O Level Notes : Chemistry - Ammonia and its Uses

Nitrogen:

Nitrogen is a gas. It is the main constituent of air (78%). It plays an important role in the formation of animal and plant protein. It is also used in the manufacture of ammonia gas, an important industrial gas, through the Haber Process. We will learn how in this chapter.

Haber Process:

Ammonia is the most important compound of nitrogen. It is produced industrially by the Haber Process.

The raw materials for the process are hydrogen and nitrogen. Hydrogen is produced industrially by cracking oil, and nitrogen from liquefaction of air. These two gases, when obtained, are combined directly in a ratio of 3:1, and are passed over an iron catalyst at a temperature of 450°C and a pressure of 200 atm.

The reaction between hydrogen and nitrogen to form ammonia is a reversible process. This means that some of the ammonia formed may revert back to nitrogen and hydrogen. So to achieve the maximum yield of ammonia at the minimum cost, the reaction conditions are very carefully controlled.

• The yield of ammonia is increased under high pressures. High pressure also increases the rate of reaction. However, maintaining high pressure is costly because expensive equipment is required.
• A lower temperature increases the yield of ammonia and reduces the decomposition of hydrogen to nitrogen. However, a lower temperature also results in a slow reaction. At industry level, we cannot afford slow reactions. Therefore the high temperature of 450°C is a compromise for a faster reaction.
• Even the high temperature and high pressure used to react the nitrogen and hydrogen result in a slow reaction, so an iron catalyst is used to speed up the reaction.

Displacement of Ammonia from its Salts:

Another way of obtaining ammonia gas is through the displacement of ammonia from its salt. For example,

NH₄Cl (s) + NaOH (aq)      NH₃ (g) + H₂O (l) + NaCl (aq)

In this reaction, you can see that ammonia gas is produced on heating. Whenever an ammonium salt is heated with an alkali, ammonia is displaced from the salt. Lets take the example of another reaction.

2NH₄Cl (s) + Ca(OH)₂ (aq)      2NH₃ (g) + 2H₂O (l) + CaCl₂ (aq)

Testing for Ammonia Gas:

Ammonia is the only common alkaline gas, so it can be identified by moist red litmus paper turning blue. However, we will learn a more specific chemical test for it now. In this test, we hold a glass rod dipped into some concentrated hydrochloric acid close to the suspected ammonia. This will give off fumes of hydrogen chloride gas which, in the presence of ammonia, form a dense, white ‘smoke’ of ammonium chloride.

In the same way, mixing a gas jar of hydrogen chloride and ammonia gas produces the same dense, white smoke, which is actually ammonium chloride.

Uses of Ammonia:

The production of ammonia is done on large scale and large quantities are produced because it has many important uses.

Manufacture of Fertilizers:

The main use of ammonia is in the manufacture of fertilizers. Approximately 75% of all ammonia produced is converted into various ammonium compounds like ammonium sulphate (NH₄)₂SO₄, ammonium nitrate NH₄NO₃, and urea NH₂CONH₂. These compounds are called nitrogenous fertilizers. They are solids for easy handling, and water soluble so that they seep into the soil to be absorbed by the roots of the plant. These fertilizers provide nitrogen to the plant, which is an important element for healthy plant growth, as it helps in making the proteins which are needed for healthy growth of stems and leaves.

There are other important elements for healthy plant growth, too, such as phosphorus, which is necessary for good root growth, and potassium, which is important in the development of flowers and food. These important elements come from other sorts of fertilizers. If a fertilizer contains only one of the important elements, it is called a straight fertilizer. If these fertilizers are mixtures of the important elements, they are called compound fertilizers.

Beside these elements, there are also trace elements needed for the healthy growth of a plant, for example calcium, magnesium, sodium, sulphur, and tiny amounts of copper, zinc, boron, manganese and iron.

But what you basically must keep in mind in this chapter is that how ammonia is used to manufacture fertilizers. These are artificial fertilizers. The nitrogen content of such a fertilizer can be calculated by percentage composition.

The use of artificial fertilizers, however, has its disadvantages. Large scale use of these causes a pollution problem eutrophication: the high solubility of the fertilizer allows it to be leached from the soil and washed into streams, and when it finally settles in still water, it causes algae to grow and removes oxygen from the water.

Manufacture of Nitric Acid:

Nitric acid is made by the catalytic oxidation of ammonia over heated platinum. Oxidising ammonia produces oxides of nitrogen which can then be dissolved in water to produce nitric acid.

Most of the nitric acid made is used to make the all-important fertilizers. Other uses of nitric acid include making explosives and making dyes.

Use as a Solvent:

Aqueous ammonia is used as a degreasing agent, as it is a good solvent of grease and fat.

Let us go through some questions now, so that we can get a clearer idea of the whole chapter.

Q1. Which of these compounds does not always contain nitrogen?

1. Protein
2. Fertilizer
3. Nitrate
4. Nitrite

The answer to that question, as most of you might have already deduced, is B, Fertilizers. I mentioned this fact earlier in my lecture that fertilizers are generally made up of ammonia, potassium, or phosphorus, not necessarily ammonia.

Now solve this question yourself.

Q2. Which of the following reactants will not produce ammonia on heating?

1. Ammonium chloride and potassium hydroxide
2. Ammonium sulphate and calcium oxide
3. Potassium hydroxide and sodium nitrate
4. Sodium hydroxide and ammonium nitrate