Heat is given out during the reaction to make ammonia heat is produced. This is called an exothermic reaction. As the reaction making ammonia is reversible, adding heat causes the back reaction to be favoured. This means that if ammonia is required (that is, the forward reaction is to be favoured) then a lower temperature should be used.
Temperature in the Haber process
Although the forward reaction is favoured by a lower temperature, too low a temperature means that the rate of the forward reaction is too slow. The particles have very little kinetic energy at low temperatures so do not move a great deal and collide infrequently. Hence a moderate temperature is used (about 450°C).
Use of a catalyst
A catalyst speeds up a reaction without altering the position of equilibrium. In the Haber process a catalyst of iron is used. The iron speeds up both forward and reverse reactions but will actually help in producing ammonia. Without the catalyst the overall production of ammonia is too slow.
Pressure in the Haber process
As 1 mole of nitrogen and 3 moles of hydrogen (a total of 4 moles of gas) react to form 2 moles of ammonia, the overall effect is a reduction in the number of moles of gas. This means that any factor that squeezes gas particles together to reduce gas volume favours the forward reaction.
The forward reaction is favoured by high pressure. A pressure of 200 atmospheres is used. If the pressure was higher the yield would increase but the cost of the apparatus to withstand higher pressures would be too great.
In short Chemical reactions are usually irreversible because of the energy put in to make them occur:
• some chemical reactions are, however, reversible
• the Haber process produces ammonia and involves a reversible reaction
• in the Haber process nitrogen reacts with hydrogen to form ammonia
• nitrogen is obtained from liquid air and hydrogen is obtained from heating steam with methane
• nitrogen and hydrogen react in the ratio 1:3 forming 2 units of ammonia
• the formation of ammonia is exothermic
• the Haber process uses a moderate temperature, a high pressure, and a catalyst
• catalysts speed up reactions without altering the position of equilibrium.
