IB syllabus > equilibrium (sl) > 8.1 

These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

7.1 - Dynamic equilibrium

7.1.1: Outline the characteristics of chemical and physical systems in a state of equilibrium. AIM 7: Spreadsheets and simulations can be used here.


When water is left to stand in the open air it slowly evaporates. This is a phenomenon that is easily appreciated as we have all seen pavements dry up after a rainstorm.

Why does the water disappear? where does it go to? How does it get there?

These are questions that can be answered by using the kinetic theory of particles.

In any body of water (or any liquid) the particles have a certain total quantity of energy, which is proportional to the absolute temperature and the total number particles. However this does not mean that all the particles have the same energy. In particles the energy is manifest by its degree of motion (i.e. the speed of the particle) - particles with large amounts of energy move fast an particles with little energy move slowly. As the particles are constantly colliding the energy is being exchanged, transferred, etc. continually between the particles.

After studying energetics we should be familiar with the idea that the total energy is distributed over all the particles according to the function described by the Maxwell Boltzmann curve.

It can be seen from the graph that even at low temperatures there are always some particles with relatively high energy. These higher energy particles are able to overcome the forces of attraction of the other particles and leave the body of the liquid as vapour (gas).

Hence, at any time there are particles leaving the liquid. As the particles with higher energy leave the average energy of the liquid bulk will decrease. Energy will then flow into the body of liquid from the outside and the original shape of the distribution curve will be retained (although now with fewer particles).

This process continues until eventually all of the liquid particles leave and we say that the liquid has evaporated.

Dynamic equilibrium

If our body of liquid is placed in a closed container the higher energy particles will continue to leave but now they will be held in the same place (the air space above the liquid surface). Occasionally, due to collisions, some of these vapour particles will lose energy and rejoin the body of liquid. As more particles gather in the air space there will be more possibility of particles losing energy and returning to the liquid.

We now have two processs happening. Liquid particles turning to vapour and vapour particles turning to liquid. Eventually the rate at which the particles move from the liquid to the vapour phase will equal the rate at which the particles move from the vapour phase to the liquid. When this situation is arrived at the concentration of vapour particles in the air space will be constant even though there is movement in both directions. We call this Dynamic Equilibrium.

To summarise:

Dynamic equilibrium is when there is change within a system in opposite directions and, as the change occurs at the same rate, then the concentrations of the two components remains constant.

A liquid in equilibrium with its own vapour is a physical example of a dynamic equilibrium as there is no change in chemical identity. This is also called a Phase Equilibrium as the particles are changing between phases.

liquid vapour

In a chemical equilibrium the forward reaction is happening at the same rate as the reverse reaction and the concentration of the reactants and products does not change (note: the concentrations are NOT equal)

reactants products


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