Sciences

Exothermic and Endothermic: Which Way the Energy Goes

Exothermic gives energy out and the thermometer rises; endothermic takes energy in and it falls. Bonds breaking cost energy, bonds forming release it.

Exothermic and endothermic reactions

An exothermic reaction gives out energy and warms its surroundings; an endothermic reaction takes energy in and cools them.

Où les élèves le rencontrent: Grade 9 chemistry, in the unit on energy changes, where energy-profile diagrams and the temperature-change practical are both examined.

La réponse en bref
An exothermic reaction transfers energy out to its surroundings, so a thermometer in the mixture reads higher. An endothermic reaction takes energy in, so the reading falls. Combustion and neutralisation are exothermic; the thermal decomposition of calcium carbonate is endothermic. Breaking bonds takes energy in, and forming bonds gives it out.

Un exemple

H₂ + Cl₂ → 2HCl: (436 + 242) − (2 × 431) = 678 − 862 = −184 kJ/mol

The bond energies are the ones a paper would supply: H–H is 436 kJ/mol, Cl–Cl is 242, and H–Cl is 431. Breaking the two reactant bonds costs 678 kJ. Forming two H–Cl bonds releases 862 kJ. More energy comes out than went in, the answer is negative, and negative means exothermic. Getting the subtraction in the right order — broken minus made — is what decides the sign.

What the thermometer actually shows

The practical test is blunt. Put a thermometer in the mixture, run the reaction and watch. If the reading rises, energy has been released into the surroundings and the reaction is exothermic. If it falls, energy has been drawn out of the surroundings and the reaction is endothermic. The one wording to be careful about is that it is the surroundings whose temperature is being measured, not the reaction's.

Two wordings are worth keeping apart. An exothermic reaction gives energy out, so the surroundings get warmer. An endothermic reaction takes energy in, so the surroundings get colder. Describing the beaker as hot or cold is a description of the surroundings, and that is what the thermometer is in.

Exothermic — burning any fuel, neutralising an acid with an alkali, most oxidation, respiration, and displacing copper from copper sulfate with zinc
Exothermic in daily use — a self-heating can and a hand warmer, which works by crystallising a supersaturated solution of sodium ethanoate
Endothermic — the thermal decomposition of calcium carbonate in a lime kiln, and the reaction between citric acid and sodium hydrogencarbonate
Endothermic in daily use — a sports injury cold pack, though there the cooling comes from ammonium nitrate dissolving rather than from a reaction
Endothermic on a large scale — photosynthesis, which takes in the energy that combustion later gives back

The two energy profiles

Draw reactants on the left, products on the right, and energy up the vertical axis. In an exothermic reaction the products sit lower than the reactants, and the drop is the energy given out; ΔH is negative. In an endothermic reaction the products sit higher, the climb is the energy taken in, and ΔH is positive.

Both diagrams have the same hump in the middle. That is the activation energy, the minimum a collision needs before anything happens, and it exists whichever way the overall change goes. This is why an exothermic reaction can still need a spark: petrol vapour releases a great deal of energy once it starts, but it will not start on its own.

Where the energy comes from

Every reaction does two things: it breaks the bonds in the reactants and makes new bonds in the products. Breaking a bond always requires energy to be supplied, so bond breaking is endothermic. Making a bond always releases energy, so bond making is exothermic.

Whether the reaction as a whole gives out or takes in energy is just the balance between those two totals. Release more in making the new bonds than you spent breaking the old ones and the surplus escapes as heat. Spend more than you release and the difference has to be drawn in from the surroundings, which is what makes the beaker feel cold.

Questions fréquentes

Is dissolving a chemical reaction?

No, it is a physical change, even though it can produce a clear temperature change. Ammonium nitrate dissolving in water cools it sharply and is often described as endothermic, which is fair as a description of the energy transfer, but no new substance is made and the process reverses on evaporation.

Are most reactions exothermic or endothermic?

Most of the ones met at school are exothermic — combustion, neutralisation, oxidation, respiration and metal displacement all release energy. Endothermic examples are few enough to be worth memorising as a short list, which is one reason exam questions keep returning to thermal decomposition and to citric acid with sodium hydrogencarbonate.

How do you tell from a ΔH value which one it is?

Negative is exothermic and positive is endothermic. The sign is written from the reaction's point of view, not the surroundings': the reaction has lost energy, so its value goes down and the number is negative. Reading the sign the other way round is the commonest error in this topic.

Does an exothermic reaction always feel hot?

Usually, but not always noticeably. Some release energy so slowly that the heat spreads away as fast as it is produced — iron rusting is exothermic and no one feels it. What defines the reaction is the direction of the energy transfer, not whether the effect is large enough for a hand to detect.

Dernière mise à jour 21 août 2026