An Introduction to Chemical Equilibria

This page looks at the basic ideas underpinning the idea of a chemical equilibrium. It talks about reversible reactions and how they behave if the system is closed. This leads to the idea of a dynamic equilibrium, and what the common term "position of equilibrium" means.

A reversible reaction is one which can be made to go in either directiclass="dynamic-rect" on depending on the conditions.

If you pass steam over hot iron the steam reacts with the iron to produce a black, magnetic oxide of iron called triiron tetroxide, Fe₃O₄.

The hydrogen produced in the reaction is swept away by the stream of steam.

Under different conditions, the products of this reaction will also react together. Hydrogen passed over hot triiron tetroxide reduces it to iron. Steam is also produced.

This time the steam produced in the reaction is swept away by the stream of hydrogen.

These reactions are reversible, but under the conditions normally used, they become one-way reactions. The products aren't left in contact with each other, so the reverse reaction can't happen.

A closed system is one in which no substances are either added to the system or lost from it. Energy can, however, be transferred in or out at will.

In the example we've been looking at, you would have to imagine iron being heated in steam in a closed container. Heat is being added to the system, but none of the substances in the reaction can escape. The system is closed.

As the triiron tetroxide and hydrogen start to be formed, they will also react again to give the original iron and steam. So, if you analysed the mixture after a while, what would you find?

You would find that you had established what is known as a dynamic equilibrium. To explain what that means, we are going to use a much simpler example

Imagine a substance which can exist in two forms – a blue form or an red form – and that each form can react to give the other one. We are going to let them react in a closed system. Neither form can escape.

Assume that the blue form turns into the red one much faster than the other way round. In any given time, these are the chances of the two changes happening:

You can simulate this very easily with some coloured paper cut up into small pieces (a different colour on each side), and a die.

The following are some real results.

Starting with 16 blue squares, and rolling the die for each, if the roll was 3 or below, the square was flipped.

A blue square was turned into an red square if I threw a 1, 2 or 3.

A red square was turned into a blue square only if I threw a 1.

After rolling for all 16 squares, I started the process all over again. The diagrams show the results of doing this 11 times (plus the original 16 blue squares).

Now, you can see the process happening once every second. Watch it until you're confident about what is going on.

You can see that the "reaction" is continuing all the time. The exact pattern of red and blue is constantly changing. However, the overall numbers of red and of blue squares remain remarkably constant – most commonly, 12 red ones to 4 blue ones.

The reaction has reached equilibrium in the sense that there is no further change in the numbers of blue and red squares. However, the reaction is still continuing. For every red square that turns blue, somewhere in the mixture it is replaced by a blue square turning red.

This is known as a dynamic equilibrium. The word dynamic shows that the reaction is still continuing.

You can show dynamic equilibrium in an equation for a reaction by the use of special arrows. In the present case, you would write it as:

It is important to realise that this doesn't just mean that the reaction is reversible. It means that you have a reversible reaction in a state of dynamic equilibrium.

The change from left to right in the equation (in this case from blue to red as it is written) is known as the forward reaction. The change from right to left is the back reaction.

In the example we've used, the equilibrium mixture contained more red squares than blue ones. Position of equilibrium is a way of expressing this. You can say things like:

If the conditions of the experiment change (by altering the relative chances of the forward and back reactions happening), the composition of the equilibrium mixture will also change.

For example, if changing the conditions produced more blue in the equilibrium mixture, you would say "The position of equilibrium has moved to the left" or "The position of equilibrium has moved towards the blue".

What happens if you started the reaction with red squares rather than blue ones, but kept the chances of each change happening the same as in the first example? This is the result of my "reaction".

Once again, you can see that exactly the same position of equilibrium is being established as when we started with the blue squares. You get exactly the same equilibrium mixture irrespective of which side of the equation you start from – provided the conditions are the same in both cases.

This is the equation for a general reaction which has reached dynamic equilibrium:

How did it get to that state? Let's assume that we started with A and B.

At the beginning of the reaction, the concentrations of A and B were at their maximum. That means that the rate of the reaction was at its fastest.

As A and B react, their concentrations fall. That means that they are less likely to collide and react, and so the rate of the forward reaction falls as time goes on.

In the beginning, there isn't any C and D, so there can't be any reaction between them. As time goes on, though, their concentrations in the mixture increase and they are more likely to collide and react.

With time, the rate of the reaction between C and D increases:

Eventually, the rates of the two reactions will become equal. A and B will be converting into C and D at exactly the same rate as C and D convert back into A and B again.

At this point there won't be any further change in the amounts of A, B, C and D in the mixture. As fast as something is being removed, it is being replaced again by the reverse reaction. We have reached a position of dynamic equilibrium.