Question 24 of 44advanced🔍 AnalyzeLong Answer5 marks

For M2+M^{2+}/M and M3+M^{3+}/M2+M^{2+} systems the E0E^{0} values for some metals are as follows: Cr2+Cr^{2+}/Cr -0.9V Cr3+Cr^{3+}/Cr2+Cr^{2+} -0.4 V Mn2+Mn^{2+}/Mn -1.2V Mn3+Mn^{3+}/Mn2+Mn^{2+} +1.5 V Fe2+Fe^{2+}/Fe -0.4V Fe3+Fe^{3+}/Fe2+Fe^{2+} +0.8 V Use this data to comment upon: (i) the stability of Fe3+Fe^{3+} in acid solution as compared to that of Cr3+Cr^{3+} or Mn3+Mn^{3+} and (ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.

(i)
Answer

The stability of Fe³⁺ can be compared with Cr³⁺ and Mn³⁺ using the E° values for M³⁺/M²⁺ couples. For Mn³⁺/Mn²⁺, the E° value is +1.5V, which is very high. The context states that Mn³⁺ is a strong oxidising agent in aqueous solutions and the +3 state of Mn is of little importance due to the much larger third ionisation energy (change from d⁵ to d⁴). This makes Mn³⁺ highly unstable.

For Fe³⁺/Fe²⁺, the E° value is +0.8V. The context mentions that the comparatively low value for Fe shows the extra stability of Fe³⁺(d⁵) due to its half-filled d⁵ configuration.

For Cr³⁺/Cr²⁺, the E° value is -0.4V. The negative value indicates Cr²⁺ is a strong reducing agent that readily oxidises to Cr³⁺, making Cr³⁺ stable.

Thus, the stability order is: Cr³⁺ > Fe³⁺ > Mn³⁺. Fe³⁺ is more stable than Mn³⁺ but less stable than Cr³⁺ in acid solution.

(ii)
Answer

The ease of oxidation of metals is determined by E° values for M²⁺/M couples. The given values are:

  • Mn²⁺/Mn: -1.2V (most negative)
  • Cr²⁺/Cr: -0.9V
  • Fe²⁺/Fe: -0.4V (least negative)

More negative E° values indicate greater ease of oxidation. Manganese has the most negative value, followed by chromium, and then iron. The context states that metals of the first series (except copper) are oxidised by 1M H⁺.

Therefore, manganese is most easily oxidised, followed by chromium, and then iron. Iron is the least easily oxidised among the three metals. The stability of half-filled d⁵ configuration in Mn²⁺ contributes to its more negative E° value.

Explanation

Part (i) tests understanding of M³⁺/M²⁺ electrode potentials and their relation to ion stability. High positive E° indicates the oxidised form (M³⁺) is unstable and acts as strong oxidising agent. Mn³⁺ has highest E° (+1.5V), making it most unstable. Fe³⁺ has moderate E° (+0.8V) with extra stability from d⁵ configuration. Cr³⁺ has negative E° (-0.4V), indicating stability. Part (ii) tests M²⁺/M potentials. More negative values indicate easier oxidation of metal. Mn (-1.2V) is most easily oxidised, Fe (-0.4V) is least easily oxidised among the three.

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