JEE Chemistry The P Block Elements Notes Set 16

Group 17: Halogens

F, Cl, Br, I and At are halogens, with configuration ns2 np5. They are very strong oxidising agents; oxidising power falls from F2 to I2.

1. Fluorine is the most electronegative element. Its electron gain enthalpy is less negative than that of Cl because of the small size of the F atom and its electron repulsion.
2. Chlorine is made by oxidising HCl with MnO2 or KMnO4, or by electrolysis of brine. Fluorine is obtained only by electrolysis of KHF2 in anhydrous HF.
3. Halogens disproportionate in cold dilute alkali to X- and OX-, and in hot conc. alkali to X- and XO3-. On acidification the product re-forms the halogen.
4. Liquid iodine conducts electricity because of self ionisation: \( \ce{3I2 <=> I3+ + I3-} \).

Hydrogen Halides

1. Acid strength: HI > HBr > HCl > HF. Boiling point: HF > HI > HBr > HCl, since HF has hydrogen bonding.
2. HF is made from CaF2 with conc. H2SO4. CaF2 must be free from silica, because \( \ce{SiO2 + 4HF -> SiF4 + 2H2O} \) would consume HF. HF cannot be kept in glass for the same reason.
3. HBr and HI cannot be made by conc. H2SO4 on the salt because H2SO4 oxidises them to Br2 and I2. Phosphoric acid is used instead.
4. HF is a weak acid, but addition of BF3, SbF5 or AsF5 makes it a strong acid. In liquid HF, HCl, H2SO4 and HNO3 behave as bases while HClO4 behaves as an acid.

Oxoacids, Bleaching Powder and Oxides

1. Hypohalous acids HOX are unstable. Acidity and thermal stability increase from HOCl to HClO4, with oxidising power decreasing: HClO > HClO2 > HClO3 > HClO4.
2. Bleaching powder is formed by \( \ce{Ca(OH)2 + Cl2 -> Ca(OCl)Cl + H2O} \) at about 40 C. It oxidises by liberating chlorine with acids and acts as a disinfectant.
3. Chlorate on heating disproportionates, \( \ce{4KClO3 -> 3KClO4 + KCl} \) at low temperature. Chlorine oxides cannot be made by direct combination; \( \ce{ClO2} \) has an odd electron and does not dimerise, and \( \ce{Cl2O6} \) consists of the ions \( \ce{ClO2+} \) and \( \ce{ClO4-} \).
4. Iodate and iodide react: \( \ce{IO3- + 5I- + 6H+ -> 3I2 + 3H2O} \).

Interhalogens, Polyhalides and Pseudohalogens

1. Interhalogens are of type AX, AX3, AX5 and AX7, with the larger halogen in the centre. No molecule has more than two types of halogen. They are more reactive than halogens (except F2) because the A-X bond is weaker.
2. IF7 is pentagonal bipyramidal, ICl3 is a dimer, and larger central atoms such as Br and I allow higher valency.
3. Polyhalide ions such as \( \ce{I3-} \), \( \ce{I5-} \) and \( \ce{I7-} \) form when halide combines with halogen or interhalogen; \( \ce{F3-} \) is not known because F lacks d orbitals.
4. Pseudohalogens such as CN-, SCN- and OCN- are univalent groups that behave like halide ions, and form acids and dimers such as cyanogen \( \ce{(CN)2} \).