the half-reactions
cathode : Zn²⁺ (aq) + 2e⁻ ---> Zn (s)
anode : Mg (s) → Mg²⁺ (aq) + 2e−
a balanced cell reaction
Zn²⁺(aq) + Mg(s)→ Zn(s) + Mg²⁺ (aq)
Further explanation
Given
Zn and Mg electrodes
Required
The half-reactions for a galvanic cell
Solution
To determine the reaction of a voltaic cell, we must determine the metal that serves as the anode and the metal that serves as the cathode.
To determine this, we can either know from the standard potential value of the cell or use the voltaic series
1. voltaic series
Li-K-Ba-Ca-Na-Mg-Al-Mn- (H2O) -Zn-Cr-Fe-Cd-Co-Ni-Sn-Pb- (H) -Cu-Hg-Ag-Pt-Au
The more to the left, the metal is more reactive (easily release electrons) and the stronger reducing agent
So the metal on the left will easily undergo oxidation and function as anode
Since Mg is located to the left of Zn, then Mg functions as anode and Zn as a cathode
2. Standard potentials cell of Mg and Zn metals :
Mg2+ + 2e– → Mg E° = -2,35 V
Zn2+ + 2e– → Zn E° = -0,78 V
The anode has a smaller E°, then Mg is the anode and Zn is the cathode.