Metals and Non-Metals - Part XIII

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Iron
Iron has chemical symbol Fe. Fe has many isotopes but Fe with mass 56 is most abundant. It has 26 protons and 30 neutrons. An atom of Fe is represented as 5626Fe. The symbol of iron is derived from its Latin name ferrum. Fe is one of the transition metals. In transition metals, electrons from few of the outer shells participate in chemical reactions. Iron is placed in the periodic table close to cobalt and nickel, as all of them show similar properties.

Since Fe has 26 protons, it also has 26 electrons. The electronic configuration of Fe is K-shell – 2 electrons, L-shell – 8 electrons, M-shell - 14 and N-shell has 2 electrons. Fe shows metallic properties and gives off its 2 electrons in the outermost shell easily to achieve a stable electronic configuration Fe2+. Sometimes Fe gives off 2 electrons from the N-shell and 1 electron from the M-shell to become Fe3+ (variable valance).  

Fe is a reactive metal and is not found in free state in nature. Fe ores are in the form of compounds of oxides, sulphides and carbonates. The ores found are :  

  • Hematite : Fe2O3

  • Magnetite : Fe3O4

  • Limonite : Fe2O3. H2O

  • Siderite : FeCO3

  • Pyrite : FeS2  

In India iron is mined from hematite ores found in Bihar, West Bengal and Karnataka. These ores are processed in factories in Jamshedpur, Bhilai, Rourkela and other places.

Extraction of Fe from hematite : The ore is concentrated by breaking the ore in smaller particles and washing in water. Sand, clay are the gangue particles which are removed. After this the ore is heated or calcined in insufficient air so that water is removed and  pure oxide of the metal remains behind. The metal-oxide is then reduced by addition of coke. Generally sand or silicon dioxide is present as an impurity. To remove this, limestone or CaCO3 is added to the ore along with coke. Limestone is able to reduce SiO2. The ore mixture is put in a blast furnace and hot air is blown from the bottom of the furnace.  

The coke is first converted to carbon mono-oxide or CO. It is CO that acts as the main reducing agent.  CO is formed in two steps in the blast furnace.  

Reduction of Fe (III) to Fe is achieved by the following reaction.  

Fe metal is a grayish liquid and flows down the blast furnace where it is collected.

The limestone or CaCO3 added in small quantities reacts in the blast furnace to become CaO. Limestone is called as flux. CaO reacts with SiO2 to form calcium silicate. This forms a fused or liquid glass like material called slag that collects at the bottom of the blast furnace. Slag floats on the molten pure iron and prevents re-oxidation of Fe.

Iron obtained from a blast furnace is called as pig iron. It has impurities such as S, C, Si or P. These can get oxidized more easily than the metal itself. When air is passed over hot molten pig iron, these non-metals get oxidized to CO2, SO2, P2O5 and can be removed easily. Thus pig iron can be purified by the oxidation method.

Physical properties of iron : Fe  is a grayish white metal. Its surface may appear brown due to rust.  Its density is high (7.86 gm/cc). Its melting point is 1539°C. Fe displays strong magnetic properties. Fe is a very ductile and malleable metal. It is a good conductor of heat and electricity.

Chemical properties of iron :
1. Valence : Fe shows variable valance. Since Fe has 2 electrons in its N-shell, it gives them off easily. Fe (II) (ferrous) has valence = 2+. Fe is able to show 3+ valence by emitting one electron from the M-shell. Fe (III) (ferric) has valence = 3 + .

2. Action of air : Fe is a reactive metal. But Fe does not react with air at ordinary temperatures. 
When heated strongly in oxygen, Fe forms Fe3O4.  

When heated in damp air, Fe forms Fe2O3 along with Fe(OH)3 which is rust.

3. Action of water : Fe does not react with water to form a hydroxide at ordinary temperatures. When steam is passed over red-hot Fe, iron  (II, III)  oxide and hydrogen are formed. The reaction is reversible.

4. Reaction with acids : Fe reacts with HCl to form Fe (II)-chloride and releases hydrogen.  

Fe reacts with dilute sulphuric acid to become ferrous sulphate, releasing hydrogen. With concentrated sulphuric acid, Fe reacts by releasing sulphur dioxide and water. (Conc H2SO4 is more oxidizing)  

When Fe fillings are added to dilute nitric acid, the following reaction occurs.  

With concentrated nitric acid, Fe forms a passive oxide layer Fe2O3 on the surface. This oxide layer prevents more reaction.

5. Action of alkalis : Pure Fe does not reacts with NaOH and KOH. It is unable to replace Na or K from the hydroxides.

6. Reaction with chlorine : Ferric chloride is formed when Fe is heated in chlorine atmosphere. The reaction is shown below.  

7. Reducing action :  Fe does not show any reducing action.

8. Reaction with sulphur : When iron fillings are heated with sulphur powder, FeS that is ferrous sulphate is formed. The reaction is shown below.  

9. Test for iron : If Fe is present as a ferrous (II) salt in a solution, it can be tested by pouring sodium hydroxide. A green precipitate of ferrous hydroxide separates out and sodium chloride solution remains behind. This is a practical test for the presence of Fe (II).  

Pouring sodium hydroxide can test if Fe is present as a ferric (III) salt also. A brown precipitate of ferric hydroxide separates out and sodium chloride solution remains behind. This is a practical test for the presence of Fe (III).  

Rust and prevention of rust : We have seen that Fe reacts with oxygen in air, in the presence of moisture to form ferric (III) oxide. This is rust. Thus rusting of iron is a slow oxidation process.  

For prevention of rust the following can be done :  

  • iron surface has to be coated with paint or lacquer so that the surface is not exposed to air and moisture.

  • Iron can be galvanized. A thin layer of Zn is applied on the iron surface. Zn itself gets oxidized and prevents Fe from the oxidation process.

  • Iron surface can be coated with another metal like Cr, tin, Ni or Al, which are more resistant to oxidation or corrosion.

  • Iron can be alloyed with Cr and Ni to obtain stainless steel that is completely rust proof.

  • Iron can be coated with other iron compounds that are already in an oxidized state; these materials can be Fe3O4 or FePO4.  

Uses of iron :

  • Fe is used for making stainless steel, which has wide applications in home and industries.

  • Fe is used for making magnets which has many used.

  • Fe is used for making alloys with which construction of bridges and houses are done.

  • Fe alloys find use in making of nuts and bolts.  

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