Here's why:
* Exothermic literally means "heat out".
* In an exothermic reaction, the energy released when new bonds form is greater than the energy required to break the original bonds. This excess energy is released as heat, causing the surroundings to warm up.
Other phrases that could be used, but are not as precise, include:
* Heat-producing
* Energy-releasing
* Releases energy to the surroundings
