3. METALS AND NON-METALS
OCCURRENCE OF METALS
- The earth’s crust is the major source of metals.
- Seawater also contains soluble salts such as sodium chloride, magnesium chloride, etc.
- The elements or compounds which occur naturally in the earth’s crust are called minerals.
- The minerals that contain a very high percentage of a particular metal are called ores. Metal can be profitably extracted from it.
Extraction of Metals
- Metals are found in the earth’s crust in the free state or as compounds.
- Based on the reactivity, metals are 3 types:
a) Metals of low reactivity: The metals at the bottom of the activity series. They are found in a free state. E.g. gold, silver, platinum and copper. Cu & Ag are also found in the combined state as sulphide or oxide ores.
c) Metals of high reactivity: Metals at the top of activity series (K, Na, Ca, Mg & Al). They are never found in nature in free state.
- Ores of many metals are oxides. This is because oxygen is very reactive and is very abundant on the earth.
- Different techniques are used to obtain metals in each category.
- There are several steps in the extraction of pure metal from ores. They are given below:
Enrichment of Ores
- Ores are usually contaminated with large amounts of impurities such as soil, sand, etc., called gangue.
- There are different separation techniques to remove the gangue based on the differences between physical or chemical properties of gangue and ore.
Extracting Metals Low in the Activity Series
- The oxides of these metals can be reduced to metals by heating. E.g., cinnabar (HgS) is an ore of mercury. When it is heated in air, it is first converted to mercuric oxide (HgO). Then it is reduced to mercury on further heating.
- Cu2S (copper ore) on heating in air is reduced to copper.
Extracting Metals in the Middle of the Activity Series
- These are present as sulphides or carbonates in nature.
- It is easier to obtain a metal from its oxide, as compared to its sulphides & carbonates. So, the metal sulphides and carbonates are first converted into metal oxides.
- The sulphide ores are converted into oxides by heating strongly in presence of excess air. This is called roasting.
- The carbonate ores are changed into oxides by heating strongly in limited air. This is called calcination.
Roasting of Zinc ore:
Calcination of Zinc ore:
- The metal oxides are then reduced to the metals by using suitable reducing agents such as carbon. E.g., when zinc oxide is heated with carbon, it is reduced to zinc.
ZnO(s) + C(s) → Zn(s) + CO(g)
- Obtaining metals from their compounds is also a reduction process.
- 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. E.g., when manganese dioxide is heated with aluminium powder, following reaction takes place
3MnO2(s) + 4Al(s) → 3Mn(l) + 2Al2O3(s) + Heat
- Here, MnO2 is reduced and Aluminium is oxidised.
- These displacement reactions are highly exothermic. So the metals are produced in molten state.
- The reaction of iron(III) oxide (Fe2O3) with aluminium is used to join railway tracks or cracked machine parts. This reaction is called the thermit reaction.
Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) + Heat
Extracting Metals towards the Top of the Activity Series
- Highly reactive metals cannot be obtained from their compounds by heating with carbon. E.g., carbon cannot reduce the oxides of Na, Mg, Ca, Al, etc. This is because these metals have more affinity for oxygen than carbon.
- These metals are obtained by electrolytic reduction.
- E.g., Na, Mg and Ca are obtained by the electrolysis of their molten chlorides. The metals are deposited at the cathode (negatively charged electrode), whereas, chlorine is liberated at the anode (positively charged electrode). The reactions are
At cathode: Na+ + e– → Na
At anode: 2Cl– → Cl2 + 2e–
- Similarly, aluminium is obtained by the electrolytic reduction of aluminium oxide.
Refining of Metals
- The metals produced by reduction are not very pure.
- The most widely used method for refining impure metals is electrolytic refining.
- Metals such as Cu, Zn, Sn, Ni, Ag, Au, etc., are refined electrolytically.
- In this process, impure metal is made the anode and a thin strip of pure metal is made the cathode. A solution of the metal salt is used as an electrolyte.
- On passing the current through the electrolyte, the pure metal from the anode dissolves into the electrolyte. An equivalent amount of pure metal from the electrolyte is deposited on the cathode. The soluble impurities go into the solution, whereas, the insoluble impurities settle down at the bottom of the anode (anode mud).
Electrolytic refining of copper