Question

Can someone help me understand exothermic and endothermic reactions?

Answer 1

An exothermic reaction occurs when the temperature of a system increases due to the evolution of heat. This heat is released into the surroundings, resulting in an overall negative quantity for the heat of reaction (qrxn<0).

An endothermic reaction occurs when the temperature of an isolated system decreases while the surroundings of a non-isolated system gains heat. Endothermic reactions result in an overall positive heat of reaction (qrxn>0).
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Answer 2

Think of the prefixes:

Exo- = Releasing. Heat is being increased, it's being released.
Endo- = Containing, absorbing . . . the heat is decreasing and absorbs heat.