NEET Chemistry Notes Hydrogen – Occurrence of Hydrogen
Occurrence of Hydrogen
Occurrence of Hydrogen
Dihydrogen (H2) is the most abundant element in the universe (70% of the total mass of the universe) and is the principal element in the solar atmosphere. The giant planets Jupiter and Saturn consist of mainly hydrogen.
Position of Hydrogen in Periodic Table
The position of hydrogen in periodic table is uncertain as it shows resemblance with alkali metals as well as with halogens. However, on the basis of electronic configuration 
it is placed above lithium in the periodic table but still, it is not considered as the member of that group. It is the lightest element known.
Isotopes of Hydrogen
- Hydrogen has three isotopes : protium
deuterium or heavy hydrogen 
and tritium 
- Tritium is radioactive and emits low energy
particles. - These three isotopes have different masses hence, their rates of reaction and equilibrium constants are different. This is known as isotopic effect.
Because of the extreme temperature of sun fusion of hydrogen atoms occurs, which liberates large amount of energy.
Allotropes of Hydrogen
Allotropes of hydrogen are ortho hydrogen (o-H2) (in which proton spin in same direction) and para hydrogen (p-H2) (in which proton spin in opposite direction). Similarly, deuterium and tritium also exhibit spin isomerism and exist in ortho and para forms.
Dihydrogen or Hydrogen Gas (H2)
Hydrogen being highly reactive, it exists in diatomic form, called the dihydrogen or hydrogen gas.
Methods of Preparation of Dihydrogen
Dihydrogen can be prepared by the following methods:
- Laboratory method In laboratory, dihydrogen is produced by the reaction of Zn with dilute H2S04.
- By the electrolysis of water In electrolysis, acidified water is electrolysed by using platinum electrodes as
- High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
- By the action of certain active metals on water or any other protic solvent.
- Certain metals like Zn, A1 reacts with alkali metals to evolve H2 as,
- Industrial methods of preparation of dihydrogen
Industrial preparation of dihydrogen involve the following processes
- In the Bosch process, H2 is prepared through the reaction of water vapours (steam) by carbon as
The CO2 is removed either by washing under pressure of 25-30 atm or by reacting with potassium hydroxide - In the Lane process, H2 is produced as
- In Nelson or Castner Kellner cell, H2 is also produced by the electrolysis of brine solution (NaCl) in them
Properties of Hydrogen
- It is colourless, tasteless, odourless gas. It is lightest and slightly soluble in water.
- It is a neutral and highly combustible gas, so in the presence of air it burns with pale blue flame to form water.
- Reducing action Hydrogen reduces the oxides of less electropositive elements but cannot reduces the oxides of alkali metals and alkaline earth metals
Uses of Hydrogen - In hydrogenation of oils.
- In the preparation of synthetic petrol.
- In oxy-hydrogen flame.
- In hydrogen-oxygen fuel cells to produce electricity.
- In a fuel cell, electrical energy is generated by the reaction of H2 and O2 without evolution of heat (cold combustion).
- Liquid hydrogen is used as rocket fuel.
- As a reducing agent in the extraction of metals.
- H2 has higher calorific value so, it is used as better fuel.
Atomic Hydrogen
- Hydrogen gas dissociates into atoms when it is subjected to an electric discharge under low pressure.
- Atomic hydrogen is very reactive and can be used as a reducing agent as well as oxidising agent.
- Reducing power of atomic hydrogen is more than that
of nascent hydrogen