Some Important Compounds of Transition Elements

Ferrous sulphate (Green vitriol), FeSO₄.7H­₂O 

 It occurs in nature as copperas and commonly known as hara Kasis.

• Preparation of Ferrous Sulphate

1. By dissolving scrap Fe in dil. H₂SO₄

2. From Kipp’s waste which contains ferrous sulphate with some free H₂SO₄; the latter is neutralised with scrap iron forming FeSO₄ and hydrogen.

3. By the action of air and water on iron pyrites. The solution is treated with scrap iron to remove H₂SO₄ and to reduce Fe₂(SO₄)₃  to FeSO₄.

• Properties of Ferrous Sulphate

1. Hydrated and anhydrous FeSO₄ are green and white in colour respectively. It is isomorphous with epsom salt, MgSO₄.7H₂O and ZnSO₄.7H₂O. It effervesces on exposure to air.

2. Light green crystals of FeSO₄ lose water and turn brown on exposure to air, due to oxidation.

3. On heating at 300°C it gives anhydrous FeSO₄ which on further heating gives Fe₂O₃ and SO₂.

4. Like other ferrous salts, it takes up HNO₃ forming brown coloured double compound, Fe(NO)SO₄, nitroso ferrous sulphate (Ring test for nitrates).

5. It decolourises acidified potassium permanganate and turns acidified dichromate green (reducing character).

6. It forms double salts with sulphates of alkali metals with general formula R₂SO₄.FeSO₄.6H₂O. With ammonium sulphate, it forms a double salt known as ferrous ammonium sulphate or Mohr’s salt, FeSO₄.(NH₄)₂SO₄.6H₂O. It does not effervesce. It ionises in solution to gives Fe²⁺, NH₄⁺ and SO₄^2– ions.

Ferric oxide, Fe₂O₃

1. It occurs in nature as haematite.

2. Fe­2O₃ is a red powder, insoluble in H₂O and not acted upon by air or H₂O

3. It is amphoteric in nature and reacts with acids and alkalies.

4. It is reduced to iron by H₂,C and CO.

5. It is used as a catalyst in the oxidation of CO to CO₂ in the Bosch process.

Ferric Chloride, FeCl₃

• Preparation of Ferric Chloride

1. Hydrated ferric chloride (FeCl₃.6H₂O) can be prepared by dissolving iron, Fe(OH)₃ or ferric oxide in dil. HCl.

2. Reaction of Fe with dry Cl₂ gives anhydrous FeCl₃,

• Properties  of Ferric Chloride

1. Anhydrous salt is yellow, deliquescent compound and highly soluble in H₂O.

2. Its aqueous solution is acidic due to hydrolysis.

3. On heating it gives FeCl₂ and Cl₂.

4. It oxidizes H₂S to S, SO₂ to H₂SO₄, SnCl₂ to SnCl₄ and Na₂S₂O₃ to Na₂S₄O₆

Copper(II) Sulphate Pentahydrate or Blue Vitriol, CuSO₄.5H₂O

• Preparation of Cupper (II) Sulphate:

In the laboratory, it is prepared by dissolving cupric oxide, cupric hydroxide or carbonate in dilute H₂SO₄.

CuO + H₂SO₄ → CuSO₄ + H₂O

Cu(OH)₂ + H₂SO₄ → CuSO₄ + 2H₂O

CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂

The solution of CuSO₄ thus obtained is concentrated and cooled when crystals of blue vitriol, CuSO₄.5H₂O separates out.

Commercially it is prepared by the action of hot dilute sulphuric acid on scrap copper in the presence of air.

2Cu + 2H₂SO₄ + O₂ → 2CuSO₄ + 2H₂O      

• Properties of Cupper (II) Sulphate?

