At 25 °C, the standard electrode potentials are +0.34 V for Cu²⁺/Cu, −0.44 V for Fe²⁺/Fe and −0.76 V for Zn²⁺/Zn. Which of these three metals is thermodynamically the most easily oxidized (most readily becomes a cation)?

Answer: C. Zinc

The standard electrode potential expresses the equilibrium between a metal and its ions relative to the standard hydrogen electrode at 0 V. The more negative (more active) the value, the stronger the tendency of the metal to give up electrons and become a cation, so of the three, zinc at −0.76 V is the most easily oxidized and copper at +0.34 V is the least (the most noble). Choosing copper reads the sign the wrong way round. Iron (−0.44 V) is more noble than zinc, so it is not as easily oxidized. Note, however, that the standard electrode potential indicates a tendency, not the actual rate of corrosion. Aluminum, for example, is very active at −1.66 V, but resists the atmosphere well thanks to its dense surface oxide film. To evaluate metal combinations in real environments, the ranking of corrosion potentials measured in seawater (the galvanic series) is used.