JEE Challenger
More from Electrochemistry
ChemMedAdv 2007 • Paper 2

Thermodynamic Instability of Mn3+ in Acidic Aqueous Solution

Comprehension Passage

Redox reactions play a pivotal role in chemistry and biology. The values of standard redox potential (E∘E^\circ) of two half-cell reactions decide which way the reaction is expected to proceed. A simple example is a Daniell cell in which zinc goes into solution and copper gets deposited.

Given below are a set of half-cell reactions (acidic medium) along with their E∘E^\circ (V\text{V} with respect to normal hydrogen electrode) values:

I2+2e−→2I−E∘=0.54 VFe3++e−→Fe2+E∘=0.77 VO2(g)+4H++4e−→2H2OE∘=1.23 VCl2+2e−→2Cl−E∘=1.36 VMn3++e−→Mn2+E∘=1.50 V\begin{aligned} \text{I}_2 + 2e^- &\rightarrow 2\text{I}^- && E^\circ = 0.54\text{ V} \\ \text{Fe}^{3+} + e^- &\rightarrow \text{Fe}^{2+} && E^\circ = 0.77\text{ V} \\ \text{O}_2(g) + 4\text{H}^+ + 4e^- &\rightarrow 2\text{H}_2\text{O} && E^\circ = 1.23\text{ V} \\ \text{Cl}_2 + 2e^- &\rightarrow 2\text{Cl}^- && E^\circ = 1.36\text{ V} \\ \text{Mn}^{3+} + e^- &\rightarrow \text{Mn}^{2+} && E^\circ = 1.50\text{ V} \end{aligned}

While Fe3+\text{Fe}^{3+} is stable, Mn3+\text{Mn}^{3+} is not stable in acid solution because

Options

A

O2\text{O}_2 oxidises Mn2+\text{Mn}^{2+} to Mn3+\text{Mn}^{3+}

B

O2\text{O}_2 oxidises both Mn2+\text{Mn}^{2+} to Mn3+\text{Mn}^{3+} and Fe2+\text{Fe}^{2+} to Fe3+\text{Fe}^{3+}

C

Fe3+\text{Fe}^{3+} oxidises H2O\text{H}_2\text{O} to O2\text{O}_2

D

Mn3+\text{Mn}^{3+} oxidises H2O\text{H}_2\text{O} to O2\text{O}_2

Correct

Topics & Concepts

ElectrochemistryStandard Reduction PotentialStability of Oxidation StatesTransition Elements

Step-by-Step Solution

From the given standard potentials:

  • E∘(Mn3+/Mn2+)=+1.50 VE^\circ(\text{Mn}^{3+}/\text{Mn}^{2+}) = +1.50\text{ V}
  • E∘(O2,H+/H2O)=+1.23 VE^\circ(\text{O}_2, \text{H}^+/\text{H}_2\text{O}) = +1.23\text{ V}
  • E∘(Fe3+/Fe2+)=+0.77 VE^\circ(\text{Fe}^{3+}/\text{Fe}^{2+}) = +0.77\text{ V}

For the oxidation of water by Mn3+\text{Mn}^{3+}:
4Mn3++2H2O→4Mn2++O2+4H+4\text{Mn}^{3+} + 2\text{H}_2\text{O} \rightarrow 4\text{Mn}^{2+} + \text{O}_2 + 4\text{H}^+
Ecell∘=1.50−1.23=+0.27 V>0E^\circ_{\text{cell}} = 1.50 - 1.23 = +0.27\text{ V} > 0
Since Ecell∘>0E^\circ_{\text{cell}} > 0, the reaction is thermodynamically spontaneous, meaning Mn3+\text{Mn}^{3+} spontaneously oxidizes water to oxygen gas and gets reduced to Mn2+\text{Mn}^{2+}.

In contrast, for Fe3+\text{Fe}^{3+}:
Ecell∘=0.77−1.23=−0.46 V<0E^\circ_{\text{cell}} = 0.77 - 1.23 = -0.46\text{ V} < 0
Thus, Fe3+\text{Fe}^{3+} cannot oxidize water, explaining why Fe3+\text{Fe}^{3+} remains stable in aqueous acidic solution.

Correct Answer: D

PreviousNext