Hydrogen ( , ) is the
chemical element with
atomic number 1. It is represented by the
symbol H. With an
atomic weight of , hydrogen is the
lightest element.
Hydrogen is the
most
abundant chemical element, constituting roughly 75 % of the
universe's elemental mass.
Stars in the
main sequence are mainly composed of
hydrogen in its
plasma state.
Naturally occuring elemental hydrogen is relatively rare on
Earth.
The most common
isotope of hydrogen is
protium (name rarely used, symbol ) with
a single
proton and no
neutrons. In
ionic
compounds it can take a negative charge (an
anion known as a
hydride and
written as H
−), or as a positively-charged
species H
+. The latter
cation is written as though composed of a bare proton,
but in reality, hydrogen cations in
ionic
compounds always occur as more complex species. Hydrogen forms
compounds with most elements and is present in
water and most
organic
compounds. It plays a particularly important role in
acid-base chemistry with many
reactions exchanging protons between soluble molecules. As the
simplest atom known, the
hydrogen atom
has been of theoretical use. For example, as the only neutral atom
with an analytic solution to the
Schrödinger equation, the study of
the energetics and bonding of the hydrogen atom played a key role
in the development of
quantum
mechanics.
Hydrogen gas (now known to be H
2), was first
artificially produced in the early 16th century, via the mixing of
metals with strong acids. In 1766-81,
Henry Cavendish was the first to recognize
hydrogen gas as a discrete substance, and that it produces water
when burned, a property which gave it later its later name, which
in Greek means "water-former". At
standard temperature and
pressure, hydrogen is a
colorless,
odorless,
nonmetallic,
tasteless, highly
flammable diatomic gas with the
molecular formula
H
2.
Industrial production is mainly from the steam reforming of natural
gas, and less often from more energy-intensive
hydrogen production methods like the
electrolysis of water . Most
hydrogen is employed near its production site, with the two largest
uses being
fossil fuel processing (e.g.,
hydrocracking) and
ammonia production, mostly for the fertilizer
market.
Hydrogen is important in
metallurgy as it
can
embrittle many metals,
complicating the design of pipelines and storage tanks. Hydrogen
has increasingly received attention as an energy-storage medium
which burns in a less-polluting way than do fossil fuels.
Properties
Combustion
Hydrogen gas (dihydrogen) is highly flammable and will burn in air
at a very wide range of concentrations between 4% and 75% by
volume. The
enthalpy of combustion for
hydrogen is −286 kJ/mol:
- 2 H2(g) + O2(g) → 2 H2O(l) +
572 kJ (286 kJ/mol)
Hydrogen gas forms explosive mixtures with air in the concentration
range 4-74% (volume per cent of hydrogen in air) and with chlorine
in the range 5-95%. The mixtures spontaneously detonate by spark,
heat or sunlight. The hydrogen
autoignition temperature, the
temperature of spontaneous ignition in air, is . Pure
hydrogen-oxygen flames emit
ultraviolet
light and are nearly invisible to the naked eye, asillustrated by
the faint plume of the
Space
Shuttle main engine compared to the highly visible plume of a
Space Shuttle Solid
Rocket Booster. The detection of a burning hydrogen leak may
require a
flame detector; such leaks
can be very dangerous.
The destruction of the Hindenburg
airship
was an infamous example of hydrogen combustion; the
cause is debated, but the visible flames were the result of
combustible materials in the ship's skin. Because hydrogen
is buoyant in air, hydrogen flames tend to ascend rapidly and cause
less damage than hydrocarbon fires. Two-thirds of the Hindenburg
passengers survived the fire, and many deaths were instead the
result of falls or burning diesel fuel.
H
2 reacts with every oxidizing element. Hydrogen can
react spontaneously and violently at room temperature with
chlorine and
fluorine to
form the corresponding hydrogen halides,
hydrogen chloride and
hydrogen fluoride, which are also
potentially dangerous
acids.
Electron energy levels
The
ground state energy level of the electron in a hydrogen atom
is −13.6
eV, which is equivalent
to an ultraviolet
photon of roughly
92
nm wavelength.
