CBSE Class 12 Chemistry Chapter 8: The d and f Block Elements NCERT Solutions

Solution:

A transition element is defined as an element that possesses incompletely filled d-orbitals either in its ground state or in any of its stable ionic states. Although a silver atom has a completely filled 4d orbital in its ground state (electronic configuration: [Kr] 4d¹⁰ 5s¹), it exhibits a +2 oxidation state. In the +2 oxidation state, silver loses one electron from the 5s orbital and one electron from the 4d orbital, resulting in the electronic configuration [Kr] 4d⁹. Since the 4d orbital is incomplete in the Ag²⁺ ion, silver is classified as a transition element.

Solution:

The enthalpy of atomization of a metallic element is largely determined by the extent of metallic bonding between its atoms. This metallic bonding is influenced by the number of unpaired electrons available for bonding. In the first transition series (Sc to Zn), most elements have unpaired electrons in their d-orbitals, which contribute to strong inter-atomic metallic bonding and consequently, high enthalpies of atomization. Zinc, however, has the electronic configuration 3d¹⁰ 4s². In this configuration, all the d-orbitals are completely filled, and there are no unpaired d-electrons. This results in weaker metallic bonding in zinc compared to other elements in the series, leading to the lowest enthalpy of atomization (126 kJ mol^-1).

Solution:

Manganese (Mn), with atomic number Z = 25 and electronic configuration 3d⁵ 4s², exhibits the largest number of oxidation states among the first transition series elements. This is because manganese has five unpaired electrons in its 3d subshell and two electrons in its 4s subshell. The availability of these unpaired electrons allows manganese to lose varying numbers of electrons, leading to a wide range of oxidation states, typically from +2 (by losing the 4s electrons) to +7 (by losing all valence electrons).

Solution:

The standard electrode potential ($E^{\theta}(M^{2+}/M)$) for a metal is determined by the overall energy changes involved in the process of converting the solid metal to its aqueous ions. This process can be broken down into three steps:

1. Sublimation: The energy required to convert the solid metal into gaseous atoms ($M_{(s)} \rightarrow M_{(g)}$), represented by the enthalpy of atomization ($\Delta_a H^{\theta}$).
2. Ionization: The energy required to remove electrons from gaseous atoms to form gaseous ions ($M_{(g)} \rightarrow M^{2+}_{(g)}$), represented by the ionization enthalpy ($\Delta_i H^{\theta}$).
3. Hydration: The energy released when gaseous ions are dissolved in water to form aqueous ions ($M^{2+}_{(g)} \rightarrow M^{2+}_{(aq)}$), represented by the enthalpy of hydration ($\Delta_{hyd} H^{\theta}$).

The overall enthalpy change is approximately $\Delta_a H^{\theta} + \Delta_i H^{\theta} + \Delta_{hyd} H^{\theta}$. A positive $E^{\theta}(M^{2+}/M)$ value indicates that the metal is less easily oxidized. For copper, the enthalpy of atomization ($\Delta_a H^{\theta}$) is relatively high, and the enthalpy of hydration ($\Delta_{hyd} H^{\theta}$) is low compared to other transition metals. This unfavorable combination of energy changes, particularly the low energy released during hydration, makes the overall process of forming Cu²⁺(aq) from Cu(s) less spontaneous, resulting in a positive standard electrode potential (+0.34 V).

Solution:

The ionization enthalpies of the first transition series (Sc to Zn) generally increase from left to right due to the gradual increase in nuclear charge and the relatively poor shielding effect of the added d-electrons. However, this increase is not perfectly regular, showing some irregularities. These irregularities can be attributed to the extra stability associated with certain electronic configurations, namely half-filled (d⁵) and fully-filled (d¹⁰) d-subshells. For example, Chromium (Cr, 3d⁵ 4s¹) has a lower first ionization enthalpy than expected. This is because losing its single 4s electron allows it to achieve a stable half-filled 3d⁵ configuration. Conversely, Zinc (Zn, 3d¹⁰ 4s²) has an exceptionally high first ionization enthalpy. This is because its electron is removed from the stable, completely filled 4s orbital, and the 3d¹⁰ configuration is very stable, making further electron removal (from 3d) difficult. Similar effects are observed for the second ionization enthalpies, where elements that achieve stable d⁰, d⁵, or d¹⁰ configurations after losing the first electron tend to have higher second ionization enthalpies.