(1) Action of Heat

1. It has 5 molecules of water of crystallisation; all of which can be removed on heating, to form colourless CuSO₄ (again coloured with H₂O).
   CuSO₄.5H₂O CuSO₄.H₂O CuSO_{4 (white ppt)}   CuO + SO₃

2. At high temperature it forms cupric oxide.

3. It forms double salts with alkali sulphates, e.g. K₂SO₄.CuSO₄.6H₂O

4. When treated with NH₄OH, it first forms precipitate of cupric hydroxide copper (II) sulphate (Schweitzer’s reagent), used for dissolving cellulose in the manufacture of artificial silk.

5. It reacts with KCN forming a complex compound K₃[Cu(CN)₄].

6. It liberates iodine from soluble iodides.

(2) Action of Alkalis

CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄

With NH₄OH it forms tetraamminecopper (II) sulphate

CuSO₄ + 4NH₄OH → [Cu(NH₃)₄]SO₄ + 4H₂O

(3) Reaction with KI

CuSO₄ + 2KI → CuI₂ + K₂SO₄

2Cul₂ → 2Cul + l₂

The liberation of iodine in this reaction is quantitative. Therefore, this reaction is used to estimate copper volumetrically.

• Uses of Cupper (II) Sulphate

1. It is used as an electrolyte in electroplating, electrotyping and refining of copper.

2. It is used in reservoirs and swimming pools to prevent the growth of weeds.

3. It is used as a fungicide under the name Bordeaux mixture, which is a mixture of CuSO₄ and slaked lime Ca(OH)₂.

4. Anhydrous CuSO₄ is used for detection of moisture in organic liquids such as alcohol, ether etc.

Silver Nitrate, AgNO₃

• Preparation of Silver Nitrate

Silver nitrate is prepared by the action of dilute nitric acid on silver and then evaporating the solution to crystallization.

3Ag + 4HNO₃ → 3AgNO₃ + NO ↑ + 2H₂O

• Properties Silver Nitrate

(1) Action of Heat

It decomposes on heating.

2AgNO₃  2AgNO₂ 2Ag + 2NO₂

On coming into contact with organic matter like skin or clothes, it is reduced to finely – divided silver, giving a black stain.

(2) Precipitation Reactions

Silver nitrat forms precipitates with some salt solutions which help in the detection of acid radicals.

Some of the precipitation reactions are:

1. NaCl + AgNO₃ → AgCl ↓ + NaNO₃

2. NaPO₄ + 3AgNO₃ → Ag₃PO₄ ↓ + 3NaNO₃

3. K₂CrO₄ + 3AgNO₃ → AgCrO ↓ + 2KNO₃

4. Na₂S + 2AgNO₃ → Ag₂S ↓ + 2NaNO₃

5. Na₂S₂O₃ + 2AgNO₃ → Ag₂S₂O₃ ↓ + 2NaNO₃

6. Na₂C₂O₄ + 2AgNO₃ → Ag₂C₂O₄ ↓ + 2NaNO₃

7. Na₃BO₃ + 3AgNO₃ → Ag₃BO₃ ↓ + 3NaNO₃

• Uses of Silver Nitrate

It is used for

1. Preparing silver halides which are used in photography.

2. For making inks and hair dyes.

3. In qualitative and quantitative analysis.

4. For silvering of glass, i.e. preparation of mirrors.
    

Halides of Silver

• Preparation of Silver Halides

Silver halides are prepared by the action of sodium or potassium halide on silver nitrate solution (except for AgF)

AgNO₃ + NaX → AgX(s) + NaNO₃   

Silver fluoride is prepared by the action of HF on silver (I) oxide. 

2HF + 2Ag₂O → 2AgF + H₂O

• Properties of Silver Halides

1. AgCl is white solid, AgBr is a pale yellow solid and AgI is a yellow solid.

2. AgF is soluble in water whereas other halides are insoluble in water. AgCl dissolves in ammonia to form a complex.

3. AgCl + 2NH₄OH → [Ag(NH₃)₂]Cl + 2H₂O

4. AgBr is partially soluble and AgI is insoluble in NH₄OH.

5. All the silver halides dissolve in potassium cyanide and Na₂S₂O₃ solution to form complexes.
   AgCl + 2KCN → K [Ag(CN)₂] + KCl
   AgCl + 2Na₂S₂O₃ → Na₂[Ag(S₂O₃)₂] + NaCl

• Uses of Silver Halides

All silver halides (particularly AgBr) are photosensitive and hence are widely used in photography.
 

