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27 Οκτ 2013 (πριν από 3 χρόνια και 11 μήνες)

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Thermodynamics

Thermodynamics

1.
How fast will it occur


Kinetics

2.
How much heat will it give off or absorb


D
H (enthalpy)

3.
Will it create more or less disorder


D
S
(entropy)

4.
Will it occur at all?
-

D
G (Gibb’s Free
Energy)

Questions We Can Ask

Thermodynamics

1.
Spontaneous Rxns


occur without an
outside source of energy

2.
Direction


Rxns are spontaneous in one
direction only (eggs breaking picture)

3.
Temperature has an effect


Ice


W慴ar (spon琮 Above 0
o
C, reverse
not spontaneous)

Spontaneous Reactions

Thermodynamics

4.
Many spontaneous rxns:


1. Exothermic


Give off heat


2. Increase disorder

Spontaneous Reactions

Thermodynamics

1.
Entropy


measure of disorder

2.
More disorder, larger the entropy

3.
D
S+

More disorder



(shuffling a deck of cards)

4.
D
S
-

Less disorder



(ordering a deck of cards)


Entropy

Thermodynamics

Boltzmann

Thermodynamics

1.
States of matter


Solid


Liquid


Gas


Plasma


2.
Dissolving
-

D
S+


NaOH(s)


Na
+
(aq) + OH
-
(aq)

Entropy

Thermodynamics

Thermodynamics

3.
Decomposing


CaCO
3
(s)


CaO(s) + CO
2
(g)

4. Increase in # gas molecules


PCl
5
(g)


PCl
3
(g) + Cl
2
(g)

Entropy

Thermodynamics

Is
D
S is positive or negative for:

1.
H
2
O(l)


H
2
O(g)

2.
Ag
+
(aq) + Cl
-
(aq)


AgCl(s)

3.
2Fe
2
O
3
(s)

4Fe(s) + 3O
2
(g)

4.
CaO(s) + CO
2
(g)


CaCO
3
(s)

5.
N
2
(g) + 3H
2
(g)


2NH
3
(g)

6.
N
2
(g) + O
2
(g)


2NO(g)

Entropy

Thermodynamics

1.
Energy is conserved

2.
Energy cannot be created or destroyed, it only
changes form

3.
D
E = q + w

4.
Battery in a motorized toy car

Chemical


Electrical

Mechanical



Work Done



Some waste heat produced

The First Law

Thermodynamics

1.
Entropy is not conserved

2.
The universe is getting more disordered.

3.
Can only create “local order.”

4. Ex: Cleaning locker


You sweat and
threw a lot of things away. Universe got
more disordered.

The Second Law

Thermodynamics

1.
The entropy of a pure crystal at absolute
zero is zero

2.
Theoretical state of perfect order

3.
Above 0 K


At 0 K


atoms wiggle

no molec.






motion

The Third Law

Thermodynamics

First

Energy is conserved

Second

Entropy is NOT conserved

Third

Pure crystal at 0 K has zero entropy

Laws of Thermodynamics

Which of the following pairs has the higher
entropy?


1.

H
2
O(s) or H
2
O(l)

2.

NaCl(s) or HCl(g)

3.

HCl(g) or Ar(g)

4.

N
2
(g) at 78 K or N
2
(g) at 298 K

5.

H
2
(g) or SO
2
(g)

6.

1 mol of HCl(g) or 2 mol HCl(g)

Thermodynamics

1.
Standard Molar Entropies


at 1 atm and 25
o
C (298
K)

2.
Unit


Joule/ mol K

3.
Standard molar entropies of element is not zero




D
H
o
f



S
o


Fe(s)

0


27.2


4. D
S
o
r

=
S
nS
o
prod



S
mS
o
reactants



Calculating Entropy

Thermodynamics

1.
Calculate
D
S
o

for:


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


2NH
3
(g)


(Ans:
-
198.3 J/ mol K, more order)


2. Calculate
D
S
o

for:


Al
2
O
3
(s) + 3H
2
(g)


2Al(s) + 3H
2
O(g)


(Ans: 180.4 J/ mol K, more disorder)

Calculating Entropy

Thermodynamics

1.
Josiah Gibbs


1
st

Ph.D. in science from a U.S.
University (Yale, 1863)

2.
“Free Energy”


Maximum amount of work you
can get from a chemical reaction

3.
D
G < 0

Rxn will occur


D
G = 0

Rxn at equilibrium


D
G > 0

Rxn will
NOT

occur

4.
D
G NEVER tells you how fast a rxn will occur


Gibbs Free Energy

Thermodynamics

Thermodynamics

Thermodynamics

1.
Combustion of Methane


CH
4

+ O
2







D
G (
-
)







CO
2

+ H
2
O



Reaction position

Gibbs Free Energy

D
G
o
r

=
S
n
D
G
f
o
prod



S
m
D
G
f
o
rxts


1.
Will the following rxn occur?


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


2NH
3
(g)


(Ans:
D
G
o
r

=
-
33.32 kJ)


2.
Will the following rxn occur?


CH
4
(g) + 2O
2
(g)


CO
2
(g) + 2H
2
O(g)


(Ans:
D
G
o
r

=
-
800.7 kJ)

Thermodynamics

D
G =
D
H


T
D
S


T must be in Kelvin


1.
Use
D
H and
D
S to determine if the following reaction
occurs spontaneously at 500
o
C.


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


2NH
3
(g)

