Explain the following with proper reason : KHF2 is well known whereas KHCl2 or KHBr2 does not exist.

Question

Explain the following with proper reason :

(a) KHF₂ is well known whereas KHCl₂ or KHBr₂ does not exist. 

(b) Ferric iodide is very unstable but ferric chloride is stable. 

(c) Fluorine does not form F₃^– (polyhalide) ion. 

(d) HF is not stored in glass bottles but kept in wax lined bottles. 

(e) HF has a greater electronegativity difference and more ionic character than HCl, HBr and HI but it is the weakest acid.

Answer 1

(a) Hydrofluoric acid exists as dimeric molecule (H₂ F₂ ) due to hydrogen bonding. It, thus, exhibits dibasic nature and forms two series of salts, KHF₂ as [K⁺ and F^– --- H –– F] and KF[K⁺ and F^– ]. HCl and HBr exists as monomeric molecules as hydrogen bonding is not present. 

(b) I ^– ions is a strong reducing agent. Iodide thus reduces ferric into ferrous, Cl ^– ion is a weak reducing agent and does not reduce Fe³⁺ to Fe2+. 

(c) No d-orbitals are present in fluorine while d-orbitals are present in other halogens. The formation of X₃^– ions involves sp³d-hybrizaion. 

(d) HF attacks glass bottles. The sodium and potassium silicates are converted into flurosilicates 

Na₂ SiO₃ + 6HF → Na₂SiF₆ + 3H₂O 

(e) The weakest acidic nature of HF is due to following three factors : 

(i) Strong H–F bond as the dissociation energy is high. 

(ii) Large heat of dehydration dues to hydrogen bonding. 

(iii) Low value of electron affinity.