Hydrogen Class 11 Notes - Chapter 9

The lightest atom is on earth is hydrogen which has a single electron. In the year 1766, Henry Cavendish was the first to discover it. The three stable isotopes of hydrogen are protium, deuterium, and tritium. The radioactive isotope amongst these three is tritium. Hydrogen is one of the most abundant elements found in the universe. Even though it resembles halogen and alkali metals, it has unique properties and has a separate position in the periodic table.

Water-Gas Shift Reaction

On an industrial scale level, Dihydrogen is prepared from petrochemicals by water-gas shift reaction. It is got in the form of byproduct on the electrolysis of brine.

Bond Dissociation Enthalpy

Dihydrogen has the highest H-H bond dissociation enthalpy for a single bond between the two atoms. The H-H bond dissociation enthalpy equals to 435.88 kJ mol–1


Even though at room temperature, dihydrogen is inactive due to high negative dissociation enthalpy, it can combine with nearly all the elements to form hydrides in suitable conditions.

Hydride can be classified into three different categories:

  • Saline or Ionic hydrides – Good reagent to prepare additional hydride compounds
  • Molecular or Covalent hydrides – Important for daily life. Example H2O
  • Non-stoichiometric or metallic hydrides – Used for ultra purification of dihydrogen

Few Important Questions

  1. How can you increase the production of dihydrogen from coal gasification?
  2. Explain auto-protolysis of water along with its significance.
  3. Explain the amphoteric nature of water with chemical reactions.
  4. Distinguish between hydration and hydrolysis.

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Practise This Question

Match the orbital overlap figures shown in column I with the description given in column II and select the correct answer using the codes given below