CHAPTER 4. THERMODYNAMICS: THE FIRST LAW

4-18
Since H is a state function
H(T ) 2
aA + bB ! cC T 2
H 9 H 9 8 H
1 2 3
aA + bB ! cC T 1
H(T ) 1
H(T ) = H + H + H(T ) + H .
2 1 2 1 3
For a reaction at constant pressure, the
substance may be calculated from the relation
H associated with changing the temperature of each
T 2
H ' n C P
dT.
T 1
Therefore, T 1
T 1
T 2
H(T 2) = H(T 1) + a C P
(A)dT % b C P
(B)dT % c C P
(C)dT.
T 2
T 2
T 1
Over a restricted range of temperatures, the heat capacities may be treated as constants, in which case
the above formula reduces to
where
H(T ) = H(T ) + C (T ! T ),
2 1 P 2 1
C j C P (i)
P = cC P(C) ! aC P(A) ! bC P(B) = .
iνi
I. Work of adiabatic expansion
The work associated with the expansion of a gas is given by
V 2
w ' !
V 1
P ex
dV
,
and for the isothermal expansion of an ideal gas, we found
a). w = !P V
ex
when P = constant
ex