Question

Consider the following reduction processes: \[ \text{Al}^{3+} + 3e^- \longrightarrow \text{Al(s)}, \quad E^\circ = -1.66\,\text{V} \] \[ \text{Fe}^{3+} + e^- \longrightarrow \text{Fe}^{2+}, \quad E^\circ = +0.77\,\text{V} \] \[ \text{Co}^{3+} + e^- \longrightarrow \text{Co}^{2+}, \quad E^\circ = +1.81\,\text{V} \] \[ \text{Cr}^{3+} + 3e^- \longrightarrow \text{Cr(s)}, \quad E^\circ = -0.74\,\text{V} \] The tendency to act as reducing agent decreases in the order:

• A. \( \text{Cr}>\text{Fe}^{2+}>\text{Al}>\text{Co}^{2+} \)
• B. \( \text{Al}>\text{Cr}>\text{Co}^{2+}>\text{Fe}^{2+} \)
• C. \( \text{Al}>\text{Cr}>\text{Fe}^{2+}>\text{Co}^{2+} \)
• D. \( \text{Al}>\text{Fe}^{2+}>\text{Cr}>\text{Co}^{2+} \)

Hint:

More negative standard reduction potential means greater tendency to lose electrons and hence stronger reducing power.

Answer 1

The Correct Option is C

To determine the tendency of substances to act as reducing agents, we compare their standard reduction potentials \((E^\circ)\).

A stronger reducing agent has a more negative standard reduction potential, because it more readily loses electrons. 

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Given standard reduction reactions:

• \[ \mathrm{Al^{3+} + 3e^- \rightarrow Al(s)}, \quad E^\circ = -1.66~\text{V} \]
• \[ \mathrm{Cr^{3+} + 3e^- \rightarrow Cr(s)}, \quad E^\circ = -0.74~\text{V} \]
• \[ \mathrm{Fe^{3+} + e^- \rightarrow Fe^{2+}}, \quad E^\circ = +0.77~\text{V} \]
• \[ \mathrm{Co^{3+} + e^- \rightarrow Co^{2+}}, \quad E^\circ = +1.81~\text{V} \]

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Comparison of reducing strength

The more negative the value of \(E^\circ\), the greater the tendency of the substance to act as a reducing agent.

Ordering the substances from most negative to most positive \(E^\circ\):

\[ \mathrm{Al} > \mathrm{Cr} > \mathrm{Fe^{2+}} > \mathrm{Co^{2+}} \]

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Final Answer

\[ \boxed{\mathrm{Al} > \mathrm{Cr} > \mathrm{Fe^{2+}} > \mathrm{Co^{2+}}} \]

This corresponds to Option C.