(a) Following are the transition metal ions of 3d series:
Ti⁴⁺, V²⁺, Mn³⁺, Cr³⁺ (Atomic numbers: Ti = 22, V = 23, Mn = 25, Cr = 24) Answer the following:
(i) Which ion is most stable in an aqueous solution and why?
(ii) Which ion is a strong oxidising agent and why?
(iii) Which ion is colourless and why?
(b) Complete the following equations:
(i) $2{\mathrm{MnO}}_4^{-}+16{\mathrm{H}}^{+}+5{\mathrm{S}}^{2-}⟶$
(ii) ${\mathrm{KMnO}}_4\stackrel{\text{ heat }}{⟶}$

(a)
(i) V²⁺ and Cr³⁺ are most stable as they both have the [Ar]3d3 or 𝑡³_2g stable electronic configuration.
(ii) Mn³⁺ ions are the strongest oxidising agent in the aqueous solution because after converting to Mn²⁺ they get stable half-filled d⁵ electronic configuration.
(iii) Ti⁴⁺ has no electron in d orbital. No d-d transition is possible in this case. Therefore, it is colourless.
(b)
(i) $2{\mathrm{MnO}}_4^{-}+16{\mathrm{H}}^{+}+5{\mathrm{S}}^{2-}\to 2{\mathrm{Mn}}^{2+}+8{\mathrm{H}}_2\mathrm{O}+5\mathrm{S}$
(ii) $2{\mathrm{KMnO}}_4\stackrel{\text{ heat }}{⟶}{\mathrm{K}}_2{\mathrm{MnO}}_4+{\mathrm{MnO}}_2+{\mathrm{O}}_2$