Occurance of Metals

Class 10; General Science
Chapter 3

1. The most unreactive metals which are not affected by air and water such as silver, gold and platinum are generally found in free state in nature Most metals however are found in combined state in the form of their oxide , carbonate or sulph ide etc. Minerals The naturally occurring compounds of metals along with other impurities are known as minerals Ores The minerals from which metals are extracted profitably and conveniently are called as ores

2. Gangue Ores contain metal compounds with some of the impurities like soil, sand, rocky material, etc. These impurities are called as gangue Metallurgy Metals can be extracted from the ores by employing different separation techniques. The process used for extraction of metals in their pure form from their ores is called metallurgy

3. Activity series K Na Ca Mg Al Zn Fe Pb Cu Hg Ag Au Decreasing order of reactivity Top ↓ ↓ ↓ ↓ ↓ Bo t t om The arrangement of metals in the decreasing order of their reactivity in the form of series is called the reactivity series of the metals Thus the most reactive metal is potassium and is placed in the top of the list and least reactive metal is gold which is placed at the bottom of the list

4. Depending on the Chemical Reactivity, Metals are divided into 4 categories. They are… 1. Highly Active Metals : They are extracted by Electrolysis. For Example, Sodium, Potassium, Calcium etc . 2. Moderately Active Metals : These metals are extracted by Reduction with Carbon. Example : Zinc, Iron etc 3. Lowly Active Metals : These metals can be extracted easily by heating. Example : Copper, Mercury etc 4. Noble or Inactive Metals : They are found in free state. Example : Gold, Silver etc

5. Extraction of M etals of H igh R eactivity Metals which are high up in the reactivity series are very reactive . For eg . sodium , potassium, calcium, aluminium, etc . are the metals o btained by electrolytic reduction Sodium, magnesium and calcium are obtained by electrolysis of their molten chlorides Sodium is extracted by electrolysis of fused sodium chloride by Down’s process. Down's electrolytic cell, consists of an iron container with a circular carbon anode. The anode is surrounded by a ring shaped iron cathode enclosed in a wire gauze which also acts as a partition and separates the two electrodes.

6. On electrolysis, chlorine is liberated at the anode and let out through an exit at the top. Sodium is liberated at the cathode and remains in the wire-gauze shell. Level of molten sodium rises and it overflows into a receiver. 2 NaCl   -- > 2Na +   + 2Cl - Anode Reaction: 2Cl - → Cl 2  + 2e - Cathode Reaction: 2Na +  + 2e - → 2Na

7. Extraction of metals of medium reactivity The metals in the middle of activity series such as iron, zinc, lead , etc . are moderately reactive . These are usually present as sulphides or carbonates in nature. It is easier to obtain a metal from its oxide, as compared to their sulphides and carbonates . The sulphide ores are first converted into oxides by heating strongly in excess of air . This process is known as roasting. An example of roasting is when Zinc sulphide is converted into zinc oxide by heating in presence of air. 2ZnS+3O 2  —> 2ZnO + CO 2

8. This process is known as roasting. The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination . The chemical reaction that takes place during roasting and calcination of zinc ores is: Roasting 2ZnS + 30 2 → 2ZnO + 2SO 2 Calcination ZnCO 3 → ZnO + C0 2 This zinc oxide is then reduced to zinc by using a suitable reducing agent such as carbon ZnO + C → Zn + CO On the other hand, the carbonate ores are changed to oxides by heating strongly in limited air. This process is known as calcination . The chemical reaction that takes place in calcination of the Carbonate ore of Zinc is: ZnCo 3  —> ZnO + CO 2 Similarly, Calcium Carbonate is converted to Calcium Oxide by calcination as follows… CaCo 3  —> CaO + CO 2

9. Zinc Oxide  is then mixed with Coke (Carbon) and ground into fine pieces and heated at 1673K in a fire clay containers.   Here the Zinc Oxide gets reduced to metallic Zinc. The carbon from coke acts as the reducing agent in this reaction. The reaction of the final step of extraction of zinc is 2ZnO + C −−> 2Zn + CO2

10. Calcination Roasting Calcination is a process in which ore is heated in the absence of air or in limited supply Roasting involves heating of ore lower than its melting point in the presence of air or oxygen. Calcination involves thermal decomposition of carbonate ores. Roasting is carried out mostly for sulfide minerals. During calcination , moisture is driven out from an ore. Roasting does not involve dehydrating an ore. Carbon dioxide is given out during calcination During roasting large amount of toxic, metallic and acidic compounds are released along with Sulpher dioxide gas Difference between Calcination & Roasting

