Iron:

It is an important and useful metal. It was known to ancient Egyptians since 4000 BC. But its extraction started since 2000 BC. In India its extraction started since 600 BC. It belongs to group VIII-B of periodic table.

Occurrence:

After Aluminum it is most abundant metal on the earth crust. It is fourth most abundant element on earth crust. In Free State it exists only in meteors. Broken piece of rock that comes from stars and burn in earth’s atmosphere is called meteor. It is found in the rocks, mineral, soils, plants and animals etc.

Some important ores of iron are:

1. Magnetite (Ferroso ferric oxide) Fe3O4. It is found in Chaghi (Baluchistan), Chitral
2. Haematite (Ferric oxide) Fe2O3 (red). It is found in Mazazri tang (Kohat), Langrial(Hazara) and huge deposits of low grade ore are found in Kalabagh.
3. Limonite (Hydrated ferric oxide) 2Fe2O3.3H2O (brown).
4. Siderite (Ferrous carbonate) FeCO3.
5. Iron Pyrite (Ferrous sulphide) FeS
6. Copper Pyrite (Fool’s Gold) CuFeS2.

Characteristics:

(i) Symbol = Fe

(ii) Atomic Number = 26

(iii) Luster = Grayish white

(iv) Group = VIII-B

(v) Period = 3rd

(vi) Melting point = 1808K or 1535^oC

(vii) Boiling point = 3023K or 2750oC
(viii) Electronic configuration = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁶
(ix) Commercial forms of Iron:
Iron is available commercially in following three forms. They differ in their carbon contents.

1. Pig iron or Cast iron. It contains 2.5 – 4.5 % carbon.
2. Wrought iron. It contains 0.12 – 0.25 % carbon.
3. Steel. It contains 0.1 – 1.5 % carbon.

(x) Oxidation states:
• Iron shows +2, +3, +4, +5 and +6 oxidation states. But the most common oxidation states of iron are +2 and +3.
• Oxidation state +2: Iron in +2 oxidation state is called ferrous ion. Ferrous ion is light or pale green in colour. It get oxidized even on exposure to air and also get oxidized even traces of dissolved oxygen are present in the solution. Ferrous ions act as reducing agent. Complexes of Fe2+ are octahedral in geometry.

• Oxidation state +3: Iron in +3 oxidation state is called ferric ion. Ferric ion is yellow or yellowish brown in colour in solution due to formation of complex ion [Fe(H2O)5(OH)]2+. When potassium ferrrocyanide is added to solution containing ferric ions Prussian blue precipitate of ferric ferro cyanide Fe4[Fe(CN)6]3 is produced. Ferric ferro cyanide is used in making blue prints and is also used as bluing agent in laundry.

Iron as catalyst in Haber’s process:

In Haber process ammonia is produced by the reaction of nitrogen and hydrogen in presence of iron catalyst.

Reaction takes place in three steps in Haber’s process :
Step-I N₂ + H₂ → N₂H₂
Step-II N₂H₂ + H₂ → N₂H₄
Step-III N₂H₄ + H₂ → 2NH₃
Dissociation of nitrogen triple is very difficult and highly endothermic so the reaction does not proceed easily without using catalyst. In first step N₂H₂ is produced which is very unstable so it dissociates as soon as it is produced.
H₂ and N₂ get adsorbed on surface of iron. H₂ immediately dissociates into H-atoms. However N₂ takes time to dissociate into N-atoms and nitrogen atoms then lock into the surface of catalyst.
H-atoms migrate across the surface of catalyst and reach to nitrogen atoms and react with them producing ammonia molecules that still attached with surface of catalyst. The ammonia molecules then detach from the surface of catalyst in the form of gas.
Iron as catalyst in reaction of Persulphate ions and iodide ions:

Persulphate ion is a powerful oxidant. Iron (II) ions and Iron (III) ions act as catalysts in the reaction of persulphate and iodide ions. Fe2+ or Fe3+
The reaction can be given as:

S2O82- + 2I- 2SO42- + I2

When Fe2+ act as catalyst:
S2O82- + 2Fe2+ → 2SO42- + 2Fe3+
2Fe3+ + 2 I- → 2Fe2+ + I2
When Fe3+ act as catalyst:
2Fe3+ + 2 I- → 2Fe2+ + I2
S2O82- + 2Fe2+ → 2SO42- + 2Fe3+

Reactions of hexa aqua iron (II) ions and hexa aqua iron (III) ions:

Reaction with water:

Reaction of both ions hexa aqua iron (II) ions and hexa aqua iron (III) ions with water produce acidic solution having pH less than 7. These are not oxidation reduction reactions as oxidation state of iron remains same.
[Fe (H2O)6]2+(aq) + H2O(l) → [Fe (H2O)5(OH) -]+(aq) + H3O+
[Fe (H2O)6]3+(aq) + H2O(l) → [Fe (H2O)5(OH) -]2+(aq) + H3O+
Fe3+ (ferric) ions give more acidic solution than Fe2+ (ferrous) ions

Reaction with alkaline solution:

Reaction of hexa aqua iron (III) ions with alkali produces precipitate of trihydroxo triaqua iron (III). This is not oxidation reduction reaction as oxidation state of iron remains same.
[Fe (H2O)6]3+(aq) + 3OH-(aq) → Fe (H2O)3(OH) -3 + 3H2O(l)
While hexa aqua iron (II) does not interact with alkali as reaction is energetically favourable.

Reaction with ammonia:

In these reactions ammonia removes protons (H+) from hexa aqua complex. These are not oxidation reduction reactions as oxidation state of iron remains same.
[Fe (H2O)6]2+(aq) + 2NH3 → Fe (H2O)4(OH) -2 + 2NH4+
Orange
[Fe (H2O)6]3+(aq) + 3NH3 → Fe (H2O)3(OH) -3 + 3NH4+
Brown
Reaction with carbonate ions:

Hexa aqua iron (II) ions on reaction with carbonate ions produce precipitate of iron(II) carbonate. As hexa aqua iron (II) ions are not strongly enough acidic so they cannot liberate CO2 gas from carbonate and simply produce precipitate of iron (II) carbonate.

[Fe (H2O)6]2+(aq) + CO32-(aq) → FeCO3(s ) + 6H2O
Hexa aqua iron (III) ions being more acidic, they on reaction with carbonate ions produce precipitate of trihydroxo triaqua iron (III) along with liberation of CO2 gas.

2[Fe (H2O)6]2+(aq) + 3CO32-(aq) → 2Fe (H2O)4(OH) -2 + 3CO2 + 3H2O

Reaction with thiocyanate ions (identification test for iron (III) ions):

Hexa aqua iron (III) ions on reaction with NaSCN or KSCN or NH4SCN, produce blood red colour solution. Thus this reaction could be used as identification test for Iron (III) ions.

[Fe (H2O)6]3+(aq) + SCN- → [Fe (H2O)5(SCN) -]2+(aq) + H2O
Deep blood red colour

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