The Nature of GasesRegardless of their chemical identity,

all gases display similar physical behavior

Examples of gasesA particle of gas can be:

A single atom He Ne Ar

A diatomic molecule H2 I2 O2

A polyatomic molecule CO2 CH4

Scientists use a model to explain the behavior of gasesKinetic Molecular Theory

Describes the behavior of the sub-microscopic particles that make up a gas

Gases are composed of tiny particles, either atoms or molecules, each of which has a mass.

Gas particles are separated by large distances relative to their size. The particles are so small and the distances between them so great that the volume of the particles themselves is negligible.

The gas particles are in constant, rapid, random motion.

Gases exert pressure because their particles frequently collide with the walls of the container and with each other. Between collisions the particles travel in straight lines.

Physical properties of gasesGases have mass

A basketball filled with air weighs more than an empty basketball.

It is easy to compress a gas Squeeze a balloon.

Gases fill their containers completely. A balloon fills evenly…all the gas does not

concentrate on one side.

Physical properties of gasesDifferent gases move through each other

quite rapidly in a process called diffusion.The smell of cooking spreads.

Gases exert pressure. Ears pop going up or down hills.

The pressure of a gas depends on its temperature.

Check tire pressure on hot or cold days!

Physical properties of gasesBecause gas particles keep colliding without

slowing down, the collisions are elastic (no energy or motion is lost)

The average kinetic energy of gases depends only on the temperature of the gases.

KE = ½ mv2

The higher the temperature, the higher the kinetic energy. Mass is constant, so particles speed up as the temperature increases.

Gas particles exert no force on each other.

How do we measure gases?Four variables

Amount of gas nVolume of gas VTemperature of gas TPressure of gas P

Amount of gasThe amount of a gas is measured in molesThe variable used to represent this is

n= number of moles

If a problem expresses the amount in grams, convert to moles using the molar mass.

n = mole = mass (g) / molar mass (g/mol)

Volume of gasThe volume of a gas is often measured in

litersThe variable used to represent this is

V = volume = liters = L

If a problem expresses the amount in other units you may convert to liters, but you don’t always need to.

Temperature of a gasThe temperature of a gas is expressed in

degrees centigrade (or Celsius) and converted to Kelvin

The variable used to represent temperature is

T = temperature = 0C = KThe Kelvin scale uses the same size degrees,

but it starts at absolute zero, -273.2 0C.To convert 0C to K, add 273.2

T (K) = 0C + 273.2

Pressure of a gasThe variable used to express pressure is

P = pressure = different units are used

Pressure exerted by air in the atmosphere is atmospheric pressure. The mass of air is attracted by the gravity of the earth.

The force exerted on one square unit of area, say one meter, equals atmospheric pressure.

Pressure of a gasPressure is calculated in units of force per unit

area.P=F/A

SI unit of force is the newtonOne newton of force exerted on one square meter of area = pascal = Pa

P = force (newton)/ area (square meter)

One pascal is small, so we often use kilopascals, kPa

STPSTP stands for

Standard Temperature and PressureStandard Temperature = 273.2 KStandard Pressure = 1 atm

You will often see this in gas law problems. It expresses the conditions of temperature and pressure described above.

Measure PressureBarometers measure pressure by noting the

change in the level of mercury in a tube caused by the force of the atmosphere

Manometers are used to measure the pressure of a gas in a closed containers

Common Units of Pressure1 pascal (Pa) = 1 newton of force / 1 square

meter of area1 kiloPascal (kPA)= 1000 pascal 1 atmosphere (atm) = 101,325 pascal (Pa)1 atm = 760 mm Hg = 760 torr1 atm = 14.70 lb/sq in (psi)1 bar = 100,000 Pa = .9869 atm