The energy levels of hydrogen can be calculated fairly accurately
using the
Bohr model of the atom, which
conceptualizes the electron as "orbiting" the proton in analogy to
the Earth's orbit of the sun. However, the
electromagnetic force attracts
electrons and protons to one another, while planets and celestial
objects are attracted to each other by
gravity. Because of the discretization of
angular momentum postulated in early
quantum mechanics by Bohr, the
electron in the Bohr model can only occupy certain allowed
distances from the proton, and therefore only certain allowed
energies.
A more accurate description of the hydrogen atom comes from a
purely quantum mechanical treatment that uses the
Schrödinger equation or the
equivalent
Feynman path integral formulation to
calculate the
probability
density of the electron around the proton.
Elemental molecular forms
There exist two different
spin
isomers of hydrogen diatomic molecules that differ by the
relative
spin of their nuclei. In the
orthohydrogen form, the spins of the
two protons are parallel and form a triplet state with a molecular
spin quantum number of 1 (½+½); in the
parahydrogen form the spins are antiparallel
and form a singlet with a molecular spin quantum number of 0 (½-½).
At standard temperature and pressure, hydrogen gas contains about
25% of the para form and 75% of the ortho form, also known as the
"normal form". The equilibrium ratio of orthohydrogen to
parahydrogen depends on temperature, but since the ortho form is an
excited state and has a higher energy
than the para form, it is unstable and cannot be purified. At very
low temperatures, the equilibrium state is composed almost
exclusively of the para form. The liquid and gas phase thermal
properties of pure parahydrogen differ significantly from those of
the normal form because of differences in rotational heat
capacities, as discussed more fully in
Spin isomers of hydrogen. The
ortho/para distinction also occurs in other hydrogen-containing
molecules or functional groups, such as water and
methylene, but is of little significance for their
thermal properties.
The uncatalyzed interconversion between para and ortho
H
2 increases with increasing temperature; thus rapidly
condensed H
2 contains large quantities of the
high-energy ortho form that converts to the para form very slowly.
The ortho/para ratio in condensed H
2 is an important
consideration in the preparation and storage of liquid hydrogen:
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.
Catalysts for the ortho-para
interconversion, such as
ferric oxide,
activated carbon, platinized
asbestos, rare earth metals, uranium compounds,
chromic oxide, or some nickel compounds, are
used during hydrogen cooling.
A molecular form called
protonated molecular hydrogen,
or , is found in the
interstellar
medium (ISM), where it is generated by ionization of molecular
hydrogen from
cosmic rays. It has also
been observed in the upper atmosphere of the planet
Jupiter. This molecule is relatively stable in the
environment of outer space due to the low temperature and density.
is one of the most abundant ions in the Universe, and it plays a
notable role in the chemistry of the interstellar medium.Neutral
triatomic hydrogen H
3
can only exist in an excited from and is unstable.
Compounds
Covalent and organic compounds
While H
2 is not very reactive under standard conditions,
it does form compounds with most elements. Millions of
hydrocarbons are known, but they are not formed
by the direct reaction of elementary hydrogen and carbon. Hydrogen
can form compounds with elements that are more
electronegative, such as
halogens (e.g., F, Cl, Br, I); in these compounds
hydrogen takes on a partial positive charge. When bonded to
fluorine,
oxygen, or
nitrogen, hydrogen can participate in a
form of strong noncovalent bonding called
hydrogen bonding, which is critical to the
stability of many biological molecules. Hydrogen also forms
compounds with less electronegative elements, such as the
metals and
metalloids, in
which it takes on a partial negative charge. These compounds are
often known as
hydrides.
Hydrogen forms a vast array of compounds with
carbon. Because of their general association with
living things, these compounds came to be called
organic compounds; the study of their
properties is known as
organic
chemistry and their study in the context of living
organisms is known as
biochemistry. By some definitions, "organic"
compounds are only required to contain carbon. However, most of
them also contain hydrogen, and since it is the carbon-hydrogen
bond which gives this class of compounds most of its particular
chemical characteristics, carbon-hydrogen bonds are required in
some definitions of the word "organic" in chemistry.
In
inorganic chemistry, hydrides
can also serve as
bridging ligands
that link two metal centers in a
coordination complex. This function is
particularly common in
group 13
elements, especially in
boranes (
boron hydrides) and
aluminium
complexes, as well as in clustered
carboranes.
