Gases

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Chapter 5

• Gases

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What are some things you remember about gases and
their behavior?
List, Pass, Amass
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Elemental States at 25oC
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Stuff You Already Know
State Property Solid
Liquid Gas Density S
hape Compressibility Thermal expansion

High High Low
Fixed Takes shape Expands
of lower part to fill the of
container container
Small Small Large
Very Small Moderate Small
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More You Remember

• In this state, the particles have sufficient
  energy to overcome all forces that attract them
  to each other.
• Each particle is completely
  separated from the others.
• This results in low densities and the fact that
  gases completely fill the container that holds
  them.

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Gas Pressure

• Gases exhibit pressure on the containers that
  hold them.
• Pressure Force / Area
• Units N/m2 a.k.a. Pascal

Standard Pressures
760 mmHg 1 atm 760 torr 1.01x105 Pa
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Manometer measures air pressure of a gas in a
contained system.
vacuum
Barometer measures ambient air pressure.
One atm
760 mmHg 29.9 inHg
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Boyles Law

• P1V1 P2V2
• Pressure and volume are inversely proportional.
• Examine the graph of the data

Derivative!
Data!
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Charles Law

• V1/T1 V2/T2
• Volume and Temperature are directly proportional.
• Check out data

All gases intersect the x axis at a temperature
of 0 Kelvin(-273 oC). At temperatures lower
than 0 Kelvin, gases would have a negative volume
(impossible, as is negative Kelvin degrees)
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Avogadros Law

• P1/n1 P2/n2

Guy-Lussacs Law
P1/T1 P2/T2 Also, at constant pressure
and temperature, the volumes of gases involved
in a chemical reaction are related by small
whole numbers. (ie 2 volumes of hydrogen and
1 volume of oxygen combine to form 2 volumes
of waterwe can use volume and moles similarly
when it comes to gas stoichiometry.)
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Ideal Gas Law

• PV nRT
• R 0.0821 Latm/Kmol
• When you hold variables constant, this equation
  can be manipulated into one of the others that
  we just talked about.
• Since ideal gases dont exist, this equation
  expresses the behavior a real gas approaches as
  pressure is lowered and temperature is increased.

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Gas Stoichiometry

• At STP, the molar volume of a gas is 22.4 L /
  mole

S.T.P. Temperature 273.15 K
(0 oC) Pressure 1 atm
Gas stoichiometry problems can involve a gas law
and/or the molar volume of a gas.
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Demo and Thoughts
Answer Now
Rearrange the ideal gas law to get an equation to
solve for the molar mass of a gas.
CRUSH THE CAN

• What is inside the can?
• Why does this happen?
• Which gas law(s) does it
• illustrate?

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Section 5.5 Daltons Law of Partial Pressure

• For a mixture of gases in a container, the total
  pressure exerted is the sum of the pressures that
  each gas would exert independently.
• Ptotal P1 P2 P3, etc

Air is a mixture of gases Pair PN2 PO2
PAr PCO2 PH2O
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• The law of partial pressures also works for
  moles.
• Ptotal (n1 n2 )(RT/V)
• In a mixture of ideal gases, it is the TOTAL
  NUMBER OF MOLES OF PARTICLES, not the identity of
  the substance, that determines the pressure.

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Partial Pressure Try Me

• Mixtures of helium and oxygen are used in scuba
  diving tanks to help prevent the bends.
• For a particular dive, 46 liters of O2 and 12
  liters of He were pumped in to a 5 liter tank.
  Both gases were added at 1.0 atm pressure at
  25oC.
• Determine the partial pressure for both gases in
  the scuba tank at 25oC. Then, determine the
  total pressure in the tank.

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• P total P He P O2
• P total 9.3 2.4
• P total 11.7 atm

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Mole Fractions

• Another way to determine the partial pressures of
  a gas.
• X n1 P1
• ntotal Ptotal

Chi, mole fraction
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When Gas Is Collected Over Water

• You must include the partial pressure of water in
  your calculations.
• The vapor pressure of water is a constant value
  at each temperature. (you would need to look this
  up on a table or it would be given to you.)

At 22oC, PH2O is 21 torr
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Section 5.6 Effusion and Diffusion

• Effusion occurs when a gas passes through a small
  opening in a barrier.
• Diffusion occurs when two gases mix randomly and
  spontaneously.

