Science
What are exothermic and endothermic reactions, and how can you tell them apart?
Quick answer
An exothermic reaction gives out heat, so the surroundings get warmer (like burning). An endothermic reaction takes in heat, so the surroundings get colder (like an instant cold pack).
In one sentence
An exothermic reaction gives out energy (usually heat) to its surroundings, which warm up; an endothermic reaction takes in energy from its surroundings, which cool down.
Explained
Every chemical reaction moves energy around. Sometimes energy flows out of the reacting chemicals into the surroundings: the beaker, the air, your hand. Sometimes energy flows in. The names tell you which: exo means "out" and endo means "in", and thermic is about heat.
fire
snow
candle
Exothermic reactions give out heat, and often light or sound too. The temperature of the surroundings goes up. Examples: burning fuels (combustion), respiration in your cells, neutralisation of an acid by an alkali, metals reacting with acids, and the iron powder in a disposable hand warmer slowly rusting.
Endothermic reactions take in heat. The temperature of the surroundings goes down, or the reaction only keeps going while you keep heating it. Examples: thermal decomposition (heating calcium carbonate to make lime), photosynthesis (which takes in light energy), citric acid reacting with sodium hydrogencarbonate (the fizz in sherbet), and electrolysis, which needs a constant supply of electrical energy.
Remember that energy is never created or destroyed. In an exothermic reaction the chemicals lose energy and the surroundings gain exactly that much. In an endothermic reaction it is the other way round.
Why does this happen? Bonds. To react, the bonds in the reactants must break, and new bonds form in the products.
- Breaking bonds takes in energy (endothermic).
- Making bonds gives out energy (exothermic).
If making the new bonds releases more energy than breaking the old ones took in, the reaction is exothermic overall. If it releases less, the reaction is endothermic overall.
Energy profile diagrams. Chemists draw the energy of the reactants and products as two levels. In an exothermic reaction the products are lower than the reactants; in an endothermic one they are higher. The gap is the overall energy change, written ΔH (delta H): negative for exothermic, positive for endothermic. Both kinds have a "hill" to climb first, the activation energy: the minimum energy particles need to react. That is why a fuel needs a match to get started even though burning gives out lots of heat.
Examples
| Reaction or process | Exothermic or endothermic? | What you notice |
|---|---|---|
| Wood or gas burning | exothermic | heat and light |
| Respiration (glucose + oxygen) | exothermic | keeps your body warm |
| Hydrochloric acid + sodium hydroxide | exothermic | the solution warms up |
| Magnesium + hydrochloric acid | exothermic | fizzing, the tube gets hot |
| Calcium oxide + water (self-heating cans) | exothermic | gets very hot |
| Heating calcium carbonate | endothermic | only reacts while heated strongly |
| Photosynthesis | endothermic | takes in light energy |
| Citric acid + sodium hydrogencarbonate | endothermic | fizzes and gets colder |
| Ammonium nitrate dissolving in water (cold pack) | endothermic | the pack goes cold |
Changes of state follow the same idea, although they are physical changes, not reactions: melting and evaporating take in energy (endothermic), while freezing and condensing give it out (exothermic).
Common mistakes
- Thinking "endothermic" means the reaction is cold. The reaction takes heat from the surroundings, which is why the thermometer reading falls.
- Saying "making bonds takes energy". It is the opposite: breaking bonds takes energy in, making bonds gives energy out.
- Getting the sign of ΔH backwards. Exothermic means the chemicals lose energy, so ΔH is negative.
- Thinking exothermic reactions start by themselves. Most still need activation energy, like a spark or a flame.
Step by step
Step by step
- Measure the starting temperaturePut the first reactant in an insulated cup (a polystyrene cup works well) and record the temperature.
- Add the second reactantStir and watch the thermometer.
- Record the highest or lowest temperatureNote how far it rises or falls.
- DecideTemperature went up: exothermic. Temperature went down: endothermic.
- Check with energyFor a calculation, find energy in (bonds broken) minus energy out (bonds made). A negative answer means exothermic.
Practice questions
Exothermic or endothermic?
Try each one, then check your answer.
- A candle burning.
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Gives out heat and light: exothermic. - Water freezing into ice.
