4. Occurence of Metals

OCCURANCE OF METALS

MINERALS :- The elements which occur naturally in the earth’s crust are called minerals.

ORES:- Minerals that contain very high percentage of particular metal andthe metal can be profitably extracted from it, such minerals are called ores.

• Metals at the bottom of the activity series are least reactive they are often found in free state . For Example –  Ag,Au, Cu.
• Metals at the ttop of the acitivity series (k,Na, Co, Mg, and Al) are so reactive that they never found in free state.
• Metals in the middle of the activity series ( Zn, Fe,Pb etc are moderately reactive . they occur as sulphates ,oxides or carbonates.
• They ore of many metals are oxide because oxygen is very reactive and is abundant on the earth.
• Steps involved in the extraction of pure metals from ores.

 Step involved in the extraction of metals from ores.

 EXTRACTION OF METALS FROM ORES :-

 1 ENCRICHMENT OF ORES    Ores are usually contaminated with large amounts of impurities such as soil, sand etc called gangue these impuriestes are removed from the ore prior to the extraction of mertal.

 2  Extraction of metals   Metals low in the activity series are very anreactive the oxides of these metals are reduced to metals by heating.

 For example           

 2HgO   →      2Hg  +  O₂

Mercury oxide is reduced to mercury on heating

The metals in the middle of the activity series ( Zn, Fe, Pb, Cu ) are moderately active. The metal sulphides  and carbontes  are converted into metal oxide. The sulphate ores are converted into oxides by heating strongly in the presence of excess air this process is known as roasting .

2 ZnS + 3O₂ →      2ZnO + CO₂

• The carbonate ores are changed into oxides by heating strongly in limited air this process is called calcination .

 Zn CO₃     →     ZnO + CO₂

• Then metal oxides are reduced to corresponding metals by using reducing agent like carbon.

 ZnO  +  C    →   Zn   +  CO

• This reaction of iron (iii) oxide ( Fe2O3) with aluminium is used to join railway tracks or cracked machine parts . this reaction is known as thermit reaction.

 Fe₂O₃    +  2Al    →  2 Fe +  Al₂O₃ + heat

 Metals high up in the reactivity series are very reactive. The metalo are obtained by electrolytic reduction. The metals are deposit at the cathode and chlore is deposited at anode.

At cathode  →   Na⁺    +  e^-    →     Na
At anode   →     2Cl^-   →    Cl₂   +  2e^-

Refining of metals

 The most widely used method for refining impure metals is electronic refining.

Electrolytic refining :-   Metals  ( Cu, Zn , Ag, Au etc ) are refined electrolytically . the impure metal is made the anode and athin strip of pure metal is made the cathode . a solution a metal salt is used as an electrolyte.

Electrolytic refining of copper.

Anode :  Impure copper

Cathode: Strip of pure copper

The insoluble impurities settle at the bottom of the anode and is called anode mud.