acids (1 Viewer)

[onebytwo]

HCl forms a strong acid. HF forms a weak acid.
But, why should this be so if we compare the structure of these two molecules, HF is so much more polar, since fluorine is so much more elctronegative than Cl, so the HF molecule is more "egg" shaped and the H atom is a lot more loosely held (or I think it is), wouldn't this mean that the HF moleule has a greater tendency to ionise completly in solution? if so why isnt it a strong acid, or "stronger" than HCl??????
Any help, please.

 

[pkc]

HCl forms a strong acid. HF forms a weak acid.
But, why should this be so if we compare the structure of these two molecules, HF is so much more polar, since fluorine is so much more elctronegative than Cl, so the HF molecule is more "egg" shaped and the H atom is a lot more loosely held (or I think it is), wouldn't this mean that the HF moleule has a greater tendency to ionise completly in solution? if so why isnt it a strong acid, or "stronger" than HCl??????
Any help, please.

The HF bond is extremely strong because of the huge dipole created by the large difference in electronegativity of these 2 atoms.

More positive next to more negative = stronger bond (harder to be torn apart by H2O neighbours).


Hope this helps.

 

[onebytwo]

why didnt i see that, it so simple.
thanks

 

[pLuvia]

The HF bond is extremely strong because of the huge dipole created by the large difference in electronegativity of these 2 atoms.

The HF bond is a hydrogen bond and these specific bonds are very strong and require a lot of energy to break

 

[rama_v]

Just for some trivia: This is an example of the first-member effect. The first member of a each period can be quite different in properties to the other members in that period.

 

[pkc]

The HF bond is a hydrogen bond and these specific bonds are very strong and require a lot of energy to break

The HF bond is not a hydrogen bond.

Hydrogen bonds are intermolecular and relatively weak. The HF bond is an intramolecular bond and as result much stronger.

 

[rama_v]

I found out that in actual fact, the reason why HF is a weak acid is much more complicated than it sounds. Really, its beyond HSC chemistry, but if you are interested:

http://www.chemguide.co.uk/inorganic/group7/acidityhx.html

 

[onebytwo]

The HF bond is not a hydrogen bond.

Hydrogen bonds are intermolecular and relatively weak. The HF bond is an intramolecular bond and as result much stronger.

i always thought the HF bond was a hydrogen bond which is of the strongest Van Der Waals forces. isnt it hydrogen to fluorine, oxygen and nitrogen represent hydrogen bonds.
correct me if im wrong

 

[pkc]

i always thought the HF bond was a hydrogen bond which is of the strongest Van Der Waals forces. isnt it hydrogen to fluorine, oxygen and nitrogen represent hydrogen bonds.
correct me if im wrong

Hydrogen bonds are one of the strongest Van Der Waals forces. (which , by the way, are far weaker than covalent or ionic bonds which HF is)

Also , by definition all Van Der Waals forces are intermolecular, not intramolecular as the HF bond is.

 

[Forbidden.]

If the bonds are strong, then it means it doesn't dissociate completely.
When it doesn't dissociate completely then it must be weak.