Acidity/Electronegativity

[HurricaneAlana]

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So, I have conflicting notes from two different sources, one of which I no longer have access to, so I was hoping I could get someone to clarify.

Acidity increases with electronegativity, which is why NH3 is more acidic than CH4.

According to this I would think that HF would be more acidic than NH3.

But I have it written that HF is a weak acid, because HF is a very strong bond.


??

Thanks!

 

[bigstix808]

So, I have conflicting notes from two different sources, one of which I no longer have access to, so I was hoping I could get someone to clarify.

Acidity increases with electronegativity, which is why NH3 is more acidic than CH4.

According to this I would think that HF would be more acidic than NH3.

But I have it written that HF is a weak acid, because HF is a very strong bond.


??

Thanks!

electronegativity is not the only thing that determines the acidity of a compound. you are correct is that Cl- is the most electronegative, but the size of the anion plays an ever bigger role, this is why HCl is a strong acid and HF is not. Cl is much larger than F and can spread out its (-) charge more effectivley making it more stable by itself in solution (I- is most stable). Your electronegativity rule is a good one and does work as you go across a row, rather than down a column (H2O vs. HF for example) because size changes less than electronegativity across a row in the periodic table.

maybe that helped...

 

[mddang]

The more you can delocalize a charge, the more stable.


You would only compare EN for similar sized atoms.

 

[phospho]

the size of the anion plays an ever bigger role, this is why HCl is a strong acid and HF is not.

That's what I always thought. But then why is HClO2 (pka=1.9) more acidic than HBrO2 (pka=2.85)?

this is what my book says: "we know that HBrO2 is less acidic than HClO2 since both acids have the same number of oxygen atoms, and chlorine is more electronegative than bromine".

thanks for your help🙂

 

[bigstix808]

That's what I always thought. But then why is HClO2 (pka=1.9) more acidic than HBrO2 (pka=2.85)?

this is what my book says: "we know that HBrO2 is less acidic than HClO2 since both acids have the same number of oxygen atoms, and chlorine is more electronegative than bromine".

thanks for your help🙂

this actually has to do with localization of charge. the fact that Cl and O are very close in electronegativity allows that molecule to "spread out" its charge as opposed to that of a Br. Also, Br is so much larger and less electronegative, the charge or electron density to be more precise, is more concentrated on the Oxygen atoms creating a slightly less stable anion than that of HCLO2. make sense??

O
||
Cl - H
||
O

-VS-

O
||
Br - H
||
O

 

[phospho]

make sense??

yes!

thank you very much for your help