Study Guides
(238,527)

United States
(119,828)

University of Texas at Austin
(3,083)

Chemistry
(139)

CH 301
(62)

Dave A.Laude
(18)

Quiz

# CH 301 Quiz #4 Notes.docx

Unlock Document

University of Texas at Austin

Chemistry

CH 301

Dave A.Laude

Fall

Description

Quiz #4 Notes
11. Theory: Statistical Thermodynamics
This study deals with the system of a molecule one at a time.
To find internal energy:
E = 1/2kT
o Energy per molecule per degree of motion freedom in a system
To find internal entropy:
S = k ln W
o Absolute entropy per molecul-23n a system
Constant k (Boltazmann constant) = 1.38 x 10 J/K for both equations
12.Calculation: Statistical Mechanics Determination of Internal Energy
Each atom has three directions to move (N atoms will have 3N ways to move)
Translation – the molecule change location
o Ex: water moving to the left because all 3 atoms move left = 3 ways all molecules
can move
Rotation – the molecule spins
o Ex: water spinning counterclockwise
Nonlinear molecules rotate 3 ways
Linear molecules rotate 2 ways
Vibration – the molecule twists
o Ex: water vibrating
Nonlinear molecules vibrate 3N – 6 ways
6 comes from 3 transitions + 3 rotations
Linear molecules vibrate 3N – 5 ways
5 comes from 3 transitions + 2 rotations
The amount of energy associated with each movement depends on temperature.
From equipartition theory
o The average energy per motion is E = 1/2kT
T = Kelvin
K = Boltzmann constant
So, the
o Translation of a molecule are 3 x 1/2kT = 3/2kT
o Rotation of a molecule are 3 x 1/2kT = 3/2kT
o Rotation of a linear molecule are 2 x 1/2kT = 2/2kT
Ex: non-linear molecule (most molecules) have
o 3/2kT + 3/2kT = 3kT of motional energy at room temperature
Ex 2: linear molecule (like CO or I ) has
2 2
o 3/2kT + 2/2kT = 5/2kT of motional energy at room temperature
What about vibration? At room temp, there isn’t enough energy to excite vibration so we
ignore them.
13.Calculation: Statistical Mechanics Determination of Entropy
As temperature K decreases, the motional energy of matter J decreases. [S decreases as well]
This is how we form perfectly ordered crystals.
S (absolute entropy) = k ln W
W = # of possible orientation = # of orientations molecules Quiz #4 Notes
Ex: CO
C = O O = C VS. C = O C = O
C = O O = C C = O C = O
2 ways to orient and four molecules of CO
There are 2 = 16 possible orientations.
-23 -23
S = k ln W = 1.38 x 10 ln 16 = 3.8 x 10 J/K for CO
H-bonding affects orientation
Ex: HCl has W <2 n
14.Theory: First Law of Thermodynamics
First law: the total energy of an isolated system is conserved.
∆Esys= q + w
q and w aren’

More
Less
Related notes for CH 301