In thermochemistry we are concerned with the exchange of energy between a chemical system and its surroundings. Sometimes chemical changes are able to bring energy into the system. These are endothermic changes. An example is the charging of a battery, in which energy from an external source becomes stored in the battery in the form of chemical potential energy. When endothermic changes occur by themselves within an uninsulated system, we often notice a cooling effect in the surroundings.
Many chemical reaction are able to release energy to the surroundings.
Such changes are described as exothermic . A typical example is the combustion of gasoline. The form of the energy absorbed or released during a change can vary. It sometimes appears as light, or electrical work, but most often occurs only as heat. When the entire energy change of a reaction involves heat, the amount of heat is called the heat of reaction and is usually represented by the symbol 'q'.