Relative Acidity: bond length VS electronegativity

[Somin Lee]

When determining relative acidity, we should consider bond length and electronegativity. When bond length increases, acidity increases as the bond gets weaker. And for electronegativity, when electronegativity increases, the acidity increases.
What confuses me is the example that Prof. Lavelle gave us during the lecture. So he explained Cl-O-H > Br-O-H > I-O-H in acidity since Cl has the highest electronegativity. Yet, this contradicts to the bond length order since I-O has the longest bond. Does that mean we should consider electronegativity first before the bond length?

[Aaron Li 1D]

When looking at bond length, you are looking at the atom that is directly bonded to the H atom, so if the bond length is shorter and stronger, the acid is weaker because it is harder of the H to separate from the compound. However, if the bond length of the atom that is directly bonded to the H atom is the same for all the given compounds, then you would look towards the other atoms of the compound that are not directly bonded to the H atom being donated. For these atoms, you look at the electronegativity to see which acid is the strongest. The compound with the higher electronegativity atoms is a stronger acid because the anion is more stabilized by the higher electron withdrawing energy.

[Iman Gauhar 3E]

Hi! When looking at relative acidity, you first look at bond length, but the bond you look at must include an atom that is bonded to an H atom. So in this example, all 3 molecules have the same O-H bond. Therefore, you must move on to judging between the electronegativities to see which acid is stronger. In this case, I has the highest electronegativity, so I-O-H has the strongest acid.

[Neeti Indiresan 3I]

In the electronegativity example, the Cl and other atoms are not bonded to the H atom, so the bond length will have no affect on the ability of H to dissociate. Therefore, it affects the acidity more based on its pull on electrons. Had it been bonded to the H instead of the O, then the bond length would have factored in.

[Heba Bounar 3K]

Hi! When determining relative acidity, you should first compare bond lengths. However, you need to compare bond lengths between Hydrogen and the element it is directly attached to. Because we are dealing with oxoacids in this case, the hydrogen is bound to oxygen in each molecule, so the bond lengths are the same and we can't compare them to determine strength. Therefore, we now have to compare electron withdrawing ability. Because Cl is the most electronegative element out of Cl, Br, and I, the Cl-O-H acid is the strongest because when it is deprotonated its conjugate base is more stable since the high electronegativity of Cl helps carry the burden of the negative charge on the oxygen. Hope this helps!