The Kinetic Theory of Gases and Ideal Gas Model
Chapter 2: The Kinetic Theory of Gases
Molecular Model of an ideal gas
Pressure, temperature, and RMS speed
Heat Capacity and Equipartition of Energy
Distribution of Molecular Speeds
Confusing Notations (don't be confused):
N
: number of molecules
n
: number of moles
N
A
: Avogadro's number: 6.02 × 10
23
.
M
: molar mass (how much mass per mole)
m
: mass of “ONE” molecule
m
total
: total mass
p
: pressure
P
: momentum
What is “mole”; and what is “molar mass”?
I have 2.5 dozens of identical coins with total weight of 300 g.
How much weight does one dozen of coins have? How much
weight does one coin have?
I have 2.5 moles of identical molecules with total weight of 300
g. How much weight does one mole of molecules have? How
much weight does one molecule have?
A “dozen” refers to “12” objects.
A “mole” refers to “6.02 x 10
23
“ objects (we use N
A
to represent
6.02 x 10
23
)
Molar mass
Single molecule mass
# of moles
# of dozens
Equation of State
From experiments:
V
is proportional to
n
V
is proportional to 1/
p
p
is proportional to
T
Unit of Pressure
Example 2.1
How to describe the status of a gas?
And what is “ideal gas”?
Ideal gas:
(1) no inter-molecular interactions (each gas
molecule does not feel the others);
(2) no volume (each gas molecule is considered as
a “point mass” particle)
Corrections for non-ideal gas (van der Waals Equation):
p-V
diagram
Ideal Gas
Non-Ideal Gas
What is the origin of the “pressure”?
What is “pressure”?
Where does “Force” come from?
Air molecules hitting/bouncing the wall
Let's look at one molecule:
What is the origin of the “pressure”?
How many molecules hitting the wall within a certain time period?
Molecules with
v
x
within a volume could hit the wall
Numbers of molecules that hit the wall:
Momentum change of one molecule that hit the wall
Sum of momentum changes of all molecules that hit the wall
What is the origin of the “pressure”?
Total force due to the molecule collisions
Pressure on the wall
Average
v
x
vs
v
What is the origin of the “pressure”?
Pressure on the wall
Average kinetic energy of ONE molecule
Total kinetic energy of ALL molecules
Kinetic energy of molecules vs Temperature
Average kinetic energy of one molecule
Total kinetic energy of all molecules
R
o
o
t
-
m
e
a
n
-
s
q
u
a
r
e
s
p
e
e
d
(
r
m
s
s
p
e
e
d
)
Summary
Average kinetic energy of single molecule on depends
on temperature
Root-mean-square speed depends on both temperature
and molecular mass
Example 2.4
(a)
What is the average kinetic energy of a gas
molecule at 20.0 °C?
(b)
Find the rms speed of a nitrogen molecule (N
2
)
at this temperature.
How do we know the specific heat
for ideal gas?
R
e
c
a
l
l
f
r
o
m
C
h
1
:
M
o
l
a
r
h
e
a
t
c
a
p
a
c
i
t
y
There are two types of Molar heat capacity
How do we know the specific heat
for ideal gas?
For ideal gas, if you add heat into the system, where does the
heat go?
Equipartition of Energy principle
Where does this “3” come from?
O
n
e
d
e
g
r
e
e
o
f
f
r
e
e
d
o
m
g
i
v
e
s
o
n
e
e
q
u
i
p
a
r
t
i
t
i
o
n
o
f
e
n
e
r
g
y
What is “degree of freedom”?
x
y
z
x
y
z
x
y
z
dof = 3
dof = 5
Monatomic vs Diatomic molecules
Translation: dof: 3
Rotation: dof: 2
Vibration: dof: 2
What is “degree of freedom”?
Kinetic energy
x
,
y
, and
z
Potential energy
(vibration)
x
,
y
, and
z
dof = 6
In general, the following
could contribute to dof.
K
i
n
e
t
i
c
e
n
e
r
g
y
R
o
t
a
t
i
o
n
a
l
e
n
e
r
g
y
V
i
b
r
a
t
i
o
n
e
n
e
r
g
y
Do all the air molecules move with the same speed?
Speeds
Solve for
v
mp
Explore the Kinetic Theory of Gases, the Molecular Model of Ideal Gases, and concepts like pressure, temperature, and heat capacity. Understand the fundamental properties of gases, including the distribution of molecular speeds and equations that describe gas behavior. Learn about the mole concept, molar mass, and equations of state, while delving into the characteristics of ideal gases and corrections for non-ideal behavior. Discover key principles through examples and explanations.
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Presentation Transcript
Chapter 2: The Kinetic Theory of Gases Molecular Model of an ideal gas Pressure, temperature, and RMS speed Heat Capacity and Equipartition of Energy Distribution of Molecular Speeds
Confusing Notations (don't be confused): N: number of molecules n: number of moles NA: Avogadro's number: 6.02 1023. M: molar mass (how much mass per mole) m: mass of ONE molecule mtotal: total mass p: pressure P: momentum
What is mole; and what is molar mass? I have 2.5 dozens of identical coins with total weight of 300 g. How much weight does one dozen of coins have? How much weight does one coin have? I have 2.5 moles of identical molecules with total weight of 300 g. How much weight does one mole of molecules have? How much weight does one molecule have? # of dozens $total= n$dozen $dozen= 12$1 A dozen refers to 12 objects. A mole refers to 6.02 x 1023 objects (we use NAto represent 6.02 x 1023) # of moles mtotal= nM M= NAm Molar mass Single molecule mass
Equation of State From experiments: V is proportional to n V is proportional to 1/p p is proportional to T ?? = ??? ? = 8.314472 ?/??? ? ? = 0.08206? ??? ??? ? ?? =?????? ?? ?? = ??? ?
