Doing an experiment of nucleophilic substitution reaction of organohalides Sn$1$ and Sn$2,$ this is the reagent used:

Iodoethane $($ethyl iodide$)$

$1-$Chlorobutane $($n$-$butyl chloride$)$

$1-$Bromobutane $($n$-$butyl bromide$)$

$2-$Chlorobutane $($sec$-$butyl chloride$)$

$2-$Bromobutane $($sec$-$butyl bromide$)$

$2-$Chloro$-2-$methylpropane $($tert$-$butyl chloride$)$

$1-$Chloro$-2-$methylpropane $($isobutyl chloride$)$

Bromobenzene

NaI is added for Sn$2$ reaction $($part A$)$

AgNO$3$ is added for Sn$1$ reaction $($part B$).$ For this experiment, we are seeing if there is a precipitation formed, and the rate of the precipitation formed.

Based on the data, do the following questions

$($a$)$ In cases where heating and$/$or an extended time was necessary for a precipitate to form, elucidate the reasons behind

these requirements.

$($b$)$ If no reaction occurred, explain the absence of precipitate in those instances