Hydrides
Compounds of hydrogen are often called
hydrides, a term that is used fairly loosely. The
term "hydride" suggests that the H atom has acquired a negative or
anionic character, denoted H
−, and is used when hydrogen
forms a compound with a more
electropositive element. The existence of
the hydride anion, suggested by
Gilbert
N. Lewis in 1916 for group I
and II salt-like hydrides, was demonstrated by Moers in 1920 with
the electrolysis of molten
lithium
hydride (LiH), that produced a
stoichiometric quantity of hydrogen at the
anode. For hydrides other than group I and II metals, the term is
quite misleading, considering the low electronegativity of
hydrogen. An exception in group II hydrides is , which is
polymeric. In
lithium
aluminium hydride, the anion carries hydridic centers firmly
attached to the Al(III). Although hydrides can be formed with
almost all main-group elements, the number and combination of
possible compounds varies widely; for example, there are over 100
binary borane hydrides known, but only one binary aluminium
hydride. Binary
indium hydride has not yet
been identified, although larger complexes exist.
Protons and acids
Oxidation of hydrogen, in the sense of removing its electron,
formally gives H
+, containing no electrons and a
nucleus which is usually composed of
one
proton. That is why is often called a
proton. This species is central to discussion of
acids. Under the
Bronsted-Lowry theory, acids are
proton donors, while bases are proton acceptors.
A bare proton, , cannot exist in solution or in ionic crystals,
because of its unstoppable attraction to other atoms or molecules
with electrons. Except at the high temperatures associated with
plasmas, such protons cannot be removed from the
electron clouds of atoms and molecules, and
will remain attached to them. However, the term 'proton' is
sometimes used loosely and metaphorically to refer to positively
charged or
cationic hydrogen attached to
other species in this fashion, and as such is denoted " " without
any implication that any single protons exist freely as a
species.
To avoid the implication of the naked "solvated proton" in
solution, acidic aqueous solutions are sometimes considered to
contain a less unlikely fictitious species, termed the "
hydronium ion" ( ). However, even in this case,
such solvated hydrogen cations are thought more realistically
physically to be organized into clusters that form species closer
to . Other
oxonium ions are found when
water is in solution with other solvents.
Although exotic on earth, one of the most common ions in the
universe is the ion, known as
protonated molecular hydrogen
or the triatomic hydrogen cation.
Isotopes
Hydrogen has three naturally occurring isotopes, denoted , and .
Other, highly unstable nuclei ( to ) have been synthesized in the
laboratory but not observed in nature.
- is the most common hydrogen isotope with an abundance of more
than 99.98%. Because the nucleus of
this isotope consists of only a single proton, it is given the descriptive but rarely used
formal name protium.
- , the other stable hydrogen isotope, is known as deuterium and contains one proton and one
neutron in its nucleus. Essentially all
deuterium in the universe is thought to have been produced at the
time of the Big Bang, and has endured since
that time. Deuterium is not radioactive, and does not represent a
significant toxicity hazard. Water enriched in molecules that
include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are
used as a non-radioactive label in chemical experiments and in
solvents for -NMR spectroscopy.
Heavy water is used as a neutron
moderator and coolant for nuclear reactors. Deuterium is also a
potential fuel for commercial nuclear
fusion.
- is known as tritium and
contains one proton and two neutrons in its nucleus. It is
radioactive, decaying into Helium-3 through
beta decay with a half-life of 12.32 years. Small amounts of tritium
occur naturally because of the interaction of cosmic rays with
atmospheric gases; tritium has also been released during nuclear weapons tests. It is used in nuclear
fusion reactions, as a tracer in isotope geochemistry, and specialized
in self-powered lighting
devices. Tritium has also been used in chemical and biological
labeling experiments as a radiolabel.
Hydrogen is the only element that has different names for its
isotopes in common use today. (During the early study of
radioactivity, various heavy radioactive isotopes were given names,
but such names are no longer used). The symbols D and T (instead of
and ) are sometimes used for deuterium and tritium, but the
corresponding symbol P is already in use for
phosphorus and thus is not available for protium.
