Study of Compounds: Ammonia notes: ICSE Class 10 Chemistry (Science Paper 2)

1. Laboratory preparation

Reactants: ammonium chloride and slaked lime (calcium hydroxide), heated.

Flask clamped slanting, mouth slightly lower, so the water formed does not run back onto the hot glass.
Drying agent: quicklime (CaO). Not concentrated H₂SO₄ or P₂O₅ (acidic, react with ammonia), not anhydrous CaCl₂ (forms CaCl₂·8NH₃).
Collection: downward displacement of air in an inverted jar (lighter than air; extremely soluble, so not over water).
Jar full? Moist red litmus turns blue at the mouth; a rod dipped in conc. HCl gives dense white fumes.

Table of drying agents for ammonia with reasons
Drying agent	Suitable?	Reason
Quicklime	Yes	Basic, does not react
Conc. H₂SO₄	No	Forms (NH₄)₂SO₄
P₂O₅	No	Acidic; reacts
Anhydrous CaCl₂	No	Forms CaCl₂·8NH₃

Formulas

Laboratory preparation (heat)
$\ce{2NH4Cl + Ca(OH)2 -> CaCl2 + 2H2O + 2NH3}$
Why sulphuric acid cannot dry ammonia
$\ce{2NH3 + H2SO4 -> (NH4)2SO4}$

Key terms

Downward displacement of air: Collection of a gas lighter than air in an inverted jar: the gas rises and pushes the air down and out.
Slaked lime: Calcium hydroxide, Ca(OH)₂.

2. Ammonia from nitrides and ammonium salts

Metal nitrides + warm water → metal hydroxide + ammonia.
Any ammonium salt + alkali, warmed → salt + water + ammonia (a test for ammonium ions).

Table of other sources of ammonia, the conditions and the other products
Source	Condition	Other product
Mg₃N₂	Warm water	Mg(OH)₂
AlN	Warm water	Al(OH)₃
(NH₄)₂SO₄	Warm with NaOH	Na₂SO₄ + H₂O

Formulas

Magnesium nitride
$\ce{Mg3N2 + 6H2O -> 3Mg(OH)2 + 2NH3}$
Aluminium nitride
$\ce{AlN + 3H2O -> Al(OH)3 + NH3}$
Ammonium salt with alkali
$\ce{(NH4)2SO4 + 2NaOH -> Na2SO4 + 2H2O + 2NH3}$
Ammonium salt with alkali
$\ce{NH4NO3 + KOH -> KNO3 + H2O + NH3}$

Key terms

Nitride: A compound of a metal with nitrogen only, such as Mg₃N₂ or AlN.

3. Manufacture: Haber’s process

Raw materials: nitrogen (from liquid air) and hydrogen, 1 : 3 by volume.
Conditions: 450–500 °C, 200–900 atm, catalyst finely divided iron, promoter molybdenum.
The reaction is reversible and exothermic; high pressure favours ammonia (4 volumes → 2 volumes).
Separation: the gases are cooled; ammonia liquefies and is drawn off. Unreacted N₂ and H₂ are recycled.

Table of the conditions of Haber’s process
Condition	Value
N₂ : H₂	1 : 3 by volume
Temperature	450–500 °C
Pressure	200–900 atm
Catalyst	Finely divided iron
Promoter	Molybdenum

Formulas

Iron catalyst, molybdenum promoter, 450–500 °C, 200–900 atm
$\ce{N2 + 3H2 <=> 2NH3}$

Key terms

Catalyst: A substance that changes the rate of a reaction without itself being used up.
Promoter: A substance that increases the efficiency of a catalyst (molybdenum in Haber’s process).

4. Physical properties; burning and catalytic oxidation

Lighter than air (vapour density 8.5); extremely soluble in water. Fountain experiment: red litmus → blue fountain (phenolphthalein → pink): very soluble, alkaline solution.
Burning: does not burn in air or support combustion; burns in oxygen with a greenish-yellow flame, giving nitrogen.
Catalytic oxidation: over platinum at about 800 °C, giving nitric oxide; exothermic, so the platinum keeps glowing. In air, 2NO + O₂ → 2NO₂ (reddish-brown).

Table comparing burning and catalytic oxidation of ammonia
Feature	Burning	Catalytic oxidation
Condition	Lit in oxygen	Pt, about 800 °C
Nitrogen product	N₂	NO
Sign	Greenish-yellow flame	Pt glows; brown fumes in air

Formulas

Burning in oxygen
$\ce{4NH3 + 3O2 -> 2N2 + 6H2O}$
Catalytic oxidation: Pt, about 800 °C
$\ce{4NH3 + 5O2 -> 4NO + 6H2O}$
Reddish-brown fumes
$\ce{2NO + O2 -> 2NO2}$

Key terms

Catalytic oxidation of ammonia: Oxidation of ammonia by oxygen over hot platinum to nitric oxide and water; the first step of Ostwald’s process.

