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Week 15
CHEM 1310  Sections L and M
1
Ch. 10  Recap
What is entropy?
Degree of disorder of a system
Equations for calculating entropy
S = k
B
ln
Ω
rigorous definition
∆
S = q
reversible
/T
For melting:
∆
S =
∆
H
fusion
/T
m
For evaporation:
∆
S =
∆
H
vap
/T
b
∆
S = nC
p
ln(T
2
/T
1
) or
∆
S = nC
V
ln(T
2
/T
1
)
Week 15
CHEM 1310  Sections L and M
2
Ch. 10  Recap
What is spontaneity?
Likelihood that a reaction will proceed without
outside influences
Spontaneity has nothing to do with the speed of
a reaction.
Both entropy and Gibbs’ free energy terms help
us to predict whether a reaction will proceed
spontaneously or not. How?…
Week 15
CHEM 1310  Sections L and M
3
Ch. 10  Recap
Entropy
as an indicator of reaction spontaneity
2
nd
Law of Thermodynamics:
in any spontaneous process there is an increase in the
entropy of the universe.
∆
S
universe
=
∆
S
system
+
∆
S
surroundings
•
If
∆
S
universe
is
positive
, then the process is
spontaneous
.
•
If
∆
S
universe
is
negative
, then the process is
nonspontaneous
.
•
∆
S
universe
is still to difficult to measure.
Need a more practical indicator!
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Week 15
CHEM 1310  Sections L and M
4
Ch. 10  Recap
Gibbs’ free energy
as an indicator of reaction
spontaneity
Equation relates thermodynamics terms
G = H  TS
expressed in terms of the system
∆
G =
∆
H  T
∆
S
How does
∆
G help us to predict rxn spontaneity?
Week 15
CHEM 1310  Sections L and M
5
Ch. 10  Recap
∆
G =
∆
H  T
∆
S
Divide both sides of the eqn by “T”
When
∆
G is
negative
,
∆
S
univ
is positive:
spontaneous
reaction
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 Spring '11
 Staff
 Chemistry

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