Nitrogen

Symbol: N
Atomic number: 7
Molecular formula: N₂
Atomic mass: 14 amu
Electronic configuration: 1s²2s²2p³
Electronegativity: 3

Position of nitrogen in periodic table:

Nitrogen lies in group 15 or VA in the modern periodic table. The elements of group 15 are nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi). Nitrogen is in the second period and is a p-block element.

Reasons for inertness of nitrogen:

The reactivity of any element in a chemical reaction is determined by the state of valence electrons. The ground state electronic configuration of the nitrogen atom is 1s²2s²2p³. 1s² and 2s² are completely filled orbitals. They do not have a significant role in a chemical reaction. Half-filled and full-filled atomic orbitals are stable towards chemical reaction. As nitrogen has half-filled p-orbitals, it is less reactive under ordinary conditions. The nitrogen molecule consists of an N to N triple bond which is a strong bond. It requires a high amount of energy to cleavage the N to N triple bond so as to carry out a chemical reaction.

Active nitrogen:

The energized nitrogen atom which can play an active role in chemical transformation is called active nitrogen. Such active nitrogen may be atomic nitrogen or excited neutral nitrogen molecule. Besides this, excited nitrogen containing molecules such as graphitized carbon nitride (g-C₃N₄) may also serve as a source of active nitrogen.

Active nitrogen can be generated by passing an electric discharge through nitrogen gas at a very low pressure of 2 mm of Hg.

Compounds of nitrogen (Ammonia- NH₃):

Manufacture of ammonia by Haber’s process:

Principle:
When a mixture of nitrogen and hydrogen in the ratio of 1:3 by volume is heated to about 450°C under a pressure of 200-900 atm in the presence of finely divided iron as a catalyst and some molybdenum as a promoter, ammonia is formed.

Conditions for high yield of ammonia:

Since the above reaction is reversible, the exothermic reaction occurs by a decrease in volume. It is favored by Le-Chatlier’s principle:

1. Optimum temperature: Since the reaction is exothermic, low temperature favors the forward reaction. But if the temperature is too low, the reaction will be slow. So optimum temperature of 450°C is used.

2. High pressure: The formation of ammonia occurs by the decrease in volume. So, high pressure of 200-900 atm is required.

3. High concentration: High concentration of nitrogen and hydrogen favors the formation of ammonia.

4. Use of catalyst: To speed up the rate of reaction, finely divided iron(catalyst) with molybdenum(promoter) is used.

5. Purity of N₂ and H₂: The reaction mixture should be free from impurities.

Laboratory preparation of ammonia:
NH₄Cl + Ca(OH)₂ → CaCl₂ + NH₃ + H₂O

Drying of ammonia:

Moist ammonia gas is dried by passing it over quick lime (CaO). Ammonia being basic cannot be dried by using conc. H₂SO₄, anhydrous CaCl_2, or phosphorous pentoxide since they react with ammonia gas.

2NH₃ + H2SO4 → (NH4)2SO4
P₂O₅ + 6NH₃ + 3H₂O → 2(NH₄)₃PO₄
CaCl₂ + 8NH₃ → CaCl₂.8NH₃

Physical properties:

1. It is a colorless gas with pungent odour.
2. Its boiling point is -33.4°C and its melting point is -78°C.
3. It is highly soluble in water.
4. It is neither combustible nor a supporter of combustion.

Chemical properties:

1. Basic nature: Ammonia is basic in nature due to the presence of lone pair of electrons in the nitrogen atom. It turns moist red litmus paper into blue and phenolphthalein solution into pink.

i. Action with water: It dissolves in water to form ammonium hydroxide. However, ammonium hydroxide does not exist. And, they get converted into ammonium ion (NH₄⁺ ) and hydroxide ion (OH^–).
NH₃ + H₂O → NH₄OH ⇌ NH₄⁺ + OH^–

ii. Action with acids:

2. Combustibility: Ammonia is neither combustible nor a supporter of combustion in normal condition. When it is heated in oxygen, it burns to give nitrogen and water.

In presence of heated platinum, ammonia is oxidized to nitric oxide by the action of air.

3. Action with CuSO₄ solution: When ammonia solution is passed into copper sulphate solution, at first a blueish white precipitate of Cu(OH)₂ is formed which dissolves in excess of ammonia forming a deep blue colored soluble complex compound, tetramine copper sulphate.

4. Action with FeCl₃ solution: Ammonia solution reacts with ferric chloride solution to form a reddish-brown precipitate of ferric hydroxide solution.

5. Action with mercurous nitrate paper: Ammonia turns mercurous nitrate paper into black. It is due to the formation of basic mercuric amidonitrate and mercury.

Applications of ammonia:

• To manufacture fertilizers like urea, ammonium sulphate, ammonium phosphate, ammonium nitrate, etc.
• As an analytical agent
• To make dyes, plastic, explosives, fibers, etc.
• Liquid ammonia is used as a refrigerant
• To remove oils and grease
• In the manufacture of nitric acid by Ostwald’s process
• In the manufacture of washing soda by Solvay process

Harmful effect of ammonia:

• Ammonia has a pungent smell and can cause irritation on the eyes, nose, throat, lungs, etc.
• Exposure to ammonia can cause nausea, skin irritation, and dizziness in humans.
• It increases the pH of the soil. It directly damages and increases the death of sensitive species like lichens and mosses.

Some Important Questions

1. What happens when ammonia is passed through CuSO₄ solution till excess?
2. Why does ammonia turn mercuric paper into black?
3. Why ammonia cannot be dried by conc. H₂SO4?