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Gases

Boyles Law and pressure/volume relationships

Boyles Law states that, for any sample of gas, its pressure multiplied by the volume it occupies is a constant providing the temperature remains constant. (ie pV = constant). This is always true for perfect gases, for which pV = nRT by definition.

The Van der Waals Equation

How can deviations from ideal behaviour be compensated for in our model of the situation? Recall that our model produced p.Vm = RT (ideal gas equation).

Partial pressure and Daltons Law

If we consider a mixture of gases in a container, then they each exert a pressure on the walls of that container. ie: one gas provides a component of the total pressure, and another gas provides another component.

Real Gases

Thus far we have concentrated on perfect gases. It is important to appreciate that no gas actually is perfect – they all deviate from ideal behaviour to some degree.

The Mean Free Path

The mean free path is, as the name suggests, the average distance a molecule can go before colliding with another molecule.

Temperature Variation of the Maxwell Distribution

Experimentally we find that the most probable speed increases as the temperature is increased, or as the moleclular mass is decreased.

Collision Frequency (z)

We treat the molecules as hard spheres (of diameter d) – like pool balls. For two molecules to collide, their centres must come within a distance d of each other.

Root Mean Square Speed

The root mean square speed, crms, can be related to macroscopic properties.

The Kinetic Model of Gases

The kinetic model is based upon 3 assumptions