How to explain exothermic and endothermic reactions

Exothermic and endothermic reactions open the thermochemistry chapter, and getting them right sets you up for the calculations that follow. The words sound similar, so the safest habit is to link each one to four things at once: the direction of heat flow, the temperature change, the sign of ΔH, and the energy level diagram. This guide gives you that framework and the examples SPM expects.

The core definitions

• An exothermic reaction releases heat energy to the surroundings. The temperature of the surroundings (usually the reaction mixture) rises. The heat of reaction, ΔH, is negative.
• An endothermic reaction absorbs heat energy from the surroundings. The temperature of the surroundings falls. The heat of reaction, ΔH, is positive.

A quick way to keep them straight: exo = exit, heat exits the reaction, so it feels hot. Endo = into, heat goes into the reaction from outside, so it feels cold. You can check the exact wording any time on our exothermic reaction and endothermic reaction glossary entries.

Why heat is released or absorbed: bonds

The deeper explanation, which higher-mark questions ask for, is about bonds:

• Breaking bonds absorbs energy (endothermic step).
• Forming bonds releases energy (exothermic step).

Every reaction breaks the bonds in the reactants and forms new bonds in the products. The overall sign depends on which is bigger:

• If more energy is released forming new bonds than is absorbed breaking old bonds, the reaction is exothermic overall (ΔH negative).
• If more energy is absorbed breaking bonds than is released forming them, the reaction is endothermic overall (ΔH positive).

Writing that comparison is what turns a definition into a full-mark explanation.

Energy level diagrams

The diagram is a favourite in Paper 2, so learn to draw both.

• Exothermic: the reactants are drawn higher than the products. The products are at a lower energy level, and the arrow for ΔH points downwards (negative). The energy lost by the chemicals is given out as heat.
• Endothermic: the reactants are drawn lower than the products. The products are at a higher energy level, and the arrow for ΔH points upwards (positive). The extra energy of the products was absorbed from the surroundings.

Always label the reactants, the products, and ΔH with its sign. An unlabelled diagram rarely gets full marks.

Examples you should know

Exothermic reactions (temperature rises):

• Combustion, burning fuels such as methane or ethanol.
• Neutralisation, an acid reacting with an alkali.
• Respiration in living cells.
• Displacement of a less reactive metal by a more reactive one.
• Rusting of iron (slow, but exothermic).
• Dissolving sodium hydroxide or concentrated sulfuric acid in water.

Endothermic reactions (temperature falls):

• Thermal decomposition, for example heating calcium carbonate to make calcium oxide and carbon dioxide.
• Photosynthesis in green plants.
• Dissolving certain salts such as ammonium nitrate or ammonium chloride in water (used in instant cold packs).
• Electrolysis of molten or aqueous compounds.

Being able to sort a list of reactions into these two groups is a common Paper 1 and Paper 2 task.

A worked exam question

When ammonium chloride is dissolved in water, the temperature of the water falls from 30 °C to 24 °C. (a) Is the process exothermic or endothermic? (b) State the sign of ΔH. (c) Explain in terms of energy.

(a) The temperature falls, so heat is absorbed from the surroundings, the process is endothermic.

(b) ΔH is positive.

(c) More energy is absorbed to break the bonds (in the ionic lattice and around the water molecules) than is released when the ions become hydrated, so there is a net absorption of heat from the surroundings, and the temperature falls.

The three parts test the observation, the sign and the reasoning, exactly the framework at the top of this guide.

Common mistakes to avoid

• Mixing up the signs. Exothermic is negative; endothermic is positive. Say it out loud until it sticks.
• Confusing “surroundings feel hot” with “the reaction gains heat”. In an exothermic reaction the surroundings gain heat from the reaction, the reaction loses it.
• Drawing the energy diagram upside down. For exothermic, reactants are higher; for endothermic, reactants are lower.
• Forgetting the bond explanation when a question asks you to explain, not just state.

Build a solid base for thermochemistry

Once exothermic and endothermic are automatic, the thermochemistry calculations, heat of neutralisation, combustion and displacement, become much easier, because the sign of ΔH falls straight out of the temperature change. If your child keeps flipping the signs or drawing the diagram the wrong way up, that is a quick fix with a teacher. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial, see how it works.