Gases

PropertiesKinetic Molecular TheoryVariables The AtmosphereGas Laws

Properties of a Gas No definite shape or volume

Gases expand to fill any container

Thus they take the shape of the container, as well as the volume!

Low Density Most of the volume of a gas is

empty space-since the gas particles want to be as far from all like particles as possible

As a result, gases are compressible

Properties of a Gas

Fluid can flow like a liquid

The Atmosphere

Composed of a mixture of gases

-Approx. 80% N2 and 20% O2

Provides conditions for life, protects from UV radiation, traps heat

Please read about the Greenhouse Effect and the Ozone Layer in your text books…you will be responsible for that information...

The Kinetic Molecular Theory

Explains the behavior of gases at the molecular level

Based on assumptions about a theoretical gas referred to as the Ideal Gas.

The Kinetic Molecular Theory1) Volume of gas particles is

essentially zero (Point Masses)

2) Particles are in constant, random motion

3) No attractive forces betw. particles and collisions are perfectly elastic No energy is lost by the

particles during collisions

The Kinetic Molecular Theory

4) The temperature of a gas is directly proportional to the average kinetic energy associated with the particles, thus the higher the temperature, the faster the particles are moving... K.E. = 1/2 mv2

Ideal vs. Real Gas

Ideal Gas Has mass but

no volume Has no

attractive forces between molecules

Real Gas Molecules do

have volume Molecules

exert an attractive force on each other when very close.

Ideal vs. Real Gas

At normal temps. & pressures, real gases behave like ideal gases

Four variables that describe a gas...

1) Temperature - the measure of the avg. kinetic energy

gases at the same temp. must have the same avg. kinetic energy

Must be in Kelvin!

TK = TC + 273

Four variables that describe a gas...

2) Volume - The amount of space occupied by the particles, measured in

liters, mL, cm3, m3, etc.

Four variables that describe a gas...

3) Pressure - The force exerted by the gas per unit of area

In gases - meas. of total force exerted by the particles colliding w/ the walls of their container. - proportional to the number of

collisions• -Incr. temp, incr. press.• - Incr. # of particles, incr. press.

usually measured in: atmospheres (atm), kPa (kilo-pascals), N/m2

Pascal - SI unit of pressure• = to 1 N/m2 • very small press. - usually use kilopascals

Four variables that describe a gas...

4) Number of particlesEqual volumes of gases under the same conditions have equal numbers of particles...

Avogadro’s PrincipleThe volume of a gas at a given

temperature and pressure is directly proportional to the number of moles of that gas

ie equal volumes of gases under the same conditions contain the same number of particles.

Molar Volume is the volume of 1 mole of any gas at STP

Standard ConditionsSTP

Standard Temperature is 273 K

Standard Pressure is…

1.0000 atm

101.325 kPa

760.0 mm Hg

760.0 torr

Note: you can use any pressure units but you can only use Kelvin temperature units!

The Gas Laws

Boyle’s Law

- the volume of a gas at constant temp. is inversely proportional to the pressure

P1 V1 = P2 V2

Read Ch. 12 Section 2

The Gas Laws

Charles’ Law - the volume of a gas at constant press. is directly proportional to the absolute (Kelvin) temp.

V1 / T1 = V2 / T2

The Gas Laws

Combined Gas Law - Combines Boyle’s & Charles’ Law

P1V1 / T1 = P2V2 / T2

Read Ch. 12 Section 2

The Gas Laws

Ideal Gas Law

PV = n R Tn = # of moles

R = 8.31 L kPa / mol K

Or 0.0821 L atm / mol K

Read Ch. 12 Section 2