In the cell Mg(s) | Mg2+(aq) || Au+(aq) | Au(s), what is the cathode and what is the cell type?

• A. Au(s) is the cathode; a galvanic cell
• B. Au+(aq) | Au(s) is the cathode; a voltaic cell
• C. Mg2+ | Mg is the cathode; a galvanic cell
• D. Au+(aq) | Au(s) is the anode; a galvanic cell

Answer: (B)

In a galvanic (voltaic) cell, the left side is where oxidation happens and the right side is where reduction occurs; electrons flow from left to right through the external circuit. Here, the left-hand electrode involves Mg metal losing electrons to become Mg2+ (Mg(s) → Mg2+(aq) + 2 e−), so that left side is the anode. The right-hand electrode involves Au+ gaining electrons to become Au(s) (Au+(aq) + e− → Au(s)), so that right side is the cathode. Since the overall reaction Mg(s) + Au+(aq) → Mg2+(aq) + Au(s) proceeds spontaneously, this is a galvanic cell. Therefore, the Au half-cell is the cathode, and the cell is galvanic.