What Is an Exothermic Reaction?
A chemical reaction that releases energy to the surroundings, usually in the form of heat, causing the temperature of the surroundings to increase.
The word exothermic comes from the Greek: exo (outside) + thermic (heat). So an exothermic reaction is literally one that sends heat outward into the surroundings.
When chemicals react exothermically, the energy stored in the chemical bonds of the products is less than the energy stored in the bonds of the reactants. This "extra" energy doesn't disappear — it is released to the surroundings as heat (and sometimes light or sound).
Think of it like a backpack getting lighter. The reactants start with a heavy load of chemical energy. After the reaction, the products carry less energy — the difference has been dropped off (released) into the surroundings as heat.
You can recognise an exothermic reaction happening around you every day — whenever something burns, a hand warmer heats up, or your body digests food, energy is being released to the surroundings.
Energy Changes and Temperature
A key observation in exothermic reactions is that the temperature of the surroundings rises. This is the most straightforward way to identify an exothermic process in the laboratory.
Everything outside the chemical reaction itself — the solvent, the container, the air around it, and ultimately everything else in the universe.
When we measure temperature change in a reaction:
- If the temperature of the solution/container goes up → the reaction is exothermic
- If the temperature of the solution/container goes down → the reaction is endothermic
It is important to say that the surroundings get hotter, not the reaction itself. The chemical system is releasing energy — the surroundings are absorbing it. Students often mix these up in exam answers!
In exothermic reactions, we say that the enthalpy change (ΔH) is negative, because the products have less energy than the reactants. You will explore this concept in more depth in MYP and IB DP Chemistry.
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