Hydrogen and Compounds

Hydrogen is the most abundant element in the universe. It is the 9th most abundant element in the earth's crust. It occurs only in the combined state except in volcanic gases where it occurs in free state.

It is the first element in the periodic table. It is the lightest of all the elements. It is the only element without neutrons i.e. Protium. It is the element with ambiguous position in the periodic table, as it resembles both I-A and VII-A group elements. Hydrogen should belong to the s-block as its electron configuration is 1s1.

The properties in which hydrogen resembles alkali metals are:

  • Like alkali metals, it has one electron in s-orbital of valence shell.
  • Like Alkali metals, it forms H+ ion.
  • Like Alkali metals it forms monoxides and peroxides.
  • Like Alkali metals, it acts as reductant.

The properties in which Hydrogen resembles Halogens are:

  • Like Halogens, it exists as diatomic molecule (H2).
  • Like Halogens, it gains one electron to attain inert gas configuration.
  • The I.P value of H is similar to that of Halogens.
  • Like halogens it forms univalent anion i.e H

Isotopes of Hydrogen

Hydrogen has 3 isotopes:

  1. Protium 1H1
  2. Deuterium 1H2
  3. Tritium 1H3

All the isotopes of Hydrogen have a) same chemical properties b) same atomic radii c) same bond lengths.

Uses of Hydrogen

Hydrogen is used:

  1. in the manufacture of chemicals
  2. in metallurgy
  3. as a source of atomic energy
  4. as a fuel

Water Gas

It contains H2 and CO in 1:1 ratio by volumes. It is prepared by passing steam over white hot coke or coal.

Ionic Hydrides

These are formed by elements of group IA, IIA (except Be and Mg) and lanthanides by heating the metal in hydrogen.

2Na + H2 → 2NaH

2K + H2 → 2KH

Ca + H2  → CaH2

Sr + H2 → SrH2

These are white colourless solids (crystalline) having high melting point and boiling point, easily decomposed by water, alcohol, CO2 (or) SO2.

Covalent Hydrides

These hydrides are formed by the combination of elements of comparatively higher electronegatively as p-block elements. (IIIA to VIIA groups)

Examples: NH3, HCl, CH4, H2O, HF, B2H6, AsH3

Metallic Hydrides

Most of the d-block elements and f block elements from metallic hydrides on reacting with hydrogen.

Water

Water is called as universal solvent because it is an excellent solvent for ionic compounds and polar covalent compounds. Purest form of natural water is rain water. Potable water means which is for drinking.

Water which gives good lather readily with soap is known as soft water. Water which does not give lather readily with soap but precipitates soap is known as hard water. When soap is added to hard water, Na+ of soap are replaced by Ca2+ or Mg2+ ions and precipitates as Ca or Mg soap.

Hardness of water is expressed in terms of ppm of CaCO3. That means the number of grams of CaCO3 (or) its equivalent part present in 1 million grams of water is known as hardness.

Temporary hardness of water is due to the presence of the bicarbonates of Ca and Mg. Permanent hardness of water is due to the presence of the chlorides and sulphates of Ca, Mg, Fe.

Hydrogen Peroxide

H2O2 was discovered by Thenard. H2O2 is also known as oxygenated water. It is a weak dibasic acid. Its molecular weight is 34 and its equivalent weight is 17.

Preparation of H2O2

In the laboratory, H2O2 is prepared by the action of ice cold dil. H2SO4 on Na2O2 (or) hydrated Barium Peroxide [BaO2.8H2O].

H2O2 can also be obtained by passing CO2 gas through a suspension of Barium peroxide in water.