The Behavior of gases Section 14.1: Properties of Gases

Why do soccer balls explode if you over pump them?

What is meant by the term compressibility? ● Compressibility is a measure of

how much the volume of matter decreases under pressure

● Example - airbags○ Airbags are much more compressible

than the dashboard○ The particles in the airbag are able to

move together, absorbing much of the impacts energy

● Under increased pressure, particles in a gas are forced together, or compressed

Modeling air at different pressuresWhich piston is at the higher pressure?

What variables are used to describe a gas?

● Pressure ○ Directly related to the number of

molecules (number of moles)○ More particles in container = more

pressure○ Used to make predictions as in, maximum

possible pressure○ If pressure in sealed container is lower

than atmospheric pressure air will rush

What variables are used to describe a gas?

● Volume○ Pressure can be increased by

decreasing the volume of the container

● Temperature○ Higher temperature means that

particles move faster, and therefore collide with side of object with more energy

The Gas LawsSection 14.2

How does a hot air balloon work?

Boyle’s lawThere is an inverse relationship between pressure and volume (assuming a constant temperature)

Practice problems - page 429

Charles’s lawThe volume of a gas is directly proportional to its kelvin temperature if pressure remains constant

Temperature must be in Kelvins!

Practice problems page 432

Guy - Lussac’s lawThe pressure of a gas is directly proportional to the Kelvin temperature, assuming that pressure remains constant

Knowing this, how could you fix this?

The combined gas lawAll three gas laws can be combined to determine what occurs when pressure, temperature and volume all vary

Ideal GasesSection 14.3

The amount of gas…...Whilst discussing the gas laws in section 14.2, we have not really spoken amount the 4th variable, the number of moles

The ideal gas law describes the physical behavior of an ideal gas - the relationship of pressure, volume, temperature and number of moles

The ideal gas constant is a set value of 8.31 (L.kPa/K.mol).

What is the difference between ideal and real gases? ● In reality ideal gases do not exist, BUT most gases sometimes can behave

like an ideal gas● In real gases, particles have a volume and can be attracted to each other

Question Ideal Gas Real Gas

Does it exist No Yes

Does it obey all gas laws? Yes Sometimes, mainly at High T and Low P

How big are the particles? So small they are insignificant

Small, but when compressed, size is important

Are there attractive forces between particles

No Yes, but due to particle speed often insignificant. Become important at lower temperatures

Gases: Mixtures and Movements

Section 14.4

What is meant by the term partial pressure?In a mixture of gases, the partial pressure is the contribution each gas makes to the total pressure

Dalton’s lawThis law states that the total pressure of a mixture of gases equals the sum of the partial pressure of the gases in the mixture

Essentially the number of particles is important for calculating pressure - the type of particle is not significant

This law assumes that volume and temperature are constant AND that each gas’s pressure is not affected by the pressures of other gases in the mixture

Percent composition and partial pressure

If you know the total pressure of a system, and the percent composition, you can calculate the partial pressure of a component gas

Diffusion vs EffusionDiffusion: The tendency for particles to move from areas of high concentration to areas of low concentration until the the concentration is uniform throughout

Effusion: The process that occurs when a gas escapes through a tiny hole in a container

How fast can particles diffuse or effuse?● The answer depends on the mass of the particle● The lower the molar mass, the faster the rate of diffusion or effusion -

Graham’s Law● This is based on the idea that if two objects with different masses have the

same kinetic energy, the lighter object will move faster● This explains why helium blimps must be airtight!