Gases

Part 1: Kinetic Molecular Theory

Kinetic Molecular Theory

describes the behavior of gases in terms of particles in motion.

makes several assumptions about the sizemotion, and energy of gas particles.

Assumptions

Particle size Particle motion Particle energy

Particle Size

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

The volume of the particles is small compared with the volume of the empty space.

Gas has Mass

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.

Elastic Collisions

Collisions between gas particles are elastic.

An elastic collision is one in which no kinetic energy is lost.

Particles move very fast

Particle energy

Kinetic energy is a function of the mass and speed of a particle.

In a sample of a single gas, all particles have the same mass but all particles do not have the same velocity.

Therefore, all particles do not have the same kinetic energy.

Kinetic Energy and Momentum

Properties of Gases

Low density Compressible Diffuses

Low density

Density – mass per unit of volume (g/cm3) Density of gasses is much lower than the

density of solids or liquids.

Compressible

Gasses can be compressed Compression – can be pushed

into a smaller volume Expansion – can be pulled into a

greater volume

Diffusion

the movement of one material through another.

particles diffuse from an area of high concentration to one of low concentration

Temperature and Pressure

Kinetic theory explains the temperature and pressure of gassesHow fast the molecules of a gas are goingHow heavy the molecules are

Temperature

Temperature is a measure of the average kinetic energy of the particles in a sample of matter.

In other words…

Temperature is related toHow fast the molecules of a gas are movingHow often the molecules hit the walls of the

container

Gas has temperature

Pressure

defined as force per unit area. Gas particles exert pressure when they

collide with the walls of their container.

In other words…

Pressure is related toHow fast the molecules of a gas are movingHow much the particles weighHow hard the molecules hit the walls of the

container

Gas has pressure

Measuring Pressure A barometer is an instrument

used to measure atmospheric pressure.

A manometer is an instrument used to measure gas pressure in a closed container.

Pressure Units SI unit of pressure is the pascal (Pa). It is

named for Blaise Pascal. One pascal is equal to a force of one

newton per square meter: 1 Pa= 1 N/m2.

Other Pressure Units engineers often report pressure as pounds

per square inch (psi). pressures measured by barometers and

manometers can be reported in millimeters of mercury (mm Hg).

Air pressure

At sea level, the average air pressure is 760 mm Hg when the temperature is 0°C.

Air pressure often is reported in a unit called an atmosphere (atm). One atmosphere is equal to 760 mm Hg or 760 torr or 101.3 kilopascals (kPa).

Temperature and Pressure

Temperature and pressure are directly proportional

As one goes up, the other goes up As one goes down, the other goes down

Temperature and Pressure

Temperature and volume

Volume and Pressure