98
Chapter 4
Liquid water has much lower entropy 16.72 cal (mol K )1
than gaseous H30 at" the same conditions.
Consequently for formation of liquid water from 0 2 and H3 at 25 C:
A
H°f
= -57800-582-18 =-68280 cal mole'1
A G / = -68280-298 (16.72- 31.21 -24.5) = -56661 cal mole'1
The heat of vaporization of 1LO at 25 C is 582 cal g'1.____________________________________________
For any chemical reaction the free energy of reaction is related to the free energy of reaction at
standard conditions by
AG = AG°
+ R T \n K
where the mass action fraction
K
for the reaction is defined by:
_ n/y
n j
(4.3)
(4.4)
y
IIS;
is the product of substrate activities where each activity is raised to the power of the
corresponding stoichiometric coefficient in the reaction to be studied, fl
P?K
is the corresponding
quantity for the products. For reactants in dilute aqueous solutions the activities can be set equal to
their molar concentrations. For gas-phase reactants at low total pressure the activities are equal to
the partial pressures in atm.
At equilibrium A G = 0 and therefore
AG° =
-R T
In
(4.5)
where
is the
equilibrium constant
for the reaction.
Example 4.2 Equilibrium constant for formation of H20
For the reaction between 0 2 and
lAH2
to form gaseous water at 25°C, 1 atm:
In
- AG°
R T
54640
^
P h 7o
298-1.987
[
p t'£ p Hi
92.28
(
1
)