The Haber process: choosing a pressure

You will be expected to explain the choice of a high pressure for the Haber process.

Look at the symbol equation for the reaction in the Haber process:

N₂(g) + 3H₂(g) 2NH₃(g)

There are 1 + 3 = 4 molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] of gas on the left of the equation, only two molecules of gas on the right. In an equilibrium [equilibrium: If the rate of the forward reaction and the rate of the back reaction in a reversible reaction are equal, the reaction is in equilibrium. ] involving gases, an increase in pressure favours the reaction which produces the smallest number of molecules. In this case, an increase in pressure favours the forward reaction, and more ammonia is produced.

There is a limit to the pressure that can be used industrially, because very high pressures require very strong and expensive equipment. This means a compromise pressure is chosen - high enough to get a good yield of ammonia, but not so high that it would add too much to the costs of the process. The pressure chosen is usually about 200 atmospheres - equivalent to about half the pressure of the water around the wreck of the RMS Titanic.

Now try a Test Bite - higher