Section 13.1
Describing the Properties of Gases
Objectives
1. To learn about atmospheric pressure and how barometers
work
2. To learn the units of pressure
3. To understand how the pressure and volume of a gas are
related
4. To do calculations involving Boyle’s Law
5. To learn about absolute zero
6. To understand how the volume and temperature of a gas
are related
7. To do calculations involving Charles’s Law
8. To understand how the volume and number of moles of a
gas are related
9. To do calculations involving Avogadro’s Law
Section 13.1
Describing the Properties of Gases
A. Pressure
Measuring Pressure
• Barometer – device that
measures atmospheric
pressure
– Invented by Evangelista
Torricelli in 1643
Section 13.1
Describing the Properties of Gases
A. Pressure
Atmospheric Pressure
– Changing weather conditions
Section 13.1
Describing the Properties of Gases
A. Pressure
Atmospheric Pressure
– Changing altitude
Section 13.1
Describing the Properties of Gases
A. Pressure
Units of Pressure
1 standard atmosphere
= 1.000 atm
= 760.0 mm Hg
= 760.0 torr
= 101,325 Pa
Section 13.1
Describing the Properties of Gases
A. Pressure
Units of Pressure
• A manometer
measures the
pressure of a gas
in a container.
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
• Robert Boyle’s experiment
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
• Graphing Boyle’s results
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
• This graph has the shape of half of a hyperbola with an
equation
PV = k
• Volume and pressure are inversely proportional.
– If one increases the other decreases.
Section 13.1
Describing the Properties of Gases
B. Pressure and Volume: Boyle’s Law
Another way of stating Boyle’s Law is
P1V1 = P2V2
(constant temperature and amount of gas)
Section 13.1
Describing the Properties of Gases
C. Volume and Temperature: Charles’s Law
• Graphing data for several gases
Section 13.1
Describing the Properties of Gases
C. Volume and Temperature: Charles’s Law
• It is easier to write an equation for the relationship if the
lines intersect the origin of the graph.
– Use absolute zero for the temperature
Section 13.1
Describing the Properties of Gases
C. Volume and Temperature: Charles’s Law
• These graphs are lines with an equation
V = bT (where T is in kelvins)
• Volume and temperature are directly proportional.
– If one increases the other increases.
• Another way of stating Charles’s Law is
V1 = V2
T1
T2
(constant pressure and amount of gas)
Section 13.1
Describing the Properties of Gases
D. Volume and Moles: Avogadro’s Law
Section 13.1
Describing the Properties of Gases
D. Volume and Moles: Avogadro’s Law
• Volume and moles are directly proportional.
– If one increases the other increases.
– V = an
– constant temperature and pressure
• Another way of stating Avogadro’s Law is
V1 = V2
n1
n2
(constant temperature and pressure)
Section 13.2
Using Gas Laws to Solve Problems
Objectives
1. To understand the ideal gas law and use it in calculations
2. To understand the relationship between the partial and total
pressure of a gas mixture
3. To do calculations involving Dalton’s law of partial
pressures
4. To understand the molar volume of an ideal gas
5. To learn the definition of STP
6. To do stoichiometry calculations using the ideal gas law
Section 13.2
Using Gas Laws to Solve Problems
A. The Ideal Gas Law
• Boyle’s Law
• Charles’s Law
• Avogadro’s Law
V = k (at constant T and n)
P
V = bT (at constant P and n)
V = an (at constant T and P)
We can combine these equations to get
Section 13.2
Using Gas Laws to Solve Problems
A. The Ideal Gas Law
Rearranging the equation gives the ideal gas law
PV = nRT
R = 0.08206 L atm
mol K
Section 13.2
Using Gas Laws to Solve Problems
B. Dalton’s Law of Partial Pressures
• What happens to the pressure of a gas as we mix different
gases in the container?
Section 13.2
Using Gas Laws to Solve Problems
B. Dalton’s Law of Partial Pressures
Dalton’s law of partial pressures
• For a mixtures of gases in a container, the total pressure
exerted is the sum of the partial pressures of the gases
present.
• Ptotal = P1 + P2 + P3
Section 13.2
Using Gas Laws to Solve Problems
B. Dalton’s Law of Partial Pressures
• The pressure of the gas is affected by the number of
particles.
• The pressure is independent of the nature of the particles.
Section 13.2
Using Gas Laws to Solve Problems
B. Dalton’s Law of Partial Pressures
Two crucial things we learn from this are:
• The volume of the individual particles is not very important.
• The forces among the particles must not be very important.
Section 13.2
Using Gas Laws to Solve Problems
B. Dalton’s Law of Partial Pressures
Collecting a gas over water
• Total pressure is the pressure of the gas + the vapor
pressure of the water.
Section 13.2
Using Gas Laws to Solve Problems
B. Dalton’s Law of Partial Pressures
Collecting a gas over water
• How can we find the pressure
of the gas collected alone?
Section 13.2
Using Gas Laws to Solve Problems
C. Gas Stoichiometry
Molar Volume
• Standard temperature and pressure (STP)
– 0oC and 1 atm
• For one mole of a gas at STP
• Molar volume of an ideal gas at STP
22.4 L
Section 13.3
Using a Model to Describe Gases
Objectives
1. To understand the relationship between laws and models
(theories)
2. To understand the postulates of the kinetic molecular
theory
3. To understand temperature
4. To learn how the kinetic molecular theory explains the gas
laws
5. To describe the properties of real gases
Section 13.3
Using a Model to Describe Gases
A. Laws and Models: A Review
Section 13.3
Using a Model to Describe Gases
A. Laws and Models: A Review
• A model can never be proved absolutely true.
• A model is an approximation and is destined to be modified.
Section 13.3
Using a Model to Describe Gases
B. The Kinetic Molecular Theory of Gases
Section 13.3
Using a Model to Describe Gases
C. The Implications of the Kinetic Molecular Theory
• Meaning of temperature – Kelvin temperature is directly
proportional to the average kinetic energy of the gas
particles
• Relationship between Pressure and Temperature – gas
pressure increases as the temperature increases because
the particles speed up
• Relationship between Volume and Temperature – volume of
a gas increases with temperature because the particles
speed up
Section 13.3
Using a Model to Describe Gases
D. Real Gases
• Gases do not behave ideally under conditions of high
pressure and low temperature.
• Why?
Section 13.3
Using a Model to Describe Gases
D. Real Gases
• At high pressure the volume is decreased
– Molecule volumes become important
– Attractions become important