Properties of a Gas

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Properties of a Gas

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Properties of a Gas

• Reading Assignment Fine, Beall Stuehr, Chapter
  4.1-4.6
• This lecture introduces the interrelated
  dependence of moles, temperature, pressure and
  volume of a confined gas.
• Based on assumptions of ideal gas behavior, these
  parameters when combined form the ideal gas
  equation, PVnRT.

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Gases
No long or short range structural order
Large intermolecular distances, compared to
molecular size
What happens when the stopcock is opened?
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Gases
Mix together in any proportions
Intermolecular collisions are elastic -- energy
is transferred through collisions
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Important Physical Properties of a Gas

• Temperature
• Pressure
• Volume
• Moles

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Temperature

• Temperature is a measure of molecular motion
  vibration, rotation and translation
• Gases move at high velocity
• (gt 1000 msec1)
• Common units
• Celsius (C), Fahrenheit (F)
• Absolute units Kelvin (K), Rankine (R)

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Temperature

• Absolute units Kelvin (K), Rankine (R)
• 0.00 C 273.15 K
• 32.00 F 491.67 R
• 100.00 C 373.15 K
• 212.00 F 671.67 R
• Isothermal - constant temperature

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Pressure
Due to molecules hitting the surface
Force per unit area
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Pressure
What do you know about
the pressure in a balloon?
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Pressure
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Barometer
What forces determine the height of the
mercury in the glass tube?
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Barometer
What forces determine the height of the
mercury in the glass tube?
atmospheric pressure
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Barometer
1.03 kgcm2 14.7 lbin2
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Barometer
33 feet
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Pressure

• Common units
• lbsin2, atmospheres (atm), mm Hg, torr, pascals
  (Nm2, Pa)
• 1.00 atm 14.7 lbsin2
• 760 mm Hg 760 torr
• 1.013 x105 Pa

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Pressure

• The actual atmospheric pressure is a function of
  the weather (high or low pressure) and altitude.
• Typical Terre Haute pressure is 730-750 mm Hg.
• Isobaric - constant pressure

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Volume

• Gases occupy the complete volume to which they
  are constrained
• Liquids and solids occupy a volume corresponding
  to their mass and density
• Isochoric - constant volume

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Moles

• A counting unit for the number of molecules being
  considered

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Physical Laws of Gases

• Boyle's Law
• Charles' Law
• Avogadro's Law
• Ideal Gas Law

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Boyle's Law

• PV constant

isothermal constant amount of gas
What does a plot of this relationship look like?
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Charles' Law

• VµT

isobaric constant amount of gas
What does a plot of this relationship look like?
What does the y-intercept indicate?
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Avogadro's Law

• Vµn

isobaric isothermal
What does a plot of this relationship look like?
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Ideal Gas Law

• PV µ nT
• PV nRT
• where R ideal gas constant
• 0.08205 Latmmol1K1

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Boyle's Law

• PV constant

How will this plot change if the same number
of moles of gas are at a higher temperature?
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Common GasesRoom Temperature, One Atmosphere
Pressure

• Monatomic He, Ne, Ar, Kr, Xe, Rn
• Polyatomic
• Elements N2, O2, O3, F2, Cl2
• Compounds CO, CO2, NO, N2O, NO2, N2O4, N2O5,
  CH4, C2H6, C2F4, etc.

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Atmospheric Composition

• Dry air
• 78 N2, 21 O2, 1Ar
• Normal air
• N2, O2, Ar, 0.03 CO2, 1.6 H2O

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Boyle's Law

• PV constant

isothermal constant amount of gas
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Charles' Law

• VµT

isobaric constant amount of gas
What does the y-intercept indicate?
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Avogadro's Law

• Vµn

isobaric isothermal
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Boyle's Law

• PV constant