Mercury (I) Chloride / Mercurous Chloride /Calomel, (Hg₂Cl₂)

• Preparation of Mercurous Chloride

It can be prepared by mixing a chloride solution with a mercury (I) salt solution.

Hg₂(NO₃)₂ + 2NaCl → Hg₂Cl₂ ↓ + 2NaNO₃ 

It can also prepared by heating a mixture of mercuric chloride and mercury in an iron vessel.

HgCl₂ + Hg  Hg₂Cl₂

• Properties of Mercurous Chloride

1. It is a white power insoluble in water but soluble in chlorine water.
   Hg₂Cl₂ + Cl₂ → 2HgCl₂

2. It decomposes on heating to HgCl₂
   Hg₂Cl₂ → HgCl₂ + Hg

3. On treatment with ammonia, if turns black due to the formation of finely divided mercury.
   Hg₂Cl₂ + 2NH₃→ Hg + Hg(NH₂)Cl + NH₄Cl

• Uses of Mercurous Chloride

1. In making standard calomel electrode and

2. As a purgative in medicine.

Mercury (II) Chloride HgCl₂

• Preparation of Mercury (II) Chloride 

1.  It is prepared by passing dry chlorine over heated mercury. 
   HgCl₂ + Hg → Hg₂Cl₂

2.  It is also obtained by treating HgO with HCl
   HgO + 2HCl → HgCl₂ + H₂O

3. Commercially, it is prepared by heating a mixture of HgSO4 and NaCl in the presence of MnO2
   HgSO₄ + 2NaCl  HgCl₂ + Na₂SO₄

• Properties of Mercury (II) Chloride

It is a white crystalline solid sparingly soluble in cold water but soluble in hot water. Its solubility can be increased by adding Cl^-.

HgCl₂ + 2Cl^– → [HgCl₄]^2–

It is readily soluble in organic solvents suggesting its covalent nature.

When treated with SnCl₂ it is reduced to mercury.

2HgCl₂ + SnCl₂ → SnCl₄ + Hg₂Cl₂

Hg₂Cl₂ + SnCl₂ → 2Hg + SnCl₄

When Cu turnings are placed in its contact a shining grey film of mercury deposits over them.

HgCl₂ + Cu → Hg + CuCl₂

• Uses of Mercury (II) Chloride 

It is used for preserving wood and hides and for making fungicides.

Mercury-II Iodide

• Preparation of Mercury (II) Iodide

  

It is prepared by treating HgCl₂ with KI.

HgCl₂ + 2Kl → Hgl₂ + 2KCl

• Properties of Mercury (II) Iodide

Mercuric iodide exists in two forms, i.e. red and yellow. The yellow form is stable above 400 K white the red form is stable below this temperature.

It readily dissolves in KI forming a complex

HgI₂ + 2KI → K₂[HgI₄]

An alkaline solution of K₂HgI₄ is called Nessler’s reagent and is used to detect the presence of NH₄⁺ with which it gives a brown precipitate due to the formation of iodide of Million’s base.

• Uses of Mercury (II) Iodide

It is used to prepare Nessler’s reagent and for making ointments for treating skin infections.

Potassium Dichromate, K₂Cr₂O₇

It is prepared from the ore called chromate or ferrochrome or chrome iron, FeO.Cr₂O₃. The various steps involved are

(a) Preparation of sodium chromate

4FeO.Cr₂O₃ + O₂ → Fe₂O₃ + 4Cr₂O₃

4Na₂CO₃ + 2Cr₂O₃ + 3O₂ → 4Na₂CrO₄ + 4CO₂

(b) Conversion of sodium chromate into sodium dichromate.

