Monday, September 16, 2013

Ortho and Para hydrogen ( Spin Isomers Of hydrogen )

        The hydrogen molecule contains two atoms, in which the nuclei of both the atoms are spinning. Depending upon the direction of the spin of the nuclei, the hydrogen are of two types.
         The protons in the nuclei of both H-atom spin in same direction is termed as ortho hydrogen.
          The protons in the nuclei of both H-atom spin in opposite direction is termed as para hydrogen.
              In the state of thermal equilibrium at room temperature dihydrogen contains 75% of ortho hydrogen and 25% of para hydrogen. At 25 K, 99% of para hydrogen and 1% of ortho hydrogen. At 0 K hydrogen contains mainly para hydrogen which is more stable. At room temperature the ratio of ortho to para hydrogen is 3:1.
              At low temperature, para hydrogen is favored energetically. The para form was originally prepared by absorbing ordinary hydrogen in activated charcoal in a quartz vessel kept at temperature of 20 K. The charcoal absorb almost pure para hydrogen. By this method pure para hydrogen can be isolated.
               The ortho form was originally prepared at low temperature, the two spin isomers are not equally absorbed on diamagnetic surfaces. Single stage separation of ortho hydrogen on alumina at 77 K, obtain a mixtures of 15 % para hydrogen and 85% of ortho hydrogen.
                Ordinary dihydrogen is an equilibrium mixture of ortho and para hydrogen.

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