Question

The standard reduction potentials at 25°C for the following half cell reactions are given against each:

\[\ce{Zn{^{2+}_{(aq)}} + 2e- -> Zn_{(s)}}\] = −0.762 V

\[\ce{Cr{^{3+}_{(aq)}} + 3e- -> Cr_{(s)}}\] = −0.740 V

\[\ce{2H+ + 2e- -> H2_{(g)}}\] = 0.000 V

\[\ce{Fe^3+ + e- -> Fe^2+}\] = 0.770 V

Which is the strongest reducing agent?

Options

• Zn

• Cr

• H_2(g)

• \[\ce{Fe{^{2+}_{(aq)}}}\]

Answer 1

Zn

Explanation:

The strongest reducing agent is the one that most readily loses electrons, i.e., has the lowest (most negative) standard reduction potential.

Given standard reduction potentials:

\[\ce{Zn{^{2+}_{(aq)}} + 2e- -> Zn_{(s)}}\] = −0.762 V

\[\ce{Cr{^{3+}_{(aq)}} + 3e- -> Cr_{(s)}}\] = −0.740 V

\[\ce{2H+ + 2e- -> H2_{(g)}}\] = 0.000 V

\[\ce{Fe^3+ + e- -> Fe^2+}\] = 0.770 V

Among these, zinc (Zn) has the most negative reduction potential, meaning it has the greatest tendency to lose electrons, making it the strongest reducing agent.