Halogens

Position of halogens in periodic table:

The elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At) belong to the group VIIA or 17 in the periodic table collectively known as halogens which means sea salt producer.

General characteristics of halogens:

1. All elements have seven electrons (ns² np⁵) in their outermost shell.
2. They exist as diatomic molecules (F₂, Cl₂, Br₂, I₂) containing a single covalent bond.
3. The m.p. and b.p. are in the order: F₂ < Cl₂ < Br₂ < I₂
4. The order of electronegativity is F₂ > Cl₂ > Br₂ > I₂
5. The order of electron affinity is : Cl₂ > F₂ > Br₂ > I₂
6. The size of halogens is in the order: F₂ < Cl₂ < Br₂ < I₂
7. Halogens are less soluble as their solubility decreases with an increase in atomic number.

Laboratory preparation of chlorine:

Chlorine is prepared in the laboratory by heating a mixture of sodium chloride, manganese dioxide and concentrated sulphuric acid.

2NaCl + 3H₂SO₄ + MnO₂ → 2NaHSO₄ + MnSO₄ + Cl₂ + 2H₂O

Laboratory preparation of bromine:

Bromine is prepared in the laboratory by heating a mixture of sodium bromide, manganese dioxide and concentrated sulphuric acid.

2NaBr + 3H₂SO₄ + MnO₂ → 2NaHSO₄ + MnSO₄ + Br₂ + 2H₂O

Laboratory preparation of iodine:

Iodine is prepared in the laboratory by heating a mixture of potassium iodide, manganese dioxide and concentrated sulphuric acid.

2KI + 3H₂SO₄ + MnO₂ → 2KHSO₄ + MnSO₄ + I₂ + 2H₂O

Chemical properties:

1. Action with water: Chlorine and bromine react with water to give hypohalous acid. Hypohalous acids are unstable.
   
   Cl₂ + H₂O → HCl + HOCl (Hypochlorous acid)
   2HOCl → 2HCl + O₂
   Br₂ + H₂O → HBr + HOBr
   2HOBr → 2HBr + O₂
   
   Iodine has little action with water but dissolves in the aqueous solution of KI.
   I₂ + KI → KI₃

2. Action with NaOH:
   
   Cl₂ + 2NaOH(cold & dil.) → NaCl + H₂O + NaClO(Sodium hypochlorite)
   
   3Cl₂ + 6NaOH(hot & conc.) → 5NaCl + 3H₂O + NaClO₃(Sodium chlorate)
   
   Br₂ + 2NaOH(cold & dil.) → NaBr + H₂O + NaBrO(Sodium hypobromite)
   
   3Br₂ + 6NaOH(hot & conc.) → 5NaBr + 3H₂O + NaBrO₃(Sodium bromate)
   
   I₂ + NaOH(cold & dil.) → NaI + HOI(Hypoiodous acid)
   
   3I₂ + 6NaOH(hot & conc.) → 5NaI + 3H₂O + NaIO₃(Sodium iodate)

3. Action with ammonia:
   
   8NH₃(excess) + 3Cl₂ → N₂ + 6NH₄Cl
   
   2NH₃ + 6Cl₂(excess) → 2NCl₃ + 6HCl
   
   8NH₃(excess) + 3Br₂ → N₂ + 6NH₄Br
   
   2NH₃ + 3Br₂(excess) → 2NBr₃ + 6HBr
   
   2NH₃ + 3I₂ → 2NI₃ + 6HI

4. Oxidizing nature: Halogens are good oxidizing agents. Their oxidizing power is in the order: F₂>Cl₂>Br₂>I₂

i. Mutual displacement:

Fluorine displaces other halogens from their aqueous salt.
F₂ + 2NaCl → 2NaF + Cl

Chlorine displaces Br₂ and I₂ from their salt solution.
Cl₂ + 2KBr → 2KCl + Br₂
Cl₂ + 2KI → 2KCl + I₂

Bromine displaces iodine from iodide salt.
Br₂ + 2KI → 2KBr + I₂

ii. with H₂S:
H₂S + Cl₂ → 2HCl + S

iii. With SO₂:
SO₂ + 2H₂O + Cl₂ → 2HCl + H₂SO₄

iv. with Na₂SO₃:
Na₂SO₃ + H₂O + Cl₂ → 2HCl + Na₂SO₄

v. with Na₂S₂O₃(sodium thiosulphate):
Na₂S₂O₃ + H₂O + Cl₂ → 2HCl + Na₂SO₄ + S

2Na₂S₂O₃ + I₂ → Na₂S₄O₆(Sodium tetrathionate) + 2NaI

5. Bleaching action: Chlorine changes colored compounds into colorless. This process is called bleaching. Bleaching action is permanent and is due to oxidation.
   
   H₂O + Cl₂ → HCl + HOCl
   HOCl → HCl + [O]
   [O] + colored substance → colorless substance

Uses of chlorine:

• To purify drinking water.
• As a bleaching agent.
• To prepare DDT, BHC, phosgene gas, chloroform, etc.

Uses of bromine:

• AgBr is used in photography.
• Manufacture of dyes, drugs, etc.
• To detect unsaturation in organic compounds.

Uses of Iodine:

• Manufacture of Iodoform.
• AgI is used in photography.
• Treatment of goitre.
• Used as an antiseptic (tincture of iodine). Tincture is iodine dissolved in ethyl alcohol in presence of little KI.

Test of Chlorine(Cl₂):

It turns starch iodide paper into blue.
Cl₂ + 2KI → 2KCl + I₂
I₂ + starch → blue

Test of Bromine(Br₂):

It turns starch iodide paper into blue.
Br₂ + 2KI → 2KBr + I₂
I₂ + starch → blue

Test of Iodine(I₂):

It turns moist starch paper into blue.
I₂ + starch → blue

Some Important Questions

1. Give the balanced chemical reaction for the preparation of chlorine in the lab. Write the action of hot conc. NaOH on bromine.
2. Arrange the halogens (Cl₂, Br₂, I₂) in terms of atomic radii, reactivity, electronegativity, electron affinity, bleaching action and oxidizing nature.