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Iron and its Properties

Iron is commonly found in the earth crust and is the second most abundant metal after aluminium. It is not found in the free state, except in meteorites.

The following are the most common iron ores: haematite (Fe₂O₃); magnetite (Fe₃O₄); iron pyrite (FeS) and siderile or spatic iron ore (FeCO₃).

Iron is also found in living matter (as a constituent of chlorophyll and hemoglobin, and as silicates(IV) in clay soil.

Extraction of Iron from Sulphide and Oxide Ores

Firstly, the iron ore is roasted in air to produce iron(III) oxide, Fe₂O₃, which is mixed with coke and limestone and heated to a very high temperature in a blast furnace. A blast of hot air is introduced into the furnace from its bottom.

The following processes take place in the course of the extraction: (a). The white hot coke burns in the hot air it comes in contact with, to form CO₂.

Great heat is released. C_(s) + O_2(g) → CO_2(g)

(b). The CO₂ formed rises upward and is reduced by the white hot coke it gets in contact with, to produce CO.

CO_2(g) + C_(s) → 2CO_(g)

(c). The CO at the prevailing high temperature (about 1000^oC) reduces the Fe₂O₃ to metallic iron and produces CO₂. 

Fe2O3(s) + 3CO(g)

2Fe(s) + 3CO2(g)

Importance of the limestone added - the limestone is decomposed at the high temperature of reaction to CaO.

CaCO_3(s) → CaO_(s) + CO_2(g)

The CaO removes acidic earthly impurities such as silicon(IV) oxide, SiO₂ and aluminium oxide, Al₂O₃ as calcium silicate and calcium aluminate respectively.

SiO_2(s) + CaO_(s) → CaSiO_3(s)

Al₂O_3(s) + CaO_(s) → CaAl₂O_4(s)

The CaSiO₃ and CaAl₂O₄ and other earthly impurities are removed as Slag. Slag can be used in making roads, and as fertilizer.

The waste gaseous mixtures out of the furnace contains mainly nitrogen, together with CO and CO₂.

Types of Iron

1. Pig Iron - this is the iron obtained directly from the blast furnace. It is very impure and contains the impurities: carbon (up to 5%), phosphorus, sulphur, silicon and manganese in different proportions depending on the ores from which it was extracted, and the operating temperature.

Properties: the melting point of the iron is reduced from 1530^oC to about 1200^oC by the presence of the impurities. it is hard and brittle - it is therefore not much of use in industry.

2. Cast Iron - this is obtained from remelting pig iron with some scrap iron and then recooled. It also contains similar impurities as the pig iron, but in lower quantity, hence, it has similar physical properties as pig iron.

It is used for small casting’s, such as fire - grates, railings, hot-water pipes, bunsen burner bases and for many other purposes where little strain is imposed.

3. Wrought Iron - this is the purest form of iron used commercially (containing only about 0.1% of carbon). It is obtained by heating cast iron with iron(III) oxide in a furnace.

The impurities (carbon and sulphur) are oxidized to their gaseous oxides by the oxygen of the iron oxide. Other impurities, such as phosphorus and silicon(IV) oxide are oxidized to tetraoxophosphate(V) and tetraoxosilicate(IV) respectively which form slag.

The resulting iron is almost pure.

Properties: wrought iron has a higher melting point than cast iron and: it is malleable, can be forged, hammered and welded when hot.

It is tough, fibrous and can withstand some strain. It is not elastic and will bend if subjected to great strain. It can not be tempered (i.e. heating and cooling to the required condition).

It is used to make nails, ornament work and the core of electromagnets (it is not permanently magnetized).

Pure iron shows the following properties:

Physical Properties

1. Appears as white solid with lustre.

2. It has a density of 7.9 g/cm³.

3. It is very malleable and ductile.

4. It has high tensile strength.

5. It has melting point of 1530^oC.

6. It’s a good conductor of heat and electricity.

7. It can easily be magnetized.

Chemical Properties

1. With air

When iron is exposed to moist air, it gradually corrodes (rusts) with the formation of hydrated iron(III).

4Fe_(s) + 3O_2(g) + xH₂O_(l) → 2Fe₂O₃ . xH₂O_(s)

The rust is Fe₂O₃. xH₂O

Note: *The rust formed is porous and does not prevent the inner layer of the iron from rusting too. This is different from the oxides of Zn and Al formed when the metals are exposed to air (these oxides on the metal surfaces, are not porous, hence, they prevent the inner layer of the respective metal from being corroded).

*Reduced or finely divided iron (iron fillings) when heated in air will combine with oxygen to form iron(II)diiron(III) oxide (also known as the magnetic oxide (Fe₃O₄).3Fe_(s) + 2O_2(g) → Fe₃O_4(s)

2. With steam

Iron reacts while red-hot with steam to produce iron(II)diiron(III) oxide and hydrogen. The reaction is reversible.

3Fe_(s) + 4H₂O_(g)  Fe₃O_4(s) + 4H_2(g)

3. With non-metals

Iron, when heated will readily combine directly with such non-metals as carbon, sulphur, phosphorus and chlorine.

2Fe_(s) + 3Cl_2(g) → 2FeCl_3(s)

Fe_(s) + S_(s) → FeS_(s)

Note: the above reaction is not possible with nitrogen.

4. With acids

With dilute HCl and H₂SO₄, iron dissolves to form the corresponding salts and hydrogen.

Fe_(s) + 2HCl_(aq) → FeCl_2(aq) + H_2(g)

Fe_(s) + H₂SO_4(aq) → FeSO_4(aq) + H_2(g)

Note:

*With dilute HNO₃, various products are possibly formed. These include iron(II) nitrate(V), ammonium nitrate(V), the oxides of nitrogen, and even ammonia.

*With Conc. HNO₃, iron will not react (i.e. it is passive). In this passive state (i.e. iron dipped into conc. HNO₃), iron no longer behaves as a piece of ordinary iron, that is, it will no longer displace copper from CuSO₄ solution nor will it be attacked by dilute HNO₃, which would normally attack it.

This passive state is believed to be due to the formation of a protective layer of Fe₃O₄ on the surface of the iron. However, if the iron is scratched, revealing the inside, and it is in the presence of dilute HNO₃, there will be a reaction.

Therefore, iron containers can be used to store conc. HNO₃.

Rusting of Iron and Protection of Iron from Corrosion:

See details here: Removal of Rust from Iron.