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.

halogen elements table

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
  1. 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)
  1. 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
  1. 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

  1. 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.


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