Lewis Acid-Base Question

[ArcherM2]

I always seem to get confused here (and everywhere else in GChem)...

Acids: Accept Electrons
Bases: Donate Electrons

so then in HCl +H2O <--> H3O+ + Cl- which is which??

HCl is then a base because it's donating it's electrons??

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[LiNk]

In this case you can think of HCl as a
Bronstead Acid = proton donor
H20 is the Bronstead Base = proton acceptor
forming Cl- conjugate base and hydronium ion conj. acid

 

[JDAD]

scroll down for the answer

 

[ArcherM2]

Good stuff, thanks a bunch...I was just confused because we all know that HCl is an ACID but I guess Lewis was just craaaaaaaazy..hahah thanks again

 

[willthatsall]

Isn't H2O donating its electrons? It donates one of the lone pairs on oxygen, making it H30+. The H+ can't donate any electrons because it doesn't have any. H2O is donating, not gaining electrons. Am I off on this?

 

[ArcherM2]

That makes more sense now that I look at another reaction...

AlCl3 + H2O <--> AlCl3- + H3O+

So it's like donating to the Red Cross, you donate but gain in the same process? Donating electrons but gaining a proton!

It all makes sense...except the fact that HCl is a base, lewis is crazy.

 

[Chlost]

Isn't H2O donating its electrons? It donates one of the lone pairs on oxygen, making it H30+. The H+ can't donate any electrons because it doesn't have any. H2O is donating, not gaining electrons. Am I off on this?

No you're right, H2O = the conj. base, it "attacks" H+ w/ it's lone pair e-'s, and HCl is the acid, donating the H+

...w/ H3O+ = to conj. acid of H2O, and Cl- = conj. base of HCl

 

[silkworm]

I am getting a little confused here. If you look at H2O it only gains a positive proton, so H3O has a positive charge. It doesn't gain electron at all. In fact, there is no electron exchange in this reaction, which means we can't judge it on a Lewis acid/base level, only on a Bronstead level.

Am I right?

 

[Kazema]

It's best to draw out what happens if you're confused.

Draw the H2O molecule with its lone pairs as well as the H-Cl. How does the H2O become H3O+? When it donates an electron to the H in H-Cl, while the H in H-Cl releases its electron from the bond to Cl, which becomes Cl- by accepting that electron.

So H2O is an electron donor and therefore a base, while HCl is an electron acceptor and therefore an acid.

All Bronsted acids are Lewis acids, and all Bronsted bases are Lewis bases. But not all Lewis acids are Bronsted acids and not all Lewis bases are Bronsted bases.

 

[silkworm]

Thanks for clearing it up.

It's best to draw out what happens if you're confused.

Draw the H2O molecule with its lone pairs as well as the H-Cl. How does the H2O become H3O+? When it donates an electron to the H in H-Cl, while the H in H-Cl releases its electron from the bond to Cl, which becomes Cl- by accepting that electron.

So H2O is an electron donor and therefore a base, while HCl is an electron acceptor and therefore an acid.

All Bronsted acids are Lewis acids, and all Bronsted bases are Lewis bases. But not all Lewis acids are Bronsted acids and not all Lewis bases are Bronsted bases.

 

[Fusion]

HCl is one of the strongest acids of all. It is not basic in the least...

There are two ways you can think of acids and bases. B-L Acids are proton donors, and Lewis acids are electron pair acceptors. HCl is a B-L acid because it has a proton to donate. Such molecules as AlCl3 and BF3 are lewis acids because they readily accept a pair of electrons from another molecule. But guess what? HCl can also be thought of as a Lewis Acid because it essentially accepts a pair of electrons when it donates ("releases") it's H+ ion. However, when a molecule donates a proton it is almost always referred to as a B-L acid.

Bottom line: acids accept electron pairs and bases donate electron pairs.

 

[JohnHolmes]

I always seem to get confused here (and everywhere else in GChem)...

Acids: Accept Electrons
Bases: Donate Electrons

so then in HCl +H2O <--> H3O+ + Cl- which is which??

HCl is then a base because it's donating it's electrons??

Negative. The lewis base and lewis acid definitions are the best, they are consistent with this reaction.

H2O, one lone pair of elections of O "donates" its electron density and abstracts a proton from H-Cl. H2O is the lewis base, HCl, the lewis acid.

It works

 

[jdgator7]

A Lewis Acid is any species with one or more empty valence orbitals (an electron pair acceptor).

A Lewis Base is any species with at least one nonbonding electron pair in its valence electron structure (an electron pair donor)

If you write out the Lewis structure, you can tell which one it is

Hope that helps

 

[Noeljan]

try this simple little quiz I found online to help you practice
http://www.cat.cc.md.us/~cminnier/quizlewis.htm