Factors Influencing Rate of a Reaction

SN2 is a bimolecular reaction. This is because as per the rate law,
Rate = k [substrate] [nucleophile]

Here, the rate determining step is dependent on both substrate and nucleophile, unlike SN1 who only involves substrate. Therefore, SN2 is proved to be bimolecular.

Yes, the phase of the reactant (solid, liquid or gas) actually affects the rate of a reaction. Gases and Liquids tend to move more freely and effectively as compared to solids, which increaes the frequency of particles colliding with each other.

This is possible because these catalysts have a lower activation energy, which allows a comparatively higher number of particles to react at a faster rate. More number of particles will mean more frequent collisions which speeds up the process.