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

Nov 27, 2015

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

#### Explanation:

$S {O}_{2} \left(g\right) + {H}_{2} O \left(l\right) \rightarrow {H}_{2} S {O}_{3} \left(a q\right)$

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, ${H}_{2} S {O}_{3}$; i.e. sulfurous acid LESS a water molecule is $S {O}_{2}$ (and anhydride literally means without/sans water).

What do you think would be the result of the same reaction with sulfur trioxide, $S {O}_{3} \left(g\right)$?

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