Block D and F Elements

8.31 (i) Of the d4 species, Cr²⁺ is strongly reducing while manganese (III) is strongly oxidizing: The +2 oxidation state becomes more stable on moving across a period i.e. the tendency of metals to give electrons becomes more. Therefore, vanadium(II) oxide and chromium(II) oxide are strong reducing agents.

As if the value of electrode potential is higher i.e. more energy required to withdraw an electron from an isolated atom, more readily it can be reduced and lesser the electrode potential more readily it can be oxidized.

The electrode potential of Cr³⁺? Cr²⁺is negative so it acts reducing agent or can undergo oxidation which makes it a more stable ion while that of Mn³⁺? Mn²⁺ is positive and it undergoes reduction acts as strong oxidizing agent. Also, Mn³⁺ has an exactly half-filled d- orbital which is highly stable.

(ii) Cobalt(II) is stable in aqueous solution but in the presence of complexing reagents, it is easily oxidized: Cobalt gets oxidized from +2 to +3 oxidation state in the presence of complexing reagents.

(iii) The d¹ configuration is very unstable in ions:The d¹ electron can be easily lost after the loss of ns electrons. Therefore, the elements having d¹ configuration undergo disproportionation reactions.

8.29 It can be observed from the above table that in the starting of 3d transition series elements like Sc, Ti, V, Cr in +2 state are not that stable in their elements in the +3 state.

In the middle Mn²⁺, Fe²⁺, Co²⁺are quite known. In fact, Mn²⁺ and Mn⁷⁺ are most stable states in Mn. Fe²⁺ is less stable when compared to Fe³⁺ which is due to fact that Fe³⁺ is able to loose one electron to acquire d⁵ state which has extra stability. Co²⁺ is less stable as compared to Co³⁺.

Ni²⁺ is the most common and stable among its +2, +3, +4 states. Cu²⁺ is more stable and is quite common as compared to Cu⁺. Towards the end, Zn forms only Zn²⁺ which is highly stable as it has 3d¹⁰ state.

As it becomes difficult to remove the third electron from d-orbital, the stability of +2 oxidation state increases from top to bottom.

8.27 (i) Reduction potential tells us the ease with which the Metal can get reduced, As E° for Cr³⁺/Cr²⁺ is negative (–0.4 V), this means that Cr³⁺ ions in solution cannot be reduced to Cr²⁺ ions i.e., Cr³⁺ ions are very stable. As a further comparison of E° values show that Mn³⁺ ions more readily than Fe³⁺ ions which means that Mn³⁺ is least stable.

So Stability of metal ions is as follows:

Mn³⁺3+

(ii) Reduction potential tells us the ease with which the Metal can get reduced or the difficulty with which they are oxidized, As the reduction potential increases in the following order Mn²⁺/Mn < Cr²⁺/Cr< Fe²⁺/Fe

So oxidation of Fe is not as easy as Cr and Mn.

So increasing order of the metals to get oxidised is as follows:

Fe < Cr < Mn

8.26 Potassium permanganate can be prepared from MnO₂. It can be done by fusing the ore with KOH in presence of an oxidizing agent like atmospheric oxygen/KNO₃ etc to give green coloured K₂MnO₄ as the product.

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O

The Product K₂MnO₄ is extracted with water and then oxidised by passing ozone/chlorine into the solution or electrolytically.

Electrolytic oxidation:

(i) Acidified KMnO₄ solution oxidizes Fe(II) ions to Fe(III) ions and water as product

MnO₄ ^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O

Fe²⁺→ Fe³⁺+ e^- } x5

Overall: MnO₄ ^- + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺

(ii) Acidified potassium permanganate oxidizes SO₂ to sulphuric acid as follows

MnO₄ ^- + 6H⁺ + 5e^- → Mn²⁺ + 3H₂O } x2

2H₂O + 2SO₂ + O₂→ 4H⁺ + 2SO₄^2- + 2e^-} x5

Overall: MnO₄ ^- + 4H₂O + 10SO₂ + 5O[₂ → 8H⁺ + 10SO₄ ^2- + 2H⁺

(iii) Acidified potassium permanganate oxidizes oxalic acid to carbon dioxide

MnO₄ ^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O} x2

C₂O₄^2-→ 2CO₂ + 2e^- } x5

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