strength of acid

[allstardentist]

we know that HI is stronger than HBr or HCl
but how come H3PO4 is stronger than H3AsO4?

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[busdent]

we know that HI is stronger than HBr or HCl
but how come H3PO4 is stronger than H3AsO4?

because Phosphorus is more electronegative than Arsenic.

 

[allstardentist]

but Cl is more electronegative than I but HI is a stronger acid than HCl

 

[busdent]

but Cl is more electronegative than I but HI is a stronger acid than HCl

in that case you look at the size. I don't know how to explain it perhaps somebody else could shed some light.

 

[Uracil]

just learn your list of strong acids...I dont think you can tell by looking at the molecule or relating electronegativity, but rather having an idea of the pKa values of some of them.

 

[Manyak222]

Yes you can.. these are oxyacids, and have different qualities than regular acids which i may or may not post after i eat this delicious pizza .

 

[allstardentist]

from my ochem class i learned that i need to look at the conjugate base to determine the strength of acid. Whichever forms the weakest conjugate base is the strongest acid.

 

[Uracil]

from my ochem class i learned that i need to look at the conjugate base to determine the strength of acid. Whichever forms the weakest conjugate base is the strongest acid.

 

[allstardentist]

got it now.. theres are two different rules for halogen acids and oxyacids.

 

[busdent]

Generally, the more electronegative the atom attached to H atom is, the stronger the acidity of molecule will turn out to be, where removal of the proton, H+, is enhanced by unevenly shared electron pair on the bond. In the case of oxyacid, HClO4, the increase of formal charge and oxidation number on the central atom, Cl, promotes even greater electron withdrawing from hydrogen atom via one of the other four oxygen atoms; making it comparatively the greatest in acidity strength.



Not directly related but something similar

 

[TL294]

For elements in the same group, you compare their bone length. For example: HI is stronger than HCl because I is less electronegative than Cl; therefore its bond with H is longer and weaker.
For element in the same row, you look at their electronegative , the one has the highest electronegative is the strongest acid.
Also, you can look at their resonance and inductive effect

 

[allstardentist]

For elements in the same group, you compare their bone length. For example: HI is stronger than HCl because I is less electronegative than Cl; therefore its bond with H is longer and weaker.
For element in the same row, you look at their electronegative , the one has the highest electronegative is the strongest acid.
Also, you can look at their resonance and inductive effect

BUT for oxyacids, the stronger acids have a more electronegative central atom even for elements in the same group. If the central atom is the same, then you look at the number of oxygens attached to the central atom. Thus, H3PO4> H3AsPO4.