Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: AhBeng on May 08, 2009, 01:21:50 AM
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"HF is weaker than HCl in water but stronger than HCl in glacial acetic acid..."
Source : http://en.wikipedia.org/wiki/Hammett_acidity_function
Ostensibly, this has to do with the leveling effect of a solvent on acids :
http://en.wikipedia.org/wiki/Leveling_effect
However, the subtle nuances of the discussion begs for clarification, and what scant available explanation on this subject leaves much to be desired in terms of depth or clarity.
For instance, regardless of how we compare the relative strengths of HF, HCl, H2O, H3O+, CH3COOH, CH3COOH2+; and no matter how we shift the basis of 'acid-base neutrality' determined by the solvent, the problem remains in explaining how we can achieve an environment (based on solvent) where the protons of HF are more fully dissociated than the protons of HCl.
Such a solvent (ostensibly glacial acetic/ethanoic acid, or more precisely its protonated cationic form?) would have to be a stronger acid than HCl but a weaker acid than HF. If the implication of this were true, meaning HF is a far, far stronger acid than HCl (in the absence of any solvent consideration), then why would HF be only partially ionized/dissociated in water while HCl is fully ionized/dissociated?
Because of the leveling effect? How exactly does this work?
Thanks in advance for any replies.
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HF seems to be a stronger acid in water than it is :o I've read that this has to do with ion pairs bonded by strong H...F hydrogen bonding:
H3O+...F-
This explains that the conductance is lower than e.g. HCl in solution of the same molarity and in the same way that HF seems to be a weaker acid than it is! Maybe this a part of an answer to your question.
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Is it related to the solubility of the acid?
HF is less soluble in water when compared with HCl
so less H+ ions are ionised