For a zinc-magnesium electrochemical cell, the half cell potentials from the standard table are:
`Mg^(2+) + 2e^- -> Mg, E = -2.38 V`
`Zn^(2+) + 2e^- -> Zn, E = -0.76 V`
From the standard half cell potential, we can see that magnesium will be reduced and zinc oxidized for the spontaneous reactions to take place.
Hence Zinc will be the cathode and Magnesium will be the anode.
The oxidation half-reaction will be:
`Zn -> Zn^(2+) + 2e^-`
The reduction half reaction will be:
`Mg^(2+) + 2e^- -> Mg`
The standard potential energy of the cell is E(magnesium) - E (zinc)
E (cell) =-2.38 - (-0.76) volts = - 3.14 volts
This is a voltaic cell, since the cell potential is negative.
Hope this helps.