Standard reduction potentials (E°) of $Pb^{2+}, Ag^+, Cu^{2+}$ and Fe are -0.13 V, +0.80 V, +0.31 V, -0.44 V respectively. Which is the strongest reducing agent?

$Fe^{2+}$

The correct answer is Option (1) → $Fe^{2+}$.

To understand why iron (Fe) is the strongest reducing agent among the given options, let us break down the concept of standard reduction potentials and how they relate to the strength of reducing agents.

Standard reduction potential (\(E^\circ\)) is a measure of the tendency of a species to gain electrons and be reduced. The more positive the value of \(E^\circ\), the greater the tendency of the species to accept electrons (i.e., undergo reduction). Conversely, the more negative the value of \(E^\circ\), the less favorable it is for the species to gain electrons. This implies that it is more likely to lose electrons and act as a reducing agent.

Reducing Agent Strength

A reducing agent is a substance that donates electrons to another species and is itself oxidized in the process. The strength of a reducing agent is inversely related to its standard reduction potential:

A lower (more negative) standard reduction potential means the species has a stronger tendency to lose electrons, making it a better reducing agent.

A higher (more positive) standard reduction potential means the species has a stronger tendency to gain electrons, making it a weaker reducing agent.

The reduction potentials of the given ions are:

\( E^\circ_{\text{Fe}^{2+}/\text{Fe}} = -0.44 \, \text{V} \)

\( E^\circ_{\text{Pb}^{2+}/\text{Pb}} = -0.13 \, \text{V} \)

\( E^\circ_{\text{Cu}^{2+}/\text{Cu}} = +0.31 \, \text{V} \)

\( E^\circ_{\text{Ag}^{+}/\text{Ag}} = +0.80 \, \text{V} \)

These values show the tendency of each metal ion to gain electrons and be reduced to its elemental form:

Fe²⁺ + 2e⁻ → Fe has a standard reduction potential of -0.44 V, meaning it does not easily accept electrons, and thus Fe tends to lose electrons and act as a strong reducing agent.

Pb²⁺ + 2e⁻ → Pb has a standard reduction potential of -0.13 V. This is less negative than iron, meaning Pb²⁺ is not as good of a reducing agent as Fe.

Cu²⁺ + 2e⁻ → Cu has a positive potential of +0.31 V, meaning Cu²⁺ is more likely to accept electrons and be reduced, making it a weaker reducing agent.

Ag⁺ + e⁻ → Ag has the highest reduction potential of +0.80 V, meaning Ag⁺ has a strong tendency to be reduced and is a very poor reducing agent.

Conclusion

The species with the most negative standard reduction potential is \(Fe^{2+}\) with \(E^\circ = -0.44 \, \text{V}\). This means \(Fe^{2+}\) has the greatest tendency to lose electrons and be oxidized, making it the strongest reducing agent among the given options.

On the other hand, Ag⁺ with \(E^\circ = +0.80 \, \text{V}\) is the weakest reducing agent, as it has a strong tendency to accept electrons (be reduced) and thus is a poor electron donor.