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: Understanding the Concept:
The standard electrode potential (\(E^{\circ}\)) of a metal ion is a measure of its tendency to be reduced. It is determined by three main energy factors: enthalpy of sublimation (\(\Delta_{sub}H\)), ionization enthalpy (\(\Delta_iH\)), and enthalpy of hydration (\(\Delta_{hyd}H\)).
Step 2: Detailed Explanation:
(I) Irregular Trend: The irregularity in the \(E^{\circ}\) values across the 3d series is due to the non-regular variation in the sublimation enthalpies and the sum of the first and second ionization enthalpies. These properties do not change smoothly across the series, causing the observed fluctuations in electrode potentials.
(II) Copper's Positive Value: Copper is the only metal in the 3d series with a positive \(E^{\circ}_{M^{2+}/M}\) value (\(+0.34\) V). This is because the high energy required to transform \(Cu(s)\) to \(Cu^{2+}(g)\) (high sublimation and high sum of \(IE_1\) and \(IE_2\)) is not compensated by the relatively low hydration enthalpy of the \(Cu^{2+}\) ion.
(III) Manganese's Highly Negative Value: The \(E^{\circ}\) for \(Mn^{2+}/Mn\) is highly negative because of the extra stability associated with the half-filled \(d^5\) electronic configuration in the \(Mn^{2+}\) ion. This stability makes the formation of \(Mn^{2+}\) from \(Mn\) energetically favourable.
Step 3: Final Answer:
The trends are explained by the balance of thermodynamic parameters (sublimation, ionization, and hydration) and electronic stability (\(d^5\) configuration).