In the reaction :
`R-Br+Cl^(-) to R -Cl+Br^(-)`
The rates of reaction of ethyl bromide (I) n-propyl bromide (II) isobutyl bromide (III) and neopentyl bromide (IV) follow the order:

Explanation

Make the Lassaigne's extract of `C_(2)H_(5)Br and C_(2)H_(5)Cl` separately. Acidify them with dil `HNO_(3) and ` add `AgNO_(3)` solution. `C_(2)H_(5)Br` gives pale yellow ppt. of AgBr partially soluble in ammonia while `C_(2)H_(5)Cl` gives white ppt. of AgCl soluble in ammonia.