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More on Diffusion, Effusion, and speed of
molecules

• The speed of diffusion is much less than the
  theoretical velocity of the gas because of
  COLLISIONS.
• The average speed of a gas that is effusing is
  proportional to the averages speed squared.

R 8.314 J/Kmol
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Even more on velocity of molecules that are
effusing and/or diffusing

• Grahams Law The rates of the gases is
  proportional to the square root of the inverse of
  the molar masses of the gases.

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Kinetic Molecular Theory

• Kinetic Molecular Theory is an attempt to
  describe the behavior of an ideal gas
• The particles are so tiny and spread out that
  their volume is negligible.
• Particles in constant motion collisions w/ walls
  cause pressure.
• Particles exert no intermolecular force.
• Average KE is directly proportional to Kelvin
  temperature.

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Can we explain each gas law using KMT?

• Boyles Law
• Gay-Lussac
• Charles
• Avogadro

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KMT tutorial

• http//www.wwnorton.com/college/chemistry/chemistr
  y3/ch/06/chemtours.aspx
• Click on Chapter 6, Molecular Speed

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Maxwell-Boltzman Distribution Curves

• Seen in wwnorton tutorial.
• Illustrates the number of particles at a given
  velocity and temperature.

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Gases are not Perfect

• Particles DO have mass and volume
• Small attractive forces between particles ARE
  sometimes significant
• Higher concentration of gas, more forces
• Under extreme pressures or at low temperature,
  gases will condense to liq./sol.

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Ideal vs Real behavior
Nitrogen at Various temp.
Various Gas Deviations
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Well, Now What?

• The ideal gas law has to be adjusted to reflect
  REAL gas behavior.

But who would want to do that?!!
I DO!!!
Johannes van der Waals Yes, hes the
intermolecular force guy, and its no coincidence!
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What Johannes Did
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Ideal gas law
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Adjusted for volume of gas
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Adjusted for intermolecular forces
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Glued Together
The Official Equation
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When should you use vanderWaals equation?

• Whenever you want to know the REAL behavior of
  the gas, not just the ideal, limiting value.
• Whenever you are working at low temperatures or
  high pressures
• Whenever you need to employ KMT to explain the
  behavior of a gas AND BE CORRECT ALL THE
  TIME.

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But how do you know the values of a and b?

• They are constants. You can look them up!

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Show Me Problem

• Calculate the pressure exerted by 0.500 mol of
  nitrogen in a 1.00L container at 25.0oC using
• The Ideal Gas Law
• The van der Waals Equation

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Try Me Problem

• Calculate the pressure exerted by 0.250 mol of
  argon in a 1.00L container at 23.0oC using
• The Ideal Gas Law
• The van der Waals Equation

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Atmospheric Chemistry

• Air is a homogeneous mixture!
• The heavier gases exist in the lower atmosphere,
  while lighter gases are present in the upper
  atmosphere.
• Radon gas found in basements because it is very
  heavy.
• Oxygen is less prevalent at the tops of
  mountains.
• Hydrogen is more prevalent as you gain altitude.
• Troposphere (closest to us) is most greatly
  impacted by our every-day routines.

Xe
H2
Kr
He
N2
Ar
H2O
O2
Ne
CO2
CH4
NO
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Causes of Air Pollution and Acid Rain

• Causes
• -Generation of electricity
• Coal burning plants
• -Transportation
• Combustion of petroleum

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Acid Rain Effects on Buildings

• Erosion of the stone
• Blackened streaks on bricks

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Why?

• Limestone (calcium carbonate) reacts with acid
  rain (sulfuric acid)
• CaCO3 H2SO4 ? CaSO4 H2CO3
• H2CO3 ? CO2 gas H2O

What's it all mean?
Calcium sulfate is soluble in water, which means
that the limestone will wash away as the rain
pours over it.
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Chemistry of Acid Rain as caused by burning coal
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Prevention of Acid Rain

• At coal plants
• Injection of limestone and air into
  combustion chamber before products are allowed
  to escape.
• (product becomes CaSO4, which is unusable, but
  non-toxic)

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Air Pollution Effects on People

• Burning eyes
• Trouble breathing
• Headaches
• Fatigue
• Photochemical smog

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Chemistry of Air Pollution as caused by excessive
traffic