Physical Characteristics of Gases

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Physical Characteristics of Gases

• Chapter 10

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Section 1

• The Kinetic-Molecular
• Theory of Matter

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Section 1 Kinetic-Molecular Theory of Matter

• Kinetic-molecular theory is based on
• All matter is made of particles that are in
  constant motion.
• Is used to explain properties of solids, liquids,
  gases.

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Section 1 Kinetic-Molecular Theory of Matter

• Solids, liquids, and gases vary due to the energy
  of the particles and the forces that act upon
  them.
• This chapter will study gases.

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Section 1 Kinetic-Molecular Theory of Matter

• Theory pertains to ideal gases - not real gases.
• An ideal gas is an imaginary gas that perfectly
  fits all the assumptions of the k-m theory.

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Section 1 Kinetic-Molecular Theory of Matter

• 5 assumptions
• 1.Gases consist of large numbers of tiny
  particles that are far apart relative to their
  size
• Much farther apart than liquids solids so they
  can be compressed.

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Section 1 Kinetic-Molecular Theory of Matter

• 5 assumptions (continued)
• 2.Collisions are elastic - there is no net loss
  of kinetic energy. Kinetic energy is completely
  transferred during collisions.

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Section 1 Kinetic-Molecular Theory of Matter

• 5 assumptions (continued)
• Kinetic energy is constant (if at same
  temperature)

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Section 1 Kinetic-Molecular Theory of Matter

• 5 assumptions (continued)
• 3.Gas particles are in constant, rapid, random
  motion. They have kinetic energy.

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Section 1 Kinetic-Molecular Theory of Matter

• 5 assumptions (continued)
• 4. No forces of attraction act on gas particles.

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Section 1 Kinetic-Molecular Theory of Matter

• 5 assumptions (continued)
• The average kinetic energy of gas particles
  depends on the temperature of the gas.
• KE mv2
• 2

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Section 1 Kinetic-Molecular Theory of Matter

• Consider KE mv2
• 2
• What does KE depend on if gases are the same
  kind?
• What does KE depend on if gases are at same
  temperature but are different kinds of gases?

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Physical Properties of Gases

• Expansion Completely fill container take its
  shape

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Physical Properties of Gases

• Fluidity Gases have the ability to flow.
  Particles can glide past each other.

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Physical Properties of Gases

• Low density Gas densities are about 1/1000 that
  of the liquid or solid phase.
• Compressibility Steel canisters contain about
  100 times the number of particles than at normal
  pressure.

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Physical Properties of Gases

• Diffusion Gases randomly move and mix by
  random motion of their particles.
• The rate of diffusion depends on the mass of the
  particles. Heavier ones move more slowly.

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• Animation of diffusion

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Physical Properties of Gases

• Effusion Gases under pressure spread out when
  released from a small opening.

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Deviations of Real Gases

• Real gases do not behave according to all the
  assumptions of the K-M theory.
• Real gases deviate most when they are under very
  high pressures and very cold temperatures.

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Reason behind this

• Both high pressure and colder temperatures force
  the atoms or molecules of a gas closer together.
• When real gases get closer together they
  experience intermolecular attractions and then
  they condense to form liquids.

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Deviations of Real Gases

• Noble gases behave more like ideal gases than any
  others.
• More polar gases behave less like ideal gases.

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Section 2

• Pressure

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Section 2 Pressure

• To describe a gas you must state 4 quantities
• Volume
• Temperature
• Number of molecules
• Pressure

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Section 2 Pressure

• Pressure is defined as the amount of force per
  unit of area.
• P force
• area

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Section 2 Pressure

• P force
• area
• Force unit is Newtons
• Area unit is square meters
• Pressure N/m2

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Section 2 Pressure

Open can
Closed Can Air Pumped Out
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Measuring Pressure

• What tool do we use to measure atmospheric
  pressure?
• Barometer!
• First built by Evangelista Torricelli (1600s)

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Measuring Pressure

• Torricelli noticed that pumps could raise water
  only 34 feet high.
• He compared density of mercury to density of
  water (14x greater)
• Predicted height that mercury could be raised
  (1/14 of 34 ft or about 30 inches).

