C3 - Ammonia

Question 1

How is ammonia made?

Answer 1

The Haber Process.

      endo

N_{2 (g)}+ 3H_{2 (g)} <===> 2NH_{3 (g)}

       exo

The nitrogen comes from the air as 78% of the air is nitrogen.

Hydrogen comes from natural gas.

Question 2

Under what conditions is ammonia made?

Answer 2

450^oC (high temperature)

200 atmospheres (moderate temperarture)

Iron Catalyst (lowers the activation energy)

These conditions are used to avoid an equilibrium (where the forward and backwards reactions are balanced). The Le Chatelier principle says if we increased the temperature the system would do the opposite and favour the cold, endothermic, backwards direction so we should use low temperatures so the reaction favours the hot, forward direction.

However, we have to compromise. If we used low temperatures then the reaction would be too slow and not have enough energy so instead we use 450^oC.

When making ammonia we start with four molecules and end with 2 molecules. In gaseous reactions, increasing the pressure favours the reaction which produces the least molecules. As this is the forward direction we should use high pressures.

However, we have to compromise. If we used high pressures it would be very expensive and dangerous so we make ammonia with 200 atmospheres of pressure.