Direct comparison
Endothermic vs exothermic
An endothermic process absorbs energy from its surroundings; an exothermic process releases energy to its surroundings, usually as heat.
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Side-by-side comparison
| Dimension | Endothermic | Exothermic |
|---|---|---|
| Energy flow | Absorbs energy from the surroundings. | Releases energy to the surroundings. |
| Effect on surroundings | Surroundings cool down (feel cold). | Surroundings warm up (feel hot). |
| Energy of products vs reactants | Products store more energy than reactants. | Products store less energy than reactants. |
| Enthalpy change (ΔH) | Positive (ΔH > 0). | Negative (ΔH < 0). |
| Bond energetics | Energy to break bonds exceeds energy released forming them. | Energy released forming bonds exceeds energy to break them. |
| Energy profile | Products sit higher than reactants on the diagram. | Products sit lower than reactants on the diagram. |
| Everyday examples | Melting ice, evaporation, photosynthesis, instant cold packs. | Combustion, respiration, neutralisation, hand warmers. |
| Memory cue | "Endo" = energy enters the system. | "Exo" = energy exits the system. |
| Common feel test | The container or mixture feels colder. | The container or mixture feels warmer. |
It comes down to breaking and making bonds
Every chemical reaction breaks bonds in the reactants and makes new bonds in the products. Breaking bonds requires energy input; forming bonds releases energy. The overall reaction is exothermic when more energy is released making new bonds than was needed to break the old ones, leaving a surplus that escapes as heat. It is endothermic when breaking the old bonds costs more than forming the new ones returns, so the reaction must draw the shortfall from its surroundings. The net difference is the enthalpy change, ΔH: negative for exothermic, positive for endothermic.
Common questions
FAQ
Is a reaction that gets cold endothermic or exothermic?+
If the mixture or container gets colder, the reaction is endothermic. It is drawing heat in from its surroundings, including from the container and your hand, so they lose warmth and feel cold. An instant cold pack works this way. A reaction that gets warmer is exothermic, releasing heat to its surroundings.
What does the sign of ΔH tell you?+
ΔH is the enthalpy change of a reaction. A negative ΔH means energy is released, so the reaction is exothermic. A positive ΔH means energy is absorbed, so the reaction is endothermic. The sign is taken from the system’s point of view: it loses energy (negative) or gains energy (positive) relative to its surroundings.
Are physical changes also endothermic or exothermic?+
Yes. Energy changes are not limited to chemical reactions. Melting and evaporation absorb energy and are endothermic; freezing and condensation release energy and are exothermic. Dissolving can be either, depending on the substance. The same logic of energy entering or leaving the system applies whether or not new substances are formed.
