IB Chemistry - Equilibrium

IB Chemistry home > Syllabus 2016 > Equilibrium > Reversible change

Syllabus ref: 7.1

Many processes can be reversed by changing the conditions. This applies to both physical and chemical change.

Kinetic theory

The name 'kinetic' comes from the Greek word for motion 'kinesis'. The kinetic theory postulates that all particles are in constant motion, vibration, rotation and translation. The motion of is caused by energy.

However this energy is not distributed evenly over all of the particles, instead it is distributed in a completely random fashion that is described statistically as a Maxwell-Boltzmann distribution.

It can be seen in the diagram that there are very few particles with low energy and the number of particles with high energy tails off in a curve. The average or mean energy of the particles is directly proportional to the absolute temperature.

When the temperature is increased, three things happen to the energy distribution of the particles.

This energy distribution is very important to understand the behaviour of particles in any system. It will be referred to regularly - it is a learning essential


Vapour and gas

Clouds are formed by water vapour accumulating into dropletsA vapour consists of particles of a substance, normally a liquid under the conditions specified, but which exists in the gaseous form.

For example: There is water vapour in air at room temperature, 25ºC.

This consists of free water molecules mixed with the molecules of air, even though the boiling point of water is much higher than 25ºC.

The reason that these molecules do not turn back to water is that they are separated enough from each other to not be attracted to one another. As the amount of water vapour in the atmosphere increases, there is more possibility of the water molecules coming together and 'precipitating' as water droplets or rain.

Clouds are simply accumulations of water droplets formed by particles of water vapour coming together.



When water is left to stand in the open, it slowly evaporates. It does so at temperatures well below the boiling point of water. How can this happen?

Evaporation is the process of turning from a liquid into a vapour. It is not boiling. Evaporation happens to liquids at all temperatures. Look at the Maxwell-Boltzmann distribution of energies over particles. You can see that there are always some particles with large amounts of energy. These 'energetic' particles escape from the attractions of the other particles and become vapour.

These energetic particles leave the body of liquid. As the energetic particles leave, the average energy of the liquid bulk decreases.

Energy then flows into the liquid from the surroundings and the original shape of the distribution curve is retained (although now with fewer particles). This continues until eventually all of the liquid particles leave and the liquid has evaporated.



When a body of air that contains water vapour is cooled down, the particles move slower on average and forces of attraction between them become more important. The particles form drops of liquid. We say that the vapour has condensed.

This is, of course, the reverse of evaporation. In one case the liquid absorbs energy and the water evaporates and in the other case the vapour loses energy and, if there are enough vapour particles in close proximity, water condenses.

water + energy vapour

vapour water + energy


Closed systems

A closed system is one in which neither matter nor energy can enter or leave.

If a body of water is sealed in a closed system, a dynamic equilibrium is established. Initially, particles of water in the liquid leave to form vapour above the body of the liquid. Occasionally, due to collisions, some of these vapour particles will lose energy and rejoin the body of liquid.

As more particles gather in the space above the liquid, there will be more possibility of particles losing energy and returning to the liquid. There is a two-way flow:

liquid water water vapour
water vapour liquid water

We now have two processes happening simultaneously. Liquid particles turning to vapour and vapour particles turning to liquid.


Dynamic Equilibrium

Eventually in the closed system example above, the rate at which the particles move from the liquid to the vapour phase equals the rate at which the particles move from the vapour phase to the liquid:

Vaporisation rate = Condensation rate

When this situation develops the concentration of vapour particles in the air space is constant even though there is movement in both directions. We call this situation "Dynamic Equilibrium".