Question

Rank the following sets of acids in order of increasing acid strength, in each case explain your ordering.

A) HCl, HBr, HI

B) HOCl,HOBr, HOI

Answer 1

A) Order of increasing acid strength

HCl < HBr < HI

H-Cl bond is stronger than H-Br and H-Br bond is stronger than H-I. So among the three bonds H-I is weakest bond so it is easily dissociated and it will give H+ ions easily. Hence HI is trong acid.

It can be explained by Hard-Soft acid-base theory. According to this theory 'hard acid prefers to bind with hord base and soft acid prefers to bind with soft base'.

hydrogen ion H+ is hard acid as it has very small size. Among the halides chloride is hard base than remaining two halides. So Cl can form strong bond with H+ and HCl is weak acid than remaining two. Iodide is soft base so it forms weak bond with H+ and HI is strongest acid.

B) Order of increasing acid strength

HOI < HOBr < HOCl

Here acid strength depends on strength of O-H bond. If O-H bond is weak then it is broken easily and acid is strong acid. Among the given halogens Cl has greater electronegativity so it decreases the electron density of oxygen which leads increase in acidic strength of HOCl. Iodide has lectronegativity so it increases electron density on oxygen which strengthen the O-H bond and acidity decreases.