Unit: Gas Laws
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Transcript of Unit: Gas Laws
Unit: Gas LawsDalton’s Law of Partial Pressures, Grahams Law,
and Real vs. Ideal Gases
Day 5 –
Notes
After today you will be able to…
• Describe Dalton’s law of partial pressures and calculate Ptotal or a partial pressure• Explain Graham’s law of
effusion and calculate the rate at which gases effuse• Explain what is meant by the
term “real” vs. “ideal” gases
Recall, gas pressure results from collisions of gas particles.•Gas pressure depends on the amount of gas and the KE of its particles.•Since particles in a mixture of gases at the same temperature contain the same average KE, the kind of particle is unimportant.
Example: Composition of Dry Air
Component
Volume
Partial Pressure
Nitrogen 78.08% 79.11 kPa
Oxygen 20.95% 21.22 kPa
Carbon dioxide
0.04% 0.04 kPa
MISC gases 0.93% 0.95 kPa
Total100.00
%101.32 kPa
“The total pressure of a
mixture of gases is equal to the sum of the individual
(partial) pressures.”
Dalton’s Law of Partial Pressures
Units of pressure must match!
Ptotal= P1 + P2 + P3…
Example: Dalton’s Law
What is the total pressure for a mixture of O2 and CO2 if PO2= 0.719 atm and PCO2= 423mmHg.
PO2= 0.719atm
PCO2= 423mmHg
x1atm
760mmHg= 546mmHg
Ptotal=546mmHg + 423mmHgPtotal=969mmHg
Thomas Graham (1846)
•Diffusion: Is the tendency of gas particles to spontaneously spread out until uniformly distributed.•Effusion: The escape of a gas through a tiny pinhole in a container of gas.–Gases with lower molar masses effuse more quickly.
“The rate of effusion of a
gas is inversely proportional to the square root
of the gas’s molar mass.”
Graham’s Law of Effusion
Always place the larger molar mass in the numerator!
Rate A √MMB
Rate B √MMA
=
Example: Graham’s Law
Which gas effuses faster, H2 or Cl2? How much faster?
Rate H2 √MMCl2
Rate Cl2 √MMH2
Rate H2 √(70.90)
Rate Cl2 √(2.02)
=
=
= 5.92x
H2 effuses 5.92x faster
than Cl2
Real vs. Ideal Gases•The gas laws we’ve learned in this unit are
based on a gas that behaves “ideally.”•An ideal gas has:–No molecular volume–No attractive forces
•In reality, there are no perfectly ideal gases. But, under most conditions, real gases will approximate ideal gas behavior.•However, under certain conditions, real gases will deviate from ideal gas behavior.
Real vs. Ideal Gases•These deviations occur for:
1. High pressure: Gas particles are pushed closer together, more attractive forces result.
2. Low Temperature: The gas is compressed, there are more attractive forces.
3. High molar mass: Higher molar mass of the molecule usually means larger volume.
4. Polar molecules: Unequal sharing of electrons creates an attraction between molecules.
Questions?Complete
WS 5