Question

Arrange the halogen acids in the decreasing order of : 

(1) thermal stability 

(2) acidic strength.

Answer 1

(1) Thermal stability : As the atomic size increases from F to I, the bond dissociation enthalpy of H-X decreases.

Thus,

The thermal stability of hydrogen halides decreases down the group in the order HF > HCl > HBr > HI

(2) Acidic strength : Since, the bond dissociation enthalpy of H-X decreases down the group, the acidic strength varies in the order HI > HBr > HCl > HF.