NEET Chemistry Notes s-Block Elements – Anomalous Properties of Lithium

NEET Chemistry Notes s-Block Elements  – Anomalous Properties of Lithium

Anomalous Properties of Lithium

Anomalous Properties of Lithium

Lithium due to its, exceptionally small size (like Mg) and high polarising power (i.e, charge/radius ratio) (like Mg) exhibits some properties which are different from those of the other members of first group but similar to that of magnesium (present diagonally in the following group, i.e. II group).
The property of showing similarity in properties with the element present diagonally opposite in the periodic table is called diagonal relationship. These properties are:
Alkali metals do not react with nitrogen except Li.

 MgCl₂ and LiCl are deliquescent and crystallise as their hydrates, LiCl  2 H₂0 and MgCl₂ 2 H₂0.
Both Li and Mg are harder and have higher melting point than the other metals in their respective groups.

Preparation and Properties of Some Compounds

Compounds of Alkali Metals

Being highly reactive, all the elements of group 1 form compounds. Although here we will discuss only some important of them

• Sodium Chloride
  (Common Salt or Rock Salt, NaCl)

Preparation

Main source is sea water which contains 2.7-7.9% of this salt. Sea water is evaporated to obtain crude salt. It is dissolved in water and insoluble impurities like CaCl₂, CaS0₄, MgCl₂ are removed by filteration HCl is passed when pure NaCl is precipitated.

Properties

•  It is white crystalline solid, soluble in water.
•  Pure NaCl is not hygroscopic. It shows hygroscopic nature due to the presence of chlorides of Ca and Mg as an impurity.

Sodium Hydroxide
(Caustic Soda, NaOH)

Preparation dirt

Sodium hydroxide is prepared commercially by the electrolysis of sodium chloride in Castner-Kellner cell (or mercury cathode cell).

The amalgam is treated with water to give sodium hydroxide and hydrogen gas.

Properties

•  Sodium hydroxide is white, translucent solid and deliquescent. It is called caustic soda because it breaks down proteins of skin to a pasty mass.
• On exposure to atmosphere it absorbs moisture and C0₂
  
• It is a strong alkali so reacts with acids, acidic oxides and amphoteric oxides to form their corresponding salts.
•  Non-metals like halogens, phosphorus, sulphur, silicon, boron are attacked by NaOH.
• It reacts with metals like Zn, Al, Sn and Pb and evolve H₂ gas, e.g.
  
• It also reacts with metallic salts to form hydroxides.
• It is used in soap manufacture, purification of bauxite, manufacture of rayon, etc.

 Sodium Carbonate
(Washing Soda, Na₂C0₃ 1oH₂0)

Preparation

Sodium carbonate is generally prepared by Solvay process.

Sodium hydrogen carbonate crystals separate out. These are heated to obtain sodium carbonate

In this process NH₃ is recovered when the solution containing NH₄Cl is treated with Ca(OH)₂.

Properties

•  It is soluble in water with the evolution of considerable amount of heat.. The solution is alkaline in nature due to hydrolysis.
  
• It is readily decomposed by acids.

 Sodium Hydrogen Carbonate
(Baking Soda, NaHC0₃)

Preparation

It is prepared as intermediate product in the Solvay ammonia soda process.

Properties

• It is white crystalline solid, sparingly soluble in water.
• Its aqueous solution is alkaline in nature due to hydrolysis
  

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