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-
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)
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.
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)
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)
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 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.
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.
It is anode surface treatment process in which we made thicker oxide layer at metal surface.
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.