An exothermic reaction is a chemical or physical reaction that releases heat. It gives net energy to its surroundings. That is, the energy needed to initiate the reaction is less than the energy that is subsequently released. When the medium in which the reaction is taking place gains heat, the reaction is exothermic. When using a calorimeter, the total amount of heat that flows into (or through) the calorimeter is the negative of the net change in energy of the system.