Chapter 5 Gases

1
Chapter 5Gases

• W. R. Murphy, Jr.
• Department of Chemistry and Biochemistry
• Seton Hall University

2
Gas Laws

• Theories of gas behavior are based on motions of
  gas particles
• particles of gas move freely through volume of
  enclosure
• collisions occur between gas particles and walls
  of enclosure
• frequency of gas collisions depend on the number
  of particles per unit volume (densityn/V)

3
Molecular speeds and energy

• Speeds of particles are related to the energies
  of the particles (KE a function of temperature)
• At a given temperature, all particles have the
  same molecular kinetic energy distribution (see
  figure 5.4)

4
Connections between speed and temperature
5
Speed

• Note - rms speed is speed from taking the square
  root of the average (mean) value of u2
• Things that depend on speed
• Effusion - movement of molecules escaping into a
  vacuum
• Diffusion - movement of one type of molecule
  through molecules of another type
• see Figure 5.6, 5.7

6
Pressure

• collective effect of molecular collisions
• force per unit area
• units of pressure
• 1 Pa 1 N/m2
• 1 atm 1.01325 ? 105 Pa
• 1 atm 760 torr 760 mmHg

7
Ideal gas assumptions

• volume occupied by molecules is negligible
  compared to volume of the container
• energies generated by forces between molecules
  are negligible compared to molecular kinetic
  energies

8
Molar mass of gases
9
Gas Mixtures - Daltons Law of Partial Pressure
10
Lower gas concentrations

• 1 ppm 1 molecule out of 106
• 1 ppb 1 molecule out of 109
• Xa, ppm, ppb all ratios of moles of a given gas
  to the total moles of gas
• ppm 106X
• ppb 109X

11
Vapor pressure

• Evaporation - process by which molecules move
  from the liquid phase to the gas phase (below bp)
• Vapor - gas that results from evaporation
• Note, puddles of water disappear because
  equilibrium is never established

12
Relative humidity

• The atmosphere normally contains less water than
  is possible at a given temperaturee. g.
  pwaterltvpwater
• Since vp is T dependent, dew and fog form when T
  drops such that vpwaterltpwater