[astrapw]

Why does the rate of an Sn2 reaction depend on the concentrations of both the substrate and the nucleophile?

[impulse29]

The rate formula for SN1 is
R=-d[Starting Material]/dt =k[Starting Material]

For SN2, it is
R=-d[Starting Material]/dt =k[E^+][Nu^-]

This shows that SN2 reaction's rate considers both the nucleophile, and the electrophile. Hope this helps.

[Yggdrasil]

For an S_N1 reaction, the rate determining step is a unimolecular reaction.  Basically, what occurs is the leaving group dissociates from the electrophile to generate a carbocation.  The rate of this depends only on the amount of your electrophile present.

For an S_N2 reaction, the rate determining step is a bimolecular reaction.  The nucleophile and electrophile collide in solution in order to react.  Therefore, the rate at which this collision occurs depends on the concentration of both electrophile and nucleophile.