Endothermic – IGCSE Chemistry Definition

Endothermic reactions absorb energy from the surroundings, causing a temperature drop. The 0620 syllabus requires you to define endothermic, identify examples, draw energy level diagrams, and explain the concept using bond energies. This is tested across Papers 1, 2, and 4.

The 0620 definition

An endothermic reaction is a reaction that takes in (absorbs) energy from the surroundings. The temperature of the surroundings decreases.

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

Examples of endothermic processes

Process	Observation
Thermal decomposition (e.g. CaCO₃ → CaO + CO₂)	Requires continuous heating
Dissolving ammonium nitrate in water	Solution gets cold
Photosynthesis	Requires light energy input
Citric acid + sodium hydrogencarbonate	Mixture cools down
Melting / boiling (changes of state)	Energy absorbed to overcome intermolecular forces

Energy level diagram

In an endothermic energy level diagram:

• The products are drawn at a higher energy level than the reactants
• The arrow between reactants and products points upward
• The energy difference (upward arrow) represents the energy absorbed from the surroundings
• The activation energy barrier is still shown as a hump above the reactants

Bond energy explanation (Supplement)

Energy is required to break bonds (endothermic step) and released when new bonds form (exothermic step). In an endothermic reaction:

Energy to break bonds > Energy released making bonds

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

Worked exam question

When ammonium nitrate dissolves in water, the temperature drops. (a) Is this process endothermic or exothermic? (1) (b) Explain what the temperature change tells you about the energy transfer. (2) (c) Draw a labelled energy level diagram for this process. (2)

Mark scheme

(a) Endothermic [1]

(b) Energy is absorbed/taken in from the surroundings/water [1]; so the temperature of the surroundings/water decreases [1]

(c) Products drawn higher than reactants [1]; arrow pointing upward between them labelled “energy absorbed” or showing positive energy change [1]

Common exam mistakes

• Confusing endothermic with exothermic. Endothermic absorbs energy (temperature goes down). Exothermic releases energy (temperature goes up). “Endo” means “into” — energy goes into the reaction.
• Drawing the energy level diagram the wrong way. For endothermic, products are higher than reactants.
• Saying “energy is created”. Energy is never created or destroyed — it is transferred from the surroundings into the reaction system.