Gases The Kinetic-Molecular Theory

Ludwig Boltzman and James Maxwell

• They each proposed a model to explain the properties of gases.

• This model is called, kinetic-molecular theory because all of the gases known to them contained molecules.

• The word kinetic means “to move” and objects

in motion have kinetic energy.

Ludwig Boltzman James Maxwell

The Kinetic-Molecular Theory

• Describes the behavior of gases in terms of particles in motion.

• This model makes several assumptions about the size, motion, and energy of gas particles.

Particle Size

• Gases consist of small particles that are separated from one another by empty space.

• Gas particles are far apart, so there is no significant attractive or repulsive forces among them.

Particle Motion• Gas particles are in constant random motion. • Particles move in a straight line until they

collide with other particles or with the walls of their container.

• Collisions between gas particles are elastic, meaning that no kinetic energy is lost.

• Kinetic energy can be transferred between colliding particles, but the total energy of the particles does not change.

Particle Energy

• 2 factors determine the kinetic energy of a particle. Mass and Velocity

• All particles do not have the same velocity but have the same mass.

Explaining the Behavior of Gases

Low Density

– Density is mass per unit volume– The kinetic-molecular theory states a great deal of

space exists between gas particles

Compression and Expansion

– When a gas is compressed the particles get closer– When a gas expands there is more air space

Diffusion and Effusion

– There are no significant attractions between gas particles

– The mixture of gases in the air diffuse until they are evenly distributed

– The rate of diffusion depends mainly on the mass of particles involved

– Mass of a gas varies from gas to gas

Diffusion and Effusion cont.

– During effusion a gas escapes through a tiny opening

– Thomas Graham did experiments to measure the rates of effusion for different gases at the same temperature

Diffusion and Effusion cont.

– Grahams law of effusion states that the rate of effusion for a gas is inversely proportional to the square root of its molar mass

– Grahams law also applies to rates of diffusion

GAS PRESSURE• PRESSURE: FORCE PER UNIT AREA

EX: Water Striders, snowshoes

MEASURING AIR PRESSURETOOLS:•BAROMETER: MEASURES ATMOSPHERIC PRESSURE

• MANOMETER: MEASURES THE PRESSURE OF A GAS IN A CLOSED CONTAINER

UNITS OF PRESSUREUnit COMPARED TO ATM

KILOPASCAL (kPa) 1 atm = 101.3 kPa

MILLIMETERS OF MERCURY (mm Hg)

1 atm = 760 mm Hg

TORR 1 atm = 760 torr

POUNDS PER SQUARE INCH (lb./in^2) or (psi)

1 atm = 14.7 psi

Atmosphere (atm)

• Pascal: N/m^2 (SI unit of pressure)• Atmosphere: 760 mm Hg

@ sea level, @ 0 degrees C

Dalton’s Law of Partial Pressures

The total pressure of a mixture of gases is equal to the sum of the pressures of all the gases in the mixture.

Ex: P (gas 1) + P (gas 2) = P (total)