3.2. Modelling a Gas

Kinetic theory describes how we can explain macroscopic properties (e.g. pressure, temperature, volume) using ideas and assumptions about the movement of molecules. When you first came across the idea of pressure, you learnt that this was due to the collisions of millions of molecules with the vessel wall, each exerting a tiny force, but if these individual forces are summed over a certain area we know this as pressure.

We can model gas behaviour as the motion of many tiny discrete particles. We make a number of assumptions to make the maths easier. This gives us a theoretical model of a gas that follows these assumptions that we call an ideal gas. Some of these assumptions are:

1. The collision between particles is elastic (no kinetic energy lost)

2. The volume of the vessel is much larger than the total volume occupied by gas molecules

3. There are no intermolecular forces between gas molecules (or in other words, the molecules kinetic energy is much larger than their potential energy)

4. The time between gas molecule collisions is much larger than the duration of the collisions.

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So, a few asusumptions here, but what does this mean in real life - or when does a real gas follow the behaviour of an ideal gas.

• Well, if the volume of the vessel must be much larger then the volume of the gas molecules, that means our gas should have a LOW DENSITY (if the gas density or pressure is higher, the volume of gas molecules relative becomes significant relative to the vessel volume).

• If the kinetic energy of the molecules needs to be much larger than the potential, we should have a HIGH TEMPERATURE (if the molecules move much slower, the effects of intermolecular attraction are more significant).
   

This Crash Course Chemistry video talks through how if we have a real gas which does not perfectly obey these assumptions we can modify our ideal gas equation to model this real-life behaviour.