26
HENRY EYRING, RICHARD P. BOYCE AND JOHN D. SPIKES
term is general and includes all forms of work—mechanical, electrical,
etc.; that is it represents the algebraic sum of all the work terms. It is
taken to be positive if work is done by the system. Likewise dQ is the
algebraic sum of all heat entering or leaving the system. It is taken to
be positive if heat is absorbed by the system.
H. APPLICATIONS OF THE FIRST LAW
In this section we summarize the most important results of the First
Law for chemical and biological systems.
Writing the First Law in the form
Q = AE + W
(20)
we assume the only work performed is mechanical work, e.g., pressurevolume work. For constant volume
Qv = AE
(21)
so that the heat absorbed at constant volume is just the increase in internal energy. At constant P, the expression becomes
QP = AE + PAV
(22)
Let
AE = E2 — E\
where E x and E 2 are the energy values for the initial and final states;
we can likewise write
AV =V 2 -
Fi
Thus Eq. 22 becomes
QP = (E 2 - EJ + P(V 2 - V 1 ) = (E 2 + PV 2 ) - (E l + PV X ) (23)
P and V are state functions; therefore E + PV is also a state function.
It is an extensive property called the heat content or enthalpy. It is
given the symbol H. Hence
H = E + PV
(24)
At constant P, we can therefore write
QP = AH
(25)
The equation for the heat of a chemical reaction is
QP = \H (products) — } H (reactants) = AH
A chemical reaction which liberates heat is termed exothermic while
one which absorbs heat from the surroundings is termed endothermic.