Chemistry » States of Matter » Motion Of Particles

# The Kinetic Theory of Gases

## Introducing Motion of Particles

We are surrounded by gases in our atmosphere which support and protect life on this planet. Everyday we breathe in oxygen and release carbon dioxide. Green plants take in the carbon dioxide and release oxygen. One way or another we are surrounded by a mix of many different gases, some that we need and some that are harmful to us.

Gas cylinders for cooking

## The kinetic theory of gases

In a previous lesson, you learnt about the kinetic theory of matter. The kinetic theory of matter says that all matter is composed of particles which have a certain amount of energy which allows them to move at different speeds depending on the temperature (energy). There are spaces between the particles and also attractive forces between particles when they come close together.

Now we will look at applying the same ideas to gases.

The main assumptions of the kinetic theory of gases are as follows:

• Gases are made up of particles (e.g. atoms or molecules). The size of these particles is very small compared to the distance between the particles.

• These particles are constantly moving because they have kinetic energy. The particles move in straight lines at different speeds.

• There are attractive forces between particles. These forces are very weak for gases.

• The collisions between particles and the walls of the container do not change the kinetic energy of the system.

• The temperature of a gas is a measure of the average kinetic energy of the particles.

From these assumptions we can define the pressure and temperature of any gas.

### Definition: Pressure

The pressure of a gas is a measure of the number of collisions of the gas particles with each other and with the sides of the container that they are in.

### Definition: Temperature

The temperature of a substance is a measure of the average kinetic energy of the particles.

If the gas is heated (i.e. the temperature increases), the average kinetic energy of the gas particles will increase and if the temperature is decreased, the average kinetic energy of the particles decreases. If the energy of the particles decreases significantly, the gas liquefies (becomes a liquid).

One of the assumptions of the kinetic theory of gases is that all particles have a different speed. However, this is only the case for a real gas. For an ideal gas we assume that all particles in the gas have the same speed.

So for an ideal gas we can simply talk about the speed of particles. But for a real gas we must use the average speed of all the particles.

### Optional Video: Temperature and Ultra-Cold Atoms

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