2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O

(c) Conversion of sodium dichromate into potassium dichromate.

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

• Properties  of Potassium Dichromate

It forms orange red crystals. It is moderately soluble in cold water but freely soluble in hot water.

1. Action of heat

When heated, it decomposed to its chromate

4K₂Cr₂O₇  4K₂CrO₄ + 2Cr₂O₃ + 3O₂

2. Action of alkalis

With alkalis it is converted into chromate which on acidifying gives back dichromate.

K₂Cr₂O₇ + 2KOH → 2K₂CrO₄ + H₂O

2K₂Cr₂O₇ + H₂SO₄ → K₂Cr₂O₇ + K₂SO₄ + H₂O

In dichromate solution the Cr₂O₇^2– ions are in equilibrium with Cr₂O₇^2– ions at pH = 4.

Cr₂O₇^2– + H₂O     2CrO₄^2– + 2Hl

        ^{orange red                            yellow}

3. Action of conc. H₂SO₄ solution

• In cold conditions
  K₂Cr₂O₇ + 2H₂SO₄ ———→ 2CrO₃ + 2KHSO₄ + H₂O

• In hot conditions
  2K₂Cr₂O₇ + 8H₂SO₄ ———→ 2K₂SO₄ + 2Cr₂(SO₄)₃ + 8H₂O + 3O₂

4. Oxidising properties

It is a powerful oxidising agent. In the presence of dil. H₂SO₄ it furnishes 3 atoms of available oxygen.

K₂Cr₂O₇ + 4H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3O

Some of the oxidizing properties of K₂Cr₂O₇ are

1. It liberates I₂ from KI
   K₂Cr₂O₇ + 7H₂SO₄ + 6Kl → 4K₂SO₄ + Cr₂(SO₄)₃ + 3l₂ + 7H₂O

2. It oxidises ferrous salts to ferric salts
   K₂Cr₂O₇ + 7H₂SO₄ + 6FeSO₄ → K₂SO₄ + Cr₂(SO₄)₃ + 3Fe₂(SO₄)₃ + 2H₂O

3. It oxidises S^-2 to S
   K₂Cr₂O₇ + 4H₂SO₄ + 3H₂S → K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3S

4. It oxidises nitrites to nitrates
   K₂Cr₂O₇ + 4H₂SO₄ + 3NaNO₂ → K₂SO₄ + Cr₂(SO₄)₃ + 3NaNO₃ + 4H₂O

5. It oxidises SO₂ to SO₄^2–
   K₂Cr₂O₇ + H₂SO₄ + 3SO₂ → K₂SO₄ + Cr₂(SO₄)₃ + 3H₂O

6. It oxidises ethyl alcohol to acetaldehyde and acetic acid.

5. Chromyl Chloride Test

When heated with conc. HCl or with a chloride in the presence of sulphuric acid, reddish brown vapours of chromyl chloride are obtained.

K₂Cr₂O₇ + 4KCl + 6H₂SO₄ → 2CrO₂Cl₂ + 6KHSO₄ + 3H₂O

Thus reaction is used in the detection of chloride ions in qualitative analysis.

• Uses of Potassium Dicromate

1. In volumetric analysis for the estimation of Fe²⁺ and I^-.

2. In chrome tanning in leather industry.

3. In photography and in hardening gelatin film. 

Potassium Permanganate, KMnO₄

• Preparation of Potassium Permanganate on a Large Scale

It is prepared from the mineral pyrolusite, MnO₂. The preparation involves the following steps

Conversion of MnO₂ into potassium manganate.

When finely powdered MnO₂ is fused with KOH. K₂MnO₄ is obtained.

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O

Oxidation of potassium manganate into permanganate

(a) Chemical oxidation

K₂MnO₄ is oxidised to KMnO₄ by bubbling CO₂ or Cl₂ or ozone into the former.

3K₂MnO₄ + 2CO₂ → 2KMnO₄ + MnO₂ + 2K₂CO₃

(b) Electrolytic oxidation

The manganate solution is electrolysed between iron electrodes. The oxygen evolved at anode converts manganate into permanganate.

