Question

Which of the following are strong acids: H2SO4, HI, HF, H3PO3, and HNO3?

Answer 1

Rough order of acidity: `HI> H_2SO_4 >HNO_3>H_3PO_3>HF`.

`HI`, `H_2SO_4`, and `HNO_3` are reasonably regarded as strong acids.

Explanation:

As physical scientists, however, we should look at actual measurements, namely the `pK_a` values of each acid in water.

`HI, pK_a = -10` (estimated)

`H_2SO_4, pK_(a1) = -3, pK_(a2)=1.99`

`HNO_3, pK_a = -1.4`

`H_3PO_3, pK_(a1) = 1.3, pK_(a2) =6.7`

`HF, pK_a = 3.17`

Importantly, the acidity of the hydrogen halides should increase down the Group, inasmuch as there is poorer overlap between the halogen and hydrogen as the halogen becomes bigger. Hydrofluoric acid (which is truly nasty stuff, and can cause horrible burns) is thus the weakest acid of the hydrogen halides, because of the strength of the `H-X` bond, and the polarizing ability of `F^-`.

Why are the `pK_(a2)` values of the polyprotic acids so high?