Chemguide: Core Chemistry 14 - 16

The effect of changing the concentration of a substance in a dynamic equilibrium

This page looks at what happens when you change the concentration of something involved in a dynamic equilibrium.

It assumes that you have already read the page about introducing reversible reactions and equilibria - in particular the last part of that page about Le Chatelier's Principle.

The reversible reaction between sulfur dioxide and oxygen

There is a simple example of the effect of changing concentration in one of the key stages in the manufacture of sulfuric acid which will be discussed at length later in the course.

That stage involves converting sulfur dioxide into sulfur trioxide.

2SO₂(g) + O₂(g)    2SO₃(g)

The oxygen in this reaction is coming from the air, and you actually use twice as much oxygen in the mixture than the equation suggests. Why?

Using Le Chatelier's Principle

Le Chatelier's Principle says:

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

Suppose you had the reaction in equilibrium using equation proportions. By doubling the amount of oxygen, you are changing the conditions. According to Le Chatelier, the position of equilibrium will move to counteract this.

So the position of equilibrium moves to the right because that uses up some of the extra oxygen - and that means that you get a better conversion of sulfur dioxide into sulfur trioxide.

How to phrase this if your examiners don't like Le Chatelier's Principle

There is nothing to stop you working it out using Le Chatelier's Principle, but just write down the result.

If you increase the concentration of something on one side of a reaction in equilibrium, the position of equilibrium will move towards the other side.

In this case, if you increase the concentration of oxygen, the position of equilibrium will move to the right, forming more sulfur trioxide.

Why does this happen?

Chemical reactions have never heard of Le Chatelier - they don't move their positions of equilibrium because Le Chatelier tells them to!

There have to be good physical reasons for the move. On the whole, the proper explanations for the changes of position of equilibrium are beyond this level, and are very unlikely to be asked in exams.

At this level, worry about what happens, not precisely why.

What happens if you lower the concentration of one of the substances in an equilibrium?

Let's go back to the general case again:

A + B    C + D

Suppose you lower the concentration of C in the equilibrium mixture.

Using Le Chatelier's Principle

According to Le Chatelier the reaction will respond by counteracting the change - making more C. The position of equilibrium will move to the right.

How to phrase this if your examiners don't like Le Chatelier's Principle

If you decrease the concentration of something in an equilibrium, the position of equilibrium will move to that side.

Use Le Chatelier to work out what happens, and then write down the result without mentioning Le Chatelier.

Turning an equilibrium into a one-way reaction

If you take the last example to extreme, you could constantly remove C from the reaction as soon as it is formed.

If you did that, it wouldn't be available to take part in the back reaction - the backward rate would fall to zero. So you now have a one-way reaction.

There are examples of that involving liquids in reactions in organic chemistry where the product you want has the lowest boiling point of anything in the mixture.

You can just distil that off and collect it as soon as it is formed. That prevents the back reaction from happening.