Environmental Engineering Reference
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para, which differ by the relative spin of their nuclei. In the orthohydrogen
form, the spins of the two protons are parallel to each other and form a triplet
state with a molecular spin quantum number of 1 ([1/2]  +  [1/2]). In the
parahydrogen form, the proton spins are antiparallel to each other and form
a singlet state with a molecular spin quantum number of 0 ([1/2] − [1/2]).
At standard temperature and pressure, H 2 gas contains about 75% of the ortho
form and 25% of the para form, known as the normal form. The ortho form
has a higher energy than the para form, and is thus unstable and cannot be
purified. The ortho/para ratio depends on temperature, and decreases with
decreasing temperature. This ratio in condensed H 2 is an important consid-
eration in the preparation and storage of liquid hydrogen (see Chapter 5),
since the conversion from ortho to para is exothermic and produces enough
heat to evaporate some of the hydrogen liquid, leading to loss of liquefied
material. The interconversion between the two forms and hydrogen cooling
are often facilitated by catalysts such as ferric oxide, activated carbon, or
some nickel compounds.
H 2 is a stable molecule but can react with a number of elements and mol-
ecules under certain conditions. One classic example is reaction with oxygen
(O 2 ) to form water. Such a reaction can be carried out by way of combustion,
which is fast and violent, as will be discussed in more detail in Chapter 8.
Among the many interesting properties of H 2 , its potential use as a clean
and renewable fuel has attractive considerable attention, especially given the
increasing demand of energy and adverse environmental impact associated
with use of fossil fuel. H 2 is an ideal fuel in several ways, including clean
byproduct, water, in energy production process and abundance, making it
potentially low cost. For example, Chapter 3 covers recent research efforts
on photoelectrochemical water splitting for hydrogen generation. However,
there are currently some major obstacles toward the practical use of hydrogen
as a fuel, including hydrogen generation, storage, transport, and utilization.
Efficiency and cost are two important factors to consider for each of these
aspects. Safety is another factor of concern.
A molecular ion, H 3 + , has been found in interstellar medium (ISM), which
is generated by ionization of H 2 from cosmic rays. It is one of the most stable
ions in the universe. The neutral form, H 3 , is not stable and can only exist
in an excited state for a short period of time.
Besides forming the hydrogen molecule, H 2 , the hydrogen element is involved
in the formation of a large number of compounds, including water, hydro-
carbons, and many important biological molecules, such as proteins and
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