Chapter 19 Chemical

Chapter 19

Chemical
Thermodynamics

John D. Bookstaver

St. Charles Community College

St. Peters, MO

2006, Prentice Hall, Inc
.

Modified by S.A. Green, 2006

Modified by D. Amuso 2011

Thermodynamics

The study of the relationships between
heat, work, and the energy of a
system.

First Law of Thermodynamics

•
You will recall from Chapter 5 that
energy cannot be created nor
destroyed
.

•
Therefore, the
total energy
of the
universe

is a
constant
.

•
Energy can
, however, be
converted
from
one form to another
or
transferred
from a system to the surroundings or
vice versa.

Second Law of Thermodynamics


The total
Entropy
of the
universe increases

in
any
spontaneous, irreversible
process.


Entropy

•
Entropy

can be thought of as a
measure of the
randomness or disorder
of a system.

•
It is related to the various modes of
motion in molecules (microstates).


Vibrational


Rotational


Translational

Entropy

Molecules have more entropy (disorder)
when:

1)
Phase Changes from: S


L


G


Example: Sublimation


CO
2
(s)


CO
2
(g)

2)
Dissolving occurs (solution forms):


Example:


NaCl
(s)


Na
+
(
aq
) +
Cl
-
(
aq
)



Entropy

3) Temperature increases


Example:


Fe(s) at 0
o
C


Fe(s) at 25
o
C

4) For Gases ONLY, when


Volume increases or Pressure decreases


Examples:


2 Liters He(g)


4 Liters He(g)



3 atm He(g)


1 atm He(g)

Entropy

5) Rx results in more molecules/moles of gas


Examples:


2NH
3
(g)


N
2
(g) + 3H
2
(g)




CaCO
3
(s)


CaO(s) + CO
2
(g)



N
2
O
4
(g)




2 NO
2
(g)



This one is difficult to predict:


N
2
(g) + O
2
(g)


2 NO(g)

Entropy

6)
When there are more moles


Example:


1 mole H
2
O(g)


2 moles H
2
O(g)


7) When there are more atoms per molecule


Examples:


1 mole Ar(g)


1 mole HCl(g)



1 mole NO
2
(g)


1 mole N
2
O
4
(g)




Entropy

8) When an atom has a bigger atomic
number


1 mole He(g)


1 mole Ne (g)





Spontaneous Processes

•
Spontaneous
processes
are those that can
proceed without any
outside intervention
.

•
The gas in vessel
B

will
spontaneously effuse into
vessel
A
, but once the
gas is in both vessels, it
will
not

spontaneously
move to just one vessel.


Spontaneous Processes

•
Processes that are spontaneous at one
temperature may be nonspontaneous at other
temperatures.

•
Above 0

C it is spontaneous for ice to melt.

•
Below 0

C the reverse process is spontaneous.

Spontaneous Processes

Processes that are
spontaneous in one
direction are
nonspontaneous in
the reverse
direction.

Spontaneous
processes are
irreversible
.

Second Law of Thermodynamics


A
reversible

process results in
no
overall change
in
Entropy

while an
irreversible, spontaneous

process
results in an
overall increase

in
Entropy
.

Reversible:

Irreversible (Spontaneous):

Third Law of Thermodynamics


The
Entropy
of a pure crystalline
substance at absolute zero is zero.


why?


Third Law of Thermodynamics



Standard Entropies

•
Standard Conditions:


298 K, 1 atm, 1 Molar


•
The values for Standard
Entropies (S
o
) are
expressed in J/mol
-
K.



•
Note: Increase with
increasing molar mass.

Standard Entropies

Larger and more complex molecules have
greater entropies.

Entropy Changes


D
S
o

=
S
n
S
o
products

-

S
m

S
o
reactants

Be careful: S
°
units are in J/mol
-
K


Note for pure elements:





Gibbs Free Energy

Use
D
G
to decide if a process is spontaneous



D
G = negative value = spontaneous



D
G = zero = at equilibrium


D
G = positive value = not spontaneous



Note: equation can be used w/o the
o
too.


D
G
o

=
D
H
o

–

T
D
S
o


Gibbs Free Energy

1.
If
D
G

is negative


D
G

= maximum amt of energy ‘free’ to do


work by the reaction


2. If
D
G

is positive


D
G

= minimum amt of work needed


to make the reaction happen

D
G
o

=
D
H
o

–

T
D
S
o


Gibbs Free Energy





In our tables, units are:


D
G
o

= kJ/mol


D
H
o


= kJ/mol


D
S
o

= J/mol
-
K


D
G
o

=
D
H
o

–

T
D
S
o


Free Energy and Temperature

•
There are two parts to the free energy
equation:



D
H

—

the enthalpy term



T
D
S


—

the entropy term


•
The temperature dependence of free
energy comes from the entropy term.

What causes a reaction to be
spontaneous?

•
Think Humpty Dumpty

•
System tend to seek:




Minimum Enthalpy


Exothermic Rx,
D
H = negative




Maximum Entropy


More disorder,
D
S = positive

•
Because:
D
G
o

=
D
H
o

–

T
D
S
o



-

= (
-
)
-

(+)

Free Energy and Temperature

By knowing the sign (+ or
-
) of
D
S and
D
H,

we can get the sign of
D
G and determine if a
reaction is spontaneous.

At Equilibrium



D
G
o


= zero Therefore:
D
H
o

=
T
D
S
o




Or
:
D
S
o

=
D
H
o


T


Use this equation when asked to calculate enthalpy of
vaporization or enthalpy of fusion.


D
G
o

=
D
H
o

–

T
D
S
o


D
G
o

=
S
n
G
o
f

products
-

S
m

G
o

f reactants

Standard Free Energy Changes


Be careful: Values for
D
G
f


are in kJ/mol

D
G


can be looked up in tables
or
calculated
from
S
°

and
D
H

.


D
G
o

=
D
H
o

–

T
D
S
o


Free Energy and Equilibrium

Remember from above:

If
D
G

is 0, the system is at equilibrium.



So
D
G

must be related to the equilibrium
constant, K (chapter 15). The
standard
free
energy,
D
G
°
, is directly linked to K
eq

by:





D
G
o

=
-

RT ln K






Where R = 8.314 J/mol
-
K








D
G
o

=
-

RT ln K







If the free energy change is a negative value,


the reaction is spontaneous, ln K must be a
positive value, and K will be a large number
meaning the equilibrium mixture is mainly products.




If the free energy change is zero,


ln K = zero and K = one.




Relationships

Free Energy and Equilibrium

Under non
-
standard conditions, we need to use
D
G
instead of
D
G
°
.







Q is the reaction quotient from chapter 15.