C3: The Production of Ammonia

Reversible reactions don't always go to completion but they can still be used efficiently in continuous processes such as the Haber process.

The Haber process is used to manufacture Ammonia. The raw materials for this process are...

• nitrogen - from fractional distallation of liquid air
• hydrogen - from natural gas and steam.

Tje purified nitrogen and hydrogen are passed over an iron catalyst at a...

• high temperature (about 450 degrees celsius)
• high pressure (about 200 atmospheres).

These reaction conditions are chosen to produce a reasonable yield quickly. Even so, only some of the hydrogen and nitrogen react together to form ammonia.

Some of the hydrogen and nitrogen reacts to form ammonia. The ammonia produced can break down back into hydrogen and nitrogen.

In a closed system, no reactants are added and no products are removed. When a reversible reaction occurs in a closed system, an equilibrium is achieved when the reactions occur at exactly the same rate in each direction. The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.

Equilibrium - the state in which a chemical reaction proceeds at the same rate as is reverse direction (the reactants are balanced)

In an exothermic reaction... 

• if the temperature is raised then the yield decreases
• if the temperature is lowered, the yield increses.

 In an endothermic reaction...

• if the temperature is raised the yield decreases
• if the temperature is lowered, the yield decreases.

In gaseous reactions, an increase in pressure favours the reaction that produces the leat number of molecules.

To maximise the yield of ammonia, you cool it down (decrease the temperature) so it will work to counteract that so will work harder in the exothermic direction. However decreasing temperature also decreases the rate so you cannot cool it down too much.

A catalyst will have no effect on the yield. It will lower the activation energy though increasing the speed of the reaction.

These factors, along with reaction rates, determine the optium conditions for the Haber process.