Most chemical reactions involve changes in energy. A chemical reaction is simply breaking bonds between atoms and making new ones. When bonds are broken in a chemical reaction, the reactants involved are colliding with enough kinetic energy to break the atoms apart. During the collision, the kinetic energy is transformed into potential energy as the atoms are rearranged and new bonds are formed. Bond breaking requires energy while bond making releases energy. Most chemical reactions, therefore, involve the overall absorption or release of energy. The energy flow that results is heat. The heat absorbed or released in a reaction depends on a quantity called enthalpy, represented by the capital letter H.

The enthalpy of a system can be defined as the amount of heat in the system at constant pressure. The enthalpy of a substance is the energy of the substance (the sum of the kinetic and potential...