Which of the following is the strongest reducing agent? (a) Fe^(3+) (b) Cr2O7^(2-) (c) Cl2

A reducing agent is a species that causes another species to be reduced, and in doing so, it itself gets oxidized (loses electrons).

The standard electrode potentials ($\text{E}^\circ$) provided are for reduction reactions. A stronger reducing agent is the reduced form of a redox couple that has a lower (more negative or less positive) standard reduction potential. This means it has a greater tendency to be oxidized.

• For A: $\text{Fe}^{3+}(\text{aq}) + \text{e}^- \rightleftharpoons \text{Fe}^{2+}(\text{aq})$, $\text{E}^\circ = +0.77 \text{ V}$
• For B: $\text{Cr}_2\text{O}_7^{2-}(\text{aq}) + 14\text{H}^+(\text{aq}) + 6\text{e}^- \rightleftharpoons 2\text{Cr}^{3+}(\text{aq}) + 7\text{H}_2\text{O}$, $\text{E}^\circ = +1.33 \text{ V}$
• For C: $\text{Cl}_2(\text{g}) + 2\text{e}^- \rightleftharpoons 2\text{Cl}^-(\text{aq})$, $\text{E}^\circ = +1.6 \text{ V}$

The lowest standard reduction potential among the given options is $+0.77 \text{ V}$ for the $\text{Fe}^{3+}/\text{Fe}^{2+}$ couple. The reduced species in this couple is $\text{Fe}^{2+}$. Therefore, $\text{Fe}^{2+}$ is the strongest reducing agent.

The strongest reducing agent is $\boxed{\text{Fe}^{2+}(\text{aq})}$.

When coupling two half-cells, the half-reaction with the higher standard reduction potential will act as the cathode (reduction), and the half-reaction with the lower standard reduction potential will act as the anode (oxidation).
Comparing A ($\text{E}^\circ = +0.77 \text{ V}$) and B ($\text{E}^\circ = +1.33 \text{ V}$):
Since $\text{E}^\circ(\text{B}) > \text{E}^\circ(\text{A})$, half-reaction B will be the cathode, and half-reaction A will be the anode.

Anode (oxidation, reverse of A):

Cathode (reduction, B):

The cell diagram notation is: Anode | Anode solution || Cathode solution | Cathode.
Since all species involved are aqueous ions, inert platinum (Pt) electrodes are used.

Anode compartment: $\text{Pt}(\text{s}) | \text{Fe}^{2+}(\text{aq}), \text{Fe}^{3+}(\text{aq})$
Cathode compartment: $\text{Cr}_2\text{O}_7^{2-}(\text{aq}), \text{H}^+(\text{aq}), \text{Cr}^{3+}(\text{aq}) | \text{Pt}(\text{s})$

The cell diagram is: