Hydrogen

Forms

Under normal conditions hydrogen gas is a mix of two different kinds of

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molecules which differ from one another by the

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relative spin of the nuclei.{{ref|uigi.com}} These two forms are

Related Topics:
Spin - Nuclei

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known as ortho- and para-hydrogen (this is different from isotopes, see

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below).

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In ortho-hydrogen the nuclear spins are parallel (form a triplet),

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while in para they are antiparallel (form a singlet).

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At standard conditions hydrogen is

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composed of about 25% of the para form and 75% of the ortho form (the

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so-called "normal" form). The equilibrium

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ratio of these two forms depends on temperature, but since

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the ortho form has higher energy (is an excited state), it cannot be stable in its pure form.

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In low temperatures (around boiling point), the equilibrium state is

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comprised almost entirely of the para form.

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The conversion process between the forms is slow, and if hydrogen is cooled down and condensed rapidly, it contains large quantities of the ortho form. It is important in preparation and storage of liquid hydrogen, since the ortho-para conversion produces more heat than the heat of its evaporation, and a lot of hydrogen can be lost by evaporation in this way during several days after liquefying. Therefore, some catalysts of the ortho-para conversion process are used during hydrogen cooling. The two forms have also slightly different physical properties. For example, the melting and boiling points of parahydrogen are about 0.1 K lower than of the "normal" form.

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