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Units of Pressure

• Several different units are used for pressure
• inches of mercury (ex. 30.4 and rising)
• mm of Hg
• atm (atmospheres)
• torrs
• Kilopascals (1 Kpa 1N/m2)

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Units of Pressure

• Standard pressure taken at sea level
• 29.9 inches of mercury
• 760 mm of Hg
• 1 atm (atmospheres)
• 760 torrs
• 101.3 Kilopascals

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Section 3

• The Gas Laws
• Mathematical relationships between volume,
  temperature, pressure and quantity of gases

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Section 3 The Gas Laws

• Boyles Law
• Charles Law
• Daltons Law of Partial Pressures
• Gay-Lussacs Law
• Combined Gas Law

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Section 3 The Gas Laws

• Boyles Law
• Relates gas volume to pressure
• Has an inverse relationship
• P ? V ?

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Section 3 The Gas Laws

• Formula for Boyles Law
• P1V1 P2V2
• What would be a gass volume if the pressure
  reduced from 98 kPa down to 60 kPa if its
  original volume was 300 Liters?

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• Simulation of Boyle's Law

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Section 3 The Gas Laws

• P1V1 P2V2
• 98 (300) 60 (V2)
• V2 490 L

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Section 3 The Gas Laws

• Charles Law
• Relates temperature to gas volume.
• Directly proportional.
• Is based on absolute zero.

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Section 3 The Gas Laws

• In 1787, Jacques Charles found that as he
  decreased the temperature of a gas 1 degree then
  the volume decreased 1/273.

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Section 3 The Gas Laws

• The volume of a fixed mass of gas at constant
  pressure varies directly with the Kelvin
  temperature.

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Section 3 The Gas Laws

• Formula for Charles Law (temperature must be in
  Kelvin)
• V1 V2
• T1 T2

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Charles Law Graph
V
T
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Problem

• If the temperature of a gas increases from 25
  degrees Celsius up to 80 degree Celsius and the
  original volume was 10 liters of gas, then what
  would the final volume be????
• 10 X
• (25 273) (80 273)

X 11.8 liters
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Gay-Lussacs Law

• Relates pressure and temperature (assumes
  constant volume)

P1 P2 T1 T2
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Combined Gas Law

• This law is used when 2 variables change
  pressure, temperature, or volume.
• P1V1 P2V2
• T1 T2

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Daltons Law of Partial Pressure

• If there is no chemical reaction occurring then
  the pressure of a mixture of gases will be equal
  to all the combined pressures of each gas.
• Equation
• PT P1 P2 P3

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Collecting Gases by Water Displacement

• Water is commonly collected by bubbling it
  through water.
• The pressure then in the container is a
  combination of both the pressure of the gas as
  well as a small amount of water vapor.

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Gas Collection
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Collecting Gases by Water Displacement

• To determine the pressure of the gas you must
  subtract the pressure of the water vapor.
• The pressure of the water varies according to the
  temperature.
• Use the chart for H2O pressure

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Vapor Pressure of Water
Temp. (0C) Vapor Pressure (torr) Temp (0C) Vapor Pressure (torr)
18 15.5 24 22.4
19 16.5 25 23.8
20 17.5 26 25.2
21 18.6 27 26.7
22 19.8 28 28.3
23 21.2 29 30.3

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Sample Problem

• Oxygen was collected under water at 20C. If the
  combined pressure was 731 torr, then what is the
  pressure of the gas within?
• H2O pressure at 20 C 17.5 torr
• Total press 731
• Minus H2O - 17.5
• Gas press 713.5 torr

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Assignment

• Page 329
• questions 39-43