Heat, Energy, and Enthalpy

Energy can be transferred to (or from) a chemical system from (or to) the surroundings in the form of heat q or work w. The first law of thermodynamics states that the change in the system's internal energy E is equal to the sum of the heat and work inputs (or outputs):

                                                                                      (1.1)

where DE = Efinal - Einitial. The first law of thermodynamics can be thought of as an extension of the mechanical law of conservation of energy to include heat. Notice that positive heat or work increases the energy of the system, negative heat or work decreases the energy. The internal energy change includes changes in the energies of atomic and molecular motion, changes in the energies of atomic and molecular interactions, and changes associated with a chemical reaction.

Heat energy flows to or from the surroundings when there is a difference in temperature, when a chemical reaction or change of state takes place, or when work is done on or by the system. Unlike P, V, T, and E, heat is not a function of the state of the system. We cannot speak of a system ''having heat"; q refers only to energy in transit.

Work is done on (or by) a chemical system when the system is compressed (or is expanded). Thus when V is constant, w = 0 and

                                                                                                  (1.2)

where the subscript reminds us that V is constant. When the pressure is constant, w = -P DV, q = ΔE - w, or qp = ΔE + P ΔV. Since E, P, and V are all variables associated with the state of the system, we can define a state function called the enthalpy,

                                                                                      (1.3)

and, for a process occurring at constant pressure, the change in enthalpy is

                                                                 (1.4)

For most chemical processes, we can think of energy and enthalpy as nearly identical since heat transfer is usually much more important than work performed. However, since the work term can be significant, we must

maintain the distinction between E and H.

We will use the joule (J) to measure energy, enthalpy, and heat. Other energy units which may be encountered are listed in Table 1.1.

Table 1.1. Some Common Energy Units