Hydrogen: Water-Gas Shift Reaction, Bond Dissociation Enthalpy (For CBSE, ICSE, IAS, NET, NRA 2022)

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The Hydrogen
  • 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.
  • Although hydrogen is placed at the top of Group 1-A in most versions of the periodic table, it is very different from the other members of the alkali metal group
  • The three stable isotopes of hydrogen are protium, deuterium, and tritium.
  • In its elemental form, hydrogen is a colorless, odorless, extremely flammable gas at room temperature, consisting of diatomic molecules of .
  • The radioactive isotope amongst these three is tritium.
  • Hydrogen is one of the most abundant elements found in the universe.
  • Molecular hydrogen boils at -253°C (20 K) , and freezes at -259°C (14 K) .
  • Even though it resembles halogen and alkali metals, it has unique properties and has a separate position in the periodic table.
  • Hydrogen typically does not form cations, but instead forms compounds through covalent bonding.

Water-Gas Shift Reaction

Water Gas Shift Reaction
  • On an industrial scale level, Dihydrogen is prepared from petrochemicals by water-gas shift reaction.
  • The water-gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:
  • As a resolution to this problem, the WGSR was combined with the gasification of coal to produce a pure hydrogen product.
  • It is got in the form of byproduct on the electrolysis of brine.

Bond Dissociation Enthalpy

Bond Dissociation Enthalpy
  • Dehydrogenate has the highest H-H bond dissociation enthalpy for a single bond between the two atoms.
  • The bond-dissociation energy is one measure of the strength of a chemical bond A – B. It can be defined as the standard enthalpy change when A – B is cleaved by homolysis to give fragments A and B, which are usually radical species.
  • The H-H bond dissociation enthalpy equals to
  • The bond dissociation enthalpy is the energy needed to break one mole of the bond to give separated atoms - everything being in the gas state.


  • Due to high negative dissociation enthalpy, dihydrogen is inactive at room temperature.
  • In chemistry, a hydride is formally the anion of hydrogen, . The term is applied loosely.
  • At suitable conditions, dihydrogen can combine with all the possible elements to form hydrides.
  • At one extreme, all compounds containing covalently bound H atoms, are called hydrides: water is a hydride of oxygen, ammonia is a hydride of nitrogen, etc.

There are three different ways to categorize the hydride:

  • 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