An exothermic reaction transfers energy from the reacting system to its surroundings, often warming them. An endothermic reaction takes energy from surroundings, often cooling them. Define the system as the reacting chemicals when choosing transfer direction. Energy is transferred or transformed, not created from nothing.
For given readings, a temperature rise is final minus starting temperature; a fall is starting minus final temperature. In a stated insulated reaction setup, warming supports an exothermic classification and cooling an endothermic one. Temperature change alone does not prove reaction, because physical changes can also transfer energy. Use the full stated chemical context.
Example 1
A stated reaction warms its surroundings from 20∘C to 28∘C. Find the rise and classify the transfer.
The rise is 28−20=8∘C. Energy went from the reaction system to the surroundings, so it is exothermic.
Example 2
A reaction takes energy from the surroundings and the nearby liquid cools. What term applies?
Endothermic: energy enters the reacting system from its surroundings.
Watch out: Energy is transferred from the reacting system to its surroundings; it is not created.