The Nature of Gases
Objectives:
• 1. Describe the nature of gases and recognize
that gases have mass, can be compressed, exert
pressure, occupy the space available to them, and
diffuse.
• 2. State the six postulates of the Kineticmolecular theory and explain how they account
for the physical properties of gases.
Key Terms:
• Diffusion, elastic, Kinetic-molecular theory
Nature of Gases
Molecules exist as monatomic, diatomic and
polyatomic molecules. Below are some basic
properties of gas molecules.
• Gases have mass
• Gases are compressible
• Gases will fill the space available to them
• Gases diffuse through each other very rapidly
• Gases exert pressure
• The pressure of a gas depends on its temperature
Kinetic Molecular Theory
•
The Kinetic-molecular Theory attempts to explain the behavior of gas molecules
based on the nature of gas. The theory is grounded on three basic assumptions:
1. All gases have mass but are very far apart
– Gas molecules are separated from each other by extremely large distances
•
•
2.
gases can be compressed
gases have low density
Gases particles are constantly moving at a very rapid rate
– explains why gases immediately fill a container
– gases move in a strait line until colliding with an object
– gases freely diffuse through each other
3.
Collisions between gas molecules are perfectly elastic
– there is no loss or gain in kinetic energy
– kinetic energy (1/2 mv2) of the gas molecules increases with added temperature
•
•
explains the expansion of gases when heated
it is important to remember that the temperature of a substance is the average KE of the molecules
Gas Pressure – closed container
• force exerted by the collisions of the gas
molecules over the surface of an object
– force/unit area
Gas Pressure – atmosphere
• Atmospheric (air) pressure is the result of the mass of the
molecules of gas and the gravity of the Earth.
– Pressure of gas at sea level is 760 mmHg - 1atm - 101.3kPa
• The millibar is used by the National Weather Service - (1013 millibars = 1
atm at sea level)
– Air pressure decreases with altitude
– Can be measured using a mercury barometer