HF; HCl; HBr; HI.Arrange the above acids in terms of strength

The order can be written as :HI>HBr>HCl>HF.

It can be explained using concept of electrovalency as well as conjugate base.

i) Fluorine is highly EN(4.0 on Pauling scale) and this EN value decreases on moving down along with increase in size.So, the bond length also increases down while bond strength decreases.So, it is easier for bond to be broken between halogen of higher Z and H than above. Hence, HI easily dissociates to give H⁺ and I^- than HBr, HCl and HF.

ii) Acidity is proportional to stability of conjugate base. In I^-, for instance, conjugate base of HI, the negative charge is distributed over larger size than say Br^- or Cl^- or F^-.Hence,stability of conjugate base increases as size of ion increases i.e. down the group.

Hence, the order in terms of strength of acids is :HI>HBr>HCl>HF