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Topic: why is HF not a strong acid?  (Read 11118 times)

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Offline LQ43

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why is HF not a strong acid?
« on: October 13, 2007, 09:24:42 PM »
Reading the other thread about boiling and melting points got me to wondering again why HF is not a strong acid?  The electronegativity difference between H-F is about 1.9 (almost ionic), H-Cl about 0.8; H-Br about 0.7, yet HCl and HBr are strong acids, easily ionized in water. With such a large electronegativity difference for H-F, why would it not be a much stronger acid than the others?
Does the difference in atomic size have a bearing on this, similar to lattice energies that are greater in similar size (small-small) ionic compounds than in non-similar (small-big)size compounds?

Offline LQ43

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Re: why is HF not a strong acid?
« Reply #1 on: October 13, 2007, 09:40:24 PM »
I think I'm answering my own question... :D

Ionization occurs for HCl (0.9)and HBr (0.8) BECAUSE there is NO H-bonding present for them and the H+---OH2 attraction is stronger. For H-F, the H-bonding already present is stronger  than the H---OH2 attraction with water. Therefore H-F does not ionize easily in water, making it a weak acid.
If someone would concur or correct, I would appreciate it  :)

Offline ryan12491

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Re: why is HF not a strong acid?
« Reply #2 on: October 13, 2007, 09:44:45 PM »
The definition of a strong acid is that it will dissociate 100%. HF does not dissociate fully into H+ and F-

Offline LQ43

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Re: why is HF not a strong acid?
« Reply #3 on: October 13, 2007, 09:51:28 PM »
Yes, thanks, I do know the definition of a strong acid, my question was WHY wouldn't H-F do that? But I think its because there is no incentive for H to dissociate if it already has stronger H-bonding present within its own kind.
« Last Edit: October 13, 2007, 10:22:09 PM by LQ43 »

Offline CT101

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Re: why is HF not a strong acid?
« Reply #4 on: October 14, 2007, 11:02:47 AM »
You're right about the H bonding, but there is another reason. HF has a (relatively) large delta G because of its ionic size (not atomic). There is a large decrease in entropy in an HF solution when compared to those of other haloacids. The small size of the fluoride ion makes the H+ F- attraction very strong. The attraction is weaker with the other halides because they're larger.

The extent of any reaction (in this case the dissociation of acids) depends on the free energy change.  Delta G= Delta H- (T x Delta S). The decrease in entropy results in the delta G for HF being around 18 Kj/mol. It plummets for HCl, the delta G is about -50 kj/mole. It's even lower for HBr and HI.

When you have the delta G, you can calculte the equilibrium constant, Ka in this instance. Delta G = -2.30RT(log Ka). When you plug in 18 for the delta G, Ka is 6.95 x 10e-4, which is close to the list value for HF. The is a small eq constant, so the reaction is not one sided (which you already know is the case for HF). Do the same calculation for HCl, and you get 5.91 x 10e8, which is a huge K value. K values of that magnitude mean the reaction goes to completion (which you already know is the case for HCl.)

Offline LQ43

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Re: why is HF not a strong acid?
« Reply #5 on: October 14, 2007, 05:14:31 PM »
thanks, that makes alot of sense too, I should have thought of the Delta G and Ka implications too with it being a weak acid. Fantastic how all the chemistry comes together!

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