Thermodynamics
Chapter 20
Thermodynamics
Prediction of whether change will occur
No indication of timeframe
Spontaneous:
occurs without external intervention
Nonspontaneous:
requires outside “push”
Entropy and Spontaneity
Driving force for a spontaneous change is an
increase in entropy of the universe
Entropy, S
:
measure of disorder
Spontaneous change implies:
more order
less order
fewer ways of arranging particles
more
Second Law of Thermodynamics
In any spontaneous change, there is always an
increase in entropy of the universe.
Units:
J
K
Entropy
1877 Ludwig Boltzmann:
k = Boltzmann constant, R/N
A
W =
no. of possible arrangements
Third Law of Thermodynamics:
The entropy of a perfect crystal at 0 K is zero.
Positional Entropy
Why does a gas expand into a vacuum?
Expanded state has highest positional probability
of states available.
Other factors in entropy
Size:
increase in S with increasing size (mass)
Molecular complexity:
increase in S with increasing complexity
Generally effect of physical state >> complexity
Reactions
For a spontaneous reaction:
NaOH
(s)
+ CO
2(g)
Na
2
CO
3(s)
+ H
2
O
(l)
S
0
64.45 213.7 139 69.94 J/K
Is the reaction spontaneous as written?
Spontaneity and S
Spontaneous:
S
univ
> 0
Nonspontaneous:
S
univ
< 0
At equilibrium:
S
univ
= 0
S
sys
can be positive if
S
surr
increases enough
Surroundings and S
univ
Surroundings add or remove heat
Exothermic:
q
sys
< 0
q
surr
> 0
so
S
surr
> 0
Endothermic:
q
sys
> 0
q
surr
< 0
so
S
surr
< 0
S
surr
and
S
sys
S
surr
:
S
surr

q
sys
S
surr
1/T
At constant pressure:
The Math
Given:
@constant P:
Multiply by
T:
Result:
Reactions and
G
G
0
:
Standard Free Energy
Reactants in standard states are
converted to products in standard states
Gibb’s Free Energy
Overall criterion for spontaneity
from the standpoint of the system
A process at constant temp. and pressure is
spontaneous in the direction
G decreases
G =
䠠

T
S
H
S
G
Spontaneous?
“Good”:
H < 0
“Good”:
S > 0
“Good”:
G < 0
“Good”:
G < 0

+

At all
temperatures


?
At low
temperatures
+
+
?
At high
temperatures
+

+
Not at any
temperature
Summary
G < 0
Spontaneous as written
G > 0
Not spontaneous as written
Reverse process spontaneous
G = 0
At equilibrium
A Closer Look…
T
区
energy not avail. for doing work
G:
E avail. as heat
–
E not avail. for work
max. work available (constant T and P)
Amount of work actually obtained depends on path
G and Work
G
Spontaneous
max. work obtainable
Nonspontaneous
min. work required
Work and path

dependence
w
max
(
w
min
)
process performed reversibly
theoretical
w
actual
<
w
max
performed irreversibly
real world
Reversible vs. Irreversible Processes
Reversible:
The universe is exactly the same as it was before
the cyclic process.
Irreversible:
The universe is different after the cyclic process.
All real processes are irreversible.
Some work is changed to heat.
Free Energy and Pressure
Q:
reaction quotient from mass action law
Free Energy and Equilibrium
K:
equilibrium constant
At
equilibrium:
G = 0
K = Q
A B
G and Extent of Reaction
A B
G
0
B
<
G
0
A
Spontaneous
C D
G
0
D
>
G
0
C
Nonspontaneous
Temperature Dependence of K
Plot lnK vs. 1/T
slope =

H
0
/R
intercept =
S
0
/R
*assumes
H
0
,
S
0
relatively T independent
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