Halogens

Occurrence

Halogens are highly reactive elements and hence do not occur in the free state or native state. They mainly occur in the combined state in form of their halide (Xminus) salts although iodine also occurs as iodate(IO₃^-)

Preparation

In principle, all halogens can be prepared by the oxidation of their corresponding halides either electrolytically or chemically by means of oxidizing agents stronger than halogens. Since fluorine is the strongest chemical oxidizing agent, therefore, it cannot be prepared by chemical oxidation of fluoride ions. Thus, only method available for preparing fluorine is the electrolytic method. All other halogens (chlorine, bromine and iodine) can, however be prepared either electrolytically or by chemical oxidation of their corresponding halides by means of oxidizing agents such as MnO2, acidified KMnO4 etc.

Properties

(i) Reaction with hydrogen: All the halogens react with hydrogen to form halogen acids (HX). The reactivity towards hydrogen, however, decreases from fluorine to iodine.

H2+ X2 →  2HX(where X = F, Cl Br or I)

(ii) Reaction with metals and non−metals: Halogens combine with most of the metals and non−metals to form their corresponding halides. Fluorine is the most reactive of all the halogens and forms compounds with all other elements except He, Ne and Ar under suitable conditions. The reactivity of other halogens decreases in the order

F2 Cl2 Br2 I2

(iii) Reaction with alkalies:

2NaOH(dil.) + X2 NaX + NaOX + H2O

Sod. hypohalite

6NaOH(conc.) + 3X2 5NaX + NaXO3 + 3H2O

Sod. halate

(where X = Cl, Br or I)(iv) Reaction with water: Because of the great affinity of halogens for hydrogen, they readily decompose water. The order of reactivity, however, follows the sequence:

F2 Cl Br2 I2

Fluorine reacts vigorously with water and oxidizes H2O to give O2 in cold and O3 in hot.

2H2O + 2F2 → 4HF + O2

3H2O + 3F2 →  6HF + O3

During these reactions, F2 is reduced to HF which being a volatile liquid fumes in air.

Chlorine and bromine react less vigorously forming a mixture of hydrohalic acid (HX) and hypohalous acid (HOX).

X2 + H2O → HX + HOX(X = Cl or Br)

(v)Oxidising agent: Since all the halogens have a strong tendency to accept electrons, they are strong oxidizing agents. Their oxidizing power decreases from F to I. Thus, fluorine is the strongest oxidizing agent of all the halogens. As such it oxidizes all other halide ions to the corresponding halogens.

F2+ 2X →  2Fminus; + X2(X = Cl, Br or I)

Hydrohalic acids (HF, HCl, HBr and HI)

(i) Physical Nature : Except HF, all others HCl, HBr, HI are gases HF is a liquid due to intermolecular hydrogen bonding

(ii) Acidic Strength : All acts as acid in their aq. solution and acidic strength varies in the order : HF < HCl < HBr < HI

HX → H+ + X-

It can be explained in terms of strength of H - X bonds which is in the order :

H- I < H - Br < H -Cl < H - F.

Preparation and Properties

(i) Less volatile acids displace more volatile acids from their salts.

2NaCl + H2SO4 →  Na2SO4 + 2HCl

HBr and HI cannot be prepared by conc. H2SO4 as they are more powerful reducing agents and reduces conc. H2SO4 to SO2

2KBr + H2SO4 →  K2SO4 + 2HBr

H2SO4 + 2HBr → Br2↑ + SO2 + 2H2O

(ii) Less volatile, non -oxidising H3PO4 is used to prepare HBr and HI

KBr + H3PO4 →  KH2PO4 + HBr↑

KI + H3PO4 → KH2PO4 + HI ↑

Properties

(i) All the three acids are reducing agents  HCl is not attacked by H2SO4.

2HBr + H2SO4 →  2H2O + SO2 + Br2 ↑

2HI + H2SO4  → 2H2O + SO2 + I2

(ii) All the three react with KMnO4 and K2Cr2O7

2KMnO4 + 16HCl → 2KCl + 2MnCl2 + 8H2O + 5Cl2 ↑

K2Cr2O7 + 14HBr → 2KBr + 2CrBr3 + 7H2O + 3Br2↑

Other reactions are similar.