Give reasons for the following:

(i) Transition elements and their compounds act as catalysts.

(ii) E° value for (Mn²⁺|Mn) is negative, whereas for (Cu²⁺|Cu) is positive.

(iii) Actinoids show irregularities in their electronic configuration.

i. Transition elements belong to the d-block of the periodic table of the elements. For a d-block element to be a transition element, it must possess an incompletely filled d-orbital.

The catalytic nature of the transition elements is because of two basic facts:

a. These elements are able to exhibit variable oxidation states. For example, copper exists as .

Due to this feature, the transition elements and their compounds are able to form complexes and hence act as good catalyst.

b. Transition elements also provide a suitable surface for the reactions to occur.

ii. is known as electrode potential and it is measured in volts. It is a measure of total energy change in a cell.

for any metal is related to the sum of enthalpy changes taking place as follows:

M(s) + Δ_aH → M(g)

M(g) + Δ_iH → M²⁺(g)

M²⁺(g) → M²⁺(aq) + Δ_hydH

where,

Δ_aH is enthalpy of atomisation

Δ_iH is enthalpy of ionisation

Δ_hydH is enthalpy of hydration

In the case of copper, it has a high enthalpy of atomisation and a low enthalpy of hydration. A high energy is required to transform and this energy is not balanced by its hydration enthalpy, that is Cu has a low hydration enthalpy. Hence, E⁰_Cu²⁺_/Cu is positive.

E⁰_Mn²⁺_/Mn on the other hand is observed to be highly negative because it difficult to reduce Mn²⁺ to Mn due to stability of half-filled d-subshell (3d⁵) in Mn²⁺.

iii. Actinoids are group of 15 elements with atomic number 89 to 113. These species show irregularities in their electronic configuration because actinoids involve gradual filling of 5f-subshell in their atoms. The energy differences between 5f, 6d and 7s subshells are very small, almost comparable. Therefore, an electron can go and occupy any of these subshells as they are close to each other.