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Physical Chemistry III01403343
Statistical Mechanics
Piti TreesukolChemistry Department
Faculty of Liberal Arts and Science
Kasetsart University : Kamphaeng Saen Campus
Chem:KU-KPSPiti Treesukol
ระบบ คื�ออะไรสภาวะของระบบ การเปลี่��ยนแปลี่ง
คืวามเสถี�ยร คื�ออะไรระบบที่��เสถี�ยรจะต้�องเป�นอย�างถี�าระบบอย �ในสภาวะที่��เสถี�ยร ม"นจะ
เปลี่��ยนแปลี่งหร�อไม�ในคืวามเป�นจร$ง ระบบที่��เราพบจะอย �
ในสภาวะไหน
2
Chem:KU-KPSPiti Treesukol
3
Introduction
Macroscopic pictureBulk materialThermodynamic & Kinetic properties
Microscopic picture Atom, Molecule, IonPosition, Energy, Momentum
Link between micro- and macro picturesStatistical method
Chem:KU-KPSPiti Treesukol
ประกาศสอบกลี่างภาคื 22 ม�นาคืม 2557 13:00-16:00 น.
สอบปลี่ายภาคื 22 พฤษภาคืม 2557 13:00-16:00 น.
4
Chem:KU-KPSPiti Treesukol
5
Properties
MassTemperaturePressureEnergyConductivityThermodynamic propertiesHeat capacityGibbs free energyEnthalpy
Etc.
Chem:KU-KPSPiti Treesukol
Extensive and Intensive properties
Extensive Properties
Intensive Properties
6
n
iitotal XXAccumulative
Average
n
ii
n
iii
total
m
XmX
Xtotal
X1 X2Xtotal
X1 X2
Chem:KU-KPSPiti Treesukol
Expectation values/Measurabl
es
Internal Propeties TemperatureT = < Ti >Ti (t)
External PropertiesTotal EnergyE = S Ei
Ei (t) 7
Chem:KU-KPSPiti Treesukol
System & Enviroment
8
Systemn, N, T, P, V,
m, etc.
Environment T, P, m
Energy
Mass
Chem:KU-KPSPiti Treesukol
9
Energy of a System
Energy of a macroscopic systemdepends on …
Energy of a microscopic systemdepends on …
A macroscopic system comprises of countless microscopic systems (x1023)
iii HE
iitotal EE
PVTEEE itotal ,,}{
,,,,,, iiimmlni zyxsl
Chem:KU-KPSPiti Treesukol
10
E1, T1 E2, T2
T1 < T2 thenE1 < E2
i
itotal EE i
itotal Tn
T1
j
jjtotal TpT
Chem:KU-KPSPiti Treesukol
State of a System
Macroscopic system!!!System composes of ???State of the system is defined by a few number of macroscopic parameters Systems with the same state may be different from each others
Properties of the system are either Acculative property orAverage property
11
Chem:KU-KPSPiti Treesukol
12
Macroscopic description
can be derived statisticaly from microscopic descriptions of a collection of microscopic systemsDescription on average*Fluctuation of microscopic properties
Microscopic properties depends on a set of parameters of each microscopic system
Macroscopic properties depend on a small set of macroscopic parameters !!!
Chem:KU-KPSPiti Treesukol
Distribution of Molecular StatesMolecules = Workers of a department
Energy level = Salary of each position
13
100,000
50,000
20,000
15,000
10,000
Total Energy / Expense = ?
Population of each level : Configuration = {3,2,0,2,1}
How many configuration is possible if the total energy was fixed?
* Nobody wants high salary (energy) because it has too much stress!!!
Chem:KU-KPSPiti Treesukol
The Distribution of Molecular States
A system composed of N molecules IF Total energy (E) is constant (Equilibrium)
Posible energy state for each molecule (ei)
Molecules in different states (i) possess different energy levels
Total energy E = Ej = (ei ni)Ej is fluctuated due to molecular collision
Constraint: Ej = EThe distribution of energy is the population of a state (there are ni molecules in i energy level){0,1,5,7,1,0}
14
Chem:KU-KPSPiti Treesukol
ExamplesTotal particle (N) = 6
15
{3,1,2,0,0,0} Etotal = 3x0 + 1x2 + 2x4 = 10
{4,0,1,1,0,0} Etotal = 4x0 + 1x4 + 1x6 = 10
{3,0,1,2,0,0} Etotal = 3x0 + 1x4 + 2x6 = 16
Chem:KU-KPSPiti Treesukol
Configuration and WeightsConfiguration
Different configurations have different population of state
Weights
Number of ways in achieved a particular configuration
16
e6
e5
e4
e3
e2
e1Conf.1 Conf. 2 Conf.3 … w.1 w. 2 w.3 …
Conf. 1
e6
e5
e4
e3
e2
e1
Chem:KU-KPSPiti Treesukol
17
Instantaneous ConfigurationPossible energy level (e0,
e1, e2 …)N moleculesn0 molecules in e0 staten1 molecules in e1 state …
The instantaneous configuration is {n0,n1,n2…}
Constraint: n0+n1+n2+… = N# ways to achieve instantaneous conf. (W)
!
