4.5.1.1 - Exothermic and endothermic reactions

Energy conservation and the surroundings

Energy is conserved in a chemical reaction, so energy transferred out of the reacting substances is gained by the surroundings.

The reaction system is the substances undergoing the chemical change. Its surroundings include the solvent in a solution, container and nearby air that can gain or lose energy. These are useful boundaries for describing a transfer; energy is not created by a reaction or destroyed when a mixture cools. The total energy in the universe after a reaction is the same as before it.

If a reacting mixture warms a thermometer, energy has been transferred from the chemical reaction to the solution and apparatus. The products have less energy than the reactants by the amount transferred to the surroundings. Saying that the reaction “makes energy” misses the conservation rule.

In an endothermic reaction the products have more energy than the reactants. That increase is supplied by the surroundings, which cool. A temperature fall is evidence for this direction of energy transfer, provided the change is due to the reaction rather than, for example, adding a colder liquid.

Worked example 1 — a temperature rise

A reacting solution starts at $21.0\,\mathrm{°C}$ and reaches $28.5\,\mathrm{°C}$. Subtract start from finish:

The positive change shows warming and supports an exothermic classification.

Worked example 2 — a temperature fall

A different reacting solution falls from $21.0\,\mathrm{°C}$ to $16.5\,\mathrm{°C}$:

The negative change shows cooling and supports an endothermic classification. Temperature change describes the direction of transfer; it is not an energy measured in joules.

Reaction types and useful applications

The direction and size of a temperature change help determine whether a reaction is suitable for a particular use.

Combustion, many oxidation reactions, and neutralisation are exothermic. Burning a fuel transfers energy to the surroundings. A real example in 2026 is the HotHands air-activated warmer, whose contents include iron powder. Opening the outer packaging lets oxygen reach the iron, and oxidation transfers energy to a user's hands. A self-heating can instead keeps its reactants separate until activated, then transfers energy through a barrier to the food. The reactants must remain separated from food, and an uncontrolled temperature rise could cause burns.

Thermal decomposition reactions are endothermic: energy must be supplied while a compound breaks down. The reaction of citric acid and sodium hydrogencarbonate is also endothermic. Some sports injury packs use endothermic reactions or dissolving processes to take in energy from the injured area. They must cool sufficiently to be useful without damaging skin. Dissolving can produce a temperature change even when the process is not described as a chemical reaction; the measurement still reveals the direction of energy transfer.

An evaluation must connect evidence to the use. The highest temperature is not automatically best. Compare the temperature reached, time taken to reach it, duration, ease of activation, ability to reuse, cost, waste and risks, using any information supplied. A reusable pack may justify a higher purchase cost when used frequently; a rapidly activated disposable pack may suit an emergency where no heating or refrigeration is available.

Consider these illustrative hand-warmer data, measured under the same conditions:

For a long outdoor activity, B's longer duration supports choosing it. Its lower maximum temperature also reduces the risk of overheating, although skin contact still needs suitable protection. A warms more strongly but lasts less time. B's resetting requirement might be inconvenient away from hot water. These are linked reasons for a choice, rather than a list of unconnected advantages.

Measuring temperature changes in solutions

A temperature-change investigation uses controlled mixing and insulation to measure the warming or cooling caused by reacting solutions.

Required practical 4 investigates variables affecting temperature change. Suitable reactions include acid with a metal, acid with a carbonate, neutralisation, and displacement of a metal. The practical is not limited to proving that a reaction is exothermic: it asks how changing a variable changes the observed temperature rise or fall.

For example, investigate the concentration of hydrochloric acid in its neutralisation with sodium hydroxide. A hypothesis is: for equal volumes and starting temperatures, increasing acid concentration increases the temperature rise while the alkali remains in sufficient supply. A more concentrated equal volume contains more acid particles that can react. Once all the alkali has reacted, further acid cannot continue increasing the amount neutralised. Choose a safe teacher-approved concentration range that explores the intended relationship.