Thermochemistry is the study of the energy changes that take place during chemical reactions. These energy changes may assume various forms. When air and natural gas are ignited, enough heat to supply a Bunsen burner is produced, but when glucose is formed during photosynthesis, light energy is absorbed in the reaction. Reactions that evolve heat are called exothermic; those that absorb heat are called endothermic.

The energy involved in a chemical reaction can be related to the differences in energy between the reactants and products. This energy is related to the heat content, or enthalpy, of the substances involved. "H" is the change in the system's enthalpy, which is also known as the heat of reaction. For example, consider hydrogen peroxide, which breaks down to form liquid water and oxygen gas:

H₂O₂(aq)H₂O(l) + 1/2O₂(g) H₂₉₈= -22.64kcal (4-10)

In this case the enthalpy...