[vhpk]

I want to ask if HBrO₄ is as strong as HClO₄, because in my text book, it is said that HBrO₄ is weaker than HClO₄ but when I learn in another book they said that HBrO₄ is as strong as HClO₄ because K_a(HBrO₄) = K_a(HClO₄) = 10⁸

[chiralic]

IMHO, HClO4 is stronger than HBrO4...because Cl is more electronegative than Br.
Central atom withdraws electron density from the oxygen, stabilizes the anion

[english]

IMHO, HClO4 is stronger than HBrO4...because Cl is more electronegative than Br.
Central atom withdraws electron density from the oxygen, stabilizes the anion

Electronegativity doesn't have much to do with it.

Think about size. 


The difference in acidity is so small, and that's probably why one reference says both acids are of equal stength, roughly.

Check the copyright of both texts.  One text may be outdated.

[Borek]

General rules when omparing oxyacids are: for the same central atom the more oxygen, the stronger the acid, for the same number of oxygen atoms, the higher electronegativity of the central atom, the stronger the acid. Thus HClO₄ should be stronger.

K_a values of such strong acids are rarely precise enough, as they are very hard to determine.

Think about size.

Can you elaborate? What is important here is how much electron density from the oxygens is sucked onto the central atom, but somehow my intuition fails when I try to connect it with the molecule size.

The difference in acidity is so small, and that's probably why one reference says both acids are of equal stength, roughly.

And that's where we agree

[english]

Hi Borek.


I was referring to just the central atoms, Br as compared to Cl.

As both oxoacids have the same number of O atoms, I was assuming that the minor change in acidity was due to the central atoms and their differing size.  Since they're in the same group, EN has little to do with acidity as compared to atomic radius.


Maybe I messed that up.  Oh well.

 

[Dan]

g english, can you explain why the smaller size of Cl stabilises the anion?

Size is a contributing factor to electronegativity anyway. So the smaller size of Cl is one of the reasons it is more electronegative than Br.

if electronegativity difference is insignificant, then surely the smaller size of Cl brings the oxygens closer to each other and destabilises the anion? This opposes the observed trend.

I suppose you could argue that Cl has smaller empty d orbitals which overlap more effectively with the O p orbitals and stabilize the charge. However, the invocation of d orbitals is pretty dodgy ground. I don't think this is a good explanation.

[chiralic]

Electronegativity doesn't have much to do with it. Think about size. 

Q:  Where is bonded the H? to Oxygen or to Halogen?

[english]

I'm sorry for the confusion.  Some references (older ones) place more emphasis on atomic size, and therefore claim that perbromic acid is stronger than perchloric acid because of this.

However, more recent texts claim the other way around, that perchloric acid is stronger than perbromic acid, due to electronegativity.


It's tough to say, merely because our trends don't always work out in more complex cases such as these.


Older texts, especially those printed around 1950-1960, as well as recent texts that reference these older ones, claim HBrO₄ as the stronger of the two.  It turns out that around that time period, 50s and 60s, HBrO₄ had not even been isolated yet.  So the attempts at understanding the acidity of such a species relative to some other known species was quite theoretical.


So there's a conflict with resources, whomever you get the information from.  And since current sources often cite these "archaic" references, there's bound to be confusion.