Chemical Resources - Making Ammonia

Most of the substances we use in our everyday lives are manufactured; very few are likely to be used in their natural state. The cost of making new and useful substances varies. Here are the factors:                                          

• Cost of starting materials           

Different starting materials cost different amounts. If unreacted materials are recycled.

• Cost of the equipment (plant) needed

Complex machinery and high pressures increase the cost of the plant.

• Labour costs and wages

The more workers needed the higher the cost. Cost may be reduced if process is automated.

• Price of the energy needed

Gas and electricity are expensive. Use of high temperatures increase energy cost.

• The speed at which the new substance is made

Catalysts increase rate of chemical reactions, reducing costs.

Ammonia is an important substance; over 140 million tonnes of it are manufactured around the world each year. It's a good example of how new substances are made.

It has a sharp smell and is often used in cleaning fluids as it reacts with grease making it easier to remove. It has other uses too such as making nitric acid and fertilisers. E.g ammonia and nitric acid react together to make ammonium nitrate which is an important source of nitrogen in artificial fertilisers.

Without fertilisers, crop yields would be much smaller as they are added to the soil to replace the minerals that plants use up as they grow. Fertilisers are so important to the world food production that they use around 80% of all the ammonia manufactured.

Ammonia is made from nitrogen and hydrogen using the Haber process:

 nitrogen + hydrogen    ammonia

     N_{2           +}            3H₂               2NH₃

The hydrogen comes from cracking oil fractions or natural gases. The nitrogen comes from air. Nitrogen is a relatively unreacted gas; the conditions needed to get it to react with hydrogen need to be chosen carefully such as:

• A high pressure
• A temperature of around 450°C
• Adding an iron catalyst (to speed up reaction)

Costs are reduced by recycling unreacted nitrogen and hydrogen.