When the cell potential is positive then the electrochemical reaction is spontaneous. The Ecell is positive when Zn serves as cathode and Mg serves as anode hence the reaction is spontaneous.
An electrochemical cell is any type of cell in which energy is produced by a spontaneous chemical reaction. Redox reactions that occur in an electrochemical cell produce energy.
An electrochemical cell becomes spontaneous only when Ecell is positive. Ecell is positive when a metal that has a more negative electrode potential serves as the anode.
Since Mg has a more negative electrode potential that Zn, it follows that a cell in which Mg is the anode and Zn is the cathode will have a positive Ecell and the reaction will be spontaneous.
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