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Preparation And Properties Of Ammonia - Practice Questions & MCQ

Edited By admin | Updated on Sep 25, 2023 25:24 PM | #NEET

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Ammonia(NH3) is considered one the most difficult concept.
10 Questions around this concept.

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EASY

Ammonia forms the complex ion $\left [ Cu(NH_{3})_{4} \right ]^{2+}$ with copper ions in alkaline solutions but not in acidic solutions. What is the reason for it?

In acidic solutions hydration protects copper ions.

In acidic solutions protons coordinate with ammonia molecules forming $NH_{4}^{+}$ ions and $NH_{3}$ molecules are not available.

In alkaline solutions insoluble $Cu(OH)_{2}$ is precipitated which is soluble in excess of any alkali.

Copper hydroxide is an amphoteric substance.

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Ammonia(NH3)

Preparation
Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter e.g., urea.

$\mathrm{NH_{2}CONH_{2}\: +\: 2H_{2}O\: \rightarrow \: (NH_{4})_{2}CO_{3}\: \rightleftharpoons \: 2NH_{3}\: +\: H_{2}O\: +\: CO_{2}}$

On a small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or calcium hydroxide.

$\mathrm{2NH_{4}Cl\: +\: Ca(OH)_{2}\: \rightarrow 2NH_{3}\: +\: 2H_{2}O\: +\: CaCl_{2}}$

$\mathrm{(NH_{4})_{2}SO_{4}\: +\: 2NaOH\: \rightarrow \: 2NH_{3}\: +\: 2H_{2}O\: +\: Na_{2}SO_{4}}$

On a large scale, ammonia is manufactured by Haber’s process.

$\mathrm{N_{2}(g)\: +\: 3H_{2}(g)\: \rightleftharpoons \: 2NH_{3}(g)}$

In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 10⁵ Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K₂O and Al₂O₃ to increase the rate of attainment of equilibrium. The flow chart for the production of ammonia is shown in the figure given below. Earlier, iron was used as a catalyst with molybdenum as a promoter.

Properties
Ammonia is a colourless gas with a pungent odour. Its freezing and boiling points are 198.4 and 239.7 K respectively. In the solid and liquid states, it is associated through hydrogen bonds as in the case of water and that accounts for its higher melting and boiling points than expected on the basis of its molecular mass. The ammonia molecule is trigonal pyramidal with the nitrogen atom at the apex. It has three bond pairs and one lone pair of electrons as shown in the structure. Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the formation of OH^–ions.

$\mathrm{NH_{3}(g)\: +\: H_{2}O(l)\: \rightleftharpoons \: NH_{4}^{+}(aq)\: +\: OH^{-}(aq)}$

It forms ammonium salts with acids, e.g., NH₄Cl, (NH₄)₂SO₄, etc. As a weak base, it precipitates the hydroxides (hydrated oxides in case of some metals) of many metals from their salt solutions. For example,

$\mathrm{ZnSO_{4}(aq)\: +\: 2NH_{4}OH(aq)\: \rightarrow \: Zn(OH)_{2}(s)\: +\: (NH_{4})_{2}SO_{4}(aq)}$

$\mathrm{FeCl_{3}(aq)\: +\: NH_{4}OH(aq)\: \rightarrow \: Fe_{2}O_{3}.xH_{2}O(s)\: +\: NH_{4}Cl(aq)}$

The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms linkage with metal ions and the formation of such complex compounds finds applications in detection of metal ions such as Cu²⁺, Ag⁺.

Uses

Ammonia is used to produce various nitrogenous fertilisers (ammonium nitrate, urea, ammonium phosphate and ammonium sulphate)
It is used in the manufacture of some inorganic nitrogen compounds, the most
important one being nitric acid.
Liquid ammonia is also used as a refrigerant.