Exothermic and Endothermic Reactions
Every reaction either releases or absorbs energy. The distinction is simple, and it was still the most-confused pair of terms in the entire Chemistry corpus — recorded going wrong more than a dozen times, including in an exam.
1. The two types
EXOTHERMIC — energy is RELEASED to the surroundings. The temperature of the surroundings RISES. ENDOTHERMIC — energy is ABSORBED from the surroundings. The temperature of the surroundings FALLS.
Memory aids that work:
EXO = EXIT — energy exits the reaction. ENDO = ENTERS — energy enters the reaction.
Tutors explicitly recommended using a memory tip here, because this exact confusion appeared again and again. Pick one and write it down at the start of the paper.
| Exothermic | Endothermic | |
|---|---|---|
| Energy | released | absorbed |
| Temperature of surroundings | increases | decreases |
| ΔH sign | NEGATIVE (−) | POSITIVE (+) |
| Products vs reactants | products are LOWER in energy | products are HIGHER in energy |
| Examples | combustion, neutralisation, respiration, most displacement and metal + acid reactions | thermal decomposition, photosynthesis, dissolving ammonium salts |
Thermal decomposition is ENDOTHERMIC — you have to keep heating it. Misidentifying it as exothermic was recorded.
Say “the temperature of the surroundings increases”, not just “it gets hot” — tutors flagged that answers should be expressed with respect to the surroundings.
2. Bond breaking and bond making
This is the most-missed concept on the page, and it explains everything else.
BREAKING bonds requires energy — it is ENDOTHERMIC. MAKING bonds releases energy — it is EXOTHERMIC.
Bond breaking ABSORBS energy — it does not release it. This exact error was recorded twice, and it inverts every subsequent conclusion. The memory hook: you have to put energy in to pull something apart; energy comes out when things snap together.
Which one wins determines the overall reaction:
If MORE energy is released making bonds than absorbed breaking them → EXOTHERMIC. If MORE energy is absorbed breaking bonds than released making them → ENDOTHERMIC.
The calculation (Extended):
ΔH = energy in (bonds broken) − energy out (bonds made)
- A negative answer means exothermic
- A positive answer means endothermic
Worked example. Bonds broken total 2500 kJ/mol; bonds made total 2800 kJ/mol.
- ΔH = 2500 − 2800 = −300 kJ/mol → exothermic
Check the sign against the type. Confusing exothermic and endothermic from the sign of ΔH was recorded — negative is exothermic, because energy has left the system.
3. Energy level diagrams
Exothermic:
- Reactants HIGH, products LOW
- The arrow for ΔH points DOWNWARDS, and ΔH is negative
Endothermic:
- Reactants LOW, products HIGH
- The arrow for ΔH points UPWARDS, and ΔH is positive
In an endothermic reaction the PRODUCTS have the higher energy. Confusion over which side is higher was recorded repeatedly — the energy absorbed has been stored in the products.
Both diagrams have a “hump” between reactants and products — the activation energy barrier.
Label your diagram fully: reactants, products, ΔH (with an arrow between the two levels), and Ea (from the reactants up to the top of the hump). Tutors flagged labelling as a specific mark.
ΔH is measured between reactants and products — NOT to the top of the hump. That distance is the activation energy, a different quantity.
4. Activation energy
Activation energy (Ea) is the MINIMUM energy that colliding particles must have for a reaction to occur.
Use the word “minimum”. Tutors flagged this term specifically — “the energy needed” is vaguer and may not score.
On the diagram: measured from the reactants up to the top of the hump.
Both exothermic and endothermic reactions have an activation energy. Confusion about comparing Ea between the two types was recorded — the barrier exists either way; what differs is where the products end up.
A catalyst provides an alternative route with a LOWER activation energy. It does not change ΔH — the reactants and products are unchanged, so only the hump gets lower.
5. Measuring energy changes practically
Method: measure the initial temperature, carry out the reaction, and measure the highest (or lowest) temperature reached. The change in temperature indicates the energy change.
You measure a TEMPERATURE change, from which the energy change is calculated. Confusing enthalpy change with temperature change — and confusing measuring volume with measuring temperature — were both recorded. They are related but not the same quantity.
Improving the experiment:
The main source of error is HEAT LOSS to the surroundings, especially from the top of the container. A recorded error missed this as the principal problem.
- Use a lid and insulation (e.g. a polystyrene cup)
- Stir constantly for even heat distribution
- Use a stop clock for consistent timing
- Repeat and average
6. Mistakes that cost marks
Swapping exothermic and endothermic — the dominant error.
Saying bond breaking releases energy.
Getting the ΔH sign the wrong way round.
Putting products higher in an exothermic diagram.
Measuring ΔH to the top of the hump.
Omitting “minimum” from the activation energy definition.
Saying a catalyst changes ΔH.
Calling thermal decomposition exothermic.
Confusing temperature change with enthalpy change.
Unlabelled energy level diagrams.
Frequently asked questions
What is an exothermic reaction? One that releases energy, so the surroundings get hotter. ΔH is negative.
What is an endothermic reaction? One that absorbs energy, so the surroundings get colder. ΔH is positive.
Is bond breaking exothermic or endothermic? Endothermic — it requires energy.
How do I calculate ΔH? Bonds broken − bonds made. Negative means exothermic.
Which is higher on an endothermic energy level diagram? The products.
What is activation energy? The minimum energy colliding particles need for a reaction to occur.
Does a catalyst change ΔH? No — it only lowers the activation energy.
Is combustion exothermic? Yes. Thermal decomposition is endothermic.
Why does the temperature fall in an endothermic reaction? Energy is taken from the surroundings.
What’s the main error in a calorimetry experiment? Heat loss to the surroundings, especially from the top.
Quick revision checklist
- I have a memory aid for exo vs endo
- I know which way the surroundings’ temperature changes for each
- I know the sign of ΔH for each
- I know breaking absorbs, making releases
- I can calculate ΔH as broken − made
- I can draw and fully label both energy level diagrams
- I know products are higher for endothermic
- I measure ΔH between reactants and products
- I can define activation energy using “minimum”
- I know a catalyst lowers Ea but not ΔH
- I can classify common reactions as exo or endo
- I can describe the practical and its main source of error
These notes cover exothermic and endothermic reactions in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and data booklet for your own exam series.
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