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Chemistry

Metals

Metals

Physical Properties of Metals

The physical properties of elements are dependent on

  • the arrangement of their atoms or molecules in crystal lattices when in solid state
  • the bonds that bind the atoms or molecules in the solid, liquid or gaseous state.

Most metals are solid at room temperature and exist as crystal lattices in which their atoms are held together by strong metallic bonds. metals have the following physical properties

  1. High melting and boiling points
  2. Characteristic lustre
  3. Malleability – can be hammered into sheets
  4. Ductility – can be drawn into a thin wire
  5. Sonorousity – give off a note when hit
  6. Hard but not brittle with great tensile strength
  7. Relatively high densities
  8. Good conductors of heat and electricity

Some metals do not exhibit all the above properties e.g

Mercury is a liquid with a melting point of -39oC. Sodium and potassium are light, soft metals with low melting points of 97oC and 63oC respectively.

Chemical Properties

  1. ionization behaviour – metallic ions have few valence electrons and so have a great tendency to form positive ions by losing electrons. i.e. they are electropositive
  2. reducing and oxidizing agents – metals are reducing agents because they donate electrons readily during chemical reactions.
  3. reaction with acids – a metal is more electropositive than hydrogen readily displaces the hydrogen ion from an acid. This is a redox reaction with the metallic ions donating electrons to form metallic ions and the hydrogen ions accepting electrons to form gaseous hydrogen.
  4. nature of oxides – most metals react with oxygen to form basic oxides which are mainly ionic compounds.Soluble basic oxides form alkalis. Some metals like aluminium and zinc form amphoteric oxides.

Occurrence of Metals

Element which have low chemical reactivity generally occur native or free or metallic state. Eg. Au, Pt, noble gas etc. element which are chemically reactive, generally occur in the combined state. Eg. Halogen, chalcogens etc. the natural materials in which the metals occur in the earth are called minerals. The minerals from which the metals is conveniently and economically extracted is called an ore. All the ores are minerals but all ores cannot be ores. Ores may be divided into four groups.

  • Metallic core (siderophile) of the earth crust contains (Mn, Fe, Co, Ni, Cu, Ru, Rb, Pd, Ag, Re, Os, Ir, Pt, Au). Entire composition of metals in the earth crust may be given as, Au(8.3%); Ca(3.6%); Na(2.8%); K(2.6%); Mg(2.1%); Ti(0.4%); Mn(0.1%); Fe(5.1%) other metals (0.1%).
  • Native ores: These ores contains metals in free state, e.g. silver, gold, platinum, mercury, copper, etc. These are found usually associated with rock or alluvial materials like clay, sand, etc. sometime lumps of pure metals are also found. These are termed nuggets. Irons is found in free state are meteorites which also have 20 to 30% nickel.
  • Sulphurised and arsenical ores: these ores consist of sulphides and arsenides in simple and complex forms of metals. Some or ores are:
MetalName of the oreCompositions 
PbGalenaPbs
ZnZinc blenderZns
AgCinnabarHgs
Feiron pyritesFes2
NiKufer nickelNiAs
CuCopper pyritesCu2s

III. Oxidized ores: In these ores, metals are present as their oxides or oxysalts such as carbonates, nitrates, sulphates, phosphates, silicates, etc.

