AS-Level Chemistry Chapter 5 States of Matter The Gaseous State

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The Gaseous State

Kinetic theory of gases

1) Assumptions made in the kinetic theory of gases as applied to ideal gases:

- The gas particles have zero intermolecular forces between them.
- The gas particles behave as point particles which have negligible volume.
- The gas particles are in constant random motion, colliding with each other and the wall of the container frequently.
- All collisions between the gas particles are perfectly elastic.
- Pressure is due to the collision of gas particles with the wall of container.

2) In the gaseous state, the particles can move freely and are far apart.

3) A gas has no fixed shape and volume, it takes the shape of container and always fills it. The volume of a gas depends on its pressure, temperature and number of moles.

Ideal gas and real gas

1) A real gas is most like an ideal gas at:

i. low pressures

- At low pressures, the distance between gas particles is large and the volume of the gas particles is negligible(very small compared to the volume of the container).
- Intermolecular forces are also negligible at low pressures.

ii. high temperatures(well above its boiling point)

- At high temperatures, the gas particles have negligible intermolecular forces between them because they have sufficient energy to overcome it.

2) However, an ideal gas does not exist, because:

i. there are intermolecular forces between the gas particles.
ii. the volume occupied by the gas particles is not zero.

3) A real gas shows biggest deviation from an ideal gas at:

i. high pressures

- At high pressures, the gas particles are packed close together, thus the volume occupied by the gas particles is not negligible.
- The intermolecular forces between them is also not negligible.

ii. low temperature

- At low temperatures, the gas particles are packed close together, thus the volume occupied by the gas particles is not negligible.
- The intermolecular forces between them is also not negligible because they do not have sufficient energy to overcome it.