The First Law of Thermodynamics
As a basis for deriving the heat source expressions needed for modeling electrochemical cells, we start from the first law of thermodynamics for a small system (volume element), stating that
(3-25)
where
δ
U
(J) is differential change of the internal energy,
δ
Q
(J) is the differential amount of heat added to the system, and
δ
W
(J) is the differential work performed by the system element on the surroundings.
Rearranging we may write
(3-26)
It should be noted that in the absence of pressure or volume changes, which are often neglected in electrochemical models, the differential internal energy equals the differential enthalpy
δ
H
(J). In addition, thermodynamic work is usually not involved in electrochemical reactions. We may hence simplify the above to
(3-27)
In order to conserve heat, the above differential heat added to the system equals the added heat drawn from the surroundings.
