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Notes on Alkali Metal

170 / May-30-2021 12:40:10 By- srijan /

Characteristics of alkali metals:

1. Alkali metals are soft metals which shows lusture.
2. They are good conductor of heat and electricity.
3. They are monovalent.
4. They shows +1 oxidation states.
5. The atomic and ionic radii of alkali metals are largest of all the other elements in their respective period.
6. The outer electronic configuration is ns1. For eg: Li:- 1s2 2s1 and Na:- 1s2 2s22p6 3s1
7. They react with halogens to form their respective hallides.
2Na + Cl2 ------> 2NaCl
8. They are good reducing agent.
9. They react with water to give alkali and hydrogen.
2Na + 2H2O ------> 2NaOH + H2
10. They form cumalgam with mercury (Hg).
M + Hg ------> MHg (M=alkali metals)

Extraction of sodium

Sodium is a strong reducing agent and is very reactive. It cannot be extracted by reduction with carbon, aluminium etc.
It cannot be extracted by usual methods due to following difficulties:
i) Sodium is strong reducing agent. So, it cannot be extracted by reduction of their oxides.
ii) Sodium cannot be extracted by electrolysis of its aqueous solution because sodium forms sodium hydroxides immediately after its contact with water.
iii) Sodium cannot be prepare from the aqueous solution of its salts by metal displacement method because it violently reacts with water.
 

Extraction by Down's process:

Sodium is extracted by electrolysis of sodium chloride itself. In this method, fused sodium chloride is electrolysed by using
graphite anode and iron cathode. Both electrodes are separated by a wire guage so that sodium and chlorine formed are not able
to react with each other.



Sodium chloride melts at high temperature i.e. 820oC which is nearer to the boiling point of sodium (Na) 880oC.
Sodium chloride is mixed with calcium chloride (CaCl2) in the ratio of 2:3 which lowers the melting point of
sodium chloride from 820oC to 600oC. The main reason of lowering melting points are:
i) At high temperature, sodium metal may vapourize forming metallic fog.
ii) The melting point of NaCl is very high and is difficult to maintain that temperature.
iii) At this high temperature, the metal obtained will be in a colloidal state and its separation is difficult.
iv) Chlorine obtained at this high temperature after electrolysis is very reactive and corrode the vessel.
On passing electricity, chlorine is liberated at graphite anode and through the outlet it escape. Sodium metal is liberated
at cathode, which rises up through the vertical pipe and is collected in a receiver.
The chemical reactions involved are:
NaCl(fuses)  Na+ + Cl-
At cathode,
Na+ + e- -----> Na
At anode,
Cl- ------> Cl + e-
Cl + Cl ------> Cl2.

Advantages of Down's Process:

i) Highly pure (99.5) sodium metal is obtained.
ii) The starting material NaCl is very cheap.
iii) Chlorine is obtained as a valuable by product
 

Properties of Na

1. Action with water: Sodium metal reacts vigorously with water giving hydrogen gas and large amount of energy.
2Na + 2H2O -------> 2NaOH + H2

2. Action with air: Sodium reacts with air to give its oxide and forms sodium hydroxide in presence of moisture, which gives sodium carbonate
when treated with carbon dioxide.
4Na + O2 ----> 2Na2O  4NaOH  2Na2CO3 + 2H2O
Na2CO3  Na2CO3.10H2O.

3. Action with air with heat: Sodium reacts with air in presence of heat to give its oxide and peroxide.
4Na + O2 ---------> 2Na2O (sodium oxide)
2Na + O2 -------> Na2O2 (sodium peroxide)

4. Action with NH3: Sodium reacts with dry ammonia to give sodamide in temperature of 300oC to 400oC.
2Na + 2NH3  2NaNH2(sodamide) + H2

5. Action with CO2:
2Na + 3CO2 -------> 2NaCO3 + C

Uses of sodium

1. It is used in sodium vapour discharge bulb.
2. It is used in manufacture of sodium peroxide, sodamide, sodium cyanide(NaCN) etc.
3. It is used in foreign element detection at laboratory.
4. It acts as catalyst in the preparation rubber.

Biological importance of Na and K
1. K+ and Na+ helps to balance the electric charge and proper functioning of nerve cells.
2. They control many enzymes reactions.
3. These ions helps to support the transport of sugar and amino acids in the cell.
4. They help to regulate water flow across the cell membrane.