The  EM+3|M+20  values for Cr, Mn, Fe and Co are – 0.41, + 1.57V, + 0.77 and + 1.97V respectively. For wihch one of these metals the change in oxidation sate from + 2 to + 3 is easiest?

Metal with lower SRP the right hand species readily undergoes oxidation. For Cr+3/Cr+2 SRP value is least. Hence Cr+2 easily changes to Cr+3Order of ease of Oxidation isCr+2 > Fe+2 > Mn+2 > Co+3