Notes-Part-2-Class-12-Chemistry-Chapter-8-Transition and Inner transition Elements-Maharashtra Board

Topics to be Learn : Part-1 Introduction Position in the periodic table Electronic configuration Oxidation states of first transition series Physical properties of first transition series Trends in atomic properties of the first transition series Topics to be Learn : Part-2 Compounds of Mn and Cr (KMnO4 and K2Cr2O7) Common properties of d-block elements Extraction of metals Topics to be Learn : Part-3 Inner transition (f-block) elements Properties of f-block elements Properties of Lanthanoids and Applications Actinoids Applications of actinoids Postactinoid elements

Compounds of Mn and Cr (KMnO₄ K₂Cr₂O₇) :

Remember : Both KMnO₄ and K₂Cr₂O₇ are strong oxidising agents.

Preparation of potassium permanganate :

Potassium permanganate (KMnO₄) is prepared in the following steps.

(i) Chemical Oxidation : When finely divided manganese dioxide (MnO₂) is heated strongly with fused caustic potash (KOH) and an oxidising agent potassium chlorate (KCIO₃) dark green potassium manganate K₂MnO₄ is obtained. (In neutral or acidic medium K₂MnO₄ disproportionates.)

3MnO₂ + 6KOH + KCIO₃  \(\underrightarrow{Δ}\) 3K₂MnO₄ + KCl + 3H₂O

The liquid is filtered through glass wool or sintered glass and evaporated. Potassium manganate crystallises as small, blackish crystals.

(ii) Oxidation of K₂MnO₄ :

(a) By electrolytic oxidation : An alkaline solution of manganite ion is electrolysed between iron electrodes separated by a diaphragm. Manganate ion (MnO₄²⁻) undergoes oxidation at anode forming permanganate ion

(MnO₄⁻). Oxygen evolved at anode converts MnO₄²⁻ to MnO₄⁻.

The overall reaction is as follows :

2K₂MnO₄ + H₂O + [O] → 2KMnO₄ + 2KOH

The electrolytic solution is filtered and evaporated to obtain deep purple black crystals of KMnO₄.

(b) BY Passing CO₂ through the solution of K₂MnO₄ :

3K₂MnO₄  + 4CO₂ + 2H₂O → 2KMnO₄ + MnO₂ + 4KHCO₃

Disproportionation of an oxidation state :

Disproportionation reaction is a chemical reaction in which atom or an ion of an element forms two or more species having different oxidation states, one lower and one higher.

Example : Manganese (Mn) shows different oxidation states +2 to +7.

When one oxidation state, lower or higher oxidation state becomes unstable as compared to other oxidation state, it undergoes disproportionation reaction.

For example, +6 oxidation state of Mn is less stable than +7 and +4.

(i) Hence, in acidic medium Mn⁶⁺ + in MnO₄²⁻ undergoes disproportionation reaction.

3MnO₄²⁻ + 4H⁺  →  2MnO₄⁻ +  MnO₂  + 2H₂O

(Mn⁶⁺)                       (Mn⁷⁺)          (Mn⁴⁺)

(ii) In neutral medium green K₂MnO₄ disproportionate to KMnO₄ and MnO₂.

3K₂MnO₄ + 4CO₂ + 2H₂O → 2KMnO₄ + MnO₂ + 4KHCO3

(Mn⁶⁺)                                        (Mn⁷⁺)     (Mn⁴⁺)

Chemical properties of KMnO₄ :

(1) KMnO₄ in acidic medium :

(i) In acidic medium iodine is liberated from potassium iodide.

10I⁻ + 2MnO₄⁻ + 16H⁺ → 2Mn²⁺ + 8H₂O + 5I₂

(ii) In acidic medium, Fe²⁺ + ion is converted to Fe³⁺ ion.

5Fe²⁺ + MnO₄⁻ + 8H⁺ →  Mn²⁺ + 4H₂O + 5Fe³⁺

(iii) In acidic medium, H2S is oxidised to S.

H₂S → 2H⁺ + S⁻²

5S⁻² + 2MnO₄⁻ + 16H⁺ → 2Mn²⁺ + 8H₂O + 5S

(iv) Oxalic acid, in acidic medium is oxidised to CO₂.

2MnO₄⁻ + 5H₂C₂O₄  +  6H⁺ → 2Mn⁺ + 10CO₂ + 8H₂O

(2) KMnO₄ in neutral or alkaline medium in neutral or weakly alkaline medium

(i) Iodide is oxidised to iodate ion.

