-> Sn1 mechanism:
-Only one reactant
-Formation of primary carbocation before reacting with OH- ions therefore OH- ions do NOT appear in the rds (rate determining step).
Changing the concentration of OH- ions does not alter the rate of reaction.
-> Sn2 mechanism:
-2 reactants present
-Both OH- ions and the Halogenoalkane are present in the rds therefore concentration of both the reactants is vital
*learning how to draw the mechanisms are compulsary
ps - Reaction rate diagrams (for zero, first and second order reactions) will always come so make sure that you learn those as well.