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Reversible Reactions

In some chemical reactions, the products can react to form the original reactants.

These are called reversible reactions.

For example, ammonium chloride is a white solid that breaks down into ammonia and hydrogen chloride gases when heated.

These gases react to form ammonium chloride.

ammonium chloride

ammonia

+

hydrogen chloride

(white solid)

(colourless gases)

The symbol  means the reaction is reversible, it can go in both directions at the same time.

Some of the gases formed immediately react to form ammonium chloride again.

This reaction is used as a test for ammonia.

If blue copper sulphate crystals are heated they turn white and give off steam. The white powder is anhydrous copper sulphate.

hydrated copper sulphate [+ heat energy]

anhydrous copper sulphate

+

water

(blue crystals)

 

(white)

 

 

If water is added to white, anhydrous copper sulphate it turns blue again. This is a test for water.

Ammonia gas is made in the Haber process. The reaction is reversible

nitrogen

+

hydrogen

   

ammonia

N2

+

3H2

 

2NH3

Equilibrium



When the rate of the forward reaction is the same as the rate of the reverse reaction it is said to be in equilibrium.

So for this reaction, at equilibrium for every two molecules of ammonia formed, two others break down into nitrogen and hydrogen again.

The relative amounts of all the reacting substances at equilibrium depend on the reaction conditions.

These are important when determining optimum conditions for industrial processes, because the manufacturer will want to make a lot of the product quickly.

Temperature



Increasing the temperature will increase the rate of any reaction, because the particles move faster and collide more frequently.

However, in reversible reactions increasing temperature will favour the endothermic reaction and increase the yield in this direction, although the rate of the forward and backward reactions will both be increased equally.

Endothermic reactions take in heat energy. Increasing the temperature will increase the yield of product and decreasing the temperature will decrease the yield.

Exothermic reactions give out heat. Increasing the temperature will decrease the yield of product, whereas decreasing the temperature will increase the yield.

In the Haber Process, increasing the temperature will decrease the yield of ammonia because the forward reaction is exothermic.

Therefore, only a moderately high temperature of around 450ºC is used, so the rate is fairly fast and the yield is reasonable.

Pressure



In gaseous reactions, an increase in pressure will favour the reaction which produces the least number of molecules, because these take up less space.

In the Haber Process increasing the pressure increases the yield of ammonia, because the forward reaction produces two molecules of gas, whereas the reverse reaction produces four molecules of gas.

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