The standard Potentials at 250 C for the half reactions are given against them below:Zn2++2e-→Zn;E0=-0.762V Mg2++2e-→Mg;E0=-2.37VWhen Zn dust is added to a solution of MgCl2

Ezn+2/Zno>EMg+2/MgoSpontaneous reduction occurs at Zn electrode.Spontaneous oxidation occurs at Mg electrode.Cell reactionMg(s)+Zn(aq)+2→Mg(aq)+2+Zn(s)Zn cannot reduce Mg+2,hence no reaction occurs.