Chemistry NCERT Exemplar Solutions Class 12th Chapter Eight

37. (D) Mn²⁺ acts as auto catalyst

Mn²⁺  formed in the reaction acts as an autocatalyst. Auto catalyst is a compound that is one of the products that are formed in the reaction itself catalyses the reaction.

Reaction: 2MnO₄^-  +  16H⁺  +  5C₂O₄^2- → 2Mn₂ +  +  10CO₂ +  8H₂O

36.  (A)i, ii

Disproportionation/redox reaction is a reaction in which one compound is oxidised as well as reduced at the same time.

(A) Cu⁺  ?  Cu²⁺  +  Cu 

  ( + 1)       ( + 2)        (0) 

(B) 3MnO₄^- +  4H⁺  ?  2MnO₄^- +  MnO₂ +  2H₂O 

  ( + 6)                          ( + 7)           ( + 4) 

35. (B)+3

Out of all +3 is the most common oxidation state for all the lanthanide elements.

34. (B) 3d⁵

Magnetic moment of an electron/dipole moment is caused by its intrinsic properties of spin and electric charge. It depends upon the number of unpaired electrons in its valence shell. The more the number of unpaired electrons, the greater will be the value of magnetic moment.

3d⁷ =  3 unpaired electrons 

3d⁵ =  5 unpaired electrons

3d⁸ =  2 unpaired electrons 

3d² =   2 unpaired electrons 

Out of all, 3d⁵ has five unpaired electrons that is maximum and hence it has the highest magnetic moment.

33.  (A) Mn₂O₇

2KMnO₄ +  2H₂SO₄ (conc.) → Mn₂O₇ +    2KHSO₄ + H₂O

Manganese heptoxide is an acid anhydride of permanganic acid ( HMnO₄). It is a dangerous oxidiser, volatile and highly reactive liquid.

32.  (B) CuF₂

(A) Ag₂SO₄  (Ag⁺ )→ 5d¹⁰6s⁰

(B) CuF₂  (Cu²⁺ )→ 3d⁹4s⁰

(C) ZnF₂  (Zn²⁺)→ 3d¹⁰4s⁰

(D) Cu₂Cl₂  (Cu⁺)→ 3d¹⁰6s⁰

Unpaired electrons present in any compound impart colour to the salt of transition metal. Only CuF₂ has unpaired electrons in its 3d orbital that's why it is white coloured in its solid-state while rest of the salts are colourless.

31.  (D) Cu

Density is mass by volume. As we move from left to right for a long period, the atomic radii decrease. Hence, volume decreases. Also, an increase in atomic masses is observed. 

So overall the density increases out of the above option from iron to copper, copper will have the highest density.

30. (A) Cu (II) is more stable .

Electronic configuration of Cuis [Ar] 3d¹⁰ 4s¹

Cu (I) - [Ar] 3d¹⁰ 4s0

Cu (II)- [Ar] 3d⁹ 4s0

Despite the fact that Cu (I)  has fully filled 3d-orbital but Cu (II) is more stable than Cu (I) due to the greater effective nuclear charge of Cu (II) as nucleus has to hold 17 electrons rather than 18 electrons like in Cu (I).

29.  (B) 26

Positive oxidation states indicate the loss of electrons from the atom. If X is in +3 oxidation state, then three electrons have been removed from it. Find the atomic number of the parent atom, we will add three in the given electronic number. i.e.

X³⁺  (Z = 23)  =  [Ar] 3d⁵

X  =  23 + 3  = 26  

28. Ionisation enthalpies are the main factor that influence the reactivity of transition elements. Higher the ionisation enthalpy, lesser is the reacting of the transition element. 

When we move along the period from Sc to Cu, a regular increase in the ionisation enthalpy is observed which results in the almost regular decrease in the reactivity of elements.