In its
nomenclatural guidelines,
the
International
Union of Pure and Applied Chemistry allows any of D, T, , and
to be used, although and are preferred.
History
Discovery and use
Hydrogen gas, H
2, was first artificially produced and
formally described by T. Von Hohenheim (also known as
Paracelsus, 1493–1541) via the mixing of
metals with
strong acids.
He was unaware that the flammable gas produced by this
chemical reaction was a new
chemical element. In 1671,
Robert Boyle rediscovered and described the
reaction between
iron filings and dilute
acids, which results in the production of
hydrogen gas. In 1766,
Henry
Cavendish was the first to recognize hydrogen gas as a discrete
substance, by identifying the gas from a
metal-acid reaction as "flammable air"
and further finding in 1781 that the gas produces water when
burned. He is usually given credit for its discovery as an element.
In 1783,
Antoine Lavoisier gave
the element the name hydrogen (from the Greek
hydro
meaning water and
genes meaning creator) when he and
Laplace reproduced Cavendish's finding that
water is produced when hydrogen is burned.
Hydrogen was
liquefied for the first
time by
James Dewar in 1898 by using
regenerative cooling and his
invention, the
vacuum flask. He
produced
solid hydrogen the next
year.
Deuterium was discovered in December
1931 by
Harold Urey, and
tritium was prepared in 1934 by
Ernest Rutherford,
Mark Oliphant, and
Paul Harteck.
Heavy
water, which consists of deuterium in the place of regular
hydrogen, was discovered by Urey's group in 1932. François Isaac de
Rivaz built the first internal combustion engine powered by a
mixture of hydrogen and oxygen in 1806.
Edward Daniel Clarke invented the
hydrogen gas blowpipe in 1819. The
Döbereiner's lamp and
limelight were invented in 1823.
The first hydrogen-filled
balloon was
invented by
Jacques Charles in 1783.
Hydrogen provided the lift for the first reliable form of
air-travel following the 1852 invention of the first
hydrogen-lifted airship by
Henri
Giffard. German count
Ferdinand von Zeppelin promoted the
idea of rigid airships lifted by hydrogen that later were called
Zeppelins; the first of which had its
maiden flight in 1900. Regularly scheduled flights started in 1910
and by the outbreak of
World War I in
August 1914, they had carried 35,000 passengers without a serious
incident. Hydrogen-lifted airships were used as observation
platforms and bombers during the war.
The first non-stop transatlantic crossing was made by the British
airship
R34 in 1919. Regular
passenger service resumed in the 1920s and the discovery of
helium reserves in the United States promised
increased safety, but the U.S. government refused to sell the gas
for this purpose.
Therefore, H2 was used in the
Hindenburg airship,
which was destroyed in a midair fire over New Jersey
on May 6, 1937. The incident was broadcast
live on radio and filmed. Ignition of leaking hydrogen is widely
assumed to be the cause, but later investigations pointed to the
ignition of the
aluminized fabric coating
by
static electricity. But the
damage to hydrogen's reputation as a
lifting
gas was already done.
In the same year the first hydrogen-cooled
turbogenerator went into service with gaseous hydrogen as a
coolant in the rotor and the stator in 1937
at Dayton
, Ohio, by
the Dayton Power & Light Co, because of the thermal
conductivity of hydrogen gas this is the most common type in its
field today. The
nickel
hydrogen battery was used for the first time in 1977 aboard the
U.S. Navy's
Navigation
technology satellite-2 (NTS-2). For example, the
ISS,
Mars Odyssey and
the
Mars Global Surveyor are
equipped with nickel-hydrogen batteries. The
Hubble Space Telescope, at the time
its original batteries were finally changed in May 2009, more than
19 years after launch, led with the highest number of
charge/discharge cycles.
Role in quantum theory
Because of its relatively simple atomic structure, consisting only
of a proton and an electron, the
hydrogen
atom, together with the spectrum of light produced from it or
absorbed by it, has been central to the development of the theory
of
atomic structure. Furthermore, the
corresponding simplicity of the hydrogen molecule and the
corresponding cation
H2+
allowed fuller understanding of the nature of the
chemical bond, which followed shortly after
the quantum mechanical treatment of the hydrogen atom had been
developed in the mid-1920s.
