An exothermic reaction releases heat to the surroundings, so the temperature of the mixture rises and ΔH is negative. An endothermic reaction absorbs heat from the surroundings, so the temperature falls and ΔH is positive.
You classify a reaction by watching whether the temperature goes up or down.
This page covers one Form 5 Thermochemistry content standard: exothermic and endothermic reactions. It is the opening idea of the whole chapter, and every later section, energy level diagrams, heat of neutralisation, displacement, precipitation and combustion, is built on it. Get the definitions, the temperature change and the sign of ΔH secure here, and the rest of the chapter becomes far easier.
Heat, the system and the surroundings
Thermochemistry is the study of the heat change that accompanies a chemical reaction. Every reaction involves the breaking of bonds in the reactants and the forming of bonds in the products. Breaking bonds absorbs energy; forming bonds releases energy. The overall reaction gives out or takes in heat depending on which of these is larger.
We call the reacting chemicals the system and everything around them, the water or solution, the container, the air, the surroundings. Heat flows between the two. The direction of that flow is exactly what the words exothermic and endothermic describe.
Exothermic reactions
An exothermic reaction releases heat to the surroundings. Because the surroundings gain heat, the temperature of the mixture rises and the container feels warm or hot. In an exothermic change, the total energy released when new bonds form is greater than the total energy absorbed to break the old bonds, so there is a net release of heat.
The heat content (enthalpy) of the products is lower than that of the reactants, so the change in heat content, ΔH, is negative.
Everyday and laboratory examples include:
- the combustion of fuels such as alcohols and hydrocarbons,
- the neutralisation of an acid by an alkali,
- the displacement of a less reactive metal from its salt by a more reactive metal,
- respiration, and the reaction of reactive metals with water or acid.
Endothermic reactions
An endothermic reaction absorbs heat from the surroundings. Because the surroundings lose heat to the system, the temperature of the mixture falls and the container feels cold. In an endothermic change, the total energy absorbed to break the old bonds is greater than the total energy released when new bonds form, so there is a net absorption of heat.
The heat content (enthalpy) of the products is higher than that of the reactants, so ΔH is positive.
Examples include:
- the thermal decomposition of substances such as metal carbonates (heating is needed continuously),
- photosynthesis,
- dissolving certain salts such as ammonium chloride or ammonium nitrate in water, which makes the solution feel cold.
The sign of ΔH, the point examiners test
ΔH stands for the change in heat content of the reaction.
- Exothermic: ΔH is negative (heat leaves the system).
- Endothermic: ΔH is positive (heat enters the system).
A very common mistake is to attach the wrong sign. Anchor it with the temperature: if the thermometer reading goes up, heat has been released, the reaction is exothermic, and ΔH is negative. If the reading goes down, heat has been absorbed, the reaction is endothermic, and ΔH is positive.
Worked example
Question. In an experiment, 50 cm³ of dilute hydrochloric acid at 28.0 °C is added to excess magnesium powder in a polystyrene cup. The highest temperature reached is 41.5 °C. State whether the reaction is exothermic or endothermic, give the temperature change, and state the sign of ΔH.
Step 1, Find the temperature change. Temperature change, Δθ = highest temperature − initial temperature Δθ = 41.5 °C − 28.0 °C = +13.5 °C
Step 2, Interpret the direction of the change. The temperature has risen by 13.5 °C, which means heat has been released to the surroundings (the solution).
Step 3, Classify the reaction. A reaction that releases heat and raises the temperature is exothermic.
Step 4, State the sign of ΔH. For an exothermic reaction, the heat content of the products is lower than that of the reactants, so ΔH is negative.
Answer. The temperature change is +13.5 °C. The reaction is exothermic because it releases heat to the surroundings, and ΔH is negative.
Practice question
Question. When ammonium nitrate is dissolved in water in a test tube, the test tube feels cold and the temperature of the water drops from 30.0 °C to 22.0 °C. State the temperature change, classify the process, and give the sign of ΔH.
Answer. The temperature change is 22.0 °C − 30.0 °C = −8.0 °C (a fall of 8.0 °C). Because the temperature falls, heat has been absorbed from the surroundings, so the process is endothermic. The heat content of the products is higher than that of the reactants, so ΔH is positive.
Exam tip
The safest way to classify a reaction is to read the temperature change first, then reason forwards: temperature up → heat released → exothermic → ΔH negative; temperature down → heat absorbed → endothermic → ΔH positive. Learn this as one linked chain so you never mix up the sign. When a question asks you to explain why a reaction is exothermic or endothermic, give the bond-energy reason: an exothermic reaction releases more energy forming bonds than it absorbs breaking bonds, while an endothermic reaction absorbs more energy breaking bonds than it releases forming bonds. Always quote the temperature with a sign and a unit, and remember that “the surroundings” in a solution experiment usually means the water or solution itself, which is why we measure its temperature.
Common mistakes to avoid
Do not say an exothermic reaction “makes the reaction hot” without saying heat is released to the surroundings, the marking scheme wants the energy transfer, not just the feel. Do not write ΔH as positive for combustion or neutralisation; these are exothermic and ΔH is negative. Do not confuse endothermic with slow: the terms describe heat direction, not speed. Finally, keep the language precise, “releases heat to the surroundings” and “absorbs heat from the surroundings” are the phrases that score.
Where this fits
This is content standard 11.1 of the Thermochemistry chapter, and it is the foundation for everything after it. The next standard shows how to draw these energy changes as energy level diagrams, and the later standards apply the same ideas to specific heats of reaction. Reinforce the definitions with the chapter revision notes and practise classifying reactions with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers drill the temperature-to-sign chain until it is automatic, because this single idea is tested throughout SPM Chemistry.
Quick recap
- Exothermic: releases heat, temperature rises, ΔH negative; products have lower heat content.
- Endothermic: absorbs heat, temperature falls, ΔH positive; products have higher heat content.
- Classify by the temperature change first, then state the sign of ΔH.
- Examples: combustion, neutralisation and displacement are exothermic; thermal decomposition and photosynthesis are endothermic.
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