HF

[Fiona Choi 1K]

I know in lecture on Monday, Professor stated that CH2FCOOH is the most acidic because it has the lowest pKa and subsequently highest Ka. The logic being that F has the highest electron withdrawing ability and stabilizes the anion the most. So wouldn't that apply to HF as well? Why is HF a weak acid in that case?

[claire smith]

In CH2FCOOH, F's pulling ability essentially takes electrostatic force away from the H atom at the end of the molecule. Conversely, in HF there are only two atoms so all of F's electronegative power pulls that Hydrogen in. HF has an extremely tight bond due to F's high electronegativity, so it would take a great deal of energy and is therefore less likely to become deprotonated. It is not just the presence of F that matters, but where on the molecule it is/what other species it interacts with.

[Ashley Presnell 1C]

Since HF has a really tight bond due to the really high electronegativity and short bond since the Fluorine atom is much smaller relative to HI which is a strong acid because it is much larger atom and has a longer, weaker bond and therefore donates the H atom much easier.

[Gavin Trent 2F]

HF is a weak acid because the bond between H and F is strong, meaning it does not easily give up its proton.

[Joseph Lee]

Well technically CH2FCOOH is also a weak acid too, so we're all talking about in terms of weak acids. However, the Flourine in CH2FCOOH delocalizes the electrons further so that in the conjugate base, when the H is taken off, the structure is more stable and its electrons are delocalized more.

[Claire Kim 1F]

I know in lecture on Monday, Professor stated that CH2FCOOH is the most acidic because it has the lowest pKa and subsequently highest Ka. The logic being that F has the highest electron withdrawing ability and stabilizes the anion the most. So wouldn't that apply to HF as well? Why is HF a weak acid in that case?

The reason why HF is a weaker acid is because acids donate and since HF has a strong bond within the molecule, it is less likely to donate. That's what makes it weak.