Question

Arrange the following acids in the order of their increasing acidity.

HF, HCI, HBr and HI

Answer 1

For this again we shall evaluate the relative basicities of their conjugate bases

F^-, Cl^-, Br^- and I^-

Here the volume available for the electron and hence delocalisation of electron increases with the increase in size, i.e. from F^- to I^-. Consequently, the attraction for a proton and hence basicity decreases in the order;

F^- > Cl^- > Br^- > I^-

Thus F^- is the strongest base and its conjugate acid HF is the weakest acid and I^- is the weakest base and its conjugate acid HI is the strongest acid of the series. Thus the order of increasing acidity of the four acids is

HF < HCl < HBr < HI