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HF is not a "strong acid" because F is the most electronegative element and holds onto H too well, so this molecule does not dissociate in water very well. It is still pretty corrosive though, despite what the term "weak acid" may suggest.
As mentioned, the high electronegativity of flourine creates an incredibly strong bond between the two atoms. Because of this, the acid cannot dissociate easily in water, hence its status as a weak acid. Note that weak acids are not necessarily closer to 7 on the pH scale, they simply cannot dissociate easily in water.
HF is weak(er) when compared to HI etc. bc of fluorine's high electronegativity and thus strong bond to hydrogen. The stronger the bond, the harder it is to break or dissociate when added to water. How strong an acid or base here is actually inversely related to the bond strength and consequently the electronegativity. Also, the smaller the bond, the stronger the bond and thus the weaker the acid/base. Thus an acid/base is strong when it has low electronegativity or long bonds.
Maddie Turk Disc 2C wrote:Why are acids with short bonds weak acids while acids with long bonds strong acids, and not the other way around?
This is because a strong acid is supposed to give off a proton quite easily, if the there is a strong bond (short bonds) then the electrons and all the rest will be held tightly. Strong acid meaning good at becoming an acid, not meaning the strength it has on pulling. Hope this helps!
You would classify a compound being a strong acid if it gives up its proton readily. F is very electronegative so it holds onto its H atom strongly and the bond cannot break easily which means that it cannot donate its proton easily. Therefore HF is a weak acid.
The shorter the bond, the stronger the bond. And the stronger the bond, the weaker the acid. HF has quite the short and strong bond due to their difference in electronegativity, so it's classified as a weak acid.
The bond between HF is very strong because F is very electronegative. A strong acid dissociates/ionizes almost completely. It would be hard for the aqueous solution to dissociate HF, therefore it is not a strong acid.
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