Identify the Lewis acid and the Lewis base in each of the following reactions:
a. AlCl3 + Cl- -> AlCl4-
b. BF3 + C5H5N -> C5H5N -> BF3
c. Co3+ + 6 NH3 -> [Co(NH3)6]3+
d. SO2 + HO- -> HSO3-

Lewis Acids are electron acceptors. Lewis bases are electron donors. The negatively charged species are the electron donors. Why? Which are the acceptors?

Lewis acid are electron acceptors. Lewis bases are electron donors. Lewis Bases will have a lone pair of electrons in their valence shell. This is big hint. Also, period 3 and up are able to expand their octet allowing them to accept more electrons, because they a empty d orbital. One last thing. Bf3 is a lewis acid because even though all of its electrons are paired, its valence shell isn't filled completely. Therefore, it is able to accept more electrons. Also it does not have a lone pair so it can't be a lewis base.

Lewis Bases: Cl-, C5H5N, NH3, HO-

Lewis acid AlCl3, BF3, Co3, SO2

We should point out that Lewis acids and bases are electron PAIR acceptors and donors, respectively.