Question

Identify the stronger acid in each of the following pairs, and give reasons for your choice:

(a) HBrO4 or HIO4

(b) HF or HI

(c) HIO2 or HIO3

(d) H3AsO4 or H2SeO4

Answer 1

Ans:

a) HBrO₄

While comparing acidic strengths in oxyacids, the larger the oxidation state of the central atom the stronger the acid is and if the oxidation states are same the more electronegative the central atom the stronger the acid is.

b) HI

pKa's of HI and HF are -10 and 3.1, Due to poor orbital overlap of H and I owing to the Iodine's much larger size than H, the electrons are not very well shared leading to an extremely ionic character to the molecule and easy dissociation of the H from it. Also, F is the most electronegative element and thus holds on to the H+ more strongly than any other element making HF a weak acid.

c) HIO₃

While comparing acidic strengths in oxyacids, the larger the oxidation state of the central atom the stronger the acid is (HIO₂ has +3 and HIO₃ has +5) because the negative charge left over after removal of H+ is stabilized by the oxygen atoms and more the number of O more is the stabilization of the conjugate base and so the stronger is the acid.

d) H₂SeO₄

The more electronegative the central atom (Se>As), the more electron density is drawn out of the O–H bond and the weaker this bond becomes, the stronger the acid is.