The Leonardo Express leaves Fiumicino at 6 and 36 minutes past the hour. The trip to Roma Termini takes 31 minutes. If you're lucky enough to breeze through passport control and luggage claim, you POSSIBLY could catch the 10:36 train (but I wouldn't count on it). If you make it, you could leave Termini at 11:30, change at Bologna Centrale, and arrive at Venezia Santa Lucia at 16:47. A safer bet would be to plan on taking the direct train that leaves Termini at 12:50 and arrives at 17:17.
The simplest thing to do would be to buy all your tickets at the station at the airport. It's possible to get Amica fares (20% discount) for Roma-Venezia by booking ahead at www.trenitalia.com/en/index.html; but registration is required on the site, a process that often is problematic. You can book up to 60 days in advance.
(continued in next post)