Reaction of HF in water

Question

I was wondering, if we put $\ce{HF}$ into a glass of water the acid will be dissolved according to this reaction:

$$\ce{HF->F- + H+}$$

Now there is a proton travelling across the water.

All $\ce{H2O}$ molecules have equal chance of getting the proton so it travels through the liquid.

Now for the reaction

$$\ce{F- + H2O ->HF + OH-}$$

the bond strength between O and H is lower than the bond strength of F and H (fluorine is has more electronegative and has a smaller atomic radius than O).

According to Bronsted-Lowry theory for acids and bases the second reaction happens more often than the first one. But how is this happening? Why does the second reaction happen at all? Oxygen is more negatively charged than fluorine so the proton would have a very small chance to reach fluorine and then the electrons would be rearranged to form the HF bond.

Off the record: HF usually tries to dissolve the "glass" in the "glass of water". Just nitpicking :D — William R. Ebenezer, May 06 '19 at 14:07
I'm sorry but after reading several times I find it difficult to process what exactly your query is. Could you please try to elaborate a tad bit clearly? I find several statements in your question slightly off. — William R. Ebenezer, May 06 '19 at 14:10
@NilayGhosh Yes it bothered me earlier, but I had made enough jokes for the day. — William R. Ebenezer, May 06 '19 at 14:30
@DonaldDuck NightWriter succeeded in clarifying formatting, but it's up to you to clarify the content. — William R. Ebenezer, May 06 '19 at 14:32
F- + H2O -> HF + OH- it is very unlikely to happen , HF -> H+ + F- is more likely to happen but , F- is a stronger base than HF is an acid. — Donald Duck, May 06 '19 at 14:35
So at the end, what is your question? I it "Why does the second reaction exist?" — Simon-Nail-It, May 06 '19 at 14:48
No why F- is a stronger base than HF is an acid?HF should be stronger than its conjugate base. — Donald Duck, May 06 '19 at 14:50
Possible duplicate of Why is HF the least acidic hydrogen halide? — Mithoron, May 06 '19 at 22:15

ANZGC FlyingFalcon · Answer 1 · 2019-05-07T09:29:26.323

2

The reason why $\ce{HF}$ doesn’t tend to disassociate in aqueous solution can be rationalized as follows:

In aqueous solution, all ions (cation or anion) are immediately solvated by a solvent “shell” that stabilizes the ion by drawing some charge density away from it. This stabilizes larger ions such as $\ce{Cl-}$ or $\ce{K+}$ more than smaller ions like $\ce{Li+}$ and $\ce{F-}$; as a matter of fact the solvation energy for $\ce{HF}$ is less than the energy needed for the $\ce{H-F}$ bond to break (bond enthalpy, if you will).

As such, $\ce{HF}$ doesn’t tend to disassociate in aqueous solution. It should be highlighted that pure, liquid $\ce{HF}$ is a much stronger acid than its aqueous form as the acid disassociates into $\ce{FHF-}$ and $\ce{H2F+}$ instead of $\ce{F-}$ and $\ce{H3O+}$.

EDIT 7 MAY

It appears that what you are asking is “why the H-F bond is weaker than the H-O bond”. If that’s what you’re asking, here’s what I have to say:

We will consider the resonance integral, $\cs{Hij}$, between atomic orbitals i and j. This resonance integral can be interpreted to be the energy of an electron associated with these 2 orbitals.

The one-electron relation (Wolfsberg-Helmholz Equation) tells us that $\cs{Hij = 0.5K(Hii +Hjj) Sij}$, and it so happens that the F orbitals are more contracted and can overlap less well. In addition, since the degree of stabilisation of the bonding MO is inversely proportional to the energy difference between the AOs, we see that there is less stabilisation from the original F 2p Orbital in a MO involving HF compared to a MO involving HO, and the HF bond is weaker.

edited May 07 '19 at 09:29

answered May 06 '19 at 15:24

ANZGC FlyingFalcon

1,085
7
19

1

Quick tip: avoid using all-caps, it gives a general appearance that you are yelling. For emphasis, it is better to use italics or bold..! – William R. Ebenezer May 06 '19 at 15:28
Ah, got it. What’s the format for italics/bold? – ANZGC FlyingFalcon May 06 '19 at 15:29
_italics_ or *italics* and **bold** – William R. Ebenezer May 06 '19 at 15:38
??????????????? – Donald Duck May 06 '19 at 15:45
I dont say it diassosciates . – Donald Duck May 06 '19 at 15:46
Just the proton as an subatomic particle has no definite position or momentum if not measured and the propability cloud can be displaced based on potential of points .So it actually swims through the solution.Arrhenius was right ! :) – Donald Duck May 06 '19 at 15:49
2

@DonaldDuck What are you taking about? Is our focus on why the ions are stronger acid/ base or is our focus on the proton "swims" through the solution? – Simon-Nail-It May 06 '19 at 16:17
The focus is at the explanation of acids/bases using quantum mechanics. – Donald Duck May 06 '19 at 17:08
2

I’m sorry, but how does QM have got anything to do with this? – ANZGC FlyingFalcon May 06 '19 at 23:56

Reaction of HF in water

1 Answers1