Kinetic theory of gases

Gas particles, in common with all particles, are in constant motion.

Gases are substances in which the force of attraction between the particles has been overcome by their energetic motion. The gas particles can simply no longer be held together by attractive forces.

As the particles fly around at high speed, they collide many times per second with each other and with the walls of any container. If the container has no walls, the gas particles spread out to fill all the available space.

To deal with gases in chemistry some assumptions are made:

• 1 The gas particles themselves occupy no volume.
• 2 The forces of attraction between particles are so small as to be negligible.
• 3 Collisions involving gas particles are perfectly elastic.

Gases which fulfil the above criteria are said to be "ideal" gases. The reality is that gas particles do have forces of attraction between one another, but the forces are negligible at normal pressures, when the gas particles are more often than not far apart, except during the moments of collision.

There is a treatment of real (non-ideal) gases in the next section.

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Gas volume

In chemistry, volumes are usually measured in litres (decimetres cubed, dm³) and centimetres cubed (cm³), rather than metres cubed (m³). The reason for this is purely practical, the metre cubed is a very large volume compared to the test tubes and flasks used in laboratories. See apparatus, section 1.62.

Gas volumes may be quoted in metres cubed (m³), litres (L), centimetres cubed (cm³) or millilitres (mL), depending on the textbook consulted.

1m³ (1 metre cubed) = 1000 dm³ (1000 decimetres cubed) = 1,000,000 cm³ (1 million centimetres cubed)

1 decimetre cubed (1 dm³) is also called 1 litre (= 1000 cm³)

1 centimetre cubed (cm³) is also called 1 millilitre (mL) as it is 1/1000 of a litre.

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Conversion between volume units

To convert from cm³ or ml to dm³ or litres divide by 1000

To convert from dm³ or litres to cm³ or ml multiply by 1000

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Gas pressure

The pressure of a gas is caused by the gas particles colliding with the walls of the container. Each small collision exerts a force on the wall. The sum of these forces over an area of the wall is called the gas pressure. The SI unit of force is the Newton and the unit for area is the metre squared (m²). Pressure is measured in Newtons per metre squared = Nm^-2. This combined unit is called the Pascal, Pa.

1 Nm^-2 = 1 Pa

The Pascal is a fairly small quantity and atmospheric pressure = 100.0 kPa (approximately) - this is the value used for gas calculations in the IB chemistry exams.

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Temperature

The SI unit of temperature is the kelvin (K), although problems are often set in degrees Celsius (ºC). It is important to ALWAYS carry out gas calculations using absolute (kelvin) temperature values.

0K is called absolute zero. It is the temperature at which particles have no energy. This temperature is equal to -273.16ºC, approximated to -273ºC. The magnitude of 1 kelvin is the same as that of 1º Celsius, therefore;

0K = -273ºC

273K = 0ºC

373K = 100ºC

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