11. Extraction of M etals of L ow R eactivity The metals at the bottom of the activity series are least reactive. They are often found in free state e.g. gold, silver and copper. But copper and silver are also found in combined state as their sulphide or oxide ores . For eg . copper is found as Cu 2 S in nature and can be obtained from its ore by just heating in air 2Cu 2 S + 3O 2 (H eat ) → 2Cu 2 O +2SO 2 2Cu 2 O + Cu 2 S ( H eat ) → 6Cu + SO 2

12. Similarly , cinnabar (HgS) is an ore of mercury. When it is heated in air, it is first converted into mercuric oxide (HgO ). Mercuric oxide is then reduced to mercury on further heating 2HgS + 3O 2 (H eat ) → 2HgO + 2SO 2 2HgO + heat → 2Hg + O 2

13. Refining of metals The metals produced by various reduction processes described above are not very pure. They contain impurities, which should be removed, to obtain pure metals. The most widely used method for refining impure metals is electrolytic refining For example, Cu (Copper) metal is purified (Refined) by electrolysis.

14. A block of impure copper is taken as an anode or positive electrode. Copper sulfate which is acidified with sulphuric acid is used as an electrolyte along with pure copper tubes as a cathode or negative electrode. In this phase of electrolysis copper sulphate divides into a positive ion of copper (Cu ++ ) and a negative ion of sulfate ( SO4 — — ). The positive copper ion (Cu ++ ) or cations travel towards the negative electrode made of pure copper where it absorbs the electrons from the cathode. Cu atom is deposited on the cathode

15. Refining of Copper

16. Corrosion of metals Corrosion is degradation of materials due to reaction with its environment. The major problem of corrosion occurs with iron, since it is used as a structural material in constructions, bridges, ship building, automobile etc. Example: When iron metal comes in contact with oxygen present in moist air, a brown coat deposited over it because of formation of iron oxide. This is called rusting of iron . When silver metal comes in contact with sulphur present in air, a black coat is deposited over it because of formation of silver sulphide. This is called corrosion or tarnishing of silver.

17. Prevention of corrosion Corrosion of metals can be prevented if the contact between metal and air is cut off. This is done in a number of ways. Some of the methods are given below: Corrosion can be prevented if the metal is coated with something which does not allow moisture and oxygen to react with it Coating of metals with paint, oil, grease or varnish prevents the corrosion of metals e. g rusting of iron can be prevented by this method

18. 3) Coating of corrosive metals with non-corrosive metals also prevents corrosion. Some of the methods by which metals can be coated with non-corrosive metals are: Galvanizing : It is process of giving a thin coating of zinc on iron or steel to protect them from corrosion. e.g. shiny iron nails, pins, etc. Tinning : It is the process of giving a coating of tin, i.e., molten tin over other metal. e.g. Cooking vessels made of copper and brass get a greenish coating due to corrosion. This greenish coating is poisonous. Therefore they are given a coating of tin to prevent corrosion

19. ALLOY An  alloy is a homogeneous mixture of two or more metallic  solutions . Sometimes, in an alloy non-metals can also be found.   The non-metals   present in an alloy may be boron, carbon, silicon or sulphur. Alloys are harder and tougher than the base metal and are resistant to corrosion . They are inert to commonly used chemicals and are magnetisable and ductile.

20. Name of the Alloy Composition Uses Brass Cu : 80% Zn : 20% In making of utensils, pipes and radiator statues etc Bronze Cu : 90% Sn : 10% In making of coins, ornaments, utensils and statues Stainless steel Fe : 82% ( Ni + Cr): 18 In making of surgical instruments, watches and utensils etc Magnalium Al : 95% Mg : 5% In making light articles and physical balance etc Duralumin Al : 95% Cu : 4% Mn : 0.5% In making parts of aeroplane and ship etc German silver Cu : 60% Zn : 20% Ni : 20% It is useful in electroplating and making of utensils Gun metal Cu : 88% Sn : 10% Zn : 2% It is useful in making of guns, machine parts and canons..etc Solder metal Pb : 50% Sn : 50% It is mainly useful to join electric wires 

21. In an alloy if one of the metals is mercury, then the alloy is known as an amalgam Pure gold, known as 24 carat gold and it is very soft. It is, therefore, not suitable for making jewellery . It is alloyed with either silver or copper to make it hard . Generally , in India 22 carat gold is used for making ornaments. It means that 22 parts of pure gold is alloyed with 2 parts of either copper or silver

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