
118 Fuel Cells
Let us consider two separate cases: (i) water existing in liquid form and
(ii)water existing in vapor form.
i. Water in liquid form
The enthalpy of formation is given as
∆∆
hh hhhhh
ff
HO(l)
f
H
f
O
=+
−+
000
22
1
.
Substituting data from Table 4.2
∆h
f
=− −+− +(,)( )( )285 830 00
1
∆h
f
J/mo
=−285 830,
with water as liquid.
The change in absolute entropy is given as
∆ss
fT
HO
T
H
T
O
=
()
−
()
−
()
00 0
22 2
1
or
∆s
f
J/mo
=− −
=−
(. )( .) (.)
.
69 950 130 678
1
2
205 148
163 302 llK.
The Gibbs free energy of formation is given as
∆∆ ∆ghTs
ff f
=−
=− −−
=−
285 830 298 15 163 302
285 830
,.
)
,
++
=−
=−
48688 4913
237 141 51
237 141 51
.
,.
,.
J/ ...