2K₂MnO₂ + H₂O + O → 2K₂MnO₄ + 2KOH 

• Properties of Potassium Permanganate

KMnO₄ exists as deep purple prisms. It is moderately soluble in water at room temperature and its solubility in water increases with temperature. 

(i) Action of heat

When heated it decomposes to K₂MnO₄.

 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂

(ii) Action of conc. H_­2SO₄

With cold conc. H₂SO₄ it gives Mn₂O₇ which on warming decomposes to MnO₂.

2MnO₂ + 2H₂SO₄ → Mn₂O₇ + 2KHSO₄ + 2H₂O

2Mn₂O₇  4MnO₂ + 3O₂

With hot Conc. H_­2SO₄ O₂ is evolved

4KMnO₄ + 6H₂SO₄ → 2K₂SO₄ + 4MnSO₄ + 6H₂O + 5O₂

(iii) Oxidising properties

KMnO₄ is a powerful oxidizing agent. The actual oxidizing action depends upon the medium i.e. acidic, basic or neutral.

In neutral solution, it acts as moderate oxidizing agent.

2KMnO₄ + H₂O → 2KOH + 2MnO₂ + 3O

Some oxidizing properties of KMnO₄ in neutral medium are

2KMnO₄ + 3Na₂S₂O₃ + H₂O → 3K₂SO₄ + 8MnO₂ + 3Na₂SO₄ + 2KOH

In strong alkaline solution, it is converted into

2KMnO₄ + 2KOH → 2K₂MnO₄ + H₂O + O

2KMnO₄ + H₂O + Kl → 2MnO₂ + 2KOH + KlO₃

In acidic medium, Mn⁺⁷ is converted into Mn⁺²

Some other reactions are

1. 2KMnO₄ + 3H₂SO₄ + 5H₂S → K₂SO₄ + 2MnSO₄ + 3H₂O + 5S

2. 2KMnO₄ + 5SO₂ + 2H₂O → K₂SO₄ + 2MnSO₄ + 2H₂SO₄

3. 2KMnO₄ + 3H₂SO₄ + 5KNO₂ → K₂SO₄ + 2MnSO₄ + 3H₂O + 5KNO₃

4. 2KMnO₄ + 3H₂SO₄ + 5C₂H₂O₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 10CO₂

5. 2KMnO₄ + 8H₂SO₄ + 10FeSO₄ → K₂SO₄ + 2MnSO₄ + 5Fe₂(SO₄)₃ + 8H₂O

6. 2KMnO₄ + 3H₂SO₄ + 10Kl → K₂SO₄ + 2MnSO₄ + 8H₂O + 5l₂

• Uses of Potassium Permanganate

1. It is used in volumetric analysis for the estimation of ferrous salts, oxalates, iodides and H₂O₂.

2. It is used as oxidizing agent in the laboratory as well as in industry.

3. It is also used as disinfectant and germicide.

Question 1: CuSO₄.5H₂O 

a. CuSO₄.H₂O

b. CuSO₄

c. CuO +SO₂

d. Cu +SO₃

Question 2: When heated with conc. HCl or with a chloride in the presence of sulphuric acid, reddish brown vapours of 

a. potassium dichromate

b. chromyl chloride 

c. chromic acid

d. chlorine

Question 3: Potassium dicromate  oxidises nitrites to 

a. nitrates

b. dinitrogen

c. nitrogen dioxide

d. nitrogen trioxide

Question 4: Hydrated ferric chloride (FeCl₃.6H₂O) can be prepared by dissolving iron, Fe(OH)₃ or ferric oxide in 

a. dil. HCl.

b. dil NaOH

c. H₂O

d. NH₃

Related Resources

• Its always helpful to have a look at past year papers of IIT JEE 

• Click here to  refer  the syllabus of chemistry for IIT JEE 

• You can also refer to general characteristics of transition elements