!
!!!
!...,,
210210
ii
n
N
nnn
NnnnW
Chem:KU-KPSPiti Treesukol
18
Examples{2,1,1}
{1,0,3,5,10,1}
122
24
!1!1!2
!41,1,2 W
81031.9!1!10!5!3!0!1
!201,10,5,3,0,1 W
Chem:KU-KPSPiti Treesukol
19
Principle of Equal a prioriAll possibilities for the
distribution of energy are equally probable
The populations of states depend on a single parameter, the temperature. If at temperature T, the total energy is 3
Energy levels: 0, 1, 2, 3
3
2
1
0
{0,3,0,0} {1,1,1,0} {2,0,0,1}
3
2
1
0
3
2
1
0
W=1 W=6 W=3
Chem:KU-KPSPiti Treesukol
20
Possible configurations for 5 molecules
State 1 5 4 4 3 3 3 3 2 2 2
State 2 1 2 1 3 1 1 1
State 3 1 1 1 3 1 1 2
State 4 1 1 1 1
State 5 1 1 1
State 6 1 1 5N 5 5 5 5 5 5 5 5 5 5 5 5 5
E 5 6 7 7 812
12 8
11
11
20
17
30
W 1 5 510
20
20
20
10
10
60
120
60 1
Energy of state j = j
Chem:KU-KPSPiti Treesukol
21
The Dominating ConfigurationSome specific
configuration have much greater weights than others
There is a configuration with so great a weight that it overwhelms all the restW is a function of all ni: W(n0, n1, n2 …)
The dominating configuration has the values of ni that lead to a maximum value of W
The number of molecule constraint :
The energy constraint :
Eni
ii Nn
ii
Chem:KU-KPSPiti Treesukol
22
Maximum & Minimum PointF is a function of x : F(x)
Maximum point:
F ’= 0 ; F ’’ < 0Minimum point:
F ’= 0 ; F ’’ > 0
1
2
3
4 5 6
7
8
9
F(x)
x
Chem:KU-KPSPiti Treesukol
23
Maximum & Minimum in 3D
F(x,y)
Chem:KU-KPSPiti Treesukol
24
Configuration is defined by a set of ni, {ni}W depends on a set of ni or {ni}
At a specific condition, several configurations may be possibleThe configuration with greatest
weight (W) will dominate and that configuration can be used to represent the system
Other configurations with less weight is negligible
Weight
Configuration
Greatest weight = Dominating Configuration
Chem:KU-KPSPiti Treesukol
Dominating ConfigurationWeight of each configuration2 energy states
Possible configurations (6 particles) :{0,6}, {1,5}, {2,4}, {3,3}, {4,2}, {5,1}, {6,0} 25
Chem:KU-KPSPiti Treesukol
Dominating ConfigurationWeight of each configuration3 energy states
Possible configurations (10 particles) :{0,0,10}, {0,1,9}, {0,2,8}, … {1,0,9}, {2,0,8}, … {1,1,8} … 26
10 particles 30 particles20 particles
Chem:KU-KPSPiti Treesukol
27
Maximum Value of W{ni}We are looking for the
best set of ni that yields maximum value of ln(W)Maximum W = W{ni,max} Maximum ln W = ln W{ni,max}
{ni,max} = ?
Chem:KU-KPSPiti Treesukol
28
Maximum Value of W{ni }{ni,max} can be determined
by differentiate
ConstraintsTotal particle (N) is constant
Total energy (E) is constant
0ln
ln
ii
i
dnn
WWd
i
iinE 0i
iidndE
i
inN 0i
idndN
Chem:KU-KPSPiti Treesukol
29
Maximum Value of W{ni }Maximum ln(W) plus
Constraints
Method of undetermined multipliers
0ln
ln
ii
i
dnn
WWd
0i
iidn
0i
idn
iii
i
iii
ii
ii
i
dnn
W
dndndnn
WWd
ln
lnln
Chem:KU-KPSPiti Treesukol
30
Stirling’s ApproximationNatural logarithmic of the
weight
Stirling’s ApproximationThe approximation for the weight
iinN
nnnNW
nnn
NnnnW
!ln!ln
!ln!ln!ln!lnln
!!!
!...,,
210
210210
xxxx ln!ln
iii
iiii
nnNN
nnnNNNW
lnln
lnlnln
If x is large
Chem:KU-KPSPiti Treesukol
31
xxxx ln!ln when x is a large number!