One of the first quantum effects to be explicitly noticed (but not
understood at the time) was a Maxwell observation involving
hydrogen, half a century before full
quantum mechanical theory arrived. Maxwell
observed that the
specific heat
capacity of H
2 unaccountably departs from that of a
diatomic gas below room temperature and
begins to increasingly resemble that of a monatomic gas at
cryogenic temperatures. According to quantum theory, this behavior
arises from the spacing of the (quantized) rotational energy
levels, which are particularly wide-spaced in H
2 because
of its low mass. These widely spaced levels inhibit equal partition
of heat energy into rotational motion in hydrogen at low
temperatures. Diatomic gases composed of heavier atoms do not have
such widely spaced levels and do not exhibit the same effect.
Natural occurrence
Hydrogen is the most
abundant
element in the universe, making up 75% of
normal
matter by
mass and over 90% by number of
atoms. This element is found in great abundance in stars and
gas giant planets.
Molecular clouds of H
2 are
associated with
star formation.
Hydrogen plays a vital role in powering
stars
through
proton-proton
reaction and
CNO cycle nuclear fusion.
Throughout the universe, hydrogen is mostly found in the
atomic and
plasma
states whose properties are quite different from molecular
hydrogen. As a plasma, hydrogen's electron and proton are not bound
together, resulting in very high electrical conductivity and high
emissivity (producing the light from the sun and other stars). The
charged particles are highly influenced by magnetic and electric
fields. For example, in the
solar wind
they interact with the Earth's
magnetosphere giving rise to
Birkeland currents and the
aurora. Hydrogen is found in the neutral
atomic state in the
Interstellar
medium. The large amount of neutral hydrogen found in the
damped Lyman-alpha systems is thought to dominate the cosmological
baryonic density of the
Universe up to
redshift z=4.
Under ordinary conditions on Earth, elemental hydrogen exists as
the diatomic gas, H
2 (for data see table). However,
hydrogen gas is very rare in the Earth's atmosphere (1
ppm by volume) because of its light weight,
which enables it to
escape from
Earth's gravity more easily than heavier gases. However,
hydrogen is the third most abundant element on the Earth's surface.
Most of the Earth's hydrogen is in the form of
chemical compounds such as
hydrocarbons and
water.
Hydrogen gas is produced by some bacteria and
algae and is a natural component of
flatus.
Methane is a hydrogen
source of increasing importance.
Production
H
2 is produced in chemistry and biology laboratories,
often as a by-product of other reactions; in industry for the
hydrogenation of
unsaturated substrates; and in nature
as a means of expelling
reducing equivalents
in biochemical reactions.
Laboratory
In the
laboratory, H
2 is
usually prepared by the reaction of acids on metals such as
zinc with
Kipp's
apparatus.
- Zn + 2 → +
Aluminium can also produce upon treatment
with bases:
- 2 Al + 6 + 2 → 2 + 3
The
electrolysis of water is a
simple method of producing hydrogen. A low voltage current is run
through the water, and gaseous oxygen forms at the
anode while gaseous hydrogen forms at the
cathode. Typically the cathode is made from platinum
or another inert metal when producing hydrogen for storage. If,
however, the gas is to be burnt on site, oxygen is desirable to
assist the combustion, and so both electrodes would be made from
inert metals. (Iron, for instance, would oxidize, and thus decrease
the amount of oxygen given off.) The theoretical maximum efficiency
(electricity used vs. energetic value of hydrogen produced) is
between 80–94%.
- 2 (aq) → 2 (g) + (g)
In 2007, it was discovered that an alloy of aluminium and
gallium in pellet form added to water could be used
to generate hydrogen. The process also creates
alumina, but the expensive gallium, which prevents
the formation of an oxide skin on the pellets, can be re-used. This
has important potential implications for a hydrogen economy, since
hydrogen can be produced on-site and does not need to be
transported.
Industrial
Hydrogen can be prepared in several different ways, but
economically the most important processes involve removal of
hydrogen from hydrocarbons. Commercial bulk hydrogen is usually
produced by the
steam reforming of
natural gas. At high temperatures
(1000–1400 K, °C;700–1100 °C or 1,300–2,000 °F),
steam (water vapor) reacts with
methane to
yield
carbon monoxide and .