Unit of Pressure Standard atmosphere (atm) 9.8692 10 6 Pounds per square inch (psi) 1.450377 10 4 Pascal Bar Torr (Pa) (bar) (Torr) 7.5006 10 3 1 Pa 1 N/m2 10 5 100 kPa 106dyn/cm2 1 bar 105 0.98692 750.06 14.50377 1.01325 105 1 atm 1.01325 1 760 14.69595 1 Torr 1 mmHg 1/760 1.315789 10 3 1.333224 10 3 1.933678 10 2 1 Torr 133.3224 6.8948 103 6.8948 10 2 6.8046 10 2 1 psi 51.71493 1 lbf /in2
Example 2.1 Suppose your bicycle tire is fully inflated, with an absolute pressure of 7.00 105 Pa at a temperature of 18.0 C. What is the pressure after its temperature has risen to 35.0 C on a hot day? Assume there are no appreciable leaks or changes in volume.
How to describe the status of a gas? And what is ideal gas ? Ideal gas: (1) no inter-molecular interactions (each gas molecule does not feel the others); (2) no volume (each gas molecule is considered as a point mass particle) Corrections for non-ideal gas (van der Waals Equation): (p+an2 V2)(V nb)= nRT
p-V diagram Ideal Gas Non-Ideal Gas
What is the origin of the pressure? p=F What is pressure ? A Where does Force come from? Air molecules hitting/bouncing the wall Let's look at one molecule: Fx= Px t
What is the origin of the pressure? How many molecules hitting the wall within a certain time period? Molecules with vx within a volume could hit the wall ? ??= ? ?? ? Numbers of molecules that hit the wall: ? ??=1 2 ? ?? ??=1 ? ?? ?? ? 2 Momentum change of one molecule that hit the wall ??= 2? ?? Sum of momentum changes of all molecules that hit the wall 1 2 ? ?? ?? ? = ? ??2? ??,?????= ? ?? ??= 2? ?? ?? ?
What is the origin of the pressure? Total force due to the molecule collisions ? = ??,????? = ? ??2? ?? ? Pressure on the wall ? =? ?= ? ??2? ? Average vx vs v v2= vx 2+ vy 2+ vz 2 (v2)av=(vx 2)av+(vy 2)av=1 2)av+(vz 2)av= 3(vx 2)av 3(v2)av (vx
What is the origin of the pressure? Pressure on the wall ? = ? ??2? ?= ?1 ?=2 3?2? ? ?=2 ? ? =2 1 2? ?2? ??? ? 3?? 3 3 Total kinetic energy of ALL molecules Average kinetic energy of ONE molecule pV=2 3EkN=2 n=N 3Ktr= nRT NA Ek=3 Ktr=3 k=R 2kT 2nRT NA
Kinetic energy of molecules vs Temperature Ek=3 2kT=1 Ktr=3 2m(v2)av 2nRT= N Ek Total kinetic energy of all molecules Average kinetic energy of one molecule Root-mean-square speed (rms speed) 2=(v2)av vrms ?2 ????= ?? 3 2kT=1 3?? ? 3?? ? 2 2mvrms ????= =
Summary Average kinetic energy of single molecule on depends on temperature Ek=3 2kT Root-mean-square speed depends on both temperature and molecular mass 3?? ? 3?? ? ????= =
Example 2.4 (a)What is the average kinetic energy of a gas molecule at 20.0 C? (b)Find the rms speed of a nitrogen molecule (N2) at this temperature.
How do we know the specific heat for ideal gas? Recall from Ch 1: Molar heat capacity C=1 dQ dT Q= nC T n There are two types of Molar heat capacity CV=1 n(dQ dT) constant volume Cp=1 n(dQ dT) constant pressure
How do we know the specific heat for ideal gas? For ideal gas, if you add heat into the system, where does the heat go? Q= Ktr Ktr=3 Q= nCV T Q= nCV T=3 2nRT 2nR T CV=3 2R
Equipartition of Energy principle Where does this 3 come from? CV=3 2)av=1 3(v2)av 2R (vx Ek=1 2m(v2)av=1 2)av+(vy 2)av+(vz 2)av)= Ek,x+Ek, y+Ek,z 2m((vx One degree of freedom gives one equipartition of energy CV=dof R 2
What is degree of freedom? Monatomic vs Diatomic molecules CV=dof R dof = 5 z 2 z y y x x z dof = 3 CV=3 y CV=5 2R x 2R
Degree of freedom vs ?? Translation: dof: 3 Rotation: dof: 2 Vibration: dof: 2
What is degree of freedom? Kinetic energy x, y, and z dof = 6 CV=6 Potential energy (vibration) x, y, and z 2R In general, the following could contribute to dof. Kinetic energy Rotational energy Vibration energy
Do all the air molecules move with the same speed? vmp vavvrms 3/2 ? ?2? ??2/2?? ? ? = 4? 2??? 3?? ? 2?? ? 8?? ?? ????= ???= ???=
Speeds 3/2 ? ?2? ??2/2?? ? ? = 4? 2??? df (v) dv = 0 Solve for vmp ???= ?? ? ?? 0 ?2? ? ?? ????= 0
