10th Class Chapter- Metals and Non-Metals
Occurrence of Metals
Maximum
metals occur in earth’s crust and some metal occur in sea water. Metals and its
compounds exist as minerals and if the percentages of metals in minerals are
large then they are known as ores.
1. Extraction of Metals
Reactivity series is very helpful in
metal extraction as metals present at the bottom of reactivity series are least
reactive so found in Free State like gold, silver and platinum found in Free-State.
Metal at top is most reactive and metals in the middle are also reactive so
found in combined form. Metals generally found as oxides, sulphides and
carbonates on earth’s crust.
Steps involved in extraction of metals from ores-
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Steps involved in Extraction of Metals from Ores |
2. Enrichment of Ore
The process
of removal of impurities or gangue from ore, before extraction of metal is
known as enrichment of ore.
Gangue is terminology used for impurities like sand,
soil etc. present in ore.
3. Extracting Metals Low in Reactivity Series (or Activity Series)
Metals present at bottom (or low
position) in activity series are very unreactive and can be obtained in pure
metallic form by just heating alone.
Example- Cinnabar (HgS), ore of Mercury (Hg)
2HgS (s) + 3O2 (g) + heat ---------> 2HgO (s) + 2SO2 (g)
2HgO (s) + Heat ------------> 2Hg (l) + O2 (g)
Similarly,
Cu2S, ore of copper (Cu)
2Cu2S + 3O2 (g)
+ Heat ---------> 2Cu2O (s) + 2SO2 (g)
2Cu2O + Cu2S + Heat ----------> 6Cu (s) + SO2 (g)
4. Extracting Metals in Middle of Activity Series
Metals in
middle like iron, zinc, lead etc. are moderately reactive and present as
sulphides or carbonates. Metals can be easily extracted from its oxides so
sulphides and carbonates are reduced to oxides. Then these metal oxides are
reduced to corresponding metal by using suitable reducing agent like carbon.
Roasting
It
is a process of converting sulphide ores into oxides by heating strongly in the
presence of excess air.
2ZnS (s) + 3O2 (g) + Heat
--------> 2ZnO (s) + 2SO2
(g)
Calcination
It is a process of converting carbonates
ores into oxides by heating strongly in the presence of limited air.
ZnCO3 (s) + Heat ----------> ZnO (s) +CO2 (g)
Oxide Reduction
Oxides of ores are reduced to metal by
using suitable reducing agent like carbon (Coke), or highly reactive metals.
ZnO (s) + C (s) ---------> Zn (s) + CO (s)
3MnO2 (s) + Al (s) -----------> 3Mn (l) + Al2O3 (s) + Heat
5. Extracting Metals at Top of Activity Series
Metals present at top in
activity series are very reactive and they are not obtained by heating their
compounds with carbon, for example Sodium, Calcium, Magnesium, Aluminium etc.
cannot be obtained by reducing with carbon as these metals have more affinity
for oxygen than carbon. So these metals are obtained by electrolytic reduction.
In
electrolytic reduction, the metals get deposited at cathode (-ve electrode) and
gas like chlorine get liberated at anode (+ve electrode)
Reaction for molten Sodium Chloride-
At Cathode :-
Na+ + e- -------> Na
At Anode :-
2Cl- -------> Cl2 + 2e-
Refining of Metals
Refining of metals are done to obtain
metals in very pure form by removing impurities present in it. Electrolytic
refining is widely used method for this purpose.
Electrolytic Refining
Electrolytic
refining is the method of obtaining very pure metals from impure metal. Metals
like copper, zinc, nickel, silver, tin, gold etc. are refined electrolytically.
In electrolytic refining, anode (+ve) is made from
impure metal and cathode (-ve) is made from thin strip of pure metal. Metal
salt solution works as an electrolyte. When we applied electric current across
the electrodes then current starts flow through electrolytic solution. Pure
metal comes out from anode and dissolve in electrolyte and equivalent amount
(i.e. to that comes from anode) of this pure metal from electrolyte solution
get deposited on cathode.
“In simple way we can say that pure metal come from
anode and get deposited on cathode by using electrolyte solution and electric
current.”
Insoluble impurities settle down below anode at bottom
and we say it as anode mud, while soluble impurities mix in electrolyte.
Corrosion
Natural
process of conversion of refined metal to its high stable form like oxides or
hydroxides of metals is known as corrosion. Corrosion is the process of gradual
destruction of any material like metals by environment and chemical reaction.
Example - Rusting of Iron
Prevention of Corrosion
There
are so many methods to prevent corrosion like-
1. Applied Coating
Applied
coating is surface treatment method. Planting, enamel application and painting
are applied coating method to prevent corrosion. These methods create barrier
between metal and environment.
2. Anodization
It
is anode surface treatment process in which we made thicker oxide layer at
metal surface.
3. Galvanization
Galvanization
is the process of coating steel and iron with very thin layer of zinc to
protect them from rusting.
Overall painting, greasing, oiling, chrome plating,
galvanizing, alloy making and anodizing are some ways for the prevention of
corrosion.
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