2. in the sn1 mechanism, the second step is attack of the nucleophile, so increasing the nucleophile concentration should speed up this step. why, then, do we say that the rate of an sn1 reaction does not depend on the concentration of the nucleophile?

The rate of reaction depends on the rate determining step of a nonelementary reaction

Explanation:

A non elementary reaction is a reaction that proceeds in a sequence of steps. One of the steps is usually the slowest step in the reaction sequence. This slowest step is called the rate determining step. In an SN1 reaction, the rate determining step is the formation of the carbocation, the concentration of the carbocation affects the rate of reaction because it is involved in the rate determining step. The attack of the nucleophile is a fast step and does not affect the rate of reaction.