Chapter 19 Chemical

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Oct 27, 2013 (3 years and 8 months ago)

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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.