[33seven]

CH₄  BH₃  NH₃  H⁺  AlCl₃

I chose all of them and the reason I did was because the definition of a Lewis acid is any species which can accept e-. The definition of a Lewis acid includes all the substances which are Bronsted-Lowry acids, so anything that is a B-L acid is also a Lewis acid. The defintion of a B-L acid is any species which can donate a proton(H⁺). So since the first 4 compounds all have H atoms, they should all be able to donate a proton. The last one I included b/c on the very next problem there was a reaction which showed it acted as a Lewis acid. Clearly my logic was faulty cuz I got it wrong, help !!!

[Jorriss]

If the question is, which are lewis acids? It doesn't necessarily matter that they are also bronsted-lowry acids.

So, we have to look for which are lewis acids explicitily. Now, a lewis acid is a substance which can accept an electron pair. But not all atoms or molecules can do this, for example, methane. Methane already has it's full octet, there is no place for those electrons to go and therefore, it is not a lewis acid.

And on the topic of B-L acids... even though methane has a proton, it is not a B-L acid. It's a relatively scale, sure, but relatively to anything - methane is not acidic.

[33seven]

I guess that makes a little more sense. So then that would mean the Lewis acids are #2,4 and 5, correct?

[nj_bartel]

Yep!