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Question #44776

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The Question

A bimetallic galvanic couple consisting of pure Chromium and pure Manganese is immersed in an aerated acidic aqueous solution (pH = 0) containing $1.0 \text{ M } \ce{Cr^{2+}(aq)}$ and $1.0 \text{ M } \ce{Mn^{2+}(aq)}$ at 298 K. The system is exposed to a continuous stream of $\ce{O2(g)}$ at 1 bar.

Given the standard reduction potentials: $E^\circ(\ce{Cr^{2+}/Cr}) = -0.91 \text{ V}$ $E^\circ(\ce{Mn^{2+}/Mn}) = -1.18 \text{ V}$ $E^\circ(\ce{Cr^{3+}/Cr^{2+}}) = -0.41 \text{ V}$ $E^\circ(\ce{Mn^{3+}/Mn^{2+}}) = +1.51 \text{ V}$ $E^\circ(\ce{O2/H2O}) = +1.23 \text{ V}$

Which of the following statements correctly describes the thermodynamic and kinetic outcomes of this system?

Options

A.
Cr acts as the anode because the formation of the kinetically inert $\ce{Cr^{3+}(aq)}$ state provides a massive thermodynamic driving force, overriding the standard $\ce{M^{2+}/M}$ potentials. Mn acts as the cathode and is protected from oxidation.
B.
Mn acts as the sacrificial anode, dissolving exclusively to $\ce{Mn^{2+}(aq)}$ because any hypothetical $\ce{Mn^{3+}(aq)}$ would spontaneously oxidize water ( $E^\circ_{\ce{Mn^{3+}/Mn^{2+}}} > E^\circ_{\ce{O2/H2O}}$ ). Concurrently, $\ce{Cr^{2+}(aq)}$ is rapidly oxidized by $\ce{O2}$ to $\ce{Cr^{3+}(aq)}$ , which is kinetically stabilized by its $t_{2g}^3$ configuration.
C.
Mn acts as the anode and dissolves to $\ce{Mn^{3+}(aq)}$ because the overall cell potential for $\ce{Mn \rightarrow Mn^{3+}}$ coupled with $\ce{O2}$ reduction is positive ( $+1.513 \text{ V}$ ). $\ce{Cr^{2+}(aq)}$ remains unoxidized due to the kinetic inertness of the $d^4$ configuration.
D.
Both metals dissolve to their $+2$ states, but neither is further oxidized to $+3$ because the activation energy required to disrupt the stable $d^5$ configuration of $\ce{Mn^{2+}}$ and the stable $d^4$ configuration of $\ce{Cr^{2+}}$ is too high for $\ce{O2}$ to overcome at 298 K.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Corrosion as Electrochemical Process|E° (M³⁺/M²⁺) Redox Potential|Variable Oxidation State Stability

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of galvanic corrosion, standard electrode potentials, and coordination chemistry (Crystal Field Theory).

The primary trap is the 'Solvent Window' exception: students often calculate the overall thermodynamic potential for a reaction (like $\ce{Mn \rightarrow Mn^{3+}}$ via $\ce{O2}$ ) and find it spontaneous, failing to realize that the intermediate/product ( $\ce{Mn^{3+}}$ ) is a stronger oxidizing agent than $\ce{O2}$ and will spontaneously oxidize the solvent ( $\ce{H2O}$ ).

Furthermore, it tests the distinction between thermodynamic stability and kinetic inertness, trapping students who confuse the lability of Jahn-Teller distorted $d^4$ complexes with the inertness of $d^3$ ( $t_{2g}^3$ ) complexes.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Electrochemistry - Class 12
Concept	Corrosion as Electrochemical Process
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free