2MnO₄⁻ + H₂O + I⁻ → 2MnO₂ + 2OH⁻ + IO₃⁻ (iodate ion)

(ii) Thiosulphate ion is oxidised to sulphate ion.

8MnO₄⁻  +  3S₂O₃²⁻   +   H₂O → 8MnO₂ + 6SO₄²⁻ + 2OH⁻

(iii) Manganous salt is oxidised to MnO₂.

2MnO₄⁻ + 3Mn²⁺ + 2 H₂O → 5MnO₂ + 4H⁺

Uses of KMnO₄ :

• An antiseptic.
• For unsaturation test in laboratory.
• In volumetric analysis of reducing agents.
• For detecting halides in qualitative analysis.
• Powerful oxidising agent in laboratory and industry

K₂Cr₂O₇ :

Preparation of potassium dichromate

Steps in manufacture of potassium dichromite from chromite ore are

• Concentration of chromite ore.
• Conversion of chromite ore into sodium chromate (Na₂CrO₄).
• Conversion of Na₂CrO₄ into sodium dichromate (Na₂CrO₇).
• Conversion of Na₂CrO₇ into K₂Cr₂O₇.

Explanation :

In the industrial production, finely powdered chromite ore (FeOCr₂O₃) is heated with anhydrous sodium carbonate (Na₂CO₃) and a flux of lime in air in a reverbatory furnace.

4(FeO.Cr₂O₃) + _8Na2CO3 + 7O₂ \(\underrightarrow{Δ}\) 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂.

Sodium chromate (Na₂CrO₄) formed in this reaction is then extracted with water and treated with concentrated sulphuric acid to get sodium dichromate and hydrated sodium sulphate :

2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + 2NaCl + Na₂Cr₂O₇.H₂O

Addition of potassium chloride to concentrated solution of sodium dichromate precipitates less soluble orange-red coloured potassium dichromate, K₂Cr₂O₇.

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

On concentrating and cooling the solution, less soluble orange coloured K₂Cr₂O₇ crystallises  out which is filtered and purified by recrystallisation.

Chemical properties of K₂Cr₂O₇ :

(i) Oxidation of I- from aq. solution of KI by acidified K₂Cr₂O₇ gives I₂. Potassium dichromate is reduced to chromic sulphate. Liberated I₂ turns the solution brown.

K₂Cr₂O₇ + 6KI + 7H₂SO₄ → 4K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3I₂

(ii) When H₂S gas is passed through acidified K₂Cr₂O₇ solution, H₂S is oxidised to pale yellow precipitate of sulphur. Simultaneously potassium dichromate is reduced to chromic sulphate, which is reflected as colour change of solution from orange to green.

K₂Cr₂O₇ + 4H₂SO₄ + 3H₂S → K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3S

Common properties of d block elements

Physical properties

• All d block elements are lustrous and shining.
• They are hard and have high density.
• Have high melting and boiling points.
• Are good electrical and thermal conductors.
• Have high tensile strength and malleability.
• Can form alloys with transition and non transition elements.
• Many metals and their compounds are paramagnetic.
• Most of the metals are efficient catalysts.

Chemical properties

• All d block elements are electropositive metals.
• They exhibit variable valencies and form colored salts and complexes.
• They are good reducing agents.
• They form insoluble oxides and hydroxides.
• Iron, cobalt, copper, molybdenum and zinc are biologically important metals
• They catalyse biological reactions.

Differences : Although most properties exhibited by d block elements are similar, the elements of first row differ from second and third rows in stabilization of higher oxidation states in their compounds. These differences are as following:

• Highest oxidation state for the first row element is + 7 as in Mn. For the second row, the highest oxidation state is + 8 as in Ru (RuO₄). For the third row, the highest oxidation state is + 8 as in Os(OsO₄).
• Compounds of Mo(V ) of 2nd row and W(VI) of 3rd row of transitional elements are more stable than Cr(VI) and Mn (VIII) of first row elements.

Extraction of metals :

Metals occur in nature :In nature, few metals occur in earth‘s crust in lice state or native state while other metals occur in the combined form.

