Hydrogen is a chemical element with symbol H and atomic number 1.
With a standard atomic weight of 7000100800000000000♠1.008, hydrogen
is the lightest element on the periodic table. Its monatomic form (H)
is the most abundant chemical substance in the Universe, constituting
roughly 75% of all baryonic mass.[note 1] Non-remnant stars are
mainly composed of hydrogen in the plasma state. The most common
isotope of hydrogen, termed protium (name rarely used, symbol 1H), has
one proton and no neutrons.
The universal emergence of atomic hydrogen first occurred during the
recombination epoch. At standard temperature and pressure, hydrogen is
a colorless, odorless, tasteless, non-toxic, nonmetallic, highly
combustible diatomic gas with the molecular formula H2. Since hydrogen
readily forms covalent compounds with most nonmetallic elements, most
of the hydrogen on Earth exists in molecular forms such as water or
Hydrogen plays a particularly important role in
acid–base reactions because most acid-base reactions involve the
exchange of protons between soluble molecules. In ionic compounds,
hydrogen can take the form of a negative charge (i.e., anion) when it
is known as a hydride, or as a positively charged (i.e., cation)
species denoted by the symbol H+. The hydrogen cation is written as
though composed of a bare proton, but in reality, hydrogen cations in
ionic compounds are always more complex. As the only neutral atom for
Schrödinger equation can be solved analytically, study
of the energetics and bonding of the hydrogen atom has played a key
role in the development of quantum mechanics.
Hydrogen gas was first artificially produced in the early 16th century
by the reaction of acids on metals. In 1766–81,
Henry Cavendish was
the first to recognize that hydrogen gas was a discrete substance,
and that it produces water when burned, the property for which it was
later named: in Greek, hydrogen means "water-former".
Industrial production is mainly from steam reforming natural gas, and
less often from more energy-intensive methods such as the electrolysis
of water. Most hydrogen is used near the site of its production,
the two largest uses being fossil fuel processing (e.g.,
hydrocracking) and ammonia production, mostly for the fertilizer
Hydrogen is a concern in metallurgy as it can embrittle many
metals, complicating the design of pipelines and storage
Electron energy levels
1.3 Elemental molecular forms
1.5.1 Covalent and organic compounds
1.5.3 Protons and acids
2.1 Discovery and use
2.2 Role in quantum theory
3 Natural occurrence
4.1 Steam reforming
4.4 Anaerobic corrosion
4.5 Geological occurrence: the serpentinization reaction
4.6 Formation in transformers
5.1 Consumption in processes
5.3 Energy carrier
5.4 Semiconductor industry
6 Biological reactions
7 Safety and precautions
10 Further reading
11 External links
Space Shuttle Main Engine
Space Shuttle Main Engine burnt hydrogen with oxygen, producing a
nearly invisible flame at full thrust.
Explosion of a hydrogen–air mixture.
Hydrogen gas (dihydrogen or molecular hydrogen) 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 is
2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)[note 2]
Hydrogen gas forms explosive mixtures with air in concentrations from
4–74% and with chlorine at 5–95%. The explosive reactions may be
triggered by spark, heat, or sunlight. The hydrogen autoignition
temperature, the temperature of spontaneous ignition in air, is
500 °C (932 °F). Pure hydrogen-oxygen flames emit
ultraviolet light and with high oxygen mix are nearly invisible to the
naked eye, as illustrated by the faint plume of the Space Shuttle Main
Engine, compared to the highly visible plume of a Space Shuttle Solid
Rocket Booster, which uses an ammonium perchlorate composite. The
detection of a burning hydrogen leak may require a flame detector;
such leaks can be very dangerous.
Hydrogen flames in other conditions
are blue, resembling blue natural gas flames.
The destruction of the Hindenburg airship was a notorious example of
hydrogen combustion and the cause is still debated. The visible orange
flames in that incident were the result of a rich mixture of hydrogen
to oxygen combined with carbon compounds from the airship skin.
