why oxdes of very reactive metals cannot be reduced with carbon ? how these metals are obtained from their ores?

The oxides of very reactive metals cannot be reduced  with carbon. This is because these metals have more affinity (more attraction) for oxygen than carbon. so carbon is unable to remove oxygen from these metal oxides and hence cannot convert them into free metals. The highly reactive metals are extracted by the electrolytic reduction of their molten chloride.

For example, sodium, magnesium and calcium are obtained by the electrolysis of their molten chlorides. The metals are deposited at the cathode (the negatively charged electrode), whereas, chlorine is liberated at the anode (the positively charged electrode).

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Conversion of ore into metal oxide
Ores concentrated by any of the above processes are still in their original oxide, carbonate, sulphide or halide forms. To obtain pure metals from these chemicals, it is better convert the metal-compounds into oxides. Oxides of metals can be reduced easily and pure metal can be obtained. To convert the concentrated ores into metal oxides, two processes are used. These processes are calcination (heating in inadequate quantity of air ) and roasting the ore (heating in adequate quantity of air).

i) Calcination : Carbonate ores are heated in absence of air. The absence of air and heat converts the CO3 into CO2 and O. The O remains with the metal as metal-oxide. Heating also expels any water content in the ore. In case these are any volatile impurities or gases trapped in the ore, they are also removed by heating.
Example of how calamine ore or zinc carbonate is converted to ZnO by calcination is shown below. 

Calcination can be done for ores containing carbonates.

ii) Roasting : Sulphide ores are roasted or heated in plenty of air. The sulphide S changes to sulphur dioxide. The metal reacts with oxygen in the air to become a metal-oxide. Heating removes gaseous and other volatile impurities.
Example below shows how zinc blende ore (zinc sulphide) is converted to ZnO by roasting. 

Other types of ores, namely metal-oxide ores and metal-chloride ores remain unaffected by calcination and roasting processes. On treating with heat, impurities and water are removed from these ores.

Reduction of metal-oxide
Reduction process is used for converting metal-oxides into metal. Metal-chloride can also be reduced directly. The reduction reaction chosen depends on the chemical reaction or reactivity of metals. Generally reduction by heat, chemical reduction or electrolytic reduction processes are utilized.

i) Reduction by heat : Metals that are unreactive, like Hg, can be reduced from their ores by heating them. Mercury ore cinnabar is actually mercury sulphide. This can be heated at 300°C so that S is removed as SO2 and HgO is obtained. Hg is a very unreactive metal. HgO dissociates into Hg and oxygen soon. The reaction is shown below. 

It is interesting to note that for the cinnabar ore, roasting and reduction processes go on one after another.

ii) Chemical reduction : Various reducing agents are used for different metal-oxides to obtain free metals. Carbon, Al, Na, Ca are some reducing agents that are put in use.

1) Reduction by carbon : Oxides of Zn, Fe, Ni, Sn, Pb are reduced by heating them with carbon. Metal-oxide is mixed with coke, a source of carbon, and heated in a furnace. Carbon reacts with oxygen and free metal is obtained. Example below shows how Zn is obtained from ZnO on reduction with coke.

Reduction by carbon cannot be done for more reactive metals like Mn, Al, Cr, etc. Cu-oxide can be reduced by coke, but Ca-oxide cannot be reduced by coke.

2) Reduction by Al : This process is called as the Thermite process. Al is more reactive than carbon. Some metal-oxides that cannot be reduced by coke are reduced by Al. Al itself attracts oxygen from the metal-oxide and becomes aluminium oxide, and this frees the metal. Mn and Cr metals oxides are extracted and reduced by Al. Example below shows what happens when manganese dioxide is heated with aluminium powder.

3) Reduction by electrolysis : Highly reactive metal-oxides and metal-chlorides are not easy to be reduced by chemical reactions. Metals such as Na, K, Mn, Ca have to be freed from their ores by electrolytic processes. These metals are so reactive that they themselves are powerful reducing agents. Molten metal-oxides or chlorides form the electrolyte in the electrolytic cell. The cathode of the cell provides the electrons needed for the metal to free itself from the metal-oxide or metal-chloride bonds.

Al2O3 is reduced at the cathode of an electrolytic cell as shown below. Al2O3 is melted and forms the electrolyte. Free Al++ ions are attracted to the negatively charged cathode. Al++ is reduced by supply of electrons at the cathode. 

Chlorides of metals like Na, Mg are melted to form electrolytes. The reactions are shown below. 

Chlorine gas is liberated at the anode. The electrolysis has be done with molten metal ores and not aqueous solutions because these metals are highly reactive and will react with water to give hydroxides and not pure metals. In the electrolysis, metals atoms get deposited on the cathode electrodes which then have to carefully removed and stored. 


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