Ordinary hydrogen gas consists of di-atomic molecules in which two H-atoms unite together by covalent bond. This is known as molecular hydrogen.
Stability of molecular hydrogen
H-H bond energy is very high i.e, 104 Kcal per mole .Due to high bond energy ,molecule of hydrogen is very stable .It does not react under ordinary conditions .Only those reactions are possible in which at least 104 Kcal per mole of energy is available.
(ii) Atomic hydrogen
Hydrogen obtained by the dissociation of molecular hydrogen is called atomic hydrogen.Atomic hydrogen is very energetic and very reactive .It has a very short life and spontaneously combine to form molecular hydrogen.
PREPARATION OF ATOMIC HYDROGEN
When molecular hydrogen is heated at 5000C,it dissociates into atomic hydrogen.(This is a theoretical approach)
H2 è 2H
H2 è 2H
REACTIONS OF ATOMIC HYDROGEN
Being very energetic ,atomic hydrogen reacts with different substances under ordinary conditions.
(i) With Oxygen
2H + O2 è H2O2 (Hydrogen per oxide)
(ii) With Sulphur
H + S è H2S
(iii) With Chlorine
2H + Cl2 è 2HCl
(iv) With Phosphorus
3 H + P è PH3 (Phophene)
(v) With Copper Oxide
CuO + 2H è Cu + H2O (reduction)
(vi) With Silver Chloride
H + AgCl è Ag + HCl (reduction)
ATOMIC HYDROGEN TORCH
Atomic hydrogen torch is a system used to obtain a flame of 5000C. Atomic hydrogen when combined, a high temperature flame is obtained which is used to weld Al-alloys and different types of steel.
When molecular hydrogen is passed through an electric discharge, which is set up between two electrodes, it dissociated into atomic hydrogen. Being short-lived atom it recombined into molecular hydrogen. Consequently a flame of about 4000C to 5000C is produced which is sufficient for welding.
H + H è H2 + Heat (104 Kcal per mole)
(iii) NASCENT HYDROGEN
Hydrogen gas liberated during a chemical reaction is always in atomic state, which is known as nascent hydrogen.
Zn+HCl è ZnCl2 +2[H]
If a substance capable to react is present then it will react with it, otherwise it recombines into molecular hydrogen.