Question

From the given data of \(E^{\circ}\) values, answer the following questions :
(I) Why \(E^{\circ}_{M^{2+}/M}\) show irregular trend in the above values ?
(II) Why is \(E^{\circ}_{Cu^{2+}/Cu}\) value exceptionally positive ?
(III) Why \(E^{\circ}_{Mn^{2+}/Mn}\) value is highly negative ?

Hint:

Standard electrode potential measures the overall feasibility of taking a solid metal into an aqueous ion state. If the energy input (sublimation + ionization) is high and the energy release (hydration) is low, the potential becomes positive.

Answer 1

Step 1: Conceptual Overview:
The standard electrode potential (\( E^{\circ} \)) of a metal ion measures its tendency to be reduced, which is influenced by several thermodynamic factors. These factors include the enthalpy of sublimation (\( \Delta_{sub}H \)), ionization enthalpy (\( \Delta_iH \)), and enthalpy of hydration (\( \Delta_{hyd}H \)). These parameters collectively affect how easily a metal can undergo reduction in an electrochemical reaction.
Step 2: Detailed Explanation:
(I) Irregular Trend: The irregular trends in the \( E^{\circ} \) values across the 3d series are due to the non-uniform changes in the enthalpy of sublimation and the combined first and second ionization enthalpies. These properties do not increase or decrease in a smooth manner across the transition metals, resulting in fluctuations in their electrode potentials. This explains the irregularity observed in the electrode potentials of metals in the 3d series.
(II) Copper's Positive Value: Copper (\( Cu \)) is the only metal in the 3d series that exhibits a positive standard electrode potential value (\( E^{\circ}_{M^{2+}/M} = +0.34 \) V). This positive value arises because the energy required to convert solid copper (\( Cu(s) \)) into the copper(II) ion (\( Cu^{2+}(g) \)) involves both a high sublimation enthalpy and a significant ionization enthalpy. However, this energy is not compensated by the relatively low enthalpy of hydration of the \( Cu^{2+} \) ion, making copper’s reduction less favorable compared to other metals in the series.
(III) Manganese's Highly Negative Value: The \( E^{\circ} \) for the \( Mn^{2+}/Mn \) couple is highly negative due to the extra stability provided by the half-filled \( d^5 \) electronic configuration in the \( Mn^{2+} \) ion. This stable configuration makes it energetically favorable for manganese to exist as \( Mn^{2+} \) rather than in the elemental form, which leads to a highly negative standard electrode potential.
Step 3: Final Conclusion:
The observed trends in the standard electrode potentials of the 3d series elements are explained by a combination of thermodynamic factors such as the enthalpy of sublimation, ionization enthalpy, and hydration enthalpy, along with the electronic stability provided by specific electron configurations (like the \( d^5 \) configuration in manganese).