Henry Le Chatelier was a French chemist who studied reversible reactions and equilibrium and he made some important observations.

Equilibrium is the point in a reversible reaction where the rate of the forward and reverse reactions are the same. We can show this as: During a reversible reaction the reactants turn into products and the products back into reactants again. The important point to remember is that these two reactions occur at the same time. The forward reaction will proceed at a given rate, labelled R_f and the back reaction which turns products back into reactants will also proceed at a given rate, labelled R_b in the example above. The rate of the forward and reverse reactions can be altered if we change the reaction conditions, e.g. for example if we alter the concentration of one of the reactants or products or change the temperature of the equilibrium mixture, but recall that at equilibrium the rate of the forward reaction, R_f and the back reaction, R_b will be the same.

Equilibrium is a very stable low energy point for a reaction, at equilibrium the reaction is nice and happy!! It's like the ball at the bottom of the curve. If you push on it so that it moves away from the bottom of the curve the ball will roll back down again, to the low energy point. Well chemical reactions are like that- if a reaction is at equilibrium (rate of forward and reverse reactions are the same) and you come along and heat it up or put it under pressure the reaction will re-adjust itself to get back to a new equilibrium position, that is a low energy state. At this new equilibrium position the amounts of reactants and products maybe different but the rate of the forward and reverse reactions will be the same e.g. Consider the reaction of nitrogen and hydrogen to make ammonia, the Haber Process.

Nitrogen_(g) + hydrogen_(g) ⇌ ammonia _(g)

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

At equilibrium the mixture contains only a small amount of ammonia and is mostly nitrogen and hydrogen. We would say the position of equilibrium lies to the left (reactants). Since ammonia is a useful substance we need to shift the position of equilibrium so that it moves more to the right (products).
The position of equilibrium for a reaction depends on several variables, these include concentration, temperature and pressure (if gases are involved in the reaction). We can adjust the amounts of reactants and products at equilibrium by simply changing the reaction conditions. The trick is to know exactly what conditions to change and that is exactly what Henri Le Chatelier figured out.
Le Chatelier's Principle is the idea that a system (reacting chemicals) at equilibrium will oppose any changes applied to it. We will use the Haber Process to try and explain Le Chatelier's principle in more detail. The equation for the Haber process is shown again below:

Nitrogen_(g) + hydrogen_(g) ⇌ ammonia _(g)

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)

N_2(g) + 3H_2(g) → 2NH_3(g)

2NH_3(g) → N_2(g) + 3H_2(g)

So if you have a mixture of nitrogen, hydrogen and ammonia in a flask at equilibrium. The position of equilibrium for this reaction lies very much to the left and unfortunately there is very little ammonia in the flask. The challenge for chemists is how to adjust this equilibrium so that the amount of the valuable product ammonia can be increased.

• What would happen if you warmed the flask? Well according to Le Chatelier's principle the reaction should oppose any change you make to it, that is it will try and remove the heat you added. How would the added heat be removed? An endothermic reaction will remove heat! The only way to do this is to force the equilibrium position even more to the left, that is to increase the back reaction since this is an endothermic reaction:

2NH_3(g) → N_2(g) + 3H_2(g)

• However this is bad news as it will reduce the amount of ammonia in the flask. So to increase the amount of ammonia instead of heating the flask, cool the mixture in the flask. Again according to Le Chatelier's Principle the system will try to generate heat, to oppose the cooling, so it will force more nitrogen and hydrogen to react to make ammonia since the forward reaction is exothermic. However we know that cooling a reaction will slow it down. So ultimately cooling it down will increase the amount of ammonia in the flask but it might take a very long time for the new equilibrium position in which there is more ammonia present to become established.
• What would happen if you increased the pressure to the position of equilibrium in the Haber process? Well first thing is that pressure will only have an effect if there are gases involved in the reaction. So since all the chemical reacting here are gases then pressure will have an effect. At room temperature and pressure 1 mole of any gas will occupy 24 litres. So for the reactants the total number of moles is 4 so this will occupy a volume of 96 litres. For the products we have 2 moles of gas, so this will occupy 48 litres at room temperature and normal pressure (standard temperature and pressure S.T.P). So obviously we can think of the reactants as the high pressure side of the equation and the products as the low pressure side. So if you increase the pressure on this reaction then it will force the equilibrium to the right, that is the equilibrium mixture will contain more ammonia. If you reduce the pressure then the equilibrium mixture will contain more reactants and less ammonia.
• Similar arguments are true if you add more reactants or products then the system will no longer be at equilibrium and it will adjust the concentrations to achieve a new equilibrium position. If you add more reactants it will force the equilibrium to the right, that is towards the products, similarly if you add more products it will force the equilibrium towards the left side, that is the amount of reactants will increase.
• How about adding a catalyst? Catalysts do not have any effect on the position of equilibrium, they will not affect the amount of reactants or products in the equilibrium mixture. What a catalyst will do is increase the rate of both the forward and reverse reaction in an equilibrium mixture. The catalyst will allow the system to achieve equilibrium faster.

Dinitrogen tetroxide

Dinitrogen tetroxide (N₂O₄) is colourless toxic gas with a very unpleasant smell. Despite the fact that it is a colourless gas it appears reddy brown since it exists in an equilibrium mixture with brown nitrogen dioxide gas. This can be shown as:

nitrogen dioxide_(g)⇌ dinitrogen tetroxide_(g)

2NO_2(g) ⇌ N₂O_4(g)

• placed a flask of this equilibrium mixture of gases in a beaker of iced water? Well according to Le Chatelier's principle the reaction should oppose any changes made to it. So in this case the change is a reduction in temperature so the position of equilibrium will shift to the right, that is make make dinitrogen tetroxide since this reaction is exothermic and will generate heat energy. This would mean the colour of the equilibrium mixture of gases in the flask should fade, since the concentration of the brown gas nitrogen dioxide is reducing.


• Using the opposite reasoning if we were to place a flask containing an equilibrium mixture of these two gases in a hot oven, what would happen to the position of equilibrium? Well according to Le Chatelier's principle the system will try to oppose the temperature rise, so it will push the equilibrium in the direction of the endothermic reaction, that is there will be more nitrogen dioxide in the equilibrium mixture. This means the flask will get darker. This is shown in the image oposite.

Le Chatelier's Principle and pressure

Key Points

• A reaction at equilibrium is one where the rate of the forward and reverse reactions are the same.
• Le Chatelier's principle explains what happens to reactions at equilibrium. According to his principle a reaction at equilibrium will change or alter the position of equilbrium in order to minimise or cancel out the change applied.

Check your understanding - Questions on Le Chatelier's Principle

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