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