Standard Energy Worksheet Page 3
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1
2
3
4o
When !G
= 0, the system is at equilibrium and both the forward and reverse
reaction are proceeding, at the same rate. This is the case during phase
o
o
o
transitions. Since !G
= 0, !H
= T!S
. This can be re-arranged:
o
o
T = !H
/ !S
which allows the determination of melting or boiling points
o
o
!S
= !H
/ T
which demonstrates that all of the heat added at the
transition temperature goes to raise the entropy of the
system
4.
Calculate the boiling point for BCl
:
3
BCl
(l) " BCl
(g)
3
3
o
o
!H
given the following information:
(kJ/mol)
S
(J/mol K)
f
BCl
(l)
-418
209
3
BCl
(g)
-395
290
3
5.
The heats of combustion of carbon, in the forms of graphite and
diamond are shown below.
o
!H
(g) ! CO
C (gr) + O
(g)
= -393.5 kJ/mol
2
2
o
!H
(g) ! CO
C(d) + O
(g)
= -394.4 kJ/mol
2
2
a) What is the heat of reaction in going from graphite to diamond
(hint: use Hess’ Law)
C (gr) ! C (d)
b) The absolute entropy of graphite is 5.69 J/mol K and for
o
diamond, 2.44 J/mol K, both at 298 K. Calculate !S
for this
reaction.
o
c) Calculate !G
for this reaction. Are diamonds
thermodynamically stable at 298 K?
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