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Gases - PowerPoint PPT Presentation

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Gases. GASES. manometers. pressure. Kinetic theory of gases. Units of pressure. Behavior of gases. Partial pressure of a gas. Pressure vs. volume. Pressure vs. temperature. Temperature vs. volume. Diffusion/effusion. Combined gas law. Ideal gas law. Properties of Gases.

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Kinetic theory of gases

Units of pressure

Behavior of gases

Partial pressure

of a gas

Pressure vs.


Pressure vs.


Temperature vs.



Combined gas law

Ideal gas law

properties of gases
Properties of Gases
  • Very low density
  • Low freezing points
  • Low boiling points
  • Can diffuse (rapidly and spontaneously spread out and mix)
  • Flow
  • Expand to fill container
  • Compressible
kinetic molecular theory of gases
Kinetic Molecular Theory of Gases
  • Particles move non-stop, in straight lines.
  • Particles have negligible volume (treat as points)
  • Particles have no attractions to each other (no repulsions, either).
  • Collisions between particles are “elastic” (no gain or loss of energy)
  • Particles exert pressure on the container by colliding with the container walls.
kinetic energy
Kinetic Energy
  • Energy due to motion
  • KE = ½ mv2
  • Temperature is a measure of average kinetic energy.
    • Temperature measures how quickly the particles are moving. (Heat IS NOT the same as temperature!)
    • If temperature increases, kinetic energy increases.
  • Which has greater kinetic energy: a 25 g sample of water at 25oC or a 25 g sample of water at -15oC?
why use the kelvin scale
Why use the Kelvin scale?
  • In the Kelvin scale, there is an absolute correlation between temperature and kinetic energy.
    • As temperature in Kelvin increases, kinetic energy increases.
  • Absolute zero: All molecular motion ceases. There is no kinetic energy.
    • 0 K
kelvin celsius conversions
Kelvin-Celsius Conversions
  • K = oC + 273.15
  • oC = K – 273.15
kelvin celsius conversions9
Kelvin-Celsius conversions
  • The temperature of liquid nitrogen is -196oC. What is this temperature in Kelvin?
  • Convert 872 Kelvin to Celsius temperature.
  • Pressure = force/area
  • Atmospheric pressure
    • Because air molecules collide with objects
  • More collisions  greater pressure
pressure units
Pressure Units
  • Atmosphere
  • Pounds per square inch (psi)
  • mm Hg
  • Torr
  • Pascal (Pa) or kilopascal (kPa)

1 atm = 14.7 psi = 760 mm Hg = 760 torr = 101.3 kPa

  • Torricelli-1643
  • Air molecules collide with liquid mercury in open dish
  • This holds the column up!
  • Column height is an indirect measure of atmospheric pressure
  • Two types: open and closed
  • Use to measure the pressure exerted by a confined gas
chapter 15 wrapup honors
Chapter 15 Wrapup (Honors)
  • At the same temperature, smaller molecules (i.e., molecules with lower gfm) have faster average velocity.
  • Energy flows from warmer objects to cooler objects.
  • Plasma
    • High energy state consisting of cations and electrons
      • Found in sun, fluorescent lights
boyle s law
Boyle’s Law
  • Pressure-volume relationships
  • For a sample of a gas at constant temperature, pressure and volume are inversely related.
  • Equation form:

P1V1 = P2V2

charles law
Charles’ Law
  • Volume-temperature relationships
  • For a sample of a gas at constant pressure, volume and temperature are directly related.
  • Equation form:
guy lussac s law
Guy-Lussac’s Law
  • Pressure temperature relationships
  • For a sample of a confined gas at constant volume, temperature and pressure are directly related.
combined gas law
Combined Gas Law
  • Sometimes, all three variables change simultaneously
  • This single equation takes care of the other three gas laws!
dalton s law of partial pressures
Dalton’s Law of Partial Pressures
  • For a mixture of (nonreacting) gases, the total pressure exerted by the mixture is equal to the sum of the pressures exerted by the individual gases.
collecting a sample of gas over water
Collecting a sample of gas “over water”
  • Gas samples are sometimes collected by bubbling the gas through water
If a question asks about something relating to a “dry gas”, Dalton’s Law must be used to correct for the vapor pressure of water!

Table: Vapor Pressure of Water

ideal gas law
Ideal Gas Law
  • The number of moles of gas affects pressure and volume, also!
    • n, number of moles
      • n  V
      • n  P
      • P  1/V
      • P  T
      • V  T

Where R is the universal gas constant

R = 0.0821 L●atm/mol●K

ideal vs real gases
Ideal vs. Real Gases
  • Ideal gas: completely obeys all statements of kinetic molecular theory
  • Real gas: when one or more statements of KMT don’t apply
    • Real molecules do have volume, and there are attractions between molecules
when to expect ideal behavior
When to expect ideal behavior?
  • Gases are most likely to exhibit ideal behavior at…
    • High temperatures
    • Low pressures
  • Gases are most likely to exhibit real (i.e., non-ideal) behavior at…
    • Low temperatures
    • High pressures
diffusion and effusion
Diffusion and Effusion
  • Diffusion
    • The gradual mixing of 2 gases due to random spontaneous motion
  • Effusion
    • When molecules of a confined gas escape through a tiny opening in a container
graham s law
Graham’s Law
  • Thomas Graham (1805-1869)
  • Do all gases diffuse at the same rate?
      • Graham’s law discusses this quantitatively.
      • Technically, this law only applies to gases effusing into a vacuum or into each other.
graham s law27
Graham’s Law
  • Conceptual:
    • At the same temperature, molecules with a smaller gfm travel at a faster speed than molecules with a larger gfm.
      • As gfm , v 
  • Consider H2 vs. Cl2

Which would diffuse at the greater velocity?

graham s law28
Graham’s Law
  • The relative rates of diffusion of two gases vary inversely with the square roots of the gram formula masses.
  • Mathematically:
graham s law problem
Graham’s Law Problem
  • A helium atom travels an average 1000. m/s at 250oC. How fast would an atom of radon travel at the same temperature?
  • Solution:
    • Let rate1 = x rate2 = 1000. m/s
    • Gfm1 = radon 222 g/mol
    • Gfm2 = helium = 4.00 g/mol
solution cont
Solution (cont.)
  • Rearrange:
  • Substitute and evaluate:
applications of graham s law
Applications of Graham’s Law
  • Separation of uranium isotopes
    • 235U
    • Simple, inexpensive technique
    • Used in Iraq in early 1990’s as part of nuclear weapons development program
  • Identifying unknowns
    • Use relative rates to find gfm
problem 2
Problem 2
  • An unknown gas effuses through an opening at a rate 3.16 times slower than that of helium gas. What is the gfm of this unknown gas?
  • Let gfm2 = x rate2 = 1

gfm1 = 4.00 rate1 = 3.16

  • From Graham’s Law,
  • Rearrange:
solution cont34
Solution, cont.
  • Substitute and evaluate: