Exothermic & Endothermic Reactions

Every chemical reaction comes with an energy change. Reactions that release heat to the surroundings, like a hand warmer, are exothermic; those that absorb heat, like a cold pack, are endothermic. In an exothermic case reactants sit higher in energy than products; endothermic is the reverse, and the surroundings warm or cool accordingly. Here you can compare both kinds side by side on energy diagrams.

Reactions Involve Energy Changes

🔥 Reactions that Heat or Cool

①Hand warmer becomes hot when shaken → releases energy (exothermic)

②Instant cold pack becomes cold → absorbs energy (endothermic)

③Every chemical reaction involves an energy change

Energy Diagrams

Energy Relation

Energy of reactants > products → energy released

Surplus energy escapes to surroundings (temperature rises)

Surrounding Temperature

💡 Examples of Exothermic Reactions

①Combustion (wood, gas) → heat and light

②Acid + metal (Mg + HCl)

③Neutralization (acid + base → water)

④Hand warmer (iron oxidation)

Exo vs Endo Comparison

Exothermic Reaction

Reactants → Products + Energy

Temperature rises | e.g. combustion, neutralization, oxidation

Endothermic Reaction

Reactants + Energy → Products

Temperature drops | e.g. photosynthesis, cold pack, dissolution

Summary

Core Comparison

Exo (release, T↑) ↔ Endo (absorb, T↓)

Conservation of energy: energy only changes form

🎯 Exam Key Points

①Exothermic: surrounding T↑ — combustion, neutralization, hand warmer

②Endothermic: surrounding T↓ — photosynthesis, cold pack

③Energy diagram: reactants > products → exothermic

④Reactants < products → endothermic

⑤Conservation of energy holds

Examples and Unit Test

Example 1

Shaking a hand warmer makes the surroundings hot. Is this an exothermic or endothermic reaction, and does it release or absorb energy?

An exothermic reaction releases energy and raises the surrounding temperature, while an endothermic reaction absorbs energy and lowers it.

The hand warmer makes the surroundings hot, so it is an exothermic reaction that releases energy.

▸ Exothermic reaction / releases energy

If the surroundings get hot it is exothermic (releases energy); if cold, endothermic (absorbs energy).

Example 2

In an endothermic reaction, how do the energies of the reactants and products compare? And give one example of an endothermic reaction.

An endothermic reaction absorbs the energy it lacks from the surroundings, so the products have more energy than the reactants.

So reactant energy < product energy, and an example is a cold pack (dissolving ammonium nitrate).

▸ Reactant energy < product energy / example: a cold pack (endothermic)

In an endothermic reaction the products have more energy than the reactants, and the surrounding temperature drops.

Unit-test style

Which statement about exothermic and endothermic reactions is correct?

①An exothermic reaction lowers the surrounding temperature

②An exothermic reaction releases energy and raises the surrounding temperature

③An endothermic reaction releases energy

④Combustion is a typical endothermic reaction

⑤In an endothermic reaction the reactants have more energy than the products

▸ ② An exothermic reaction releases energy and raises the surrounding temperature

An exothermic reaction releases energy, so the surrounding temperature rises.

An exothermic reaction makes the surroundings hot (①), an endothermic reaction absorbs energy (③), combustion is exothermic (④), and in an endothermic reaction the products have more energy (⑤).

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Oxidation & Reduction

Photosynthesis & Respiration

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