The reaction of benzyl bromide (C6H5CH2Br) with azide ion (N…

The reaction of benzyl bromide (C6H5CH2Br) with azide ion (N3-) is shown below.  When the benzyl bromide concentration is doubled and azide concentration is held constant, the reaction rate is observed to increase by a factor of two.  When the azide concentration is doubled and the benzyl bromide concentration is held constant, the rate of the reaction doubles.  What is the correct rate law for this reaction?                                                C6H5CH2Br  +  N3- ——>  C6H5CH2N3  +  Br- I.   Rate = k [C6H5CH2Br ]2 [N3- ]2 II.  Rate = k [C6H5CH2Br ]2 [N3- ] III. Rate = k [C6H5CH2Br ]4 [N3- ]2 IV. Rate = k [C6H5CH2Br ] [N3- ]

The rate of precipitate formation increases for the reaction…

The rate of precipitate formation increases for the reaction of 1-bromobutane with NaI as the concentration of NaI is increased.  When the concentration of NaI is increased in the reaction of 2-chloro-2-methylpropane, no rate change is observed.  Why is there a change in rate in the first case and not in the second?