I'm doing the same exam
I would think of Silicon like Carbon.
Si02, like C02 has a double bond, but two lone pairs of electron and two bonding pairs, which are more repulsive making it non linear. So it is a non polar molecule because of the difference in electronegativety. As it is non polar, it there are dipole-dipole forces acting, which are stronger than the Van der Waals forces acting on SiCl4, a polar molecule. Does that make sense?
The other one seems to be a standard reactivity question
Trend in reactivityi increases because as you go down the group
-Increasing atomic radius and electron shielding means there is less attraction acting on the outer shell electrons
-These factors outweigh increasing nuclear charge.
-This increases the first ionisation energy, meaning less energy is needed to lose and electron
-Which makes it more reactive, and it is easier to lose two electrons in the reaction
What year is the paper? I'd look up the markscheme if I were you.