0
$\begingroup$

I am having trouble with this question:

Identify the Lewis acid and Lewis base in each of the following reactions:

$\ce{SO2(g) + H2O(l) = H2SO3(aq)}$

What I tried:

A Lewis acid is an electron pair acceptor.

A Lewis base is an electron pair donor.

The other problems of this type involved cations or anions, so I was able to identify the cation as the Lewis acid and the anion as the Lewis base.

However, this question does not involve ions. So I tried to look at the lone pairs on the molecules. Since they both have lone pairs, I figured since $\ce{SO2}$ has more lone pairs, it would act as the Lewis base. Also in the final molecule, the hydrogen atoms would be bonded to two separate oxygen atoms, so I thought of them as migrating to the lone pairs on the $\ce{SO2}$ oxygen atoms.

However, the textbook says that $\ce{SO2}$ is the Lewis acid and $\ce{H2O}$ is the Lewis base.

Why would this be the case?

$\endgroup$
  • $\begingroup$ Clearly it is $\ce{H2O}$ that donates an electron pair to from the bond? So it is the Lewis base. $\endgroup$ – Gert Dec 9 '17 at 21:25
  • $\begingroup$ Can you explain why it must be $\ce{H2O}$ that donates the electron pair(s) and not $\ce{SO2}$? $\endgroup$ – user6615434 Dec 9 '17 at 22:07
  • $\begingroup$ I'll try and answer that tomorrow. $\endgroup$ – Gert Dec 9 '17 at 22:48
1
$\begingroup$

The oxygen in $\ce{H2O}$ already has its orbitals ($2s^2$,$2p^4$) filled and since oxygen is in the second row of the periodic table it can't form hypervalent molecules (That means that the oxygen would have more than 8 valence electrons).
Sulfur on the other hand is in the third row of the periodic table and can therefore form such molecules.

Because of this it is clear, that the $\ce{H2O}$ can't act as a Lewis acid as it can't have any more electrons. Therefore it has to be the Lewis-base which leaves $\ce{SO2}$ as the Lewis-acid.

$\endgroup$
  • $\begingroup$ I think what you say is misleading. Because you are saying that water cannot act as a Lewis acid. However, water is amphoteric and can act as a Lewis acid, as it has an electron deficient hydrogen atom capable of accepting an electron pair. $\endgroup$ – Tan Yong Boon Dec 10 '17 at 10:47
  • $\begingroup$ I think you are referring to hydrogen bonds aren't you? In that case water is indeed acting as a Lewis acid but I'm not sure if one would consider this as a real Lewis acid-vase-reaction... See quora.com/Can-water-be-classified-as-a-lewis-acid?share=1 $\endgroup$ – Raven Dec 10 '17 at 11:10
0
$\begingroup$

Molecules that contain polar multiple bonds can function as Lewis acid because the central atom is electron deficient with a vacant orbital that can accept an electron pair. When $\ce{SO_2}$ dissolves in water , it forms the weak diprotic acid $\ce{H_2SO_3}$ (sulfurous acid) from Lewis acid/Lewis base reaction: $$\ce{O\bond{=}S-O +H_2O<=>H_2SO_3}$$ enter image description here

The $O$ atom of an $\ce{H_2O}$ molecule donates a lone pair to the $S$ of $\ce{SO_2}$,forming a new $\ce{S\bond{-}O}$sigma bond and breaking $\ce{S\bond{-}O}$ π bond

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.