When h2co3 reacts to form co2 and h2o, what is the net change in the number of shared electron pairs?

The answer to this question would be: zero

The net change is zero because the number of shared electron pairs is not changed. In H2CO3 there are 2 pairs of shared electron in O-H bond, 2 pairs of shared electron in C-OH bond and 2 pairs of shared electron in C-O bond. So the total pairs is 6

In H2O there are 2 pairs of shared electron in O-H bond and in CO2 4 pairs of shared electron in C-O bond. The total pair is still 6