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Question #44280

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The Question

Consider the isoelectronic series of aqueous ions: \ce{Cu^+}, \ce{Zn^{2+}}, and \ce{Ga^{3+}} at 298 K. Given the standard reduction potentials: $E^\circ(\ce{Cu^+/Cu}) = +0.52\text{ V}$ $E^\circ(\ce{Cu^{2+}/Cu^+}) = +0.15\text{ V}$ $E^\circ(\ce{Zn^{2+}/Zn}) = -0.76\text{ V}$ $E^\circ(\ce{Ga^{3+}/Ga}) = -0.53\text{ V}$ An experimenter places a $1.0\text{ M}$ aqueous solution of each ion into separate containers made of pure metallic Gallium (\ce{Ga}). Which of the following statements correctly identifies the stable solution(s) and the primary thermodynamic reason for the $E^\circ(\ce{M^{n+}/M})$ trend from \ce{Zn} to \ce{Ga}?

Options

A.
Only the \ce{Zn^{2+}} solution can be safely stored. \ce{Cu^+} is reduced by \ce{Ga} to \ce{Cu}. The $E^\circ$ of \ce{Ga^{3+}} is less negative than \ce{Zn^{2+}} because the $3d^{10}$ electrons in \ce{Ga} poorly shield the nuclear charge, causing a massive increase in hydration enthalpy that dominates the ionization energy.
B.
Only the \ce{Zn^{2+}} solution can be safely stored. \ce{Cu^+} undergoes spontaneous disproportionation. \ce{Ga^{3+}} is easier to reduce than \ce{Zn^{2+}} because the sharp increase in the sum of ionization energies ( $\sum IE$ ) from \ce{Zn} to \ce{Ga} outweighs the corresponding increase in exothermic hydration enthalpy per mole of electrons.
C.
None of the solutions can be safely stored. \ce{Ga} spontaneously reduces both \ce{Cu^+} and \ce{Zn^{2+}}. The non-monotonic trend in $E^\circ$ arises because \ce{Ga^{3+}} undergoes hydrolysis to form \ce{[Ga(OH)6]^{3-}}, altering its standard state reduction potential.
D.
Both \ce{Zn^{2+}} and \ce{Ga^{3+}} solutions can be safely stored. \ce{Cu^+} is stable kinetically but thermodynamically unstable. \ce{Ga^{3+}} has a less negative $E^\circ$ than \ce{Zn^{2+}} due to the inert pair effect stabilizing the $+1$ oxidation state of \ce{Ga}, making \ce{Ga^{3+}} highly eager to gain electrons.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

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Concepts Behind This Question

Isoelectronic Energy Ordering|Nernstian Redox Equilibrium|Gibbs Energy and Spontaneity

Why This Question Is Tricky

Common Mistakes

This question tests the 'Thermodynamic Trinity' (Sublimation, Ionization, Hydration) behind standard reduction potentials, specifically targeting the anomaly in the $3d^{10}$ isoelectronic series.

Students often memorize that higher oxidation states or higher $Z_{eff}$ lead to predictable trends, but fail to account for the non-linear scaling of $\sum IE$ versus $\Delta H_{hyd}$ .

The trap includes the hidden disproportionation of \ce{Cu^+}, which invalidates any simple redox calculation with the Ga container, distinguishing rote calculators from true chemical thinkers.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Structure of Atom - Class 11 Part 1
Concept	Isoelectronic Energy Ordering
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free