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Section 1: Gases
Gases expand, diffuse, exert pressure, and can be compressed
because they are in a low-density state consisting of tiny,
constantly moving particles.
K
What I Know
W
What I Want to Find Out
L
What I Learned
• 9(C) Describe the postulates of kinetic molecular theory.
• 4(C) Compare solids, liquids, and gases in terms of compressibility,
structure, shape, and volume.
• 9(A) Describe and calculate the relations between volume, pressure,
number of moles, and temperature for an ideal gas as described by
Boyle's law, Charles' law, Avogadro's law, Dalton's law of partial
pressure, and the ideal gas law.
• 11(A) Understand energy and its forms, including kinetic, potential,
chemical, and thermal energies.
• 2(H) Organize, analyze, evaluate, make inferences, and predict
trends from data.
Copyright © McGraw-Hill Education
Gases
Essential Questions
• How is the kinetic-molecular theory used to explain the
behavior of gases?
• Why does mass affect the rates of diffusion and effusion?
• How is gas pressure measured and how is the partial pressure
of a gas calculated?
Copyright © McGraw-Hill Education
Gases
Vocabulary
Review
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• kinetic energy
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Copyright © McGraw-Hill Education
kinetic-molecular theory
elastic collision
temperature
diffusion
Graham’s law of effusion
pressure
barometer
pascal
atmosphere
Dalton’s law of partial
pressures
Gases
The Kinetic-Molecular Theory
Kinetic-molecular theory explains the different properties of solids, liquids, and
gases. Atomic composition affects chemical properties. Atomic composition also
affects physical properties. The kinetic-molecular theory describes the behavior
of matter in terms of particles in motion.
Gases consist of small particles
separated by empty space. Gas
particles are too far apart to
experience significant attractive
or repulsive forces. Gas particles
are in constant random motion.
Collisions between gas particles
are elastic. An elastic collision
is one in which no kinetic energy
is lost.
Copyright © McGraw-Hill Education
Gases
The Kinetic-Molecular Theory
Kinetic energy of a particle depends on mass and velocity.
Temperature is a measure of the average kinetic energy of the particles in a
sample of matter.
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Gases
Explaining the Behavior of Gases
Great amounts of space exist between gas particles. Compression reduces the
empty spaces between particles.
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Gases
Explaining the Behavior of Gases
Gases easily flow past each other because there are no significant forces of
attraction. Diffusion is the movement of one material through another. Effusion is
a gas escaping through a tiny opening.
Graham’s law of effusion states that the rate of effusion for a gas is inversely
proportional to the square root of its molar mass.
Graham’s law also applies to diffusion.
Copyright © McGraw-Hill Education
Gases
GRAHAM’S LAW
SOLVE FOR THE UNKNOWN
•
Use with Example Problem 1.
RateNH3
=
RateHCl
Problem
Ammonia has a molar mass of 17.0 g/mol;
hydrogen chloride has a molar mass of 36.5
g/mol. What is the ratio of their diffusion
rates?
Response
ANALYZE THE PROBLEM
You are given the molar masses for ammonia
and hydrogen chloride. To find the ratio of the
diffusion rates for ammonia and hydrogen
chloride, use the equation for Graham’s law
of effusion.
KNOWN
UNKNOWN
molar massHCl = 36.5 g/mol
ratio of diffusion
rates = ?
molar massNH3 = 17.0 g/mol
Copyright © McGraw-Hill Education
State the ratio derived from Graham’s
law.
•
molar massHCl
molar massNH3
Substitute molar massHCl = 36.5 g/mol
and molar massNH3 = 17.0 g/mol.
36.5 g/mol
= 1.47
17.0 g/mol
The ratio of diffusion rates is 1.47.
EVALUATE THE ANSWER
A ratio of roughly 1.5 is logical because
molecules of ammonia are about half as
massive as molecules of hydrogen
chloride. Because the molar masses have
three significant figures, the answer also
does. Note that the units cancel, and the
answer is stated correctly without any units.
Gases
Gas Pressure
Pressure is defined as force per unit
area. Gas particles exert pressure
when they collide with the walls of
their container. The particles in the
earth’s atmosphere exert pressure in
all directions called air pressure.
There is less air pressure at high
altitudes because there are fewer
particles present, since the force of
gravity is less.
Torricelli invented the barometer.
Barometers are instruments used to
measure atmospheric air pressure.
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Gases
Gas Pressure
Manometers measure gas pressure in a closed container.
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Gases
Units of Pressure
The SI unit of force is the newton (N). One pascal(Pa) is equal to a force of
one Newton per square meter or N/m2. One atmosphere is equal to 760 mm
Hg or 101.3 kilopascals.
Copyright © McGraw-Hill Education
Gases
Dalton’s Law of Partial Pressure
Dalton’s law of partial pressures states that the total pressure of a mixture
of gases is equal to the sum of the pressures of all the gases of the mixture.
The partial pressure of a gas depends on the number of moles, size of the
container, and temperature and is independent of the type of gas. At a given
temperature and pressure, the partial pressure of 1mol of any gas is the
same. Partial pressure can be used to calculate the amount of gas produced
in a chemical reaction.
Ptotal = P1 + P2 + P3 +...Pn
Copyright © McGraw-Hill Education
Gases
THE PARTIAL PRESSURE OF A GAS
Use with Example Problem 2.
Problem
KNOWN
UNKNOWN
A mixture of oxygen (O2), carbon dioxide
(CO2), and nitrogen (N2) has a total
pressure of 0.97 atm. What is the partial
pressure of O2 if the partial pressure of
CO2 is 0.70 atm and the partial pressure
of N2 is 0.12 atm?
PN2 = 0.12 atm
PO2 = ? atm
PCO2 = 0.70 atm
Ptotal = 0.97 atm
SOLVE FOR THE UNKNOWN
•
Response
ANALYZE THE PROBLEM
You are given the total pressure of a
mixture and the partial pressure of two
gases in the mixture. To find the partial
pressure of the third gas, use the
equation that relates partial pressures to
total pressure.
Copyright © McGraw-Hill Education
State Dalton’s law of partial pressures.
Ptotal = PN2 + PCO2 + PO2
•
Solve for PO2.
PO2 = Ptotal - PCO2 - PN2
•
Substitute PN2 = 0.12 atm, PCO2 = 0.70
atm, and Ptotal = 0.97 atm.
PO2 = 0.15 atm
Gases
The Partial Pressure of a Gas
EVALUATE THE ANSWER
Adding the calculated value for the partial
pressure of oxygen to the known partial
pressures gives the total pressure, 0.97
atm. The answer has two significant
figures to match the data.
Copyright © McGraw-Hill Education
Gases
Review
Essential Questions
• How is the kinetic-molecular theory used to explain the behavior of
gases?
• Why does mass affect the rates of diffusion and effusion?
• How is gas pressure measured and how is the partial pressure of a
gas calculated?
Vocabulary
• kinetic-molecular
theory
• elastic collision
• temperature
• diffusion
Copyright © McGraw-Hill Education
• Graham’s law of
effusion
• pressure
• barometer
• pascal
• atmosphere
• Dalton’s law of
partial pressures
Gases