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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) + O2(g) → CO2(g)

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

CO2(g) + C(s) → 2CO(g)

(c). The CO at the prevailing high temperature (about 1000oC) 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.

CaCO3(s) → CaO(s) + CO2(g)

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

SiO2(s) + CaO(s) → CaSiO3(s)

Al2O3(s) + CaO(s) → CaAl2O4(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 1530oC to about 1200oC 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).

Properties of pure Iron (Physical and Chemical)

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 1530oC.

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) + 3O2(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) + 2O2(g) → Fe3O4(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)  Fe3O4(s) + 4H2(g)

3. With non-metals

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

2Fe(s) + 3Cl2(g) → 2FeCl3(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) → FeCl2(aq) + H2(g)

Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(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.

Steel

Steel is produced from iron by a process which involves the removal of all impurities from molten pig iron by oxidation, and the addition of known quantities of carbon and other elements (depending on the type of steel desired).

Ordinary steel is mainly a mixture of iron and about 0.1 - 1.5% carbon, while alloy steel contains certain other elements, such as nickel and chromium in different proportions, depending on the properties of the steel desired.

Advantage of Steel over Iron

(1). Steel can be tempered. This means steel can be repeatedly heated and cooled to attain the desired properties (i.e. the desired hardness, toughness, strength and elasticity). These properties are greater in steel than in iron.

(2). Various kinds of steel can be made by adding different elements in different proportions to meet certain desired properties.

Example, stainless steel is made by adding chromium and nickel to ordinary steel - stainless steel is resistant to rusting (unlike iron).

(3). Steel is harder and tougher than iron. For example, tungsten steel is a very hard steel used for making cutting and drilling tools; manganese steel is also very hard and tough and is used in rock drills.

(4). Molybdenum steel is resistant to acid while iron is attacked by acids.

(5). Steel gives an attractive surface, iron does not.

Test for Iron(II) Ions, Fe2+

The presence of iron(II) can be identified and confirmed using any of the following tests:

1. With NaOH Solution - a dirty green precipitate is formed if a few drops of NaOH solution is added to a solution of iron(II) salt.

The precipitate is insoluble in excess NaOH. If exposed to air, the precipitate will gradually turn brown.

Note: *The dirty green precipitate is iron(II) hydroxide.

Fe2+(aq) + 2NaOH(aq) → Fe(OH)2(s) + 2Na+(aq)

*On exposure to air, the iron(II) is oxidized by the oxygen of the air to iron(III)

4Fe(OH)2(s)+2H2O(l) + O2(g) → 4Fe(OH)3(aq)

*The solution of common soluble iron(II) salts is usually greenish in colour.

*With aqueous ammonia, rather than with NaOH, the same observation as above will be obtained.

2. With Potassium hexacyanoferrate(III) solution, K3Fe(CN)6 - addition of few drops of K3Fe(CN)6 solution to a solution of iro(II) salt produces a deep blue precipitate.

Fe2+(aq) + K3Fe(CN)6(aq) → KFe[Fe(CN)6](s) + 2K+(aq)

Test for Iron(III) Ions, Fe3+

The following tests will confirm the presence of Fe3+:

1. With NaOH - a reddish brown or yellow precipitate is formed when a few drops of NaOH solution is added unto a solution of iron(III) salt. The precipitate is insoluble in excess NaOH.

Fe3+(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3Na+(aq)

Note:

*The reddish brown or yellow precipitate is iron(III) hydroxide.

* Similar observation as above will be obtained if aqueous ammonia solution is used instead of NaOH.

* Iron(III) salts solutions are usually yellow or reddish brown in colour.

2. With potassium hexacyanoferrate(II) solution, K4Fe(CN)6 - a blue precipitate is formed when a few drops of K4Fe(CN)6 is added to a solution of iron(III) salt.

Fe3+(aq) + K4Fe(CN)6(aq) → KFe[Fe(CN)6](s) + 3K+(aq)

3. With potassium thiocynate solution, KSCN.

A blood-red precipitate is formed when a few drops of KSCN solution is added to a solution of iron(III) salt.

Fe3+(aq) + 3KSCN(aq) → Fe(SCN)3(aq) + 3K+(aq)

Rusting of Iron and Protection of Iron from Corrosion:

See details here: Removal of Rust from Iron.
 

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