1. Lecture 66 – Lecture 67 – Lecture 68 Gases Ozgur Unal
NIS – CHEMISTRY
Lecture 66 – Lecture 67 – Lecture 68
Gases
Ozgur Unal

2. The Kinetic – Molecular Theory
About 1860, chemists Ludwig Boltzmann and James Maxwell independently proposed a model to explain the properties and the behavior of gases  kinetic molecular theory
The word kinetic comes from a Greek word meaning to move.
Objects in motion have energy called kinetic energy.
The kinetic – molecular theory describes the behavior of matter in terms of particle motion.
This theory makes some assumptions about the size, motion and energy of gas particles.

3. Particle Size
Gases consist of small particles that are separated from one another by empty space.
The volume of particles is small compared with the volume of the empty space.
Because the gas particles are far apart, they experience no significant attractive or repulsive forces.

4. Kinetic energy = KE = m*v2 / 2
Particle Motion
Gas particles are in random, constant motion.
Particle move in a straight line until they collide with other parcitles, or with the walls of the container.
Collisions between gas particles are elastic.
An elastic collision is one in which no kinetic energy is lost.
Kinetic energy can be transferred between colliding particle, but the total kinetic energy of the two particles does not change.
Kinetic energy = KE = m*v2 / 2
m: mass of the particle
v: speed of the particle
Temperature is a measure of the average kinetic energy of the particles in a sample of matter.

5. Compression and Expansion of Gases
Gases can be compressed and expanded.
Expanding a gas decreases the density of the gas.
Compressing a gas increases its density.
Can you provide more examples where we compress or expand a gas?
Gases have much lower densities compared to solids and liquids.
Density of solid gold: 19.3 g /mL
Density of Cl2 gas: 2.95 x 10-3 g/mL
The empty space between gas particles is huge!

6. Diffusion Check this out!
Diffusion is the term used to describe the movement of one material through another.
Diffusion of gas particles takes place from high concentration to low concentration.
Example: When your mama cooks
food, you can smell the food because
the gas particles form the food diffuses
in the house and reach your nose.

7. Rate of Effusion α (molar mass)-0.5
Effusion is aprocess related to diffusion.
During effusion, a gas escapes through a
tiny opening.
Example: Gas leaking from a fuel gas tank
Do you think different gases effuse at the same rate?
In 1846, Thomas Graham conducted experiments to measure the rates of diffusion for different gases.
He found the following relationship between the effusion rate and the molar masses of the gases.
Rate of Effusion α (molar mass)-0.5
This relationship is called Graham’s Law of Effusion.

8. Diffusion and Effusion
According to the Graham’s law of effusion, gas particles with lower molar masses effuse faster than gas particles with higher molar masses.
Graham’s law also applies to diffusion rates.
Example: O2 has a molar mass of 32 g/mol and N2 has a molar mass of 28 g/mol  N2 diffuse (effuse) faster than O2
We can use Graham’s law of effusion to compare the diffusion rates of particles.
Rate A / Rate B = (molar mass B / molar mass A)0.5
Example: Ammonia has a molar mass of 17 g/mol, HCl has a molar mass of 36.5 g/mol. What is the ratio of their diffusion rates?

9. Pressure is defined as force per unit area. Do gases have pressure?
Gas Pressure
Pressure is defined as force per unit area.
Do gases have pressure?
Gas particles also apply pressure when they collide with the walls of their container.
Example: As you push the piston in a syringe, there are more and more collisions between the gas particles and the piston  gas pressure increases
Air Pressure:
Gas particles in the air move in every direction  pressure in all directions
This pressure is called atmospheric or air pressure.

10. Air pressure decreases as you go to higher latitudes. Why?
Gas Pressure
Air pressure decreases as you
go to higher latitudes.
Why?
Atmospheric pressure can be measured by an instrument called barometer.
Check out Figure 12.5!!
A manometer is an instrument used to
measure gas pressure in a closed container.
Check out Figure 12.6!!

11. Using the definition of pressure can you guess the unit of pressure?
Gas Pressure
Using the definition of pressure can you guess the unit of pressure?
The unit of pressure in SI is Pascal (Pa).
Pa is equal to N/m2.
The pressure at sea level at 0 oC is kPa.
One atmosphere is equal to kPa or 760 mm Hg (Torr).
Check out Table 12.1!!

12. Gas Pressure Dalton’s Law of Partial Pressures: Remember Dalton?
When Dalton studied gases, he found that each gas in a mixture exerts pressure independently of the other gases present.
Dalton’s law of partial
presures states that the
total pressure of a mixture
of gases is equal to the
sum of the pressures of
all the gases in the mixture.
Ptotal = P1 + P2 + P Pn

13. Gas Pressure
Example: A mixture of oxygen, CO2 and nitrogen has a total pressure of 0.97 atm. What is the partial pressure of ozygen if the partial pressure of CO2 is 0.7 atm and the partial pressure of N2 is 0.12 atm?