Most chemical reactions involve the breaking and formation of chemical bonds. It takes energy to break a chemical bond but energy is released when chemical bonds are formed. If more energy is released than consumed, then the chemical reaction evolves heat and is said to be exothermic. On the other hand if the reaction consumes more energy than it releases then the reaction absorbs heat and is said to be endothermic. Most reactions are exothermic.
The amount of heat generated by a reaction can be quantified. The quantity that specifies the amount of heat is known as the "enthalpy" . If is positive then the reaction is endothermic and if it is negative the reaction is exothermic. For example, consider the combustion of methane:
This equation tells us that the combustion of methane is exothermic and releases 883 kilojoules of energy (in the form of heat) for every mole of methane (CH4). Given this information we can determine how much heat will evolve if we burn a certain mass of methane: For example: Suppose we combust 20 g of methane, How much heat will evolve (in kJ)? The procedure is the usual stoichiometry: