4 Postulates of Kinetic Theory

1) Spherical molecules in constant, random straight-line motion

2) “Elastic” collisions3) Point masses4) No interactions

Ideal Gas

Gas that obeys all 4 assumptions of the kinetic theory all of the time. It doesn’t exist. It’s a model.

Real Gas

Most real gases obey the kinetic theory most of the time.

Which assumptions of the kinetic theory hold up?

1. Spherical molecules in random, straight-line motion

2. “Elastic” collisions

Which assumptions of the kinetic theory break

down?

1. Point masses2. No interactions

When do the assumptions of the kinetic theory

break down?

When the gas molecules are close to each other.

When are the gas molecules close to each

other?

At high pressure & low temperature.

When are the gas molecules far apart from

each other?

At low pressure & high temperature.

How do the gas molecules act when they are far apart from each

other?

Good!

What are the properties of gases?

1. Have mass2. Take the shape & volume of their

container3. Compressible4. Diffuse (spread spontaneously)5. Exert Pressure

Pressure

Force/Area

Pressure results from?

Collisions of the gas molecules with the walls of the

container.

Pressure depends on?(microscopically)

# of impacts per unit time and force of each impact

Pressure depends on?(macroscopically)

# of gas molecules per unit volumeAnd

temperature

Units of Pressure?

1 atm = 760 mm Hg =

760 torr = 101.3 kPa

STP

P = 1 atmT = 0C = 273 K

Molar Volume at STP

The volume of 1 mole of any gas at STP = 22.4 Liters

Temperature

A measure of the avg. kinetic energy of the particles of a

substance.

4 variables needed to completely describe a

gas-phase system?

1.Temperature2.Pressure3.Volume4.# of moles

Boyle’s Law

For a fixed mass and temperature, the pressure-

volume product is a constant.

Boyle’s Law

Volume of a gas varies inversely with the pressure.

Boyle’s Law

PV = k where k = a constant

Boyle’s Law

P1V1 = P2V2

Graph of Boyle’s Law

Hyperbola – it’s an inverse relationship!

Graph of Boyle’s Law, Pressure vs. Volume

Double the pressure

Volume goes to ½ the original volume

Triple the pressure

Volume goes to 1/3 the original volume

Halve the pressure

Volume goes to 2 X the original volume

Quadruple the pressure

Volume goes to 1/4 of the original volume

What does the graph of a direct relationship look

like?

Which temperature scale has a direct relationship to molecular velocity?

Kelvin: 0 K means 0 speed.

Absolute Temperature Scale

Kelvin: 0 K means 0 speed.

Graph of Volume vs. Kelvin Temperature

It’s a direct relationship.

Math expression of relationship between

volume & Kelvin temperature

V1/T1 = V2/T2

Charles’ Law

Verbal expression of Charles’ Law

Volume of a gas varies directly with the Kelvin temperature.

What happens to the volume when the Kelvin temperature is doubled?

The volume doubles!

What happens to the volume when the Kelvin temperature is tripled?

The volume triples!

What happens to the volume when the Kelvin temperature is halved?

The volume is halved!

What happens to the Kelvin temperature when the

volume is halved?

It’s halved!

Graph of Pressure vs. Kelvin Temperature

It’s a direct relationship.

Math expression for relationship between

pressure & Kelvin temperature.

P1/T1 = P2/T2

Gay-Lussac’s Law

Verbal expression of relationship between

pressure & Kelvin temperature.

The pressure of a gas varies directly with the Kelvin temperature.

What happens to the pressure when the Kelvin temperature is doubled?

The pressure is doubled.

What happens to the pressure when the Kelvin temperature is halved?

The pressure is halved.

What happens to the pressure when the Kelvin temperature is tripled?

The pressure is tripled.

What happens to the Kelvin temperature when the pressure is doubled?

The Kelvin temperature is doubled.

Combined Gas Law

P1V1 = P2V2 T1 T2

Same pressure & volume units on both sides. Temperature must be in Kelvins.

What is Avogadro’s Principle?

Equal volumes of gases at the same temperature & pressure have equal numbers of molecules.

These 2 boxes have the same:

a) Mass c) # of molecules

b) Density d) # of atoms

He N2

What is Dalton’s Law of Partial Pressures?

Ptot = P1 + P2 + P3 + P4 + …

At the same temperature, which of the following gases diffuses most

rapidly?

He Ne Ar Kr Xe

Lightest is fastest!

At the same temperature, which of the following gases diffuses most

slowly?

He Ne Ar Kr Xe

Heaviest is slowest!