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enter image description here

Here the central chlorine atom is best described as $\ce{sp^3}$ hybridized. This makes sense; the molecule is pyramidal. Also, due to the molecule's pyramidal geometry, this rules out the possibility of effective resonance due to less overlap between the $\ce{sp^3}$ lobes, which are angled, as opposed to the parallel pi lobes.

enter image description here

This explains quite well the difference in acidity between nitric acid and chloric acid; nitric acid is stronger than chloric acid. At first it seems difficult to differentiate between the acidities of both; the nitrate anion also has three peripheral oxygen atoms, which provide a similar (negative) inductive effect by delocalizing electron density.

enter image description here

With regard to electronegativity, the chlorate anion seems like it should be more slightly stable than the nitrate anion, implying that chloric acid is stronger than nitric acid; chlorine is more electronegative than nitrogen (if only slightly so - difference is 0.12 on the Pauling scale).

Oxidation state analysis suggests that the two should be similar in acidity; both the chlorine and the nitrogen have oxidation states of +5.

So it seems that the best rationalization of why nitric acid is stronger than chloric acid is the "better" (overlapping pi orbitals) resonance form of the nitrate anion.

With respect to the above rationalization,

1) How, exactly, do parallel pi orbitals enable "better" resonance? I get the principle of maximum overlap. If I had to guess, I would say that more overlap enables greater distribution of electrons among the relevant orbitals ... is there a more concise and scientific way of putting this? Is this even correct?

2) Are parallel pi orbitals necessary for resonance? I got the impression it is from organic class. Or are parallel pi orbitals only necessary for a special form of resonance - conjugation? Is the picture on the bottom referring to conjugation?

3) Are there any other ways to rationalize the difference in strength between these two acids?

enter image description here

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    $\begingroup$ I have no definitive answer, but two things I'd note: (1) don't ignore solvation; nitrate is smaller than chlorate, which likely means better charge stabilization by solvent molecules (especially protic), (2) resonance is an abstraction, not a real phenomenon; it's a convenient device that allows electron delocalization to be represented within VB theory. Conjugation refers to systems of overlapping $p$ AOs that combine to form alternating $\pi$-bonds. This requires planar geometry, and probably isn't fully applicable to chlorate. MO theory is likely necessary for an adequate explanation. $\endgroup$ – Greg E. Jun 22 '14 at 5:45
  • $\begingroup$ How does size exactly affect charge stabilization? I'm not understanding the underlying mechanism. $\endgroup$ – Dissenter Jun 22 '14 at 5:51
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    $\begingroup$ A larger molecule requires a larger shell of solvent molecules to solvate it. In the case of an anion in a protic solvent, solvation is stabilization of the negative charge by intermolecular bonding with the $\delta^{+}$ of hydrogen atoms. This need for a larger shell may have an undesirable enthalpic effect (typically if the charge is localized and inaccessible due to bulky groups, probably not the case here), as well as an undesirable entropic one, because the shell of solvent molecules is highly ordered. Hence, the change in free energy associated with solvation becomes less favorable. $\endgroup$ – Greg E. Jun 22 '14 at 6:04
  • $\begingroup$ Ah, I see, thank you! Good considerations, and ones I'm sure my prof would appreciate. Also to be clear, you mean that steric factors can cause the solvation energy to be less exo/possibly endothermic? $\endgroup$ – Dissenter Jun 22 '14 at 6:06
  • $\begingroup$ Also is water a protic solvent? Finally I'm trying to think through how the mechanism behind the enthalpy part works. The entropy part makes sense though. $\endgroup$ – Dissenter Jun 22 '14 at 6:10
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The two ions you are comparing to each other are very different. In $\ce{ClO3-}$ there are two more electrons than in $\ce{NO3-}$. This is also the reason why the former pyramidalizes, while the latter does not.

In nitrate the negative charge can be delocalised over the whole molecule, since there is one molecular orbital with $\pi$ symmetry. One basic requirement for these orbitals is, that they are antisymmetric with respect to a mirror plane, that includes the bonding axis.

This MO scheme would also be thinkable for the chlorate ion, but here you have two more electrons, so you have to fill an antibonding orbital. The charge can no longer be delocalised as well in this molecule. The conclusion is, that the symmetry of the whole molecule is lowered and the $\pi$ orbitals vanish. A lone pair is always more stable in an orbital with high $\ce{s}$ contribution, which leaves only the $\ce{p}$ orbitals of chlorine for bonding with oxygen.

Resonance is a model which is necessary to describe bonding situations, that cannot be described by one Lewis structure alone. It is therefore more a rule of thumb than any evidence. $\pi$ bonds (or orbitals) are not necessary for valid resonance structures. They are however necessary for ([negative] hyper-)conjugation.

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First, we must see $pK_a's$, too see which factors which we must account: $$\begin{array}{c|c|} &\ce{HNO3}&-1.4\\ &\ce{HClO3}&-1\\ \end{array}$$ The strength of a Bronsted-Lowry acid depends upon the extent to which the proton can be separated from it and transferred to a base.Removing the proton involves breaking a bond tp the proton, and it involves making the conjugate base more electrically negative.

Relationship between structure and acidity can be seen through factors:

  • Hybridisation
  • Inductive Effects(Electronegativity)

There is also an effect of the solvent on acidity.

Now let's compare both acids:

$$\begin{array}{c|c|c} &acid&EN&\text{Hybridisation}\\ &\ce{HNO3}&N\equiv3.0^*&sp^2&\\ &\ce{HClO3}&Cl\equiv3.0^*&sp^3&\\ \end{array}$$ $^*$may vary a little

So Hybridisation suggests N to be more acidic as %s character is more and thus the negative charge on O will be better stabilised; But Solvation is different due to size. Both have almost same electronegativity.In water essentialy both ionize to 100%.So you must here only cosider solvation to be major factor.


Parallel pi-orbitals are necessary for resonance; consider pyridine where there is no resonance of N's lone pair as N is $sp^2$ which makes its lone pair in the plane of benzene(type) ring and thus this leaves a lone pair of electrons available on nitrogen.

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  • $\begingroup$ But the chlorate anion can resonance despite its lack of p orbitals. $\endgroup$ – Dissenter Jun 23 '14 at 6:24
  • $\begingroup$ @Dissenter Actually, it can't. The order of Cl-O bonds is 1. And nitrogen in nitric acid and nitrate-anion participates in exactly 4 bonds $\endgroup$ – permeakra Jun 23 '14 at 8:45
  • $\begingroup$ "The chlorate ion cannot be satisfactorily represented by just one Lewis structure, since all the Cl-O bonds are the same length (1.49 Å in potassium chlorate[2])." upload.wikimedia.org/wikipedia/commons/thumb/d/d2/… $\endgroup$ – Dissenter Jun 23 '14 at 13:46
  • $\begingroup$ I think you mean that parallel pi-orbitals are necessary for conjugation. $\endgroup$ – Dissenter Jun 23 '14 at 13:54
  • $\begingroup$ yes necesarry... $\endgroup$ – RE60K Jun 23 '14 at 15:47

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