Destroyer General Chem Q113

[Meas]

Which can function as Lewis acid?
AlCl3
BF3
H2O
NH2-
AlCl3 & BF3

Ans: AlCl3 & BF3

- I can see that H2O is NOT a Lewis acid since it already has 6 electron pairs from oxygen.
- NH2- is also not a Lewis acid b/c it is negative (which means it already has nonbonding electrons)
- I don't really understand why AlCl3 & BF3 can be Lewis acid. Shouldn't a Lewis acid have a + charge in order to accept electrons?

Thanks in advance!

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[SupahShyGuy]

I like to think of Lewis Acids as e- pair Acceptors. They're in group III which explains the AlCl3 and BF3, they have only 3 valence shell electrons. As you can see, group III elements such as Al and B will not have a full octet of electrons, and in this case, they both have 6. But these will happily accept 2 more electrons from Lewis Bases (electron pair donors) to make an more favorable octet.

For example...
AlCl3 + :NH3 = AlCl3NH3. I believe this forms a coordinate covalent bond? Perhaps someone could correct me if I'm wrong? But use a Periodic Table to easily find out the valence shell electrons.

The key to this problem is knowing the definition of the various Acids and bases. Be familiar with the Bronsted-Lowry, Lewis, and Arrhenius definitions. They will most likely be asked on the actual DAT

 

[SupahShyGuy]

Also, Lewis acids do not have to have a positive charge to accept electrons. Simply think of it as fulfilling the octet rule.

 

[Meas]

Thx!