- + → CO + 3
This reaction is favored at low pressures but is nonetheless
conducted at high pressures (2.0 MPa, 20 atm or
600
inHg) since high pressure is the most
marketable product. The product mixture is known as "
synthesis gas" because it is often used
directly for the production of
methanol and
related compounds.
Hydrocarbons other
than methane can be used to produce synthesis gas with varying
product ratios. One of the many complications to this highly
optimized technology is the formation of coke or carbon:
- → C + 2 H2
Consequently, steam reforming typically employs an excess of .
Additional hydrogen can be recovered from the steam by use of
carbon monoxide through the
water gas shift reaction,
especially with an
iron oxide catalyst.
This reaction is also a common industrial source of
carbon dioxide:
- CO + → +
Other important methods for production include partial oxidation of
hydrocarbons:
- 2 + → 2 CO + 4
and the coal reaction, which can serve as a prelude to the shift
reaction above:
- C + → CO +
Hydrogen is sometimes produced and consumed in the same industrial
process, without being separated. In the
Haber process for the
production of ammonia, hydrogen is
generated from natural gas.
Electrolysis of
brine to
yield
chlorine also produces hydrogen as a
co-product.
Thermochemical
There are more than 200 thermochemical cycles which can be used for
water splitting, around a dozen of
these cycles such as the
iron oxide
cycle,
cerium oxide-cerium
oxide cycle,
zinc zinc-oxide
cycle,
sulfur-iodine cycle,
copper-chlorine cycle and
hybrid sulfur cycle are under
research and in testing phase to produce hydrogen and oxygen from
water and heat without using electricity. A number of laboratories
(including in France, Germany, Greece, Japan, and the USA) are
developing thermochemical methods to produce hydrogen from solar
energy and water.
Applications
Large quantities of are needed in the petroleum and chemical
industries. The largest application of is for the processing
("upgrading") of fossil fuels, and in the production of
ammonia. The key consumers of in the petrochemical
plant include
hydrodealkylation,
hydrodesulfurization, and
hydrocracking.
has several other important uses. is used as a hydrogenating agent,
particularly in increasing the level of saturation of unsaturated
fats and
oils (found in items such as
margarine), and in the production of
methanol. It is similarly the source of hydrogen in
the manufacture of
hydrochloric
acid. is also used as a
reducing
agent of metallic
ores.
Hydrogen is highly soluble in many
rare earth and
transition metals and is soluble in both
nanocrystalline and
amorphous
metals. Hydrogen
solubility in metals
is influenced by local distortions or impurities in the
crystal lattice.. These properties may be
useful when hydrogen is purified by passage through hot
palladium disks, but the gas serves as a
metallurgical problem as hydrogen solubility contributes in an
unwanted way to
embrittle
many metals, complicating the design of pipelines and storage
tanks.
Apart from its use as a reactant, has wide applications in physics
and engineering. It is used as a
shielding
gas in
welding methods such as
atomic hydrogen welding.
H
2 is used as the rotor coolant in
electrical generators at
power stations, because it has the highest
thermal conductivity of any
gas. Liquid H
2 is used in
cryogenic research, including
superconductivity studies. Since is
lighter than air, having a little more than of the density of air,
it was once widely used as a
lifting gas
in balloons and
airships.
In more recent applications, hydrogen is used pure or mixed with
nitrogen (sometimes called
forming gas)
as a tracer gas for minute leak detection. Applications can be
found in the automotive, chemical, power generation, aerospace, and
telecommunications industries. Hydrogen is an authorized food
additive (E 949) that allows food package leak testing among other
anti-oxidizing properties.
Hydrogen's rarer isotopes also each have specific applications.
Deuterium (hydrogen-2) is used in
nuclear fission applications as a
moderator to slow
neutrons, and in
nuclear
fusion reactions. Deuterium compounds have applications in
chemistry and biology in studies of reaction
isotope effects.
Tritium (hydrogen-3), produced in
nuclear reactors, is used in the production
of
hydrogen bombs, as an isotopic
label in the biosciences, and as a
radiation source in luminous paints.
