Chapter 9 · ICSE Class 10 Chemistry (Selina Concise)

Study of Compounds: Ammonia

Ammonia feeds the world: most of it goes into fertilisers. Learn how it is made in the laboratory and in industry.

💨 Chapter 9

The Haber process for ammonia Nitrogen and hydrogen in the ratio one to three are compressed to about two hundred atmospheres and passed over an iron catalyst at four hundred and fifty to five hundred degrees Celsius. The ammonia is cooled and liquefied, and unreacted gases are recycled. N₂ + H₂ mixed in the ratio 1 : 3 purified gases Compressed to about 200 atmospheres high pressure favours more ammonia Reactor: iron catalyst, 450 to 500 °C N₂ + 3H₂ ⇌ 2NH₃ (promoter: molybdenum) Cooled: ammonia liquefies unreacted N₂ and H₂ are recycled to the reactor Liquid ammonia is drawn off and stored
9.1

Laboratory Preparation

Ammonia is made by heating ammonium chloride with calcium hydroxide: 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃. It is dried with quicklime (CaO) and collected by downward displacement of air, since it is lighter than air (vapour density 8.5) and very soluble in water.

1

Why is quicklime used to dry ammonia and not concentrated sulphuric acid?

Ammonia is basic and would react with the acid (and with calcium chloride or phosphorus pentoxide); quicklime is basic and does not react with it

2

Why is ammonia collected by downward displacement of air?

It is lighter than air and too soluble in water to be collected over water

3

How is ammonia gas identified?

It has a pungent smell, turns moist red litmus blue, and gives dense white fumes with hydrogen chloride

9.2

The Haber Process

Industrially, ammonia is made by the Haber process: N₂ + 3H₂ ⇌ 2NH₃ (a reversible, exothermic reaction). The conditions are about 200 atmospheres, 450 to 500 °C, and a finely divided iron catalyst with molybdenum as a promoter. Unreacted gases are recycled.

1

Why does high pressure favour the formation of ammonia?

Four volumes of reactants (1 of N₂ and 3 of H₂) form only two volumes of ammonia, so pressure favours ammonia

2

What volume of ammonia is formed from 30 litres of hydrogen with excess nitrogen?

3 volumes of H₂ give 2 volumes of NH₃, so 20 litres

9.3

Properties and Uses

Ammonia is extremely soluble in water, giving a weakly alkaline solution. It burns in oxygen but not in air: 4NH₃ + 3O₂ → 2N₂ + 6H₂O. Over hot platinum it is oxidised to nitric oxide: 4NH₃ + 5O₂ → 4NO + 6H₂O. It also reduces hot copper(II) oxide. It is used to make fertilisers and nitric acid.

1

What happens when ammonia is passed over heated copper(II) oxide?

The black oxide is reduced to reddish-brown copper, and nitrogen and steam form: 3CuO + 2NH₃ → 3Cu + N₂ + 3H₂O

2

Name two uses of ammonia.

Making fertilisers such as urea and ammonium sulphate, and manufacturing nitric acid

Practice

Exercises

1. Write the equation for the laboratory preparation of ammonia.

2. Name the drying agent used for ammonia.

3. Why is ammonia collected by downward displacement of air?

4. How does ammonia affect moist red litmus paper?

5. State the conditions used in the Haber process.

6. Which substance acts as a promoter in the Haber process?

7. What is the vapour density of ammonia? (N = 14, H = 1)

8. What volume of ammonia is formed from 30 litres of hydrogen with excess nitrogen?

9. Write the equation for ammonia burning in oxygen.

10. Why does the fountain experiment work with ammonia?

Next chapter — Study of Compounds: Nitric Acid →