Halogens readily decomposes water. This tendency decreases on moving down the group. Fluorine decomposes water very energetically to give oxygen and ozone,
2H2O + 2F2 ——> 4HF + O2 ; 3H2O + 3 F2 ———> 6 HF + O3
Oxgygen
Ozone
Fluorine gives fumes in moist air. This is due to the formation of HF, which is a liquid and can absorb moisture to form liquid droplets and therefore, gives fumes with moist air. Chlorine and bromine react less vigorously,
Cl2 + H2O ——> HCl + HClO
Hypochlorous acid
Br2 + H2O ——> HBr + HBrO
Hypobromous acid
In the presence of sunlight, HClO (hypochlorous acid) HBrO (hypobromous acid) liberate oxygen.
2HClO ——> 2HCl + O2
; 2HBrO ——> 2HBr + O2
Iodine is only slightly soluble in water. However, it dissolves in 10% aqueous solution of Kl due to the formation of I3–
ions.
I2 + Kl  Kl2 + I– I3–
Complexion
Reduction
Form covalent halides.
–200oC
H2 + F2 —————> 2HF
(very violent)
Sunlight
H2 + Cl2 —————> 2HCl
Heat
H2 + Br2 —————> 2HBr
pt. catalyst
Heat
H2 + I2 2HI (poor yield)
Acidic strength in aqueous solution is in the order,
HI > HBr > HCl < HF.
Reducing character of hydrides follow the order,
HI > HBr > HCl > HF.
? Boiling point HF > HI > HBr > HCl. Thermal stability,
H – F > H – Cl > H – Br > H – I.
HCl is also called Muriatic acid.
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