Exothermic – IGCSE Chemistry Definition

Exothermic reactions release energy to the surroundings, causing a temperature rise. Alongside endothermic reactions, they form the core of the chemical energetics section of the 0620 syllabus. You must define the term, give examples, draw energy level diagrams, and apply bond energy calculations.

The 0620 definition

An exothermic reaction is a reaction that gives out (releases) energy to the surroundings. The temperature of the surroundings increases.

In terms of bond energies (Supplement): the energy released when new bonds form in the products is greater than the energy required to break bonds in the reactants. The overall energy change is negative.

Examples of exothermic reactions

Reaction	Observation
Combustion (burning fuels)	Flame, heat, light released
Neutralisation (acid + alkali)	Solution temperature rises
Reaction of metals with acids	Solution warms up
Oxidation / rusting	Slow heat release
Respiration	Energy released for living processes
Freezing / condensation	Energy released as intermolecular forces form

Energy level diagram

In an exothermic energy level diagram:

• The products are drawn at a lower energy level than the reactants
• The arrow between reactants and products points downward
• The energy difference (downward arrow) represents the energy released to the surroundings
• The activation energy is shown as a hump above the reactants that must be overcome to start the reaction

Bond energy explanation (Supplement)

Energy to break bonds < Energy released making bonds

Overall energy change = energy to break bonds - energy released forming bonds = negative value

For example, in the combustion of hydrogen: 2H₂ + O₂ → 2H₂O

Breaking: 2 H-H bonds + 1 O=O bond (energy absorbed) Making: 4 O-H bonds (energy released)

The energy released by forming 4 O-H bonds exceeds the energy needed to break the reactant bonds, so the reaction is exothermic.

Worked exam question

Methane burns in oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O. (a) Is this reaction exothermic or endothermic? (1) (b) Describe the energy change in terms of bond breaking and bond making. (2) (c) What would you observe about the temperature of the surroundings? (1)

Mark scheme

(a) Exothermic [1]

(b) Energy is needed to break bonds in CH₄ and O₂ [1]; more energy is released when new bonds form in CO₂ and H₂O / energy released forming bonds is greater than energy needed to break bonds [1]

(c) The temperature increases / the surroundings get hotter [1]

Common exam mistakes

• Saying “exothermic reactions do not need energy to start”. Most exothermic reactions still need activation energy to get going — a match to light a fuel, for example.
• Confusing the direction of the energy level diagram. Exothermic = products lower than reactants. The arrow points downward.
• Stating “heat is created”. Energy is released (transferred to the surroundings), not created. Energy conservation applies.