Heat is the transfer of energy that results from the difference in temperature between a system and its surroundings. At a molecular level, heat is the transfer of energy that makes use of or stimulates disorderly molecular motion in the surroundings. For instance, when a hydrocarbon fuel burns, the energy released in the reaction stimulates the surrounding atoms and molecules into more vigorous random motion, and we refer to this escape of energy as heat. Heat is not stored: Heat is energy in transit.
The measurement of quantities of energy transferred as heat is called calorimetry. Such a measurement is commonly made by observing the rise in temperature caused by the process being studied and interpreting that rise in terms of the heat produced. Calorimetry is used to measure the changes in internal energy and enthalpy that accompany chemical reactions. The field of study is called thermochemistry, and it is used to assess the efficacy of fuels, the energy flow in chemical plants, and the strengths of chemical bonds. Measurements of the heat produced or absorbed by chemical reactions are central to thermodynamics, and to assessments of whether or not a particular reaction will tend to occur.
In thermodynamics, the quantity of energy transferred as heat as a result of a chemical reaction is identified with the change in the internal energy of the system if the transfer takes place without change in the system's volume, and with the change in enthalpy of the system if the transfer takes place at constant pressure. The energy or enthalpy change accompanying a chemical reaction that is inaccessible to measurement may be determined by using Hess's law, which states that the enthalpy change accompanying a chemical reaction can be regarded as the sum of the enthalpy changes of the reactions into which the overall reaction may be divided. Hess's law is no more than a special application of the first law of thermodynamics.
The source of heat as a fuel burns is the energy released when the bonds characteristic of the reactants are replaced by the bonds characteristic of the products. Energy is released when hydrocarbons burn because of the great strengths of the oxygen–hydrogen and oxygen–carbon bonds that are formed in the products (water and carbon dioxide), replacing the relatively weak carbon–hydrogen and carbon–carbon bonds of the fuel. Ultimately, the energy of burning fuel is the energy released as the electrons and atomic nuclei settle into more favorable arrangements (just as nucleons do in the much more exothermic processes accompanying nuclear rearrangements).
Although the term "heat energy" is commonly encountered in casual conversation, strictly speaking there is no such entity. The term is commonly used in place of the more precise term "energy of thermal motion," where thermal motion is random molecular motion, as in the motion of molecules in a gas. Nor is heat stored: Only energy is stored, and heat is one of the modes by which it may be increased or extracted.
Atkins, Peter, and de Paula, Julio (2002). Atkins' Physical Chemistry, 7th edition. New York: Oxford University Press.
Smith, Crosbie (1998). The Science of Energy: A Cultural History of Energy Physics in Victorian Britain. Chicago: University of Chicago Press.