(a) A blackish brown coloured solid (A) which is an oxide of manganese, when fused with alkali metal hydroxide and an oxidizing agent like KNO3, produces a dark green coloured compound (B). Compound (B) on disproportionation in neutral and acidic solution gives a purple coloured compound (C). Identify A, B and C and write the reaction involved when compound (C) is heated to 513 K.

(b)

(i) E0 M3+ / M2+values for the first series of transition elements are given below.

Answer the question that follows:

Identify the two strongest oxidizing agents in the aqueous solution from the above data.

ii) Copper (I) ion is not known in aqueous solution

iii) The highest oxidation state of a metal is exhibited in its oxide.

OR

(a) Write balanced equations to represent what happens when

(i)Cu2+ is treated with KI.

(ii) Acidified potassium dichromate solution is reacted with iron (II) solution.

(ionic equation)

(b)

i) The figure given below illustrates the first ionization enthalpies of first, second and third series of transition elements. Answer the question that follows

Which series amongst the first, second and third series of transition elements have

the highest first ionization enthalpy and why?

ii) Separation of lanthanide elements is difficult. Explain.

iii) Sm2+, Eu2+ and Yb2+ ions in solutions are good reducing agents but an

aqueous solution of Ce4+ is a good oxidizing agent. Why?

(a)

• (A) must be manfanese dioxide MnO₂ which is blackish brown in colour and when fused with alkali metal hydroxide and an oxidizing agent like KNO3, it produces (B) K₂MnO₄ which is a dark green compound which on disproportionation in neutral and acidic solution gives (C) which is KMnO₄ , the purple coloured compound.

•

• Compound (C) on heated to 513 K decomposes back to potassium managanate (B) and and manganese dioxide (A).

•

b) (i)

• The two strongest oxidizing agents in the aqueous solution are Mn³⁺ and Co³⁺

• Because they have the most positive E₀ values which indicates higher potential to be reduced and oxidizing other substances. Rest of the elements are good reducing agents.

(b) (ii)

• Stability of an ion in aqueous solution highly depends upon hydration energy of the ion which is the capacity to attrat water molecules; which is more negative in the case for Cu(II) than Cu(I) which more than compemsates the second ionization enthalpy of copper.

• In aqueous solution Cu⁺ undergoes oxidation and reduction simultaneously

• And the relative stability of Cu and Cu²⁺ ions can be seen by their standard reduction potentials.

• Cu²⁺(aq) + e^-→ Cu(s), E⁰_red =0.52

• Cu²⁺ + 2e^-→ Cu(s), E⁰_red = 0.47

Having higher electrode potential makes Cu(I) very unstable in aq soln and therefore it readily disproportionates.

(b) (iii)

• Oxygen is possesses a very high eletrotronegativity and has a considerably small size which allows it to be a very good oxidizing agent (probably the best alongside F ) .

• Due to these properties O can oxidize metals to their highest oxidation states and that is why The highest oxidation state of a metal is exhibited in its oxide.

OR

(a) (i)

• When Cu2+ is treated with KI solution CuI (white) is precipitated alongside production of I₂ solution.

(ii)

• K₂Cr₂O₇ is a good oxidizing agent therdfore when it is introduced in Fe(II) solution it readily oxidizes it to Fe(III) solution and changes its colour from green to yellowish brownand used for identification of Fe(II).

• Cr₂O₇^2- + 6 Fe²⁺ + 14H⁺→ 2Cr³⁺ + 6 Fe³⁺ + 7H₂O [ E° = 0.56 V ]

(b) (i)

• The 3^rd series of transition elements with 5d electrons have the highest first ionization enthalpy.

• The speciality of these elements is that they possess 4f electrons which has very weak shielding abilities there failed to shield the interaction between nuclear charge and valence electrons Which results in greater effective nuclear charge acting on valence electrons and hence higher ionization potentials.

(b)(ii)

• The extraordinary properties of lanthanides are they resemble each other in both physical and chemical properties so closely that it is not an easy job to separate them.

(b) (iii)

• In lanthanide series of elements the most cpmmon and stable oxidation state is +3.

• Hence , for these ions Sm²⁺, Eu²⁺ and Yb²⁺ it is easier and sponmtaneous to loose electrons and converted to 3+ statesand gets oxidised and reduce others , whereas for Ce ⁺⁴ in aq solution gain +3 state by gaining electrons and gets reduced while oxodizing others.