Block D and F Elements

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

8.24 Potassium dichromate is prepared from chromite ore (FeCr₂O₄) by the following steps:

Step1: Preparation of sodium dichromate in the reaction of Chromite ore with sodium hydroxide and oxygen gas.

4FeCr₂O₄ + 16NaOH + 7O₂? 8Na₂CrO₄ + 2Fe₂O₃ + 8H₂O

Step2: Conversion of Sodium Chromate on reaction with concentrated Sulfuric acid gives Sodium dichromate as a product.

2Na₂CrO₄ + conc. H₂SO₄? Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Step3: Sodium dichromate on reaction with potassium chloride converts to potassium dichromate as a product.

Na₂Cr₂O₇ + 2KCl? K₂Cr₂O₇ + 2NaCl

As potassium dichromate is less soluble than sodium chloride so, potassium dichromate is obtained in form of orange crystals.

Dichromate ion exists in equilibrium with chromate ion at around pH. However, by changing the pH they can be interconverted.

8.21 (i) Transition metals and many of their compounds show paramagnetic behavior- Paramagnetic behaviour is shown by transition metals as paramagnetism is due to the presence of unpaired electrons which have a magnetic moment associated with its spin and angular momentum, as the orbital angular momentum is satisfied in the first transition series. So the paramagnetic is only due to the unpaired electrons.

(ii) The enthalpies of atomisation of the transition metals are high- Enthalpies of atomization is the enthalpy change when 1 mol of gaseous atoms is formed from its element in its defined physical state under standard conditions (298.15K, 1 atm). In case of the transition metals enthalpies are high due to high effective nuclear charge and a large number of valence electrons which also leads to strong metallic bonding.

Eg: 515 KJ/mol for Vanadium,473 KJ/mol for Titanium

(iii) The transition metals generally form coloured compounds- Transition metals are generally coloured because of absorption of radiations which fall in the visible region as on obtaining energy electrons jump from one d-orbital to another.

(iv) Transition metals and their many compounds act as good catalyst- Transition metals and their compounds act as good catalysts due to its ability to show variable oxidation state and form complexes, another reason is that they provide a suitable surface for reaction to take place.

Eg: Vanadium Oxide in contact process, Finely divided iron in Haber's Process.