H2 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
Depiction of a hydrogen atom with size of central proton shown, and
the atomic diameter shown as about twice the
Bohr model radius (image
not to scale)
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 91 nm wavelength.
The energy levels of hydrogen can be calculated fairly accurately
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 atomic electron and proton are held together by
electromagnetic force, while planets and celestial objects are held 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,
Dirac equation or even the Feynman path integral formulation to
calculate the probability density of the electron around the
proton. The most complicated treatments allow for the small
effects of special relativity and vacuum polarization. In the quantum
mechanical treatment, the electron in a ground state hydrogen atom has
no angular momentum at all—illustrating how the "planetary orbit"
differs from electron motion.
Elemental molecular forms
See also: Spin isomers of hydrogen
First tracks observed in liquid hydrogen bubble chamber at the
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
(1⁄2+1⁄2); in the parahydrogen form the spins are
antiparallel and form a singlet with a molecular spin quantum number
of 0 (1⁄2–1⁄2). 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 because 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
The uncatalyzed interconversion between para and ortho H2 increases
with increasing temperature; thus rapidly condensed H2 contains large
quantities of the high-energy ortho form that converts to the para
form very slowly. The ortho/para ratio in condensed H2 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.
Further information: Category:
Covalent and organic compounds
While H2 is not very reactive under standard conditions, it does form
compounds with most elements.
Hydrogen can form compounds with
elements that are more electronegative, such as halogens (e.g., F, Cl,
Br, I), or oxygen; in these compounds hydrogen takes on a partial
positive charge. When bonded to fluorine, oxygen, or nitrogen,
hydrogen can participate in a form of medium-strength noncovalent
bonding with the hydrogen of other similar molecules, a phenomenon
called hydrogen bonding that is critical to the stability of many
Hydrogen also forms compounds with less
electronegative elements, such as metals and metalloids, where it
takes on a partial negative charge. These compounds are often known as
Hydrogen forms a vast array of compounds with carbon called the
hydrocarbons, and an even vaster array with heteroatoms that, because
of their general association with living things, are 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 because 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. Millions of
hydrocarbons are known, and they are usually formed by complicated
synthetic pathways that seldom involve elementary hydrogen.
Main article: Hydride
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
Gilbert N. Lewis in
1916 for group 1 and 2 salt-like hydrides, was demonstrated by Moers
in 1920 by the electrolysis of molten lithium hydride (LiH), producing
a stoichiometry quantity of hydrogen at the anode. For hydrides
other than group 1 and 2 metals, the term is quite misleading,
considering the low electronegativity of hydrogen. An exception in
group 2 hydrides is BeH
2, which is polymeric. In lithium aluminium hydride, the AlH−
4 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, more than 100 binary borane hydrides are known, but only one
binary aluminium hydride. Binary indium hydride has not yet been
identified, although larger complexes exist.
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
Protons and acids
Further information: Acid–base reaction
Oxidation of hydrogen removes its electron and gives H+, which
contains no electrons and a nucleus which is usually composed of one
proton. That is why H+ 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, H+, 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 "H+" 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" (H
3O+). However, even in this case, such solvated hydrogen cations are
more realistically conceived as being organized into clusters that
form species closer to H
4. Other oxonium ions are found when water is in acidic solution
with other solvents.
Although exotic on Earth, one of the most common ions in the universe
is the H+
3 ion, known as protonated molecular hydrogen or the trihydrogen
Main article: Isotopes of hydrogen
Hydrogen discharge (spectrum) tube
Deuterium discharge (spectrum) tube
Protium, the most common isotope of hydrogen, has one proton and one
electron. Unique among all stable isotopes, it has no neutrons (see
diproton for a discussion of why others do not exist).
Hydrogen has three naturally occurring isotopes, denoted 1H, 2H and
3H. Other, highly unstable nuclei (4H to 7H) have been synthesized in
the laboratory but not observed in nature.
