To determine the strongest reducing agent, we need to understand the concept of standard reduction potential (E°). The standard reduction potential measures the tendency of a species to gain electrons and be reduced. A more positive E° value indicates a greater tendency to be reduced, meaning it is a stronger oxidizing agent. Conversely, a more negative E° value indicates a lesser tendency to be reduced and a greater tendency to lose electrons, meaning it is a stronger reducing agent.

The given standard reduction potentials are:

$E_{{\mathrm{Cl}}_2/{\mathrm{Cl}}^{-}}^{◦}=1.36\text{V}$

$E_{{\mathrm{Cr}}^{3+}/\mathrm{Cr}}^{◦}=-0.74\text{V}$

$E_{{\mathrm{Cr}}_2{O}_7^{2-}/{\mathrm{Cr}}^{3+}}^{◦}=1.33\text{V}$

$E_{{\mathrm{MnO}}_4^{-}/{\mathrm{Mn}}^{2+}}^{◦}=1.51\text{V}$

To identify the strongest reducing agent, we look at the species that is most easily oxidized. This corresponds to the half-cell with the most negative standard reduction potential. The more negative the E° value, the stronger the reducing agent.

Let's list the reduction potentials for the species involved as reducing agents. A reducing agent is the reduced form in a half-cell. We need the E° for the reduction of the oxidized form to the reducing agent species.

For the options:

• Cr: This is the reduced form in the half-cell Cr³⁺/Cr. Its tendency to act as a reducing agent is indicated by the negative E° value of -0.74 V for Cr³⁺/Cr. A more negative value means Cr is a good reducing agent.
• Cr³⁺: This is the oxidized form in multiple half-cells. To act as a reducing agent, it would need to be oxidized to a higher state, but the given potentials are for its reduction. The reduction potential for Cr₂O₇²⁻/Cr³⁺ is +1.33 V, which is positive, meaning Cr³⁺ is not a good reducing agent; it is rather stable.
• Mn²⁺: This is the reduced form in the half-cell MnO₄⁻/Mn²⁺. The reduction potential is +1.51 V, which is very positive, meaning Mn²⁺ has little tendency to lose electrons and is a weak reducing agent.
• Cl⁻: This is the reduced form in the half-cell Cl₂/Cl⁻. The reduction potential is +1.36 V, which is positive, meaning Cl⁻ is a weak reducing agent.

Comparing the E° values relevant to the reducing ability:

The most negative E° is for the Cr³⁺/Cr couple: -0.74 V. This means Cr has the strongest tendency to lose electrons and be oxidized, making it the strongest reducing agent among the options.

Therefore, the strongest reducing agent is Cr.

Related Topics and Formulae

Standard Reduction Potential (E°): A measure of the tendency of a chemical species to acquire electrons and thereby be reduced. It is measured in volts (V) under standard conditions (1 M concentration, 1 atm pressure, 25°C).

Strong Reducing Agent: A species that readily donates electrons. It has a highly negative standard reduction potential.

Strong Oxidizing Agent: A species that readily accepts electrons. It has a highly positive standard reduction potential.

The relationship is inverse: the stronger the reducing agent, the weaker its conjugate oxidizing agent, and vice versa.