6C19 parts a and c

[LBacker_2E]

I am confused when you use which rule to identify which compound is the stronger acid. In lecture we mainly discussed bond length and anion stability, but the textbook solutions only justified their reasoning with bond length to say that HCl was stronger than HF (in part a). I thought you would use this same reasoning (for part c) when determining if HBrO2 or HClO2 was a stronger acid, but the book used electronegativity as the reasoning which gives you the opposite answer compared to using bond length. For part c, HClO2 was said to be the stronger acid because Cl is more electronegative than Br, but F is more electronegative than Cl and HCl was still the stronger acid.

Do you only use electronegativity to determine relative acidity when there are O's present in the compound?

[LBacker_2E]

I had the same questions for parts d and f, and also why do you not consider different amount of H's in part d?

[Edmund Zhi 2B]

The reason why we use electronegativity to determine that HClO is a stronger acid than HBrO is because the hydrogen isn't bonded to the halogen, it's bonded to the oxygen. Now, considering electronegativity, we see that the more electronegative atom will delocalize the electrons/charge more, causing the O-H bond to become weaker the more electronegative the halogen is. Now, we say that HCl is a stronger acid than HF because now we are considering acids in which the hydrogen is directly attached to the halogen, not the oxygen in contrast to oxoacids. Now, when we consider electronegativity, we are considering a DIRECT LINK between the halogen and the hydrogen. Since F is more electronegative than Cl, it holds on to the proton tighter than Cl. The bond is stronger, harder to break, and less likely to dissociate. This is why the acid strengths of group 17 in order are: HF < HCl < HBr < HI. According to Chem_Mod in 2011,

"HF is a weaker acid because of strong hydrogen bonding with water molecules that takes a great amount of energy to break. Also, there is a substantial loss in entropy when HF dissociates. This is due to the F- anion (with its high charge density) instilling a high degree of order in the solution. Generally, greater entropy is favored in a system (more on that when you cover thermodynamics)."

But I think this is too complex and out of our current scope... I would mostly ignore it, but I thought I'd share

[LBacker_2E]

Thank you, your explanation was really helpful!