1H 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
2H, the other stable hydrogen isotope, is known as deuterium and
contains one proton and one neutron in the nucleus. 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.
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
Heavy water is used as a neutron moderator
and coolant for nuclear reactors.
Deuterium is also a potential fuel
for commercial nuclear fusion.
3H 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. It is so radioactive that it can
be used in luminous paint, making it useful in such things as watches.
The glass prevents the small amount of radiation from getting out.
Small amounts of tritium are produced naturally by 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 in
specialized self-powered lighting devices.
Tritium has also been
used in chemical and biological labeling experiments as a
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 their own names, but such names
are no longer used, except for deuterium and tritium. The symbols D
and T (instead of 2H and 3H) are sometimes used for deuterium and
tritium, but the corresponding symbol for protium, 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 (IUPAC) allows any of D, T, 2H, and 3H to be used, although
2H and 3H are preferred.
The exotic atom muonium (symbol Mu), composed of an antimuon and an
electron, is also sometimes considered as a light radioisotope of
hydrogen, due to the mass difference between the antimuon and the
Muonium was discovered in 1960. During the muon's
7000220000000000000♠2.2 µs lifetime, muonium can enter into
compounds such as muonium chloride (MuCl) or sodium muonide (NaMu),
analogous to hydrogen chloride and sodium hydride respectively.
Discovery and use
Main article: Timeline of hydrogen technologies
Robert Boyle discovered 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 naming the gas from
a metal-acid reaction "inflammable air". He speculated that
"inflammable air" was in fact identical to the hypothetical substance
called "phlogiston" and further finding in 1781 that the gas
produces water when burned. He is usually given credit for the
discovery of hydrogen as an element. In 1783, Antoine Lavoisier
gave the element the name hydrogen (from the Greek ὑδρο- hydro
meaning "water" and -γενής genes meaning "creator") when he
and Laplace reproduced Cavendish's finding that water is produced when
hydrogen is burned.
Antoine-Laurent de Lavoisier
Lavoisier produced hydrogen for his experiments on mass conservation
by reacting a flux of steam with metallic iron through an incandescent
iron tube heated in a fire. Anaerobic oxidation of iron by the protons
of water at high temperature can be schematically represented by the
set of following reactions:
Fe + H2O → FeO + H2
2 Fe + 3 H2O → Fe2O3 + 3 H2
3 Fe + 4 H2O → Fe3O4 + 4 H2
Many metals such as zirconium undergo a similar reaction with water
leading to the production of hydrogen.
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
François Isaac de Rivaz built
the first de Rivaz engine, an internal combustion engine powered by a
mixture of hydrogen and oxygen in 1806.
Edward Daniel Clarke
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
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 6 May 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 and commercial hydrogen airship travel ceased.
Hydrogen is still used, in preference to non-flammable but more
expensive helium, as a lifting gas for weather balloons.
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. In the dark
part of its orbit, the
Hubble Space Telescope
Hubble Space Telescope is also powered by
nickel-hydrogen batteries, which were finally replaced in May
2009, more than 19 years after launch and 13 years beyond their
Role in quantum theory
Hydrogen emission spectrum lines in the visible range. These are the
four visible lines of the Balmer series
Because of its 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, study
of the corresponding simplicity of the hydrogen molecule and the
corresponding cation H+
2 brought 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 H2 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 H2 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.
Antihydrogen (H) is the antimatter counterpart to hydrogen. It
consists of an antiproton with a positron.
Antihydrogen is the only
type of antimatter atom to have been produced as of 2015.
NGC 604, a giant region of ionized hydrogen in the Triangulum Galaxy
Hydrogen, as atomic H, is the most abundant chemical element in the
universe, making up 75% of normal matter by mass and more than 90% by
number of atoms. (Most of the mass of the universe, however, is not in
the form of chemical-element type matter, but rather is postulated to
occur as yet-undetected forms of mass such as dark matter and dark
energy.) This element is found in great abundance in stars and gas
giant planets. Molecular clouds of H2 are associated with star
Hydrogen plays a vital role in powering stars through the
proton-proton reaction and the
CNO cycle nuclear fusion.