5. Reducing nature; reactions with chlorine and hydrogen chloride

Hot metal oxides are reduced to the metal; ammonia is oxidised to nitrogen. Copper(II) oxide: black → reddish-brown, with water droplets. Lead(II) and lead(IV) oxides give grey lead.
Chlorine: with excess ammonia → NH₄Cl (white fumes) + N₂; with excess chlorine → NCl₃ (yellow, explosive) + HCl.
Hydrogen chloride: dense white fumes of NH₄Cl.

Table of reagents and the products they give with ammonia
Reagent	Product with ammonia
Hot CuO	Cu, H₂O, N₂
Cl₂ (NH₃ in excess)	NH₄Cl, N₂
Cl₂ in excess	NCl₃, HCl
HCl	NH₄Cl

Formulas

Copper(II) oxide
$\ce{3CuO + 2NH3 -> 3Cu + 3H2O + N2}$
Lead(II) oxide
$\ce{3PbO + 2NH3 -> 3Pb + 3H2O + N2}$
Lead(IV) oxide
$\ce{3PbO2 + 4NH3 -> 3Pb + 6H2O + 2N2}$
Excess ammonia
$\ce{8NH3 + 3Cl2 -> 6NH4Cl + N2}$
Excess chlorine
$\ce{NH3 + 3Cl2 -> NCl3 + 3HCl}$

Key terms

Reducing agent: A substance that removes oxygen from another substance (or gives electrons), being itself oxidised.
Nitrogen trichloride: NCl₃, a yellow, highly explosive oily liquid formed by ammonia with excess chlorine.

6. Aqueous ammonia

Ammonia dissolves to give ammonium hydroxide, a weak alkali. It neutralises acids and precipitates metal hydroxides:

Solution	Precipitate	Excess NH₄OH
FeCl₃	Reddish-brown Fe(OH)₃	Insoluble
FeSO₄	Dirty green Fe(OH)₂	Insoluble
Pb(NO₃)₂	Chalky white Pb(OH)₂	Insoluble
Zn(NO₃)₂	White gelatinous Zn(OH)₂	Dissolves (colourless)
CuSO₄	Pale blue Cu(OH)₂	Dissolves (deep blue)

Table of metal ions identified with ammonium hydroxide
Ion	Identified by
Fe³⁺	Reddish-brown precipitate
Fe²⁺	Dirty green precipitate
Zn²⁺	White, soluble in excess
Pb²⁺	White, insoluble in excess
Cu²⁺	Deep blue solution in excess

Formulas

Neutralisation
$\ce{2NH4OH + H2SO4 -> (NH4)2SO4 + 2H2O}$
Reddish-brown precipitate
$\ce{FeCl3 + 3NH4OH -> Fe(OH)3 + 3NH4Cl}$
Dirty green precipitate
$\ce{FeSO4 + 2NH4OH -> Fe(OH)2 + (NH4)2SO4}$
Pale blue precipitate
$\ce{CuSO4 + 2NH4OH -> Cu(OH)2 + (NH4)2SO4}$

Key terms

Ammonium hydroxide: The weak alkali formed when ammonia dissolves in water, NH₄OH.

7. Uses, refrigerants and tests

Uses: fertilisers (urea, ammonium sulphate, nitrate, phosphate); nitric acid (Ostwald’s process); explosives; refrigerant in ice plants and cold stores (high latent heat of vaporisation); cleansing agent (removes grease).
Refrigerants: CFCs release chlorine in the upper atmosphere that destroys the ozone layer; they are replaced by non-ozone-depleting refrigerants (HFCs, hydrocarbons, ammonia).
Tests: pungent smell; moist red litmus turns blue; dense white fumes with a rod dipped in conc. HCl; with copper sulphate solution, a deep blue solution in excess.

Table of tests for ammonia
Test	Result
Moist red litmus	Turns blue
Rod dipped in conc. HCl	Dense white fumes
CuSO₄ solution	Pale blue precipitate, then deep blue solution

Key terms

CFCs: Chlorofluorocarbons: older refrigerants that release chlorine in the upper atmosphere and deplete the ozone layer.

Exam tips

Common mistakes

Naming concentrated sulphuric acid or calcium chloride as the drying agent for ammonia.
Writing "upward displacement" for ammonia (it is downward).
Mixing up the catalysts: iron (Haber) and platinum (catalytic oxidation).
Saying burning in oxygen gives nitric oxide (it gives nitrogen).
Saying lead hydroxide dissolves in excess ammonium hydroxide (only zinc and copper hydroxides do).

How marks are usually awarded

Equations (1 mark): balanced, with conditions (Pt, 800 °C; iron, 450–500 °C, 200–900 atm) where asked.
Observations (1 mark): colour of precipitate or solid, and its behaviour in excess.
Reasons (1 mark): drying agent, method of collection, slanting flask.

1. Laboratory preparation

Formulas

Key terms

2. Ammonia from nitrides and ammonium salts

Formulas

Key terms

3. Manufacture: Haber’s process

Formulas

Key terms

4. Physical properties; burning and catalytic oxidation

Formulas

Key terms

5. Reducing nature; reactions with chlorine and hydrogen chloride

Formulas

Key terms

6. Aqueous ammonia

Formulas

Key terms

7. Uses, refrigerants and tests

Key terms

Exam tips

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