Chemistry Electrochemistry MDCAT 2019
PMDC Verified Question 56 of 73
\( Mg^{2+} + 2e^- \rightarrow Mg \quad E^\circ = -2.37V \)
\( Zn^{2+} + 2e^- \rightarrow Zn \quad E^\circ = -0.76V \)
\( 2H^+ + 2e^- \rightarrow H_2 \quad E^\circ = 0.000V \)
\( Cu^{2+} + 2e^- \rightarrow Cu \quad E^\circ = +0.34V \)

Keeping in view the values of standard reduction potential given above, which one of the following would you select as a feasible redox chemical reaction?
A
\( 2Au + 6H^+ \rightarrow 2Au^{3+} + 3H_2 \)
B
\( 2Cl^- + I_2 \rightarrow Cl_2 + 2I^- \)
C
\( Mg + 2H^+ \rightarrow Mg^{2+} + H_2 \)
D
\( Cu + Zn^{2+} \rightarrow Cu^{2+} + Zn \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \( Mg + 2H^+ \rightarrow Mg^{2+} + H_2 \)
Concept:

A spontaneous (feasible) redox reaction occurs when a metal with a highly negative reduction potential is oxidized while transferring its electrons to a species with a less negative (or positive) reduction potential.

Solution:

  • Let's evaluate Option C: Magnesium solid reacting with Hydrogen ions.


  • Magnesium has a very negative reduction potential (\( -2.37 \text{ V} \)), making it an extremely strong reducing agent (it wants to oxidize).


  • Hydrogen has a higher reduction potential (\( 0.00 \text{ V} \)).


  • Therefore, Mg will easily displace Hydrogen from acidic solutions, undergoing oxidation while \( H^+ \) is reduced to \( H_2 \) gas. The cell potential is vastly positive (\( +2.37 \text{ V} \)), making it highly feasible.


Why other options are incorrect:

In option D, Cu (\( +0.34 \text{ V} \)) cannot reduce \( Zn^{2+} \) (\( -0.76 \text{ V} \)). Options A and B include elements (Au, Cl, I) whose potentials are not even listed, though scientifically Au cannot displace H+, and I2 cannot oxidize Cl-.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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