Which is stronger acid HCl or HI

  • An acid is a substance that can produce a hydronium ion in solution according to Arrhenius theory or an acid is a proton donor according to Bronsted-Lowry theory.
  • So the easier it is to release a proton in an aqueous solution, the stronger is the acid.
  • Now if we compare the bond strength between HI and HCl we find that the HI bond is weaker as Iodine is much larger than chlorine and cannot form an effective overlap between the s orbital of Hydrogen  and the p orbital of Iodine, so relatively the H-Cl bond is stronger due to the better overlap of the sigma bonds.
  • Since the H-Cl bond is stronger bond it is a weaker acid, more difficult to break the H-Cl bond as compared to H-I.
  • So it would be easier to release a Hydrogen ion from HI than from HCl
  • This logic may be applied to similar oxy-acids too.