Which of the following is the strongest reducing agent given the standard reduction potentials Cu2+/Cu (+0.34 V), Ag+/Ag (+0.80 V), and Fe2+/Fe (-0.44 V)?

• A. Cu2+
• B. Ag+
• C. Cu
• D. Fe

Answer: (D)

The strength of a reducing agent is tied to how easily it is oxidized—that is, how readily it donates electrons. In standard conditions, the tendency to be reduced is given by the standard reduction potential: the more negative the value, the less willing the species is to gain electrons, so it’s a stronger reducer when it is the species being oxidized. Among the given couples, iron has the most negative reduction potential (-0.44 V), so metallic iron is the most readily oxidized and therefore the strongest reducing agent. Copper and silver have higher reduction potentials (+0.34 V and +0.80 V, respectively), meaning they are more inclined to stay reduced and are weaker reducing agents by comparison.