the surroundings, so the temperature of the surroundings rises. ΔH is negative. Examples: combustion, neutralisation, most oxidation reactions.
Endothermic reactions take energy in from the surroundings, so the temperature falls. ΔH is positive. Examples: thermal decomposition, photosynthesis, dissolving ammonium nitrate.
Energy level diagrams
- Exothermic: products are lower than reactants; ΔH is drawn as a downward arrow.
- Endothermic: products are higher than reactants; the arrow points up.
- The activation energy is the hump from the reactants to the peak — the minimum energy needed for a reaction to occur.
Both types of reaction have an activation energy. An exothermic reaction is not one that starts by itself.
Bonds explain everything
- Breaking bonds takes energy in — always endothermic.
- Making bonds gives energy out — always exothermic.
Compare the two:
- If more energy is released making bonds than was used breaking them, the reaction is exothermic.
- If breaking costs more than making releases, it is endothermic.
ΔH = energy to break bonds − energy released making bonds
Think of it like this
Bond energy is a deposit system. You pay to break bonds apart and get paid when new ones form. Exothermic reactions are the ones where you end up in profit.
Worked examples
Method, step by step
In a reaction, breaking the bonds in the reactants requires 1350 kJ/mol and forming the bonds in the products releases 1600 kJ/mol. Calculate ΔH and state the type of reaction.
- 1ΔH = energy in − energy out
- 2ΔH = 1350 − 1600
ΔH = −250 kJ/mol, so the reaction is exothermic.
Common misconceptions
- Saying an exothermic reaction "gets colder". The reaction releases energy, so the surroundings get hotter.
- Thinking endothermic reactions have no activation energy. Both types do.
- Getting the sign of ΔH backwards. Exothermic is negative.
In the exam
- Label energy level diagrams fully: reactants, products, activation energy and ΔH. Unlabelled diagrams lose easy marks.
- When explaining an energy change, use the phrase "energy is released when bonds are formed" explicitly.