Hydrogen and S-Block Element: Diagonal Relationship between Lithium and Magnesium (For CBSE, ICSE, IAS, NET, NRA 2022)

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Diagonal Relationship between Lithium and Magnesium

  • The similarity between first member of one group and the second member of succeeding group is called diagonal relationship.
  • The lithium show properties similar to magnesium and Beryllium and aluminum show similar properties.

Similarity in their physical and chemical properties are listed below:

  • Both elements have nearly the same values of electronegativities.
  • The melting and boiling points of the two elements are comparable.
  • Lithium and magnesium possess nearly the same degree of hardness.
  • Both the elements form nitrides when heated directly with nitrogen.
  • Both the elements form carbides when heated directly with carbon.
  • Both the elements form normal oxides when heated in air.
  • The carbonates, nitrates and hydroxides of both the elements undergo thermal decomposition to their respective oxides.
  • The halides of both the elements are appreciably soluble in organic solvents.
  • Oxyacid salts of both the elements are sparingly soluble in water.

Anomalous Behaviour of Lithium

  • Lithium salts of large polarizable anions are less stable than those of other alkali metals.
  • Solubility differences: The lithium salts of anions of high charge density are less soluble than those of the other alkali metals, for example .
  • Complex formation: Lithium forms more stable covalent bonds than the other alkali metals and therefore forms more stable complex compounds.
  • Lithium reacts only very slowly with water.
  • Lithium forms stable salts with anions of high charge density owing to their high lattice energy. For example, in air lithium forms the normal oxide, whereas the others form higher oxides such as peroxides and superoxide՚s.
  • Lithium compounds are more covalent. Thus, the halides are more soluble in organic solvent and the alkyls and aryls are more stable than those of the other alkali metals.

Sodium Hydroxide

Manufacture of sodium hydroxide by the Castner-Kellner process:

Sodium Hydroxide
  • A saturated solution of sodium chloride (brine) flows through the cell (Fig) in the same direction as a shallow stream of mercury which constitutes the cathode, the anode consists of a number of titanium blocks.
  • On electrolysis chlorine is discharged at the anode and sodium at the cathode, where it dissolves in the mercury and is removed from the cell.
  • The sodium amalgam is passed through water where the sodium reacts to form 50 per cent sodium hydroxide solution of high purity and the mercury is then returned to the cell.
  • The reaction being catalysed by the presence of iron grids.
  • The product are sodium hydroxide, chlorine and hydrogen.
Cathode and Anode

Reaction of Sodium Hydroxide

  • It dissolves readily in water with vigorous evolution of heat.
  • In aqueous solution this is completely dissociated.
  • It neutralize acids due to hydroxide ions which is strong base.

It displaces ammonia from ammonium salts when heated.

In aqueous solution it reacts with many salts and precipitates the corresponding basic hydroxide, e. g.

The hydroxides of the less electropositive metals are amphoteric and dissolve in an excess of alkali, e. g. the hydroxides of aluminum (III) , lead (II) , tin (II) and zinc (II) .

Uses of Sodium Hydroxide

  • Sodium hydroxide is used in the laboratory for absorbing carbon dioxide and other acidic gases.
  • It is used in the manufacture of soap and sodium format in industry.

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