Chemguide: Core Chemistry 14 - 16

The effect of changing the temperature in a dynamic equilibrium

This page looks at what happens when you change the temperature of the mixture 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 involving NO₂ and N₂O₄

Nitrogen dioxide, NO₂, is a dark brown gas.

Dinitrogen tetroxide, N₂O₄, is formed when two NO₂ molecules join together. On its own, this is a colourless gas.

At ordinary temperatures, this equilibrium is set up:

N₂O₄(g)    2NO₂(g)    ΔH = +56.9 kJ/mol

A typical equilibrium mixture will appear a medium brown colour due to the mixture of the two gases.

Notice that I have given a measure of the energy change during this reaction. You can ignore the units for the moment, but you should notice that the value is positive.

The figure given is for the forward reaction. The positive sign shows that heat is absorbed - it's an endothermic change.

If you have read the introductory page I mentioned at the top of this page, you will realise that the back reaction will be exothermic. The number will be the same, but the sign will be negative.

What would you expect to happen if the mixture was cooled?

Le Chatelier says that if you change a condition, the system will respond by countering the change. How can it do that?

If you cool it, it can counter by favouring the exothermic reaction (the formation of N₂O₄) because this evolves heat. So the mixture will become lighter in colour, because it will contain more colourless gas and less brown gas.

What would you expect to happen if the mixture was heated?

This time the reaction can counter the change by favouring the endothermic reaction - the one that absorbs heat. That is the forward reaction, and the mixture will become darker brown.

The video shows this happening:

How can you say what happens without mentioning Le Chatelier?

Use Le Chatelier's Principle to work out what happens, and then write that down without mentioning Le Chatelier.

• If you decrease the temperature, it will shift the position of equilibrium in the direction of the exothermic change.

• If you increase the temperature, it will shift the position of equilibrium in the direction of the endothermic change.

The reversible reaction used in the manufacture of ammonia

Ammonia, NH₃, is manufactured from nitrogen and hydrogen and involves this equilibrium:

N₂(g) + 3H₂(g)    2NH₃(g)    ΔH = -92 kJ/mol

The forward reaction this time is exothermic (a negative value for ΔH). Would the yield of ammonia be improved by a low temperature or a high temperature?

Work it out before you go on.

A low temperature shifts the position of equilibrium in the direction of the exothermic change. (Using Le Chatelier, if you decrease the temperature the reaction will counter this by favouring the reaction which produces heat.)

So you would expect the reaction to be done at a low temperature.

But it isn't! It is actually done at about 400 - 450°C.

The problem is that although you might in theory get a really good yield of ammonia at a low temperature, you might have to wait for ever to get it because the reaction would be so slow as to be non-existent.

400 - 450°C is a compromise temperature. It produces a fast reaction and gives a yield of about 15% of ammonia in the equilibrium mixture.

That doesn't sound much, but the ammonia can be extracted from the reaction mixture, and the remaining unreacted gases passed through the reactor again (and again and again!).

We will look at this reaction in some more detail later on in the course.