Page 54 - Basic physical chemistry for the atmospheric sciences

40 Basic physical chemistry

o
2. 1 4 . Determine the molar standard enthalpy (or heat) f reaction
for:

given that the values of �H�x for the reactions:

C(s) + Oz(g) � C02(g)
Hz(g) + !Oz(g) � H 2 0(l)
and,

C H ig) + 202(g) � C0 2(g) + 2H20(l)
1
l
are 393 .5 , - 285.6, and - 889.9 kJ mo1 - , respective y .
-
2. 1 5 . Assuming the truth of the second law of thermodynamics,
as expressed by Eq. (2. 2 0), prove that heat will not flow
unaided from a cold to a hot body in an isolated system.
2. 1 6 . State whether the following reactions are likely to result in
an increase or decrease in the entropy of the system:
(a) 3H2(g) + Nz(g)- 2NH 3 (g)
(b) 2S02(g) + Oz(g)- 2S0 3(g)
(c) 2C2H 6(g) + 702(g)- 4C02(g) + 6H20(l)
2 . 1 7 . U s e the Gibbs-Helmholtz equation in the form of Eq. (2 .28)
to determine the effect of temperature (T) on the spontaneity
of chemical reactions for which (a) dh is negative and ds is
positive, (b) dh is positive and ds is negative, (c) dh and ds
are both negative, and (d) dh and ds are both positive.
2 . 1 8 . In formulating the first law of thermodynamics i n Section
2 . 1 we tacitly assumed that the only external work that a
system can do is the work of expansion (p da . However, a
)
system may also perform external work by other means
(e. . , chemical or electrical). Therefore, in general, the first
g
law of thermodynamics for a unit mass of a system should
be written as

dq = d u + dwtotal

where dw101a1 is the total external work done by the system.
If da is the external work done by a unit mass of a system
over and above any p da work, that is
da = d w101a1 - p da,