• Elements in free state or native state : The metals which are non-reactive with air, water, CO₂ and non-metals occur in free state or native state. For example, gold, platinum, palladium occur in free state. Metals like Cu, Ag and Hg occur partly in the free state.
• Combined form : The metals which are reactive occur in the combined state with other elements forming compounds like oxides, sulphides, sulphates, carbonates, silicates, etc.

Mineral : A naturally occuring substance found in the earth’s crust containing inorganic salts, solids, siliceous matter etc. is called a mineral.

Ore : The mineral which contains high percentage of the metal and from which the metal can be extracted economically is called an ore.

Metallurgy : The process of extracting pure metal from its ore is known as metallurgy.

Different methods are used for their extraction depending on the nature of a metal and its ore.

• Pyrometallurgy: It is the process of reducing ore to metal at high temperatures utilising reducing agents such as carbon, hydrogen, aluminium, and so on.
• Hydrometallurgy : It is the technique of extracting metals from aqueous solutions of their salts using a suitable reducing agent.
• Electrometallurgy : It is a method that extracts metal from molten (fused) metallic compounds using electrolytic reduction.

Gangue : The earthly and undesired impurities of various substances like sand (SiO₂), metal oxides, etc. present in the ore are called gangue or matrix.

Steps Involved in Process of Extraction :

• Concentration of ores : It is the process in which unwanted impurities (gangue) are removed. It is also called benefaction or dressing of an ore
• Conversion of ores into oxides or other reducible compounds of metals
• Reduction of ores to obtain crude metals.
• Refining of metals giving pure metals.

Common methods of concentration of an ore :

The concentration of an ore involves different methods depending upon the differences in physical properties of compounds or the metal present and the nature of the gangue.

The common methods of concentration of ore are as follows :

• Gravity separation or hydraulic washing : This can be carried out by two processes : (a)Hydraulic washing by using Wilfley’s table method (b) Hydraulic classifier methods.
• Magnetic separation
• Froth floatation process.
• Leaching.

The method chosen for concentration depends upon the nature of the ore.

Process :

Leaching : It is a (chemical) process used in the concentration of an ore by extracting soluble material from an insoluble solid by dissolving in a suitable solvent. This method is used in the concentration process of oresof Al, Ag, Au, etc.

Roasting : It is a process of strongly heating a concentrated ore in the excess of air below melting point of metal, to convert it into oxide form. It is used for a sulphide ore. Example : ZnS ore on roasting forms ZnO.

ZnS + O₂ \(\underrightarrow{Δ}\) ZnO + SO₂

Smelting : The process of extraction of a metal from its ore by heating and melting at high temperature is called smelting. Reduction of ore is carried out during smelting.

Know This : Flux : A flux is a chemical substance which is added to the concentrated ore during smelting in order to remove the gangue or impurities by chemical reaction forming a fusible mass called slag. Slag : It is a waste product formed by combination of a flux and gangue (or impurities) during the extraction of metals by smelting process.

Calcination : It is a process in which the ore is heated to a high temperature below the melting point of the metal in the absence of air or limited supply of air in a reverberatory furnace. It is generally used for carbonate and hydrated oxides to convert them into anhydrous oxides.

Extraction of Iron from Haematite ore using Blast furnace :

• Composition of Haematite ore : Fe₂O₃ + SiO₂ + Al₂O₃ + phosphates

• SiO₂ and Al₂O₃ are the impurities present in the haematite ore

• Iron is extracted from haematite by its reduction using coke and limestone.
• Coke and CO acts as a reducing agents.
• Carbon in the limestone is reduced to carbon monoxide. Carbon and carbon monoxide together reduce Fe₂O₃ to metallic iron.
• Limestone acts as flux, it combines with the gangue material to form molten slag.

Reactions :

Fe₂O₃ + 3C → 2Fe + 3CO.

Fe₂O₃ + 3CO → 2Fe + 3CO₂.

Use of limestone in the extraction of iron :

• The ore of iron contains acidic gangue or impurity of Silica SiO₂
• To remove silica gangue, basic flux like calcium oxide CaO, is required, which is obtained from the decomposition of limestone, CaCO₃.

CaCO3 → CaO + CO₂

• Silica reacts with CaO and forms a fusible slag of CaSiO₃.

SiO₂ + CaO \(\underrightarrow{Δ}\)  CaSiO

• Therefore in the extraction of iron, lime is used.
• Extraction of iron is carried out in Blast furnace

The extraction process involves following steps :

Concentration of an ore : The powdered ore is concentrated by gravity separation process by washing it in a current of water. The lighter impurities (gangue) are carried away leaving behind the ore.