1.67%
Chem:KU-KPSPiti Treesukol
32
0ln
0ln
ln
ii
iii
i
n
W
dnn
WWd
N
nNn
n
W ii
i
ln1ln1lnln
0ln ii
N
n ieN
ni
jj
jiji eNenNNen
j
jee 1
j i
jj
ii n
nn
n
NN
n
W lnlnln
1lnln
ln
N
n
NN
n
N
n
NN
iii
1ln
lnln
ln
i
j i
jjj
i
j
j i
jj
n
n
nnn
n
n
n
nn
Eq. 1 is possible if (and only if) …
Eq. 1
ii n
N
NN
n
NN
1ln
ei is relative
energy
Chem:KU-KPSPiti Treesukol
33
The Boltzmann DistributionThe populations in the
configuration of the greatest weight depend on the energy of the state
The fraction of molecules in the state i (pi) is
i
i
i
i
e
e
N
n
Z
e
N
np
ii
i
jj
i
j
i
eg
eZ
The Molecular Partition Function
(Z,q,Q)Sum over energy level (j)
degeneracy
kT
1
Sum over all states (i)
Boltzmann constant = 1.38x10-23 J/K
***
Chem:KU-KPSPiti Treesukol
34
The Molecular Partition Function
An interpretation of the partition functionat very low T ( T0) b ∞
at very high T ( T∞) b 0
The molecular partition function gives an indication of the average number of states that are thermally accessible to a molecule at the temperature of the system
j
jjegZ
kT
1
000
lim0lim gZeTT
i
Ze
TT
i
00lim1lim
Chem:KU-KPSPiti Treesukol
Uniform Energy Levels Equally spaced non-degenerate
energy levels
Finite number
Infinite number
35
x
SS
SxxxxS
xxS
1
1
132
2
e0= 0 e1= e e2= 2e e3= 3e …
e3
e2
e1
e0
e n
i
ieZ
e
eee
eee
eee
eZi
i
1
1
1
132
32
210
Infinite # of energy levels
Finite # of energy levels
Chem:KU-KPSPiti Treesukol
36
What are the possible states of particles at high temperature?High-energy states?Low-energy states?All states?
Chem:KU-KPSPiti Treesukol
37
The Possibility *The possibility of molecules in the state with energy ei (pi)
The possibilities of molecules in the 2-level system
i
i
eeZ
epi
1
Z of infinite # of energy levels*
e
ep
11
ep
1
10
As T the populations of all states (pi’s) are equal.
Chem:KU-KPSPiti Treesukol
38
The possibilities of molecules in the infinite-level system* ep 10
eep 11
22 1 eep
As T the populations of all states are equal.
Chem:KU-KPSPiti Treesukol
39
Temperature
Chem:KU-KPSPiti Treesukol
40
ExamplesVibration of I2 in the ground,
first- and second excited states (Vibrational wavenumber is 214.6 cm-1)
036.1226.207
6.2141
1
cm
cm
kT
hc
1226.207
15.2982,1,0
cmhc
kT
KTandvfor
081.0
229.0
645.0
)1(
2
1
0
p
p
p
eep vv
Relative energy
Chem:KU-KPSPiti Treesukol
41
Approximations and Factorizations
In general, exact analytical expression for partition functions cannot be obtained. Closed approximation expressions to estimate the value of the partition functions are required for each systems
Energy levels of a molecule in a box of length X
,2,18 2
22
nmX
hnEn
08 12
2
1 mX
hE
2
22
81
mX
hnn Relative
energy
Chem:KU-KPSPiti Treesukol
42
Translational Partition Function
Partition function of a molecule in a box of length X
1
12
n
nX eq ,2,1
81
2
22 n
mX
hnn
The translation energy levels are very close together, therefore the sum can be approximated by an integral.