The
triple point temperature of
equilibrium hydrogen is a defining fixed point on the
ITS-90 temperature
scale at 13.8033
kelvins.
Energy carrier
Hydrogen is not an energy resource, except in the hypothetical
context of commercial
nuclear fusion
power plants using
deuterium or
tritium, a technology presently far from
development. The Sun's energy comes from nuclear fusion of
hydrogen, but this process is difficult to achieve controllably on
Earth. Elemental hydrogen from solar, biological, or electrical
sources require more energy to make it than is obtained by burning
it, so in these cases hydrogen functions as an energy carrier, like
a battery. Hydrogen may be obtained from fossil sources (such as
methane), but these sources are unsustainable.
The
energy density per unit
volume of both
liquid
hydrogen and
compressed
hydrogen gas at any practicable pressure is significantly less
than that of traditional fuel sources, although the energy density
per unit fuel
mass is higher. Nevertheless, elemental
hydrogen has been widely discussed in the context of energy, as a
possible future
carrier of energy on an economy-wide
scale. For example,
sequestration
followed by
carbon capture
and storage could be conducted at the point of production from
fossil fuels. Hydrogen used in transportation would burn relatively
cleanly, with some
NOx emissions, but without
carbon emissions. However, the infrastructure costs associated with
full conversion to a hydrogen economy would be substantial.
Semiconductor industry
Hydrogen is employed to saturate broken ("dangling") bonds of
amorphous silicon and
amorphous carbon that helps stabilizing
material properties. It is also a potential
electron donor in various oxide materials,
including
ZnO,
SnO2,
CdO,
MgO,
ZrO2,
HfO2,
La2O3,
Y2O3,
TiO2,
SrTiO3, LaAlO
3,
SiO2,
Al2O3,
ZrSiO
4, HfSiO
4, and SrZrO
3.
Biological reactions
H
2 is a product of some types of
anaerobic metabolism and is
produced by several
microorganisms,
usually via reactions
catalyzed by
iron- or
nickel-containing
enzymes
called
hydrogenases. These enzymes
catalyze the reversible
redox reaction between
H
2 and its component two protons and two electrons.
Creation of hydrogen gas occurs in the transfer of reducing
equivalents produced during
pyruvate
fermentation to
water.
Water splitting, in which water is
decomposed into its component protons, electrons, and oxygen,
occurs in the
light reactions in all
photosynthetic organisms. Some such
organisms—including the alga
Chlamydomonas reinhardtii and
cyanobacteria—have evolved a second
step in the
dark reactions in which
protons and electrons are reduced to form H
2 gas by
specialized hydrogenases in the
chloroplast. Efforts have been undertaken to
genetically modify cyanobacterial hydrogenases to efficiently
synthesize H
2 gas even in the presence of oxygen.
Efforts have also been undertaken with genetically modified
alga in a
bioreactor.
Safety and precautions
Hydrogen poses a number of hazards to human safety, from potential
detonations and fires when mixed with air
to being an
asphyxant in its pure,
oxygen-free form. In addition,
liquid hydrogen is a
cryogen and presents dangers (such as
frostbite) associated with very cold liquids.
Hydrogen dissolves in some metals, and, in addition to leaking out,
may have adverse effects on them, such as
hydrogen embrittlement. Hydrogen gas
leaking into external air may spontaneously ignite. Moreover,
hydrogen fire, while being extremely hot, is almost invisible, and
thus can lead to
accidental burns.
Even interpreting the hydrogen data (including safety data) is
confounded by a number of phenomena. Many physical and chemical
properties of hydrogen depend on the
parahydrogen/orthohydrogen ratio
(it often takes days or weeks at a given temperature to reach the
equilibrium ratio, for which the data is usually given). Hydrogen
detonation parameters, such as critical detonation pressure and
temperature, strongly depend on the container geometry.
See also
Notes
References
- 286 kJ/mol: energy per mole of the combustible material
(hydrogen)
- IUPAC Compendium of Chemical Terminology, Electronic version,
Hydrogen Bond
- § IR-3.3.2, Provisional Recommendations, Nomenclature of
Inorganic Chemistry, Chemical Nomenclature and Structure
Representation Division, IUPAC. Accessed on line October 3,
2007.
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