Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

The standard reduction potentials at \(298 \mathrm{~K}\) for the following half- reactions are given against each \(\mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Zn}(\mathrm{s})-0.762\) \(\mathrm{Cr}^{3+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Cr}(\mathrm{s}) \quad-0.740\) \(2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{H}_{2}(\mathrm{~g}) \quad 0.000\) \(\mathrm{Fe}^{3+}(\mathrm{aq})+2 \mathrm{e} \rightleftharpoons \mathrm{Fe}^{2+}\) (aq) \(0.770\) Which is the strongest reducing agent? (a) \(\mathrm{H}_{2}(\mathrm{~g})\) (b) \(\mathrm{Cr}(\mathrm{s})\) (c) \(\mathrm{Zn}(\mathrm{s})\) (d) \(\mathrm{Fe}^{2+}(\mathrm{aq})\)

Step-by-Step Solution

Answer

(c) \( \mathrm{Zn}(s) \) is the strongest reducing agent.

Step 1: Understand Reduction Potentials

The reduction potential of a half-reaction indicates its tendency to gain electrons and undergo reduction. More negative values mean a weaker tendency to undergo reduction and a stronger tendency to undergo oxidation, making it a better reducing agent.

Step 2: Compare Reduction Potentials

We are given the following reduction potentials: \( \mathrm{Zn}^{2+} + 2 \mathrm{e}^- \rightleftharpoons \mathrm{Zn}, E^0 = -0.762 \) V, \( \mathrm{Cr}^{3+} + 2 \mathrm{e}^- \rightleftharpoons \mathrm{Cr}, E^0 = -0.740 \) V, \( 2 \mathrm{H}^+ + 2 \mathrm{e}^- \rightleftharpoons \mathrm{H}_2, E^0 = 0.000 \) V, \( \mathrm{Fe}^{3+} + 2 \mathrm{e}^- \rightleftharpoons \mathrm{Fe}^{2+}, E^0 = 0.770 \) V.

Step 3: Identify the Strongest Reducing Agent

The reducing agents correspond to the elements on the right side of the given half-reactions: \( \mathrm{Zn}(s), \mathrm{Cr}(s), \mathrm{H}_2(g), \mathrm{Fe}^{2+}(aq) \). A strong reducing agent has a more negative reduction potential. Comparing potentials: - \( \mathrm{Zn}(s) : -0.762 \) V - \( \mathrm{Cr}(s) : -0.740 \) V - \( \mathrm{H}_2(g) : 0.000 \) V - \( \mathrm{Fe}^{2+} : 0.770 \) V. The most negative one corresponds to Zn(s).

Step 4: Conclusion

From the comparison, \( \mathrm{Zn}(s) \)'s potential of \(-0.762\) V is the most negative. Therefore, \( \mathrm{Zn}(s) \) is the strongest reducing agent.

Key Concepts

Standard Reduction Potential

Standard reduction potential helps us understand how easily a species can gain electrons, which is central to determining redox behavior in chemical reactions. It is measured in volts and is assigned to half-reactions like Zn: \( \mathrm{Zn}^{2+} + 2\mathrm{e}^- \rightarrow \mathrm{Zn}(\mathrm{s}) \) with a value of \(-0.762\) V. These values are determined under standard conditions: \(298\,\mathrm{K}\), \(1\, \mathrm{atm}\), and \(1 \mathrm{M}\) concentrations.The rule of thumb is that a more positive potential indicates a greater ability to gain electrons, meaning the species is more inherently ready to be reduced. On the contrary, more negative values suggest a weaker inclination to accept electrons, making such species better candidates for oxidation.When you order these half-reaction potentials from less negative to more negative, you can efficiently identify which substances are stronger reducing agents. This ranking is crucial in predicting outcomes of redox reactions.

Reducing Agent

When discussing reducing agents, it is important to note that they are substances that lose electrons in a redox reaction. This process causes them to undergo oxidation while reducing another species. The power of a reducing agent can be gauged by its standard reduction potential. The more negative this potential, the stronger the reducing agent.Characteristics of a Strong Reducing Agent:- It easily donates electrons.- It commonly features more negative standard reduction potentials.- Often found on the right-hand side of a half-reaction equation, as they serve as the product of reduction.Using the given exercise, Zn(s) with a potential of \(-0.762\,\mathrm{V}\) is clearly the strongest reducing agent since it has the most negative standard reduction potential. In contrast, with less negative potentials or even positive values, the reactivity of the other substances in terms of reducing capability decreases.

Oxidation-Reduction

Redox reactions are a type of chemical reactions that involve a transfer of electrons between two species. In this process, one substance gets oxidized, meaning it loses electrons, and another one gets reduced by gaining those electrons. Key Concepts in Redox:

• Oxidation: The loss of electrons by a molecule, atom, or ion.
• Reduction: The gain of electrons by a molecule, atom, or ion.
• Oxidizing Agent: The substance that accepts electrons and gets reduced.
• Reducing Agent: The substance that donates electrons and gets oxidized.

Understanding these terms is essential in classifying species in reactions and predicting products. For instance, in the exercise, Zn is acting as a reducing agent, becoming oxidized while facilitating the reduction of another species.