Simple Help with Lewis Base & acids

[DrBowtie]

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I got this wrong on my test and I feel pretty stupid, but also puzzled.

SO3 + H2O = H2SO4
In the reaction the lewis acid is ____ and the Lewis Base is ____.

I put H2O as the acid and SO3 as the base. Can anyone explain the correct answer? Thanks

 

[scrubsRcomfy]

i'm chem-******ed, and i sat and stared at this for a while, and i may very well still be wrong but here goes...

sulfur's atomic orbital is [Ne]3s23p4, and forms covelent bonds between itself and three other oxygen molecules (= resonance structure somewhere between 2 single bonds and 1 double bond), which helps it to fill sulfur's p-orbital. at this point, sulfur doesn't want to give electrons because it already has stability. however, it can accept two electrons to completely fill an s-orbital. accept electrons= lewis acid. the hydrogens that end up attached to H2SO4 are therefore not actually taking electrons away from the original SO3.

does that make sense?

 

[nrddct]

whats the answer?

 

[DrBowtie]

whats the answer?

I don't know the answer. So I guess I am looking for the correct answer with explanation.

 

[sanche60]

I believe H2SO4 is the acid and SO3 is the base. This makes sense using the bonsted lowry definition of conjugate base pairs.i.e. the acid is the proton donor(lowry) or an electron-pair acceptor(lewis definition). The base is just the opposite of the acid definition.(proton acceptor and electron pair donor)

 

[Amandeep]

Hi, I'm going to try to give this a shot:

I think that ....
SO3 would be the Lewis Acid
H20 would be the Lewis Base

Because...
One of the S=0 double bond's pi electron pair accepts a proton from the H20, leaving an hydroxide ion OH-. We all know that OH- is a Lewis base that donates its electrons to SO3H forming sulfuric acid.

That's my best guess...I could be horribly wrong...What do you all think?

 

[nrddct]

I think sancho is right. H2SO3 is a weak acid so its conjugate SO3 is a strong base. We all know that H2SO4 is a strong acid. Its trying to answer the question in terms of Lewis acid and bases that is throwing me off.

Lewis acids accept elections. Bronsted acids donate protons.

 

[sanche60]

im wrong, SO3 is the lewis acid and h20 is the lewis base. The lone electron pairs of the oxygen are donated to the sulfur. Sulfur ends up with more bonds and is thus the electron acceptor and the oxygen of the water donates one of its lone electron pairs. thats what i got from the kaplan comprehensive review via the reaction BF3+NH3----->BF3NH3 BF3 is the lewis acid because it accepts the lone pair of electrons from the Nitrogen of NH3.

 

[DrBowtie]

See...with all of the homework we did, conjugate bases and acids never came up in context of Lewis. All of the Lewis problems were very obvious so I guess the test question threw me for a loop.

 

[DrBowtie]

im wrong, SO3 is the lewis acid and h20 is the lewis base. The lone electron pairs of the oxygen are donated to the sulfur. Sulfur ends up with more bonds and is thus the electron acceptor and the oxygen of the water donates one of its lone electron pairs. thats what i got from the kaplan comprehensive review via the reaction BF3+NH3----->BF3NH3 BF3 is the lewis acid because it accepts the lone pair of electrons from the Nitrogen of NH3.

Thanks for the response.

 

[booji]

Just a tip, a good way to remember Lewis Acids and Lewis Bases is as electrophiles (electron/-ve charge lovers) and nucleophiles (positive charge lovers) respectively. In this case, H2O, with the lone pair of electrons on the oxygen will act as a nucleophile (+ve charge lover). The SO3 consequently acts as a Lewis Acid (an electrophile).

 

[jmnykrkts]

Just a tip, a good way to remember Lewis Acids and Lewis Bases is as electrophiles (electron/-ve charge lovers) and nucleophiles (positive charge lovers) respectively. In this case, H2O, with the lone pair of electrons on the oxygen will act as a nucleophile (+ve charge lover). The SO3 consequently acts as a Lewis Acid (an electrophile).

I find this to be confusing at times. How is the O in H2O a nucleophile if it has 8 valence electrons and a core charge of zero (2 lone pairs and 2 pairs of bonding electrons)? Wouldn't this make oxygen unreactive? I understand that the lone pairs can react -because they are lone pairs- but aren't atoms always merely trying to achieve their most stable electronic configuration?
Or is it just that the sulfur atom wants those electrons even more than oxygen?

I'm confusing myself right now. I have been studying for too long and I fear this is a simple concept that my tired brain can't be bothered to deal with right now 🙁

 

[NapeSpikes]

How I think of it--and I'm sure it's totally simplified, but it works--is that Sulfur still has room on it's valence, since it can form five bonds. Only four is taken up in SO3. The O from H2O just has electrons sitting there, but it cannot form five bonds, so it cannot accept more electrons. It's got a packed house, and therefore can't be a lewis acid, but it doesn't lose anything when it joins up with SO3. O still has the octet, and S is satisfied with its new packed house as well. So the S accepts two electrons, making it a lewis acid. And everyone is happy.