Which species will reduce Ag+ but not Fe2+?

• A. Co^2+
• B. H2
• C. K
• D. Cr

Answer: (B)

The ability of a species to reduce another species is judged by standard redox potentials. If the reduction potential of the species being reduced (Ag+/Ag) is higher than the reduction potential of the reducing agent’s couple, the overall reaction is spontaneous.

For reducing Ag+, hydrogen gas can do it via H2 -> 2H+ + 2e-. The Ag+/Ag couple has E° = +0.80 V, while the H+/H2 couple has E° = 0.00 V. The overall cell potential is 0.80 − 0.00 = +0.80 V, which is favorable, so hydrogen will reduce Ag+.

Check whether hydrogen would reduce Fe2+. The relevant reduction is Fe2+ + 2e- -> Fe, with E° = −0.44 V. The corresponding overall reaction with hydrogen would have E°cell = (+0.80) − (0.00) for Ag+ but for the Fe2+/Fe pair it becomes −0.44 − 0.00 = −0.44 V, which is not favorable. Thus hydrogen will not reduce Fe2+.

So hydrogen can reduce Ag+ but not Fe2+, matching the correct choice.