Acid/Base Comparison: Understanding Conjugate Stability and Electronegativity

[Qube]

Homework Statement

http://i.minus.com/jhPgz49XLoB3Z.png

Homework Equations

Strength of acid depends on conjugate stability.

The Attempt at a Solution

Why can't the answer also be 4? Both have the same number of oxygens attached, each of which exerts an inductive effect. The only difference is one has Cl and the other has Br. One would think that the conjugate base of bromous acid would be more stable because despite Br's slight lack of electronegativity versus Cl (2.8 vs. 3.0), Br is much bigger than Cl, and is therefore able to spread out negative charge over a bigger surface area.

 

[Christian0412]

Try thinking of (4) a bit differently. Chlorine is more electronegative than Bromine (Electronegativity increases up and right on PT). This implies that Cl "loves" its electrons a bit more than Br, and wants to hold on to them. Because of this, the conjugate base of chlorous acid is a bit more stable as an ion than that of bromous acid.

A simpler way of looking at it is:
Both are oxyacids. The greater the electronegativity of the nonoxygen atom, the greater the acid strength. (General Chemistry rule)

http://www.ualr.edu/rebelford/chem1403/q1403/X5/17-9/17-9.htm
This also may help!

 

[Qube]

Thank you, but I have no use for rules in chemistry. Rules are simply a crutch; one must consider all aspects of the problems rather than relying on some "rule" that may or may not work. Speaking of oxoacids and your rule, consider phosphoric acid and phosphorous acid; both are oxoacids, and both have the same central atom (so both central atoms obviously have the same electronegativity). Yet the two differ in strength. This is due not to central atom EN differences but rather differences in the resonance structures of their conjugate bases.

 

[Borek]

Thank you, but I have no use for rules in chemistry.

Then you have to treat chemistry as a set fact of disjointed facts to memorize.

Rules are simply a crutch; one must consider all aspects of the problems rather than relying on some "rule" that may or may not work.

Agreed, but see above.

TBH, I don't like most of these rules, but in most cases even limping with a crutch is better then just lying in the middle of nowhere. And your guesses from the first post are not better than these rules are.

Speaking of oxoacids and your rule, consider phosphoric acid and phosphorous acid; both are oxoacids, and both have the same central atom (so both central atoms obviously have the same electronegativity). Yet the two differ in strength. This is due not to central atom EN differences but rather differences in the resonance structures of their conjugate bases.

The general rule is: when comparing two homologous acids the one with the higher electronegativity of the central atom is stronger, the one with the higher number of oxygen atoms (ie, higher oxidation number of the central atoms) is stronger.

Hence, first part of the rule tells us HOCl is stronger than HOBr, second part tells us phosphoric acid is stronger than phosphorous acid, disclaimer at the beginning tells us to get lost and not even try to use the rule to compare HOCl with H₃PO₄.

 

[Christian0412]

This rule is for homolous acids.

When comparing phosphorous acid and phosphoric acid, again you must compare the stability of its conjugate base. The differences in determining relative stabilities is no longer comparing the electronegativities between nonoxygen atoms (not simply because the nonoxygen atoms are the same) but more because there are a different number of oxygen atoms. Again, the "general rule" is that the greater the resonance, the greater the stability of the ion or molecule.

Now, if we want to get into the stability of resonance due to hybrid orbitals, then that is another topic. But to keep ideas simple and on topic, we can use certain rules that the describe behavior of molecular bonding due to ideas that can be complex - such as resonance. Since phosphoric acid has "more resonance" through is P-O bonds, its conjugate base is more stable. Thus, phosphoric acid being a stronger acid when compared to phosphorous acid.

 

[Qube]

When comparing phosphorous acid and phosphoric acid, again you must compare the stability of its conjugate base. The differences in determining relative stabilities is no longer comparing the electronegativities between nonoxygen atoms (not simply because the nonoxygen atoms are the same) but more because there are a different number of oxygen atoms. Again, the "general rule" is that the greater the resonance, the greater the stability of the ion or molecule.

Now, if we want to get into the stability of resonance due to hybrid orbitals, then that is another topic. But to keep ideas simple and on topic, we can use certain rules that the describe behavior of molecular bonding due to ideas that can be complex - such as resonance. Since phosphoric acid has "more resonance" through is P-O bonds, its conjugate base is more stable. Thus, phosphoric acid being a stronger acid when compared to phosphorous acid.

Well, the data show that phosphorous acid is stronger than phosphoric acid, so we must revisit and revise our premise here.

 

[Borek]

Sigh, naming of the phosphoric acids is misleading.

Phosphorous acid is not homologous to phosphoric acid, check their structures.

 

[Qube]

Right, in phosphorous acid, we have a hydrogen stuck not to an oxygen but to the central phosphorous atom.