(Ans:
D
G = 61 kJ)


2. At what temperature will it become spontaneous?

Gibbs Free Energy

Thermodynamics

3. Use
D
H and
D
S to determine if the following
reaction occurs spontaneously at 25
o
C?



2SO
2
(g) + O
2
(g)


2SO
3
(g)


(Ans:
D
G =
-
140.1 kJ)

4. At what temperature will it become non
-
spontaneous?

Gibbs Free Energy

Thermodynamics

D
G =
D
H


T
D
S



D
H

D
S

D
G

-

+

Always spontaneous

-

-

Spont at low temps

+

+

Spont at high temps

+

-

Never spontaneous

Gibbs Free Energy

Thermodynamics

1.
A certain rxn is exothermic and becomes
more ordered. Will the rxn occur at any
temperature?

2.
A certain rxn is endothermic and becomes
less ordered. Will it occur?

Gibbs Free Energy

Thermodynamics

3. A certain rxn is exothermic and occurs at
any temperature. What does this reveal?


Gibbs Free Energy

A solution of sodium chloride is added to a
solution of silver(I)nitrate.

a.
Write the net ionic reaction

b.
Predict and explain the sign of
D
G (a reaction
does

occur)

c.
Predict and explain the sign of
D
S

d.
Predict and explain the sign of
D
H

e.
High
or low temperature spontaneity?

Thermodynamics

D
H


Is heat produced or must heat a rxn


DH
o
r

=
S
n
DH
o
prod



S
m
DH
o
rxts

D
S


more or less order


D
S
o
r

=
S
n
D
S
o
prod



S
m
D
S
o
rxts

Overview

Thermodynamics

D
G


Tells you whether a rxn will occur
spontaneously


Considers
D
H,
D
S and temperature




D
G =
D



T
D
S


Does NOT tell you speed

Overview

Thermodynamics

Free Energy and K

D
G = 0 at equilibrium

D
G =
D
G
o
+ RT lnQ


=
D
G
o
+ RT lnK

D
G
o

=
-
RT lnK

or

K = e
-
D
G/RT


D
G
o

negative

K>1

D
G
o

zero


K=1

D
G
o

positive

K<1



Thermodynamics

Calculate the value of K for the following equation
if
D
G
o

=
-
33.32 kJ
:

N
2
(g) + 3H
2
(g)


2NH
3
(g)


D
G
o

=
-
RT lnK

lnK =
D
G
o

=

-
33,320 J



-
RT


(
-
8.314 J/mol
-
K)(298K)

lnK = 13.4

K = e
13.4

= 6.6 X 10
5


Example 3

Thermodynamics

Calculate
D
G
o

and K for the following reaction at
298 K.

H
2
(g) + Br
2
(g)


2HBr(g)






ANS:
-
109.6 kJ/mol, 1.6 X 10
19


Example 4

Thermodynamics

Free Energy and K

D
G
o

is at 298 K (25
o
C)

Can calculate
D
G at other temperatures using:


D
G =
D
G
o
+ RT lnQ


D
G
o
= Free energy change at 25
o
C

R = 8.314 J/mol
-
K

T = Temperature in Kelvin (Absolute temperature)

Q = Reaction Coefficient

Thermodynamics

Calculate the
D
G at 298 K for a reaction mixture of
1.0 atm of N
2
, 3.0 atm of H
2
, and 0.50 atm of
NH
3
.


N
2
(g) + 3H
2
(g)


2NH
3
(g)


Example 1

Thermodynamics

Q =

[NH
3
]
2


=

(0.50)
2

=

0.0093



[N
2
][H
2
]
3



(1.0)(3.0)
3

D
G
o

=
-
33.32 kJ


D
G =
D
G
o
+ RT lnQ

D
G =
-
33,320 J

+ (8.3.14 J/mol
-
K)(298 K)(ln0.0093)

D
G =
-

44.9 kJ/mol




(larger driving force for making NH
3
)

Thermodynamics

Calculate the
D
G at 298 K for a reaction mixture of
0.50 atm of N
2
, 0.75 atm of H
2
, and 2.0 atm of
NH
3
.






(ANS:
-
26.0 kJ/mol)

Example 2

40.
a) As
4



b) 1 mol H
2
O(g)


c) 0.5 mol CH
4

d) 100 g Na
2
SO
4
(aq)

42.
a)
D
S


(fewer moles gas)


b)
D
S+ (more moles gas)


c)
D
S
-

(fewer moles gas)


d)
D
S


(formation of solid)

50.
a) 15.9


b)
-
147.2

c)
-
207.2

d)
-
221.5

54. a)
D
H
-


b)
D
S+

c)
D
G =
-
32.2 kJ (spon)

56.
a)
-
2116.4 kJ

b) 218.1 kJ c)
-
895.0 kJ


d)
-
240.6 kJ

58.
a)
-
190.5 kJ

b) 69.1 kJ c) 192.7 kJ


d) 40.8 kJ


62. a)
-
795 kJ

b) +519 kJ, 3200K

74.
a)
D
G becomes more negative, more spon


b)
D
G becomes more positive, less spon


c)
D
G becomes more negative, more spon

76. a)
D
G
o

= 29.2 kJ


b)
D
G =
-
9.3 kJ

78.
a) K = 2 X 10
-
14


b) K = 1.6 X 10
14


c) K = 3.5 X 10
24