Exothermic vs Endothermic

Quick Answer

Exothermic releases heat to the surroundings. Endothermic absorbs heat from the surroundings.

What Each One Really Means

The word 'exo' means outside and 'endo' means inside. So exothermic means heat goes out, and endothermic means heat comes in. That is the easy part. The confusion starts when students hear 'exothermic' and imagine something hot. Not necessarily. Exothermic means heat is leaving the system. It does not mean the substance is hot to touch. Combustion is exothermic, yes, and it does feel hot. But some exothermic reactions feel warm only briefly, because the heat escapes quickly. The opposite mistake happens with endothermic reactions. Students imagine them being cold, but that is only because heat is being drawn out of the surroundings. The reaction mixture itself cools down, but the surroundings are just donating heat. The sign convention is the part that traps people. In thermodynamics, we usually define q as positive when heat enters the system. So endothermic has positive q, exothermic has negative q. This is the opposite of how most people intuitively think about 'releasing heat'. So be careful. Another point students always mix up is exothermic versus exergonic. They are not the same. Exothermic is about heat only. Exergonic is about Gibbs free energy, which includes entropy. A reaction can be endothermic and still spontaneous if the entropy increase is large enough. That is why dissolving ammonium nitrate in water feels cold, but it still happens on its own. It is endothermic but exergonic.

Side-by-Side Comparison

Memory Device

Exo is exit. Endo is enter. Heat leaving or heat coming in.

Related Topics