Several categories of heats of reaction describe the enthalpy changes in reactions. The enthalpy of physical change describes the change in enthalpy when a substance changes state. Such enthalpy changes include the enthalpy of vaporization, the enthalpy of melting (or fusion), and the enthalpy of sublimation. Enthalpies can also be associated with chemical reactions. The enthalpy of combustion (DHc) describes the heat released when a sample is completely burned (at standard conditions). The enthalpy of formation (DHf) gives the heat released when a molecule is formed from its elements. Reaction enthalpies are generally given at standard state conditions: the reaction proceeds with 1 mol of reactants and products at 1 atm and 25 degrees celsius. Reactions occurring at standard conditions are denoted by the degree symbol ¬°.
Reactions can generally be reduced to a series of simpler steps such that the final product of the steps is the product of the original reaction. Such a sequence of steps does not necessarily give an accurate reaction mechanism. However, the sum of the enthalpies of the individual steps is equal to the enthalpy of the overall reaction. This result is known as Hess's law. (Hess's law holds as long as the conditions of the
reaction remain constant.) Enthalpies of reactions are generally only given for one
particular form of the equation. For example, the enthalpy of formation of silver chloride is -127.1 kJ/mol. However, energy is required to split silver chloride into
