Predict feasibility of the reaction between Fe3+ and Br-
We will discuss predicting feasibility of redox reaction between Fe3+ and Br- by using standard reduction potential.
Standard reduction potential is a measure of electron gaining power of an electrode. It may be positive or negative.
Positive electrode potential shows that the species is oxidising agent which can pull electrons from another regents. For the redox reaction to be possible, oxidising agent should be more powerful, it means it should be positive.
Fe3+ +e = Fe2+ E0 = +0.77 v
Br- = Br2 + 2e E0 = 1.09v
Since Fe3+ has lower potential, that is not enough to pull electrons from Bromide ion. So, the reaction is not feasible.