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- Learn important concepts of the chapter.

Physical properties and uses of Dihydrogen

Physical properties:
  1. It is a colourless, tasteless and odourless gas
  2. It is the lightest substance known.
  3. It is slightly soluble in water since its molecules are non-polar.
Uses of dihydrogen are:
  1. Commercial fixation of nitrogen from the air in the Haber process
  2. Hydrogenation of fats and oils
  3. Methanol production, hydrocracking, and hydrodesulfurization
  4. Rocket fuel
  5. Welding

Preparation of hydrogen by electrolysis of water

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During electrolysis of pure water, at the negatively charged cathode, a reduction reaction takes place, with electrons from the cathode being given to hydrogen cations to form hydrogen gas (the half-reaction balanced with acid):

Cathode (reduction):

Anode (oxidation):

Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen: 


Laboratory preparation of dihydrogen

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Hydrogen is prepared in the laboratory by the action of acids on metals. Dilute sulphuric acid containing 1 volume of concentrated acid to 5 volumes of water, or dilute hydrochloric acid containing 1 volume of concentrated acid to 4 volumes of water, is added to granulated zinc. Zinc sulphate or zinc chloride is formed in solution and the hydrogen that is evolved is collected over water in a trough.

Since hydrogen is very much lighter than air, it may be collected by upward displacement.

Define Hydrides and give examples

Dihydrogen combines with a number of elements to form binary compounds called hydrides. Types of hydrides are Ionic hydrides, Metallic hydrides and Molecular hydrides.
Ionic hydrides: , , etc.
Metallic hydrides: , , etc.
Molecular hydrides: ,


Heavy water

Heavy water () is used in certain types of nuclear reactors, where it acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile uranium-235 than with uranium-238, which captures neutrons without fissioning (D = Deuterium).
Methods of preparation of heavy water are:
  1. By prolonged electrolysis: This method is based upon the principle that when ordinary water is electrolysed, protium is liberated much more readily than deuterium. As the electrolysis continues, the concentration of heavy water in ordinary water increases and pure is eventually obtained.
  2. By fractional distillation: At normal atmospheric pressure, the boiling points of ordinary and heavy water are 373 K and 374.42 K. This small difference helps in the preparation of heavy water.

Preparation of hydrogen peroxide

From sodium peroxide:

From Barium peroxide:
(a) By the action of dilute sulphuric acid:

(b) By the action of carbon dioxide:
(c) By the action of phosphoric acid:

Uses of hydrogen peroxide

  1. It acts as a bleaching agent for delicate material.
  2. It is used in the production of epoxides, inorganic chemicals like sodium perborate.
  3. It is used as an antiseptic for washing wounds.
  4. It is used in the laboratory for detecting the presence of chromium, titanium, etc.