Endothermic vs. Exothermic: What's the Difference?

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Read the comparison below, or explore it visually. Create a visual lesson comparing endothermic and exothermic using available credits or plan allowance.

An endothermic reaction absorbs heat from its surroundings, so they get colder, while an exothermic reaction releases heat, so they get warmer. The difference comes from chemical bonds: breaking bonds takes energy and forming bonds releases it. If forming the new bonds releases more energy than breaking the old ones took, the reaction is exothermic; if less, it is endothermic.

At a glance

	Endothermic	Exothermic
Heat flow	Absorbed from the surroundings	Released to the surroundings
Temperature change	Surroundings get colder	Surroundings get warmer
Enthalpy change	Positive: products hold more energy	Negative: products hold less energy
Bond energy balance	Bond breaking costs more than bond making	Bond making releases more than breaking
Examples	Photosynthesis, decomposing limestone	Burning fuel, respiration, neutralisation

Which should you use?

Endothermic

Endothermic processes explain instant cold packs, where a salt such as ammonium nitrate dissolves in water and draws heat out of the pack, and why plants need sunlight to photosynthesise.

Exothermic

Exothermic reactions explain the heat of an LPG flame on a kitchen stove, and the hissing warmth when quicklime is mixed with water to make limewash.

Frequently asked questions

Is photosynthesis endothermic or exothermic?

Photosynthesis is endothermic. Plants absorb light energy to build glucose from carbon dioxide and water, storing that energy in chemical bonds. Respiration runs the overall process in reverse and is exothermic, releasing the stored energy for the cell to use.

Do exothermic reactions need heat to start?

Many do. A match or petrol must be heated first to supply activation energy that breaks the first bonds. Once the reaction is running, the heat it releases keeps it going, which is why a fire spreads by itself.

Is melting ice an endothermic reaction?

Melting ice is an endothermic process, but it is a physical change, not a chemical reaction. The ice absorbs heat to loosen the bonds holding water molecules in a crystal, while the molecules themselves stay unchanged as H2O.