6C. 19 part C

[Mansi_1D]

For number 19 part c in Focus 6C, it asks which of the two pairs, HBrO2 and HClO2 is a stronger acid. The corrected answer is HClO2 and I wanted to why HClO2 is stronger because Br and Cl are in the same group so wouldn't you look at the bond strength/ atom size to determine acid strength? Br is larger than Cl so wouldn't it be easier to remove a proton from HBrO2 than HClO2?

[Brian Tangsombatvisit 1C]

In both HBrO2 and HClO2, the H+ atom is bonded to an O, not Cl and Br. If the H+ were bonded to the Cl or Br, then you would be correct in that the H-Br is a weaker bond than H-Cl, making it a stronger acid. However, this is not the case. To determine the acid that is stronger, we would have to look instead at the resulting stability of the anion after the acid donates its proton since both acids have the same O-H bond. HClO- would be a more stable anion than HBrO- since Cl is more electronegative, pulling more of the electron density away from the negatively charged O once it donates the proton. Because of this, HClO2 is the stronger acid.