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Unit 5: Gases and Gas Laws

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760 mm Hg = 760 torr. Measuring Pressure. The first device for measuring atmospheric ... P = 1 atmosphere, 760 torr, 101.3 kPa. T = 0 C, 273 Kelvins ... – PowerPoint PPT presentation

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Title: Unit 5: Gases and Gas Laws


1
Unit 5 Gasesand Gas Laws
2
The Nature of Gases
  •  Gases expand to fill their containers
  •  Gases are fluid they flow
  •  Gases have low density
  •  1/1000 the density of the equivalent liquid or
    solid
  •  Gases are compressible
  •  Gases effuse and diffuse

3
Kinetic Energy of Gas Particles
At the same conditions of temperature, all gases
have the same average kinetic energy.
m mass
v velocity
4
Kinetic Molecular Theory
  • Particles of matter are ALWAYS in motion
  • Volume of individual particles is ? zero.
  • Collisions of particles with container walls
    cause pressure exerted by gas.
  • Particles exert no forces on each other.
  • Average kinetic energy µ Kelvin
    temperature of a gas.
  •  

5
Pressure
  •  Is caused by the collisions of molecules with
    the walls of a container
  •  is equal to force/unit area
  •  SI units Newton/meter2 1 Pascal (Pa)
  •  1 standard atmosphere 101.3 kPa
  •  1 standard atmosphere 1 atm
  • 760 mm Hg 760 torr

6
Measuring Pressure
The first device for measuring atmospheric
pressure was developed by Evangelista Torricelli
during the 17th century.
The device was called a barometer
  •   Baro weight
  •   Meter measure

7
Units of Pressure
8
Standard Temperature and PressureSTP
  •   P 1 atmosphere, 760 torr, 101.3 kPa
  •   T 0C, 273 Kelvins
  •   The molar volume of an ideal gas is 22.42
    liters at STP

9
Converting Celsius to Kelvin
Gas law problems involving temperature require
that the temperature be in KELVINS!
Kelvins ?C 273
C Kelvins - 273
10
Boyles Law
Pressure is inversely proportional to volume
when temperature is held constant.
11
A Graph of Boyles Law
12
Charless Law
  •  The volume of a gas is directly proportional to
    temperature, and extrapolates to zero at zero
    Kelvin.
  • (P constant)

Temperature MUST be in KELVINS!
13
A Graph of Charles Law
14
Gay Lussacs Law
The pressure and temperature of a gas are
directly related, provided that the volume
remains constant.
15
A Graph of Gay-Lussacs Law
16
The Combined Gas Law
The combined gas law expresses the relationship
between pressure, volume and temperature of a
fixed amount of gas.
Boyles law, Gay-Lussacs law, and Charles law
are all derived from this by holding a variable
constant.
17
Avogadros Law
  • For a gas at constant temperature and pressure,
    the volume is directly proportional to the number
    of moles of gas (at low pressures).
  • V an
  • a proportionality constant
  • V volume of the gas
  • n number of moles of gas

18
Standard Molar Volume
Equal volumes of all gases at the same
temperature and pressure contain the same number
of molecules. - Amedeo Avogadro
19
Ideal Gases
Ideal gases are imaginary gases that perfectly
fit all of the assumptions of the kinetic
molecular theory.
  • Gases consist of tiny particles that are far
    apart
  • relative to their size.
  • Collisions between gas particles and between
  • particles and the walls of the container are
  • elastic collisions
  • No kinetic energy is lost in elastic collisions

20
Ideal Gases (continued)
  • Gas particles are in constant, rapid motion.
    They
  • therefore possess kinetic energy, the energy
    of
  • motion
  • There are no forces of attraction between gas
  • particles
  • The average kinetic energy of gas particles
  • depends on temperature, not on the identity
  • of the particle.

21
Real Gases Do Not Behave Ideally
Real gases DO experience inter-molecular
attractions
Real gases DO have volume
Real gases DO NOT have elastic collisions
22
Deviations from Ideal Behavior
23
Ideal Gas Law
  • PV nRT
  •  P pressure in atm
  •  V volume in liters
  •  n moles
  •  R proportionality constant
  • 0.08206 L atm/ molK
  •  T temperature in Kelvins

Holds closely at P lt 1 atm
24
Gas Density
so at STP
25
Daltons Law of Partial Pressures
For a mixture of gases in a container, PTotal
P1 P2 P3 . . .
This is particularly useful in calculating the
pressure of gases collected over water.
26
Gas Stoichiometry 1
If reactants and products are at the same
conditions of temperature and pressure, then mole
ratios of gases are also volume ratios.
3 H2(g) N2(g) ?
2NH3(g)
3 moles H2 1 mole N2 ?
2 moles NH3
3 liters H2 1 liter N2 ?
2 liters NH3
27
Gas Stoichiometry 2
How many liters of ammonia can be produced when
12 liters of hydrogen react with an excess of
nitrogen?
3 H2(g) N2(g) ?
2NH3(g)
12 L H2
L NH3
2
L NH3
8.0
L H2
3
28
Gas Stoichiometry 2
How many liters of ammonia can be produced when
12 liters of hydrogen react with an excess of
nitrogen?
3 H2(g) N2(g) ?
2NH3(g)
12 L H2
L NH3
2
L NH3
8.0
L H2
3
29
Gas Stoichiometry 3
How many liters of oxygen gas, at STP, can be
collected from the complete decomposition of 50.0
grams of potassium chlorate?
2 KClO3(s) ? 2 KCl(s) 3 O2(g)
50.0 g KClO3
1 mol KClO3
3 mol O2
22.4 L O2
122.55 g KClO3
2 mol KClO3
1 mol O2
L O2
13.7
30
Gas Stoichiometry 4
How many liters of oxygen gas, at 37.0?C and
0.930 atmospheres, can be collected from the
complete decomposition of 50.0 grams of potassium
chlorate?
2 KClO3(s) ? 2 KCl(s) 3 O2(g)
50.0 g KClO3
1 mol KClO3
3 mol O2
n mol O2
0.612 mol O2
122.55 g KClO3
2 mol KClO3
16.7 L
31
Diffusion
Diffusion describes the mixing of gases. The
rate of diffusion is the rate of gas mixing.
32
Effusion
Effusion describes the passage of gas into an
evacuated chamber.
33
Grahams LawRates of Effusion and Diffusion
Effusion
Diffusion
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