Important ores of this groups are listed below,

Oxides

Haemalite                                            Fe2O3

Magnetite                                            Fe2O4

Limonite                                              Fe2O3.3H2O

Bauxite                                                Al2O3.2H2O

Corundum                                            Al2O3

Diaspore                                              Al2O3.H2O

Chromite                                             FeO.Cr2O3

Chromeochre                                       FeO.Cr2O3

Tinstone (Cassiterite)                           Cr2O3

Chrysoberyl                                         BeO.Al2O3

Cuprite (Rubby copper)                       Cu2O

Pyrolusite                                            MnO2

Zincite                                                 Zno

Rutile                                                  TiO2

Llmenite                                              FeO.TIO2

Carbonates

Magnesite                                            MgcO3

Lime stone                                          CaCO3

Dolomite                                             CaCO3.MgCO3

Calamine                                             ZnCO3

Malachite                                            CuCO3.Cu(OH)2

Azurite                                               Cu(OH)2.2CuCO3

Cerussite                                             PbCO3

Siderite                                               FeCO3

Nitrates

Chile saltpeter                                     NaNO3

Salt petre                                             KNO3

Sulphates

Epsom salt                                           MgSO4.7H2O

Barites                                                 BasO4

Gypsum                                               CaSO4.2H2O

Glauber’s salt                                      Na2SO4.10H2O

Anglesite                                             PbSO4

Schonite                                              K2SO4.MgSO4.6H2O

Polyhalite                                            K2SO4.MgSO4.CaSO$.2H2O

Phosphates and Silicates

Lepidolite                                            (Li, Na, K)2Al2(SiO3)(F,OH)2

Petalite                                                            liAl(Si2O5)2

Triphylite                                             (Li, Na)3PO4,(Fe, Mn)3(PO4)2

  • Halide ores : Metallic halides are few in nature, chlorides are most common . for example,

Comman salt NaCl.

Extraction of Metals

General Principles

Metals found in combined forms exist as positive ions.  During extraction, the metallic ions can be reduced to their corresponding metal atoms. This can be done electrolytically or by chemical and thermal methods. The method chosen depends on the stability of the ore which in turn depends on the position of the metal in the activity series.

  1. Mining of ore containing rock

The composition of rock around the world varies greatly and locations with metal bearing ore have been sought ever since man was able to extract metals. Nowadays the search is still going on for important deposits of rock with high percentages of the mineral in question. This search is now taking place under the sea and in other inhospitable environments.

Recently, for example, rock containing an appreciable percentage of rare earth elements has been discovered under the pacific ocean. This is a particularly important discovery as virtually 99% of known working deposits are in china and rare earths are essential in the manufacture of the strong neodymium magnets needed for the computer industry

  1. Separation, purification or preparation of useful ore

Very few metal ores occur in a pure enough form to be used directly in the extraction process. The first stage is to separate the useful ore from the rock. This may not be necessary in some cases, for example, the extraction of iron, but essential in the extraction of aluminium.

This separation may be physical, such as floatation, or chemical such as digestion of the required compound in a strong base or acid followed by re-precipitation and filtration.

Most ores are either oxides or sulfides. The sulfides are usually converted to oxides by roasting in air. This tends to release sulfur in the form of sulfur(IV) oxide, a pollutant and acidic gas. However, it is also a useful gas in that it is used for the manufacture of sulfuric acid by the contact process.

  1. Extraction of metal from ore

Metals are all electropositive and need to be reduced to become metallic elements. Hence, all extraction processes use reduction. For the less reactive metals chemical reduction suffices, but for the more reactive metals electrochemical reduction is needed.

  1. Purification of metal

Metals that are extracted by reductive processes usually need to be further processed to make them industrially useful.

Uses of Metals

Metals are very useful to people. They are used to make tools because they can be strong and easy to shape. Iron and steel have been used to make bridges, buildings, or ships.

Some metals are used to make items like coins because they are hard and will not wear away quickly. For example copper (which is shiny and red in color), aluminium (which is shiny and white), gold (which is yellow and shiny), and silver and nickel (also white and shiny).

Some metals, like steel, can be made sharp and stay sharp, so they can be used to make knives, axes or razors.

Rare metals with high value, like gold, silver and platinum are often used to make jewellery. Metals are also used to make fasteners and screws. Pots used for cooking can be made from copper, aluminium, steel or iron. Lead is very heavy and dense and can be used as ballast in boats to stop them from turning over, or to protect people from ionizing radiation.

ASSESSMENT (POST ANSWERS BELOW USING THE BOX)

  1. Most metals exist in nature as
    a. crusts
    b. alloys
    c. ores
    d. felspar
  2. Most metals are malleable with high densities and have high boiling points except
    a. Zn
    b. K
    c. Sn
    d. Ca
  3. Method adopted in extracting a particular metal from its ore depends on
    a. the fragile nature of the metal
    b. the location of the ore in the earth’s crust
    c. the stability of the ore which depends on the position of the ore in E.C.S
    d. the availability of power in the country
  4. Metals found in combined forms exist as ……… ions
    a. positive
    b. negative
    c.  neutral
    d. none of the above

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