The activity or electrochemical series is an arrangement of elements according to their order
of reactivity.
If we look at the reactions, one thing that we must note is that the reactions that can occur are those in which an element that is higher in the series displaces another element that is lower in the series.
Br is higher in the electrochemical series than I so it can displace it. Ni is higher than Ag in the electrochemical series hence it can displace it.
