Hydrogen and it's types

 Hydrogen

• Discovered by Henry Cavendish in 1766 and name was given by Antonie Lavoisier.

Position in the periodic table: Group IA

Atomic no: 1

Symbol: H

Molecular formula: H2

Atomic mass: 1.008

Electronic configuration: 1S1

• It resembles the alkali metals (group IA) as it can be oxidized to (H+ )ion. Also, it resembles the halogens (group VIIA) as it forms hydride ion (H- ). Hence the position of hydrogen in the periodic table is controversial/anomalous. 

Types/Forms of Hydrogen 

• Molecular hydrogen (H2 ): It is a diatomic molecule, remains in gaseous state at room temperature. It is commonly obtained by the displacement reaction of dilute acids (HCl or H2 SO4 ) with active metals such as Mg, Zn, Al, etc.

Mg + 2HCl(dil.) → MgCl2 + H2↑

Zn + H2 SO4 (dil.) → MgSO4 + H2↑

• Nascent hydrogen [H]: The ‘newly born’ hydrogen is called nascent hydrogen. The hydrogen obtained during the chemical reaction in contact with the substance to be reduced is called nascent hydrogen. In this state, it contains high energy and very reactive.

Q. How can you show that nascent hydrogen is much powerful reducing agent than molecular hydrogen?

ANS: The nascent hydrogen is more reactive and hence more powerful reducing agent than molecular hydrogen.

When hydrogen gas (H2 ) is passed through acidified KMnO4 or acidified K2Cr2O7 or acidified FeCl3 solution, no one of these get reduced. For e.g.

KMnO4 + H2 SO4 + H2 → No reaction

K2Cr2O7 + H2 SO4 + H2 → No reaction

FeCl3 + HCl + H2 → No reaction

But when nascent hydrogen is generated insitu in its proper place by adding zinc metal to the acidified KMnO4 or acidified K2Cr2O7 or acidified FeCl3 solution, all of them get reduced. For e.g.

i) Zn + H2 SO4 → ZnSO4 + 2[H]

  2KMnO4 (pink) + 3H2 SO4 + 10[H]+ → K2SO4 (colorless) + 2MnSO4 + 8H2O 

ii) Zn + H2 SO4 → ZnSO4 + 2[H]

   K2Cr2O7 (orange) + 4H2 SO4 + 6[H] → K2SO4 (colorless) + Cr2 (SO4)3 + 7H2O

iii) Zn + 2HCl → ZnCl2 + 2[H]

     FeCl3 (yellow) + [H] → FeCl2 (light green) + HCl

• Atomic hydrogen: 

When a stream of hydrogen gas under pressure is passed through an electric arc struck between tungsten rods, atomic hydrogen is produced.

This atomic hydrogen is highly unstable and combine readily to form molecular hydrogen liberating large amount of heat.

      2000 C

H2 ↔ 2H (atomic hydrogen) - 104 Kcal

Uses:

• It is used as reducing agent.
• It is used as atomic hydrogen torch for welding metals of very high melting point.

 

Ortho and para hydrogen

When two hydrogen atoms combine to form a molecule, the spins of the two electrons are always anti-parallel [Pauli’s exclusion principle+. However, the spins of the two nuclei (protons) may either be in same direction (parallel) or in opposite direction (anti-parallel).

• When the spin of the nuclei are in the same direction, the hydrogen molecule is called ortho hydrogen.
• When the spin of the nuclei are in the opposite direction, the hydrogen molecule is called para hydrogen. 

  

Isotopes of Hydrogen

Isotopes are the atoms of the same element having same atomic number but different mass number. There are three isotopes of hydrogen.

1. Protium or ordinary hydrogen
2. Deuterium or heavy hydrogen
3. Tritium or radioactive hydrogen

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