221.

A blackish brown coloured solid ‘A’, when fused with alkali hydroxide in presence of air produces a dark green coloured compound ‘B’. When electrolytic oxidation in alkaline medium gives a dark purple coloured compound. Identify A, B and C and write the reaction involved.

222.

What happens when an acidic solution of the green compound ‘B’ is allowed to stand for sometime? Give the equation involved. What is the type of reaction called?

223. Write complete chemical equations for oxidation of Fe²⁺ by Cr₂O₇^2– in acid medium.

224. Write complete chemical equations for oxidation of S₂O₃^2– by MnO₄^– in neutral aqueous solution.

225. How would you account for the following situations?
The transition metals generally form coloured compounds.

226. How would you account for the following situations?
With 3d⁴ configuration, Cr²⁺ acts as a reducing agent but Mn³⁺ acts as an oxidising agent. (Atomic masses, Cr = 24, Mn = 25).

227. How would you account for the following situations?
The actinoids exhibit a larger number of oxidation states than the corresponding lanthanoids. 

228. (i) Write the electronic configuration of Cu²⁺, Z = 29 for Cu.

(ii) Discuss the oxidation states of lanthanides.

(iii) What is the trend in the ionic radii of transition elements?

(iv) Account for the fact that the second ionisation energies of both chromium and copper are higher than those of the next element.

(v) Why is manganese more stable in the +2 state than the +3 state while the reverse is true for iron?

229.

Give reasons:
(i) The d-orbital of Cu is completely filled (3d¹⁰4s¹) still it is considered as transition metal but Zn (3d¹⁰4s²) is not.

(ii) Zinc salts are colourless at room temperature but nickel salts are coloured.

(iii) The atomic radii of europium and ytterbium do not fit into the concept of lanthanide contraction but ionic radii fit in.

(iv) The atomic radius of Cu is greater than that of Cr but ionic radius of Cr²⁺ is greater than that of Cu²⁺.

(v) Cu²⁺ is colourless while Cu²⁺ is blue in aqueous solution.

230.

Give reasons:
(i) Scandium (At. No. 21) is a transition element but calcium (At. No. 20) is not.
(ii) The transition metal ions such as Cu⁺ Ag⁺ and Sc³⁺ are colourless.
(iii) The melting points of transition metals of 3d series are much higher than the melting point of zinc of the same period.
(iv) The radius of Fe²⁺ is less than that of Mn²⁺.
(v) Actinides cations are coloured.