The standard potentials at 25oC for the following half reactions are given against them Zn2++2e−→Zn, Eo=−0.762 VMg2++2e−→Mg, Eo=-2.37 VWhen zinc dust is added to a solution of magnesium chloride

Zn+MgCl2→ZnCl2+Mg∴ Ecello=EZn+2/Zno-EMg+2/Mgo=-0.762−-2.37                                                     =+1.608 V Here, Ecello is  positive so  Zn+2 is reduced by Mg but Zn can't reduces Mg+2 ions