Study of Compounds: Ammonia — ICSE Class 10 Chemistry Important Questions

Answer: (b) Finely divided iron.

Finely divided iron acts as the catalyst (with molybdenum as promoter) in $N_2+3H_2\rightleftharpoons 2NH_3$N₂+3H₂⇌ 2NH₃.

Answer: (b) Ammonia. $Mg_3N_2+6H_2O\rightarrow3Mg(OH)_2+2NH_3\uparrow$Mg₃N₂+6H₂O→3Mg(OH)₂+2NH₃↑. Metal nitrides such as $Mg_3N_2$Mg₃N₂ and $AlN$AlN give ammonia with warm water.

Answer: (a) Black solid turns reddish-brown.

$3CuO+2NH_3\rightarrow 3Cu+3H_2O+N_2$3CuO+2NH₃→ 3Cu+3H₂O+N₂. Black $CuO$CuO is reduced to reddish-brown copper, showing ammonia is a reducing agent.

Answer: (a) A pale blue precipitate that dissolves to a deep blue solution.

First $CuSO_4+2NH_4OH\rightarrow Cu(OH)_2\downarrow+(NH_4)_2SO_4$CuSO₄+2NH₄OH→ Cu(OH)₂↓+(NH₄)₂SO₄ (pale blue ppt); with excess it dissolves forming the deep blue complex $[Cu(NH_3)_4]^{2+}$[Cu(NH₃)₄]^2+.

Answer: (a) Ammonia is lighter than air (so collected by downward displacement of air, i.e. an inverted jar) and is highly soluble (so cannot be collected over water); R correctly explains A.

(i) $AlN+3H_2O\rightarrow Al(OH)_3+NH_3\uparrow$AlN+3H₂O→ Al(OH)₃+NH₃↑

(ii) $(NH_4)_2SO_4+2NaOH\xrightarrow{\Delta}Na_2SO_4+2H_2O+2NH_3\uparrow$(NH₄)₂SO₄+2NaOH→ (Δ)Na₂SO₄+2H₂O+2NH₃↑

Any ammonium salt gives ammonia when warmed with an alkali.

Dense white fumes of ammonium chloride are formed.

$NH_3+HCl\rightarrow NH_4Cl$NH₃+HCl→ NH₄Cl

This is the standard test used to identify ammonia gas.

A dry round-bottomed flask is filled with dry ammonia and fitted with a stopper carrying a jet tube and a dropper containing water. A small amount of water is squeezed in from the dropper.

Observation: Ammonia dissolves rapidly, creating a partial vacuum, so water rushes up the jet tube and gushes out as a fountain inside the flask.

It demonstrates that ammonia is extremely soluble in water. If red litmus (or phenolphthalein) is added to the water, the fountain appears blue (or pink) because the ammonia solution ($NH_4OH$NH₄OH) is alkaline.

Conditions: pressure $\approx200\text{ atm}$≈200 atm; temperature $450\text{-}500^\circ C$450-500° C; catalyst finely divided iron with molybdenum as promoter.

$N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)$N₂(g)+3H₂(g)⇌ 2NH₃(g) (exothermic).

Since the forward reaction is exothermic, a low temperature would favour a higher yield of ammonia, but at low temperature the reaction becomes very slow. A moderate temperature of about $450^\circ C$450° C is therefore chosen as a compromise: it gives a reasonable yield of ammonia at an acceptably fast rate.

(i) With heated copper(II) oxide: $3CuO+2NH_3\xrightarrow{\Delta}3Cu+3H_2O+N_2$3CuO+2NH₃→ (Δ)3Cu+3H₂O+N₂

Black $CuO$CuO is reduced to reddish-brown copper; ammonia is oxidised to nitrogen.

(ii) With chlorine:

Ammonia in excess: $8NH_3+3Cl_2\rightarrow6NH_4Cl+N_2$8NH₃+3Cl₂→6NH₄Cl+N₂. Dense white fumes of ammonium chloride form.

Chlorine in excess: $NH_3+3Cl_2\rightarrow NCl_3+3HCl$NH₃+3Cl₂→ NCl₃+3HCl. Nitrogen trichloride, a yellow, highly explosive oily liquid, forms.

In both reactions the nitrogen of ammonia is oxidised, so ammonia acts as a reducing agent.

Reactants: an ammonium salt (ammonium chloride) heated with slaked lime.

$2NH_4Cl+Ca(OH)_2\xrightarrow{\Delta}CaCl_2+2NH_3\uparrow+2H_2O$2NH₄Cl+Ca(OH)₂→ (Δ)CaCl₂+2NH₃↑+2H₂O

Collection: Ammonia is lighter than air and very soluble in water, so it is collected by the downward displacement of air in an inverted gas jar (not over water).

Drying agent: the gas is dried by passing it over quicklime ($CaO$CaO).

Unsuitable agents:

• Concentrated $H_2SO_4$H₂SO₄ and $P_2O_5$P₂O₅ are acidic, so they react with basic ammonia: $2NH_3+H_2SO_4\rightarrow(NH_4)_2SO_4$2NH₃+H₂SO₄→(NH₄)₂SO₄.
• Anhydrous $CaCl_2$CaCl₂ cannot be used because it forms the compound $CaCl_2\cdot8NH_3$CaCl₂·8NH₃.

(a) Uses of ammonia:

1. Manufacture of nitric acid by the Ostwald process.
2. Manufacture of nitrogenous fertilisers such as urea and ammonium sulphate.
3. As a refrigerant in ice plants and in the manufacture of washing soda (Solvay process); also in cleaning agents.

(b)(i) Iron(III) chloride: a reddish-brown precipitate of iron(III) hydroxide forms and is insoluble in excess.
$FeCl_3+3NH_4OH\rightarrow Fe(OH)_3\downarrow+3NH_4Cl$FeCl₃+3NH₄OH→ Fe(OH)₃↓+3NH₄Cl

(b)(ii) Zinc sulphate: a white gelatinous precipitate of zinc hydroxide forms, which dissolves in excess ammonium hydroxide to give a colourless solution of the complex.
$ZnSO_4+2NH_4OH\rightarrow Zn(OH)_2\downarrow+(NH_4)_2SO_4$ZnSO₄+2NH₄OH→ Zn(OH)₂↓+(NH₄)₂SO₄
$Zn(OH)_2+4NH_4OH\rightarrow[Zn(NH_3)_4](OH)_2+4H_2O$Zn(OH)₂+4NH₄OH→[Zn(NH₃)₄](OH)₂+4H₂O

(i) Ammonia ($NH_3$NH₃); it turns moist red litmus blue (it is alkaline).

(ii) Ordinary combustion in oxygen:
$4NH_3+3O_2\rightarrow 2N_2+6H_2O$4NH₃+3O₂→ 2N₂+6H₂O

(iii) Catalytic oxidation over heated platinum:
$4NH_3+5O_2\xrightarrow{Pt,\ 800^\circ C}4NO+6H_2O$4NH₃+5O₂→ (Pt, 800° C)4NO+6H₂O

This is the first step of the Ostwald process for manufacturing nitric acid.

(iv) $2NH_4OH+H_2SO_4\rightarrow(NH_4)_2SO_4+2H_2O$2NH₄OH+H₂SO₄→(NH₄)₂SO₄+2H₂O. Ammonium sulphate, a fertiliser, is formed: aqueous ammonia neutralises acids.