Question

Arrange the following in the order of the property indicated for each set :
(i) F₂, Cl₂, Br₂, I₂ (Increasing bond dissocation energy)
(ii) HF, HCl, HBr, HI (decreasing acid strength)
(iii) NH_3, PH₃, AsH_3, SbH₃, BiH₃ (Decreasing base strength)

Answer 1

[Hint : (i) F₂ has exceptionally low bond dissociation enthalpy. Lone pairs in F₂ molecule are much closer to each other than in Cl₂ molecule. Stronger electron-electron repulsions among the lone pairs in F₂ molecule make its bond dissociation enthalpy exceptionally low.
(ii) Acid strength depends upon H – X bond dissociation enthalpy. As the size
of ‘X’ atom increases, bond dissociation enthalpy of H – X decreases.
(iii) Electron availability on the central atom ‘E’ in EH₃ decreases down the group.]