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1
Chapter 10
Spontaneity,
Entropy, &
Free Energy
o
Entropy
o
Isothermal Processes
o
ΔS of Physical Reactions
o
2
nd
Law of Thermo
o
Free Energy
o
Hess’s Law/ 3
rd
Law of
Thermo
o
Equilibrium
1
Big Idea:
The change in free
energy of a reaction
indicates whether a
reaction is
spontaneous.
In any
spontaneous process
there is always an
increase in the entropy
of the universe.
Chapter 10:
Spontaneity, Entropy, and Free Energy
Entropy
Entropy (S):
Entropy is
a measure of how
energy and matter
can be distributed in a
chemical system.
2
# of molecules
of left side
# of ways of
arranging
(microstates)
4
3
2
1
0
Chapter 10:
Spontaneity, Entropy, and Free Energy
Entropy
In General:
Entropy
increases from solid to liquid to gas corresponding
to an increase in positional probability.
Entropy
increases when you dissolve a solid in liquid
corresponding to an increase in positional probability.
The larger the
volume
the larger the positional probability
and the greater the entropy (n constant).
The larger the
pressure
the smaller the positional
probability and the lower the entropy (n constant).
The larger the
molecule
the larger the number of relative
positions of the atoms resulting in a greater positional
probability and a greater entropy.
The higher the
temperature
the greater the range of
energies, therefore the larger the entropy.
3
Chapter 10:
Spontaneity, Entropy, and Free Energy
Student Question
Entropy
The normal boiling point of the element
rubidium is 688°C.
The following equilibrium is
important in rubidium vapor at that
temperature: 2Rb(g)
⇌
Rb
2
(g) + heat.
Predict
the signs of ΔH° and ΔS° for this reaction
assume the reaction occurs at constant
pressure.
a)
ΔH° is negative ΔS° is positive
b)
ΔH° is negative ΔS° is negative
c)
ΔH° is positive ΔS° is positive
d)
ΔH° is positive ΔS° is negative
4
Chapter 10:
Spontaneity, Entropy, and Free Energy
Student Question
Entropy
Predict which of the following reactions has a
negative entropy change.
I.
CH
4
(g) + 2O
2
(g)
CO
2
(g) + 2H
2
O(l)
II.
NH
3
(g) + HCl(g)
NH
4
Cl(s)
III.
2KClO
4
(s)
2KClO
3
(s) + O
2
(g)
a)
II and III
b)
III
c)
II
d)
I
e)
I and II
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Chapter 10:
Spontaneity, Entropy, and Free Energy
Entropy
Phase Change:
The condition (for a given
pressure, and temperature) at which two
different phases are in dynamic
equilibrium.
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This note was uploaded on 01/18/2012 for the course CHEM 1B taught by Professor Watts during the Winter '08 term at UCSB.
 Winter '08
 Watts
 Reaction

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