Question

Attempt any five of the following: (a) Cu\(^+\) is not stable in aqueous solution. Comment.

Hint:

Transition metals show catalytic activity, variable oxidation states, and colored ions due to partially filled d-orbitals. Scandium qualifies despite forming colorless Sc\(^{3+}\).

Answer 1

(a) Cu\(^+\) is unstable in aqueous solution because it undergoes disproportionation: \[ 2\text{Cu}^+ \rightarrow \text{Cu}^{2+} + \text{Cu} \] This is thermodynamically favored as Cu\(^+\) has a tendency to be both oxidised to Cu\(^{2+}\) and reduced to Cu.
This instability arises due to the positive value of the standard electrode potential for the disproportionation reaction.
\[ \boxed{\text{Cu}^+ \text{ is unstable and disproportionates in aqueous solution.}} \] (b) Out of Cr\(^{2+}\) and Fe\(^{2+}\), which is a stronger reducing agent and why?
Solution:
(b) Cr\(^{2+}\) is a stronger reducing agent than Fe\(^{2+}\) because:
- Cr\(^{2+}\) readily loses an electron to form the more stable Cr\(^{3+}\) (having half-filled t\(_{2g}^3\) configuration).
- Fe\(^{2+}\) does not gain as much stability on oxidation.
\[ \boxed{\text{Cr}^{2+} \text{ is stronger reducing agent due to stability of Cr}^{3+}} \] (c) Actinoid contraction is greater from element to element than lanthanoid contraction. Why?
Solution:
(c)Actinoid contraction is more pronounced because:
- Poor shielding of 5f electrons compared to 4f electrons in lanthanoids.
- Greater increase in effective nuclear charge across the series.
\[ \boxed{\text{5f electrons shield poorly → stronger contraction across actinoids}} \] (d) KMnO\(_4\) acts as an oxidising agent in acidic medium. Write the ionic equation to support this.
Solution:\ (d) In acidic medium, KMnO\(_4\) gets reduced from Mn\(^{7+}\) to Mn\(^{2+}\). The ionic redox equation is: \[ \text{MnO}_4^- + 8\text{H}^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} \] \[ \boxed{\text{KMnO}_4 \text{ acts as a strong oxidising agent in acidic medium.}} \] (e) Name the metal in the first transition series which exhibits \(+1\) oxidation state most frequently.
Solution:
(e) The metal is **Copper (Cu)**. Cu shows +1 oxidation state frequently due to configuration: \[ \text{Cu: } [\text{Ar}]\,3d^{10}\,4s^1 \Rightarrow \text{Cu}^+ = [\text{Ar}]\,3d^{10} \] A completely filled d-orbital provides extra stability. \[ \boxed{\text{Cu exhibits } +1 \text{ oxidation state most frequently.}} \] (f) Transition metals and their compounds are good catalysts. Justify.
(f) Solution:
Transition metals act as good catalysts due to:
- Variable oxidation states → allow redox cycling
- Ability to form complexes with reactants → intermediate formation
- Provide surface area for adsorption
\[ \boxed{\text{Transition metals facilitate alternate paths with lower activation energy.}} \] (g) Scandium forms no coloured ions, yet it is regarded as a transition element. Why?
Solution:
(g) Scandium forms Sc\(^{3+}\) with 3d\(^0\) configuration → no unpaired electrons → no d-d transitions → hence, no colour.
However, it is a transition element because:
- It has an incompletely filled d-subshell in its ground state (Sc: 3d\(^1\)4s\(^2\))
\[ \boxed{\text{Transition elements are defined by ground or ionic d-orbital incompleteness.}} \]