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Gives out energy as the molecules are pulled into fixed places: exothermic (a physical change, not a reaction). - Photosynthesis in a leaf.
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Takes in light energy: endothermic. - The temperature of a solution rises from 19 °C to 31 °C.
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It rose by 31 − 19 = 12 °C: exothermic. - The temperature falls from 22.0 °C to 16.5 °C.
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It fell by 22.0 − 16.5 = 5.5 °C: endothermic. - ΔH = −890 kJ/mol.
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Negative ΔH: exothermic. - ΔH = +178 kJ/mol.
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Positive ΔH: endothermic. - On the energy diagram, the products are lower than the reactants.
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The chemicals lost energy: exothermic. - Bonds broken take in 679 kJ; bonds made give out 864 kJ. Find ΔH.
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ΔH = 679 − 864 = −185 kJ (exothermic) - Bonds broken take in 864 kJ; bonds made give out 679 kJ. Find ΔH.
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ΔH = 864 − 679 = +185 kJ (endothermic) - H₂ + Br₂ → 2HBr. Bond energies: H–H 436, Br–Br 193, H–Br 366 kJ/mol. Find ΔH.
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Broken: 436 + 193 = 629. Made: 2 × 366 = 732. ΔH = 629 − 732 = −103 kJ/mol (exothermic) - CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies: C–H 413, O=O 498, C=O 805, O–H 464 kJ/mol. Find ΔH.
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Broken: 4 × 413 + 2 × 498 = 1652 + 996 = 2648. Made: 2 × 805 + 4 × 464 = 1610 + 1856 = 3466. ΔH = 2648 − 3466 = −818 kJ/mol (exothermic)
Word problems
Energy changes in real life
Try each one, then check your answer.
- A sports cold pack goes cold when you squeeze it. Is the change inside exothermic or endothermic, and where does the heat go?
Show answer
Endothermic: the chemicals take heat from your skin and the surroundings, so the pack feels cold. - A student adds magnesium to acid. The temperature goes from 20.5 °C to 34.0 °C. Find the change and name the type.
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34.0 − 20.5 = 13.5 °C rise: exothermic. - 50 g of water warms up by 6 °C. Using Q = m × c × ΔT with c = 4.2 J/g °C, how much heat did it take in?
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Q = 50 × 4.2 × 6 = 1260 J (1.26 kJ) - Why does a gas cooker need a spark if burning gas is exothermic?
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The gas particles need the activation energy to start reacting. After that, the heat given out keeps the reaction going. - A self-heating can of coffee uses calcium oxide and water. Should the reaction be exothermic or endothermic? Explain.
Show answer
Exothermic: it must give out heat to warm the coffee.
Questions people ask
What is the difference between exothermic and endothermic reactions?
Exothermic reactions give out heat, so the surroundings get warmer and ΔH is negative. Endothermic reactions take in heat, so the surroundings get colder and ΔH is positive.
How do you identify exothermic and endothermic reactions?
Measure the temperature before and after. If it goes up, the reaction is exothermic; if it goes down, it is endothermic. A reaction that only happens while you keep heating it is endothermic too.
What are everyday examples of each?
Exothermic: burning wood or gas, respiration, hand warmers, fireworks. Endothermic: instant cold packs, photosynthesis, baking powder or sherbet fizzing, thermal decomposition.
How are exothermic and endothermic reactions similar?
Both are chemical reactions in which bonds break and form, both transfer energy between the chemicals and the surroundings, both need activation energy to start, and in both the total energy is conserved.
What does an exothermic or endothermic reaction graph look like?
On an energy profile diagram, an exothermic reaction goes up a small hill (the activation energy) and ends lower than it started. An endothermic reaction climbs the hill and ends higher than it started.
How do you calculate the energy change of a reaction?
Add up the bond energies of all bonds broken and subtract all bonds made: ΔH = energy in − energy out. For H₂ + Cl₂ → 2HCl, (436 + 243) − (2 × 432) = −185 kJ/mol, so it is exothermic.
Quick quiz
Exothermic and endothermic reactions
Check what you know with 5 quick questions.
Question 1 of 5
A reaction in a test tube makes the tube feel cold. What kind of reaction is it?
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