Roasting : The concentrated ore is heated strongly in a limited current of air. During this, moisture is removed and the impurities like S, As and phosphorus are oxidised to gaseous oxides which escape.

S + O2 \(\underrightarrow{Δ}\)  SO₂ ↑

2As + 3O₂ → As₂O₃ ↑

P₄ + 5O₂ → 2P₂O₅ ↑

FeO present in the ore is oxidised to Fe₂O₃.

4FeO + O₂ → 2 Fe₂O₃

After roasting, the ore is sintered to form small lumps. .

Reduction (or smelting) : The roasted or calcined ore is then reduced by heating in a blast furnace.

Construction of Blas furnace :

• The blast furnace is a tall cylindrical steel tower about 25 m in height and has a diameter about 5- 10 m lined with fire bricks inside.
• Blast furnace has three parts : (i) the hearth, (ii) the bosh and (iii) the stack.
• At the top, there is a cup and cone arrangement to introduce the ore and at the bottom, tapping hole for withdrawing molten iron and an outlet to remove a slag.
• The roasted ore is mixed with coke and limestone in the approximate ratio of 12:5:3.
• A blast of hot air at about 1000 K is blown from downwards to upwards by layers arrangement. The temperature range is from bottom 2000 K to 500 K at the top.
• The charge of ore from top and the air blast from bottom are sent simultaneously.

There are three zones of temperature in which three main chemical reactions take place.

(i) Zone of combustion : This is 5 - 10 m from the bottom. The hot air blown through the tuyers, oxidises coke to CO which is an exothermic reaction, due to which the temperature of furnace rises.

C + O₂ → CO Δ H = —220 kJ

Some part of CO dissociates to give finely divided carbon and O₂.

2CO → 2C + O₂

The hot gases with CO rise up in the furnace and heats the charge coming down. CO acts as a fuel as well as a reducing agent.

(ii) Zone of reduction (22-25 m near the top) : At about 900 K, CO reduces Fe₂O₃ to spongy (or porous) iron.

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Carbon also reduces partially Fe₂O₃ to Fe.

Fe₂O₃ + 3C → 2Fe + 3CO

 (iii) Zone of slag formation (20 m unit) : At 1200 K limestone, CaCO₃ in the charge, decomposes and forms a basic flux CaO which further reacts at 1500 K with gangue (SiO₂, Al₂O₃) and forms a slag of CaSiO3 and Ca₃AlO₃.

CaCO₃ → CaO + CO₂

CaO + SiO₂ → CaSiO₃

12CaO + 2Al₂O₃ → 4Ca₃AlO₃ + 3O₂

The slag is removed from the bottom of the furnace through an outlet.

(iv) Zone of fusion (15 m ht): The impurities in ore like MnO₂ and Ca₃(PO₄)₂ are reduced to Mn and P while SiO₂ is reduced in Si. The spongy iron moving down in the furnace melts in the fusion zone and dissolves the impurities like C, Si, Mn, phosphorus and sulphure. The molten iron collects at the bottom of furnace. The lighter slag floats on the molten iron and prevents its oxidation.

The molten iron is removed and cooled in moulds. It is called pig iron or cast iron. It contains about 4 % carbon.

Refining :

Refining of metals : The purification of impure or crude metals by removing metallic and nonmetallic impurities is known as refining of metals.

• Pure iron can be obtained by electrolytic refining or other method.
• Methods of refining of metals : (i) Electro refining (ii) Liquefaction (iii) Distillation (iv) Oxidation

The choice of extraction technique is governed by the following factors :

• Nature of ore
• Availability and cost of reducing agent. (Generally cheap coke is used).
• Availability of hydraulic power.
• Purity of metal required.
• Value of by products. For example, SO₂ obtained during roasting of sulphide ores is important for the manufacture of H₂SO₄.

Commercial forms of iron are : (i) Cast iron (ii) wrought iron (iii) steel

Remember :

• Iron melts at a very high temperature (1800 K). On addition of carbon its melting point decreases depending upon percentage of carbon.
• Mechanical properties of steel can be modified by addition of small amounts of suitable elements such as manganese, chromium, sulfur, nickel etc. These elements are called alloying elements and steels are alloy steels.
• Iron possesses a high degree of magnetism below 1042 K. This is known as ferromagnetism.