01
1 22
dnedneq nnX
Make substitution: x2=n2be and dn = dx/(be)1/2
Xh
mdxeq x
X
2/1
2
2/12/11
0
2/11 2
2
2
Transitional partition function
Chem:KU-KPSPiti Treesukol
43
When the energy of a molecule arises from several different independent sourcesE = Ex+Ey+Ezq = qxqyqz
A molecule in 3-d box
)()()(,,
Zn
Yn
Xnnnn zyxzyx
zyx
nnn
qqq
eeeqz
Zzn
y
Yyn
x
Xxn
)()()(
XYZh
mq
2/3
2
2
Chem:KU-KPSPiti Treesukol
44
is called the thermal wavelength
The partition function increases with The mass of particle (m3/2)The volume of the container (V)
The temperature (T3/2)
XYZh
mq
2/3
2
2
3
Vq
2/1
2/1
22 mkT
h
mh
Chem:KU-KPSPiti Treesukol
45
ExampleCalculate the translational
partition function of an H2 molecule in 100 cm3 vessel at 25C
About 1026 quantum states are thermally accessible at room temperature
26
311
34
31077.2
1012.7
1000.1
m
mVq
m
KJKkg
Js
mkT
h
11
2/112327
34
2/1
1012.7
2981038.1106605.1016.22
10626.6
2
Chem:KU-KPSPiti Treesukol
46
The Internal Energy and
EntropyThe molecular partition function contains all information needed to calculate the thermodynamic properties of a system of independent particlesq Thermal wave function
The Internal Energy **
i
iinE
i
iie
q
NE
d
dee
i
ii
d
dq
q
NE
ed
d
q
Ne
d
d
q
NE
ii
ii
Boltzmann distribution q
eNn
i
i
Chem:KU-KPSPiti Treesukol
47
Total energy
ei is relative energy (e0=0)E is internal energy relative
to its value at T=0The conventional Internal
Energy (U)
e3
e2
e1
e0
e
Relative energy
3e
2e
e
0
d
dq
q
NnE
iii
A system with N independent molecules • q=q(T,X,Y,Z,…)
EUU )0(
V
q
q
NUU
)0(
V
qNUU
ln
)0(
Only the partition function is required to determine the internal energy relative to its value at T=0.
***
Chem:KU-KPSPiti Treesukol
48
ExampleThe two-level partition function
e
d
d
e
N
d
dq
q
NE 1
1
e
N
e
eNE
11
0
0.1
0.2
0.3
0.4
0.5
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
kT/
E/N
At T = 0 : E 0all are in lower state (e=0)
As T : E ½ Netwo levels become equally populated
Chem:KU-KPSPiti Treesukol
49
The value of bThe internal energy of monatomic ideal gas
For the translational partition function
nRTUq
q
NUU
V
23)0()0(
3
Vq
d
dVV
Vq
VV
4333
1
222
1
2 2/12/12/1
2/1
m
h
m
h
d
d
d
d
32
3
Vq
V
2
3)0(
2
3)0(
3
3 NU
V
VNUU
V
A
A
N
Rk
kTnRT
nN
nRT
N
NnRT
1
2
3
2
3
This result is also true for general cases.
Chem:KU-KPSPiti Treesukol
1 amu = ? g12C 1 mol = 12 g12C 1 atom = 12 amu12C 1 mol = 6.02x1023 atom
1 amu = 1g/6.02x1023 =1.66x10-27 kg
50
Chem:KU-KPSPiti Treesukol
51
Temperature and PopulationsWhen a system is heated,The energy levels are
unchangedThe populations are changed
0 0.2 0.4 0.6 0.8
e10
e9
e8
e7
e6
e5
e4
e3
e2
e1
e0 0 0.2 0.4 0.6 0.8
e10
e9
e8
e7
e6
e5
e4
e3
e2
e1
e0
Increase T
HEAT
2
22)(
81
mX
hnX
n
Chem:KU-KPSPiti Treesukol
52
Volume and PopulationsTranslational energy levels
When work is done on a system, The energy levels are changed
The populations are changed
2
22)(
81
mX
hnX
n
decrease V
e10
e9
e8
e7
e6
e5
e4
e3
e2
e1
e0 0 0.2 0.4 0.6 0.8
e5
e4
e3
e2
e1
e00 0.2 0.4 0.6 0.8
WORK
Chem:KU-KPSPiti Treesukol
53
The Statistical EntropyThe partition function
contains all thermodynamic information.Entropy is related to the disposal of energy
Partition function is a measure of the number of thermally accessible states
Boltzmann formula for the entropy
As T 0, W 1 and S 0
WkS ln ***
Chem:KU-KPSPiti Treesukol
54
A change in internal energy
When the system is heated at constant V, the energy levels do not change.From thermodynamics,
Entropy and Weight
i
iii
iii
ii dndndUdUnUU )0()0(
i
iidndU
iii
iiirev
dnkT
dUdS
dnTdSdqdU
iii
i i
ii
dnkdnn
WkdS
n
W
ln
0ln
Wdkdnn
WkdS i
i i
lnln
qNk
T
UUS
WkS
ln0
ln
Chem:KU-KPSPiti Treesukol
55
Calculating the EntropyCalculate the entropy of N
independent harmonic oscillators for I2 vapor at 25ºCMolecular partition function:
The internal energy:
The entropy:
eq1
1
11)0(
e
N
e
eNq
q
NUU
V
e
eNkS
qNkT
UUS
1ln1
ln)0(
Entropy
0
5
10
15
20
25
30
35
0 1000 2000 3000 4000 5000
T(K)
S(J
K-1
mo
l-1)
Chem:KU-KPSPiti Treesukol
56
Entropy and Temperature
What do we know from the graph?T increases, S increasesWhat else?
Entropy
0
5
10
15
20
25
30
35
0 1000 2000 3000 4000 5000
T(K)
S(J
K-1
mo
l-1)