Question

Which is the Lewis acid, and which is the Lewis base in the following equation: SO2(g) + H2O(l) --> H2SO3(aq)?

Answer 1

The Lewis acid is the electron pair acceptor; the Lewis base is the electron pair donor.

Explanation:

`SO_2(g) + H_2O(l) rarr H_2SO_3(aq)`

While it is not immediately obvious, it is clear that the sulfur species has accepted a lone pair from the water molecule; so sulfur dioxide is the Lewis acid in this example. A more old-fashioned treatment is to designate sulfur dioxide as the acid anhydride of sulfurous acid, `H_2SO_3`; i.e. sulfurous acid LESS a water molecule is `SO_2` (and anhydride literally means without/sans water).

What do you think would be the result of the same reaction with sulfur trioxide, `SO_3(g)`?

`SO_3(g) + H_2O(l) rarr ??`