Ortho and Para hydrogen
The nucleus of the hydrogen atom spins about an axis like a top. When two hydrogen atoms combine, they form molecular hydrogen.
Thus depending on the direction of the two protons in the nucleus the following two types of hydrogen molecules are known. Hydrogen molecule in which both the protons in the nuclei of both H-atoms are known to spin in same direction is termed as ortho hydrogen. If the protons in the nuclei of both H-atoms spin in opposite direction, it is termed as parahydrogen.
At room temperature ordinary hydrogen consists of about 75% ortho and 25% para form. As the temperature is lowered, the equilibrium shifts in favour of para hydrogen. At 25K. There is 99% para and 1% ortho hydrogen. The change in the proportion of the two forms of hydrogen requires a catalyst such as platinum or atomic hydrogen or silent electric discharge.
The para form was originally prepared by absorbing ordinary hydrogen in activated charcoal in a quartz vessel kept at a temperature of 20K. The charcoal absorbs almost pure para hydrogen. By this method, pure para hydrogen can be isolated.
Conversion of para into ortho hydrogen
Ortho hydrogen is more stable than para hydrogen. The para form is transformed into ortho form by the following methods.
1. By treatment with catalysts like platinum or iron
2. By passing an electric discharge
3. By heating to 800oC or more.
4. By mixing with paramagnetic molecules like O2, NO, NO2.
5. By mixing with nascent hydrogen or atomic hydrogen.
Properties: Ortho and para hydrogen are similar in chemical properties but differ in some of the physical properties.
(i) Melting point of para hydrogen is 13.83K while that of ordinary hydrogen is 13.95 K.
(ii) Boiling point of para hydrogen 20.26K while that of ordinary hydrogen is 20.39K.
(iii) The vapour pressure of liquid para hydrogen is higher than that of ordinary liquid hydrogen.
The magnetic moment of para hydrogen is zero since the spins neutralise each other while in the case of ortho , it is twice than that of a proton.
Para hydrogen possesses a lower internal molecular energy than ortho form.