IB Chemistry home > Syllabus 2016 > Fundamental concepts > The kinetic theory

Syllabus ref: 6.1

The particulate theory of matter first was postulated by Democritus in the fifth century BC. He decided through inductive thought that all matter was composed of tiny indivisible particles, which he called "atomos".

The theory was re-discovered by John Dalton at the end of the eighteenth century AD in the course of his investigations into gases.

Historically, this is all very interesting, but what you actually need to know is the motion, forces and distance between the particles in solids, liquids and gases.


The particulate theory of matter

All of the available evidence points to the fact that all matter is made of particles and that they are in constant motion. The observable properties of matter are a function of these particles and the way that they are joined together.

The particles of solids and liquids must be close together as both states of matter are not easily compressible, while gas particles are far apart as gases are highly compressible.


The particular motion

The kinetic theory describes the motion of the particles in solids, liquids and gases. At any temperature above absolute zero the particles have energy and this is expressed as motion (kinetic energy).

The energy is distributed over all of the particles in a random manner. This results in a statistical sharing out of the energy, called a Maxwell-Boltzmann distribution:

The average (mean) energy of the particles is always proportional to the temperature of the sample.


The degrees of freedom

The way that particles are allowed to move is called their degree of freedom. The particles in a solid are only able to vibrate about fixed positions within a structure, while the particles in a liquid are able to vibrate, rotate and displace (translate) to a limited degree.

Particles in a gas have freedom of motion in terms of vibration rotation and translation.

At room temperature gas particles move very fast and collide very often. As they all move in different directions with different speeds the square root of mean square speed is used as a measure of their average velocity.

The quantity root mean square speed is defined as vrms = √(3RT/Mr), Where

For example, the root mean square speed of ammonia at room temperature = √(3 * 8.314 * 298/0.017) = 660 metres per second


The distance between particles

Solids and liquids are very difficult to compress, while gases are very easy to compress. This shows that the particles in solids and liquids are very close together, while the particles in gases are very far apart.

However, as the gas particles are travelling very fast, they collide very frequently. The average distance between collisions is called the "mean free path" and can be calculated from first principles.


Summary of the kinetic theory

The characteristics of the particles and their motion on a particulate scale defined the macroscopic physical properties.

  motion distance forces between particles shape volume
solids vibration about fixed points very close together strong fixed shape fixed
liquids vibration, rotation and limited translation very close together strong adopt the shape of the bottom of the container fixed
gases vibration, rotation and translation very far apart negligible expand to fill the container not fixed