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Nitric acid and ammonia

 

A major use of ammonia is in the manufacture of fertilisers. Some ammonia is first converted into nitric acid, HNO3.

Nitric acid is used as raw material for production of plastics, fabrics, dyes, detergents, paints and explosives, as well as fertilisers.

 

Production of ammonia

The Haber Process is used for the manufacture of ammonia from nitrogen and hydrogen.

The Haber Process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic.

A flow scheme for the Haber Process looks like this:

Haber Process

 

According to Le Chatelier's Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again.

In order to get as much ammonia as possible in the equilibrium mixture, you need as high a pressure as possible. 200 atmospheres is a high pressure, but not amazingly high. In addtion, at each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.

Recycling

 

Production of nitric acid from ammonia

Ammonia, NH3, is oxidised in the presence of a platinum/rhodium catalyst to form nitrogen monoxide:

4NH3 + 5O2 → 4NO + 6H2O

The nitrogen monoxide is then further oxidised to form nitrogen dioxide:

2NO + O2 → 2NO2

The nitrogen dioxide is then dissolved into water in the presence of oxygen to form concentrated nitric acid:

4NO2 + 2H2O + O2 → 4HNO3

 

 

Production of fertilisers

The most effective fertilisers contain nitrogen because plants need it in order to produce proteins. Phosphorus and potassium are also essential elements commonly found in fertilisers.

Many fertilisers are ammonium salts. They can also be made from ammonium hydroxide (ammonia solution). If ammonium hydroxide is used, water is also produced in the neutralisation reaction.

Ammonia + nitric acid → ammonium nitrate

Ammonium hydroxide + nitric acid → ammonium nitrate + water

 

Questions

1. There are a number of ways that the choice of conditions affects the Haber process:

• their effect on the position of equilibrium;

• their effect on the rate of the reaction;

• their effect on the economics of the process.

By considering each of these (where relevant), explain the choice of the conditions respect to

- The choice of temperature

- The choice of pressure

- The use of the catalyst.

 

2. Look at this reaction: 2 NO(g)  +  O2(g)    2 NO2(g)

Increasing the pressure would favour the formation of which gas?

A. Nitrogen monoxide

B. Nitrogen dioxide

 

3. Which substance is oxidized in the first stage of the process to make nitric acid?

A. Oxigen

B. Nitrogen monoxide

C. Ammonia

 

Activities

1. Read the information in button text about fertiliser and make a discussion about the way Industrial sector produce fertilisers (base your discussion according to the next questions).

- Are fertiliser the only way to prevent the spread of plagues?

- What do you think about natural products? Such as, essential oils

2. What will happen if we mix ammonia and nitric acid? 

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