 So remember exo means outside so heat goes to the outside so in these reactions the surroundings get hotter and Endo or inside heat goes to the inside the chemicals themselves and that heat doesn't actually go towards making the chemicals hotter Instead it's stored as chemical energy in the bonds of the newly formed molecules So energy disappears from the surroundings gets locked up in the chemical bonds and the surroundings get colder If you have trouble remembering which is which perhaps think of a biology analogy Insects spiders crabs creatures like that. They have an exo skeleton. This means their hard supportive structures are on the outside So exo means outside Whereas vertebrates including us have skeletons on the inside and the proper name for this is an endo skeleton So endo means inside