<Li> Where an exothermic reaction causes heating of the reaction vessel which is not controlled, the rate of reaction can increase, in turn causing heat to be evolved even more quickly . This positive feedback situation is known as thermal runaway . An explosion can also result from the problem . </Li> <P> Heat production or absorption in either a physical process or chemical reaction is measured using calorimetry . One common laboratory instrument is the reaction calorimeter, where the heat flow into or from the reaction vessel is monitored . The technique can be used to follow chemical reactions as well as physical processes such as crystallization and dissolution . </P> <P> Energy released is measured in Joule per mole . The reaction has a negative ΔH (heat change) value due to heat loss . e.g.: - 123 J / mol </P>

When a reaction generates heat the reaction is termed endergonic