We speak of an exothermic reaction when the standard enthalpy difference is negative. If the reaction takes place under isobaric conditions (constant pressure), this is accompanied by a release of energy in the form of heat to the environment. Whether a reaction is exothermic or endothermic does not depend on whether a reaction is voluntary (spontaneous, exergonic) or endergonic. © 2017 SUBSTRATEC. All Rights Reserved.