Chemical systems possess both kinetic and potential energy.

• The kinetic energy of a system refers to how fast the atoms, ions, or molecules are moving. Temperature measures the average kinetic energy of the particles of a system. Kinetic energy is represented by the symbol E_k.
  
  
• The potential energy of a chemical system refers to the energy stored in the bonds between atoms and in the bonds between entities. Chemical bonds are not all the same. Some bonds have more potential energy than others. Potential energy is represented by the symbol E_p.
  
  

The total of the kinetic and potential energy within a chemical system is known as its enthalpy and is symbolized by H. In other words, enthalpy represents the total chemical energy possessed by a chemical system.

The enthalpy of any particular chemical system is difficult to quantify directly. However, scientists can readily evaluate how enthalpy changes within a chemical system as a chemical reaction proceeds. In other words, the enthalpy of the products can be compared with the enthalpy of the reactants. The difference between the enthalpy of the products and the enthalpy of the reactants is the "enthalpy change" and is symbolized as ΔH.

In measuring enthalpy change, variables must be controlled and certain assumptions must be made, as follows:

Enthalpy changes in chemical reactions must be measured at the same conditions of temperature and pressure, such as SATP. Recall that enthalpy represents the total kinetic and potential energy in a chemical system. However, if the reactants and products of a chemical reaction are at the same temperature and pressure, we can assume that the kinetic energy of the system does not change. Therefore, any enthalpy change that occurs within a chemical system is due solely to a change in the potential energy of that chemical system.