Gases and Kinetic Molecular
Theory: Chapter 12
Chem 101
Fall 2004
Chapter Outline
• Comparison of Solids, Liquids, and Gases
• Composition of the Atmosphere and Some Common
Properties of Gases
• Pressure
• Boyle’s Law: The Volume-Pressure Relationship
• Charles’ Law: The Volume-Temperature Relationship;
The Absolute Temperature Scale
• Standard Temperature and Pressure
• The Combined Gas Law Equation
Chem 101
Fall 2004
Chapter Outline
• Avogadro’s Law and the Standard Molar Volume
• Summary of Gas Laws: The Ideal Gas Equation
• Determination of Molecular Weights and Molecular
Formulas of Gaseous Substances
• Dalton’s Law of Partial Pressures
• Mass-Volume Relationships in Reactions Involving Gases
• The Kinetic-Molecular Theory
• Diffusion and Effusion of Gases
• Real Gases: Deviations from Ideality
Chem 101
Fall 2004
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Diffusion and Effusion of Gases
• Diffusion is the intermingling of gases.
• Effusion is the escape of gases through tiny holes.
Chem 101
Fall 2004
Diffusion and Effusion of Gases
• The rate of effusion is inversely proportional to
the square roots of the molecular weights or
densities.
R1
=
R2
M2
M1
or
R1
=
R2
D2
D1
Chem 101
Fall 2004
Diffusion and Effusion of Gases
• Calculate the ratio of the rate of effusion of He to that
of sulfur dioxide, SO2, at the same temperature and
pressure.
R He
=
R SO 2
=
M SO 2
M He
641
. g / mol
4.0 g / mol
= 16 = 4 ∴ R He = 4 R SO 2
Chem 101
Fall 2004
2
Real Gases: Deviations from Ideality
• Real gases behave ideally at ordinary temperatures
and pressures.
• At low temperatures and high pressures real gases
do not behave ideally.
• The reasons for the deviations from ideality are:
• The molecules are very close to one another, thus their
volume is important.
• The molecular interactions also become important.
Chem 101
Fall 2004
Real Gases: Deviations from Ideality
• van der Waals’ equation accounts for the behavior
of real gases at low temperatures and high
pressures.

n 2a
P +
 ( V − nb ) = nRT
V2 

Chem 101
Fall 2004
Real Gases: Deviations from Ideality
• The van der Waals constants a and b take into
account two things:
• a accounts for intermolecular attraction
• b accounts for volume of gas molecules
• At large volumes a and b are relatively small and
van der Waal’s equation reduces to ideal gas law
at high temperatures and low pressures.
Chem 101
Fall 2004
3
Real Gases: Deviations from Ideality
• What are the intermolecular forces in gases that
cause them to deviate from ideality?
• For nonpolar gases the attractive forces are
London Forces
• For polar gases the attractive forces are dipoledipole attractions or hydrogen bonds.
Chem 101
Fall 2004
Real Gases: Deviations from Ideality
• Calculate the pressure exerted by 84.0 g of
ammonia, NH3, in a 5.00 L container at 200. oC
using the ideal gas law.
Chem 101
Fall 2004
Real Gases: Deviations from Ideality
• Solve the previous example using the van der
Waal’s equation.
Chem 101
Fall 2004
4
Real Gases: Deviations from Ideality
• Solve the previous example using the van der
Waal’s equation.
n = 4.94 mol
a = 4.17
L2 atm
b = 0.0371 L
mol
mol 2

n 2a 
 P + 2  (V - nb ) = nRT ∴
V 

nRT
n 2a
P=
−
V - nb V 2
Chem 101
Fall 2004
Next Class: Chapter 13:
Liquids and Solids
• Start Reading Chapter 13
Chem 101
Fall 2004
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