Hydrofluoric acid

[Pwyll Pendefig Dyfed]

I recently saw a question about the difference between strong and weak acids. The question stated that hydrochloric acid is known as a strong acid as it dissociates completely, whereas a weak acid would reach a dissociated equilibrium (or words to that effect).

This is true for acids such as ethanoic acid of course, the question used hydrofluoric acid as an example. I was under the impression that Hydrofluoric acid is a strong acid, and the electronegativity of fluorine would mean that it would readily form the fluoride ion.

Is there a mistake in this question, or is there a reason why some could consider hydrofluoric acid a weak acid?

[mississippichem]

I recently saw a question about the difference between strong and weak acids. The question stated that hydrochloric acid is known as a strong acid as it dissociates completely, whereas a weak acid would reach a dissociated equilibrium (or words to that effect).

This is true for acids such as ethanoic acid of course, the question used hydrofluoric acid as an example. I was under the impression that Hydrofluoric acid is a strong acid, and the electronegativity of fluorine would mean that it would readily form the fluoride ion.

Is there a mistake in this question, or is there a reason why some could consider hydrofluoric acid a weak acid?

 

This question gets a lot of students.

 

The answer is quite counter intuitive, but there are several ways to analyze this. Hydrofluoric acid is a weak acid, unlike the other binary halo-acids. The "best" reason is the that the aqueous solvation shell that forms around a dissociated fluoride ion is highly organized; so much so that it creates a positive negative [math] \Delta S [/math] [change in entropy term] for the thermodynamics of the solubility equilibrium. Positive changes in entropy are not thermodynamically favored.

 

Also, HF in concentrated solution can begin to polymerize somewhat as a result of very strong hydrogen bonding interactions: [ce][HFHFHF]_{n}[/ce].

 

EDIT: Negative entropy change, not positive.

Edited March 6, 2011 by mississippichem

[Pwyll Pendefig Dyfed]

This question gets a lot of students.

 

The answer is quite counter intuitive, but there are several ways to analyze this. Hydrofluoric acid is a weak acid, unlike the other binary halo-acids. The "best" reason is the that the aqueous solvation shell that forms around a dissociated fluoride ion is highly organized; so much so that it creates a positive [math] \Delta S [/math] [change in entropy term] for the thermodynamics of the solubility equilibrium. Positive changes in entropy are not thermodynamically favored.

 

Also, HF in concentrated solution can begin to polymerize somewhat as a result of very strong hydrogen bonding interactions: [ce][HFHFHF]_{n}[/ce].

 

Ahhh, thanks.

 

Would that bit in bold be linked to the size of the fluoride ion?

Chlorine doesn't exhibit hydrogen bonding (even though it's very electronegative - more than Nitrogen IIRC which does exhibit H-bonding) but Fluorine does - kind of thing?

Edited March 5, 2011 by ceribethlem

[insane_alien]

you can think of it this way, the flourine ion is smaller than all the other halogen ions meaning that the potential at the 'surface' is greater. This makes it more difficult for the hydrogen ion to escape and it holds water molecules very tightly.

 

if you plot the pKa values of the haloacids you'll see a general trend of the further down the group, the stronger the acid. hydroiodic acid is the strongest of them and has the largest ionic radius.

 

it would be hydroastatic acid but the electro negativities are so close that the hydrogen can sometimes come away as the hydride ion and so the whole mess ends up as free hydrogen and astatine. but the dissociation constant is the highest.

[adianadiadi]

"The positive entropy change is thermodynamically not favored" is not correct.

According to second law of thermodynamics, all the spontaneous processes involve increase in entropy of universe.

[mississippichem]

"The positive entropy change is thermodynamically not favored" is not correct.

According to second law of thermodynamics, all the spontaneous processes involve increase in entropy of universe.

 

True, I should have said negative entropy change. Good catch

[Horza2002]

You can also think of it that fluorine forms very strong bonds (again related to its size). The H-F bond is much stronger than the H-Cl which is stronger then H-Br which is stronger than H-I....thats another way to look at it.

[Gugan]

Fluoroantimonic acid is the strongest acid created so far. It's formula is HSbF6.

 

Fluoroantimonic acid (HSbF₆) is a mixture of hydrogen fluoride and antimony pentafluoride in various ratios. The 1:1 combination forms the strongest known superacid, which has been demonstrated to protonate even hydrocarbons to afford carbocations and H₂.^[2]

 

The reaction of hydrogen fluoride (HF) and SbF₅ is exothermic. HF, being an Lewis base, attacks the molecules of SbF₅ to give an adduct. In the fluoroantimonic molecule, the anion is coordinated to the hydrogen, although the anion is formally classified as noncoordinating, because it is both a very weak nucleophile and a very weak base.

 

Despite the proton being called effectively "naked," it is in fact always attached to a fluorine through a very weak dative bond, similar to the hydronium cation.^[3] However, the weakness of this bond accounts for the system's extreme acidity. Fluoroantimonic acid is 2×10¹⁹ (20 quintillion) times stronger than 100% sulfuric acid.^[4]

 

[Cap'n Refsmmat]

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