Sunday, August 14, 2011

Chemistry X: Extracting Metals in the Middle and Low of the Activity Series

Extracting metal low in activity series:- 
The metals lows in the reactive series are very less reactive. The oxides of such metals can be reduced to metals by heating alone e.g. mercury is obtained from its ore, cinnabar (HgS), by the process of heating.  
2HgS(s) + 3O2 -----Heat--------->  2HgO(s) + 2SO2(g)
2HgO(s)           -------------->   2Hg(l) + O2(g)
Copper can also be obtained in a similar manner from its sulphide ore (Cu2S).
 2Cu2S + 3O (g) --------Heat----------->  2u2O(s) + 2S2O (g)                           
2Cu2O + Cu2S ------- Heat ----------> 6Cu(s) + SO2 (g)

Extracting metals to  the middle  of activity series
Metals like iron, zinc, lead etc. occur as sulphides or carbonates in nature. First the ore is converted into its oxide form This is done by either roasting or by calcination. 
(a)  Roasting:  The sulphide ores are converted into oxides by heating strongly in the presence of excess air. This process is known as roasting.
2Zns(s) + 3O2 (g)---- heat --------> 2ZnO +2SO2 (g)
2Pbs (s) + 3O2 (g)---- heat --------> 2PbO +2SO2 (g)
4Fes (s) +7O2 (g)---- heat --------> 2Fe2O3 +4SO2 (g)
(b) Calcination:-   The carbonate ores are changed into oxides by heating strongly in limited air. This process is known as calcination.
Al2O3.2H20(s) -----> Al2O3(s) + H2O(i)
CaCO3   ---------> CaO + CO2
ZnCO3(s) -----heat--------> ZnO(s) +CO2(g) 
(c) SMELTING  : The metal oxide is reduced to metal by using suitable reducing agent such as Carbon.  For example, when zinc oxide is heated with carbon, it is reduced to metallic zinc.
ZnO(s) + C(s) → Zn(s) + CO(g)
Besides using carbon (coke) to reduce metal oxides to metals, sometimes displacement reactions can also be used. The highly reactive metals such as sodium, calcium, aluminium, etc., are used as reducing agents because they can displace metals of lower reactivity from their compounds.
For example, when manganese dioxide is heated with aluminium powder, the following reaction takes place –       3MnO2(s) + 4Al(s) → 3Mn(l) + 2Al2O3(s) + Heat
The reaction of iron(III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the thermit reaction. 
Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) + Heat
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