Throughout the universe, hydrogen is mostly found in the atomic and
plasma states, with properties quite different from those of 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, H2. 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, mostly 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, as is methane, itself
a hydrogen source of increasing importance.
A molecular form called protonated molecular hydrogen (H+
3) is found in the interstellar medium, where it is generated by
ionization of molecular hydrogen from cosmic rays. This charged ion
has also been observed in the upper atmosphere of the planet Jupiter.
The ion is relatively stable in the environment of outer space due to
the low temperature and density. H+
3 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 H3 can exist only in an excited form and is
unstable. By contrast, the positive hydrogen molecular ion (H+
2) is a rare molecule in the universe.
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.
Hydrogen can be prepared in several different ways, but economically
the most important processes involve removal of hydrogen from
hydrocarbons, as about 95% of hydrogen production came from steam
reforming around year 2000. Commercial bulk hydrogen is usually
produced by the steam reforming of natural gas. At high
temperatures (1000–1400 K, 700–1100 °C or
1300–2000 °F), steam (water vapor) reacts with methane to
yield carbon monoxide and H
4 + H
2O → CO + 3 H
This reaction is favored at low pressures but is nonetheless conducted
at high pressures (2.0 MPa, 20 atm or 600 inHg). This
is because high-pressure H
2 is the most marketable product and pressure swing adsorption (PSA)
purification systems work better at higher pressures. 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:
4 → C + 2 H
Consequently, steam reforming typically employs an excess of H
2O. 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 + H
2O → CO
2 + H
Other important methods for H
2 production include partial oxidation of hydrocarbons:
4 + O
2 → 2 CO + 4 H
and the coal reaction, which can serve as a prelude to the shift
C + H
2O → CO + H
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
In the laboratory, H
2 is usually prepared by the reaction of dilute non-oxidizing acids on
some reactive metals such as zinc with Kipp's apparatus.
Zn + 2 H+ → Zn2+ + H
Aluminium can also produce H
2 upon treatment with bases:
2 Al + 6 H
2O + 2 OH− → 2 Al(OH)−
4 + 3 H
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 in the range 80–94%.
2O(l) → 2 H
2(g) + O
An alloy of aluminium and gallium in pellet form added to water can be
used to generate hydrogen. The process also produces 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, as hydrogen can be produced on-site and does
not need to be transported.
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(IV) oxide–cerium(III) 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.
Under anaerobic conditions, iron and steel alloys are slowly oxidized
by the protons of water concomitantly reduced in molecular hydrogen (H
2). The anaerobic corrosion of iron leads first to the formation of
ferrous hydroxide (green rust) and can be described by the following
Fe + 2 H
2O → Fe(OH)
2 + H
In its turn, under anaerobic conditions, the ferrous hydroxide (Fe(OH)
2) can be oxidized by the protons of water to form magnetite and
molecular hydrogen. This process is described by the Schikorr
2 → Fe
4 + 2 H
2O + H
ferrous hydroxide → magnetite + water + hydrogen
The well crystallized magnetite (Fe
4) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)
This process occurs during the anaerobic corrosion of iron and steel
in oxygen-free groundwater and in reducing soils below the water
Geological occurrence: the serpentinization reaction
In the absence of atmospheric oxygen (O
2), in deep geological conditions prevailing far away from Earth
atmosphere, hydrogen (H
2) is produced during the process of serpentinization by the anaerobic
oxidation by the water protons (H+) of the ferrous (Fe2+) silicate
present in the crystal lattice of the fayalite (Fe
4, the olivine iron-endmember). The corresponding reaction leading to
the formation of magnetite (Fe
4), quartz (SiO
2) and hydrogen (H
2) is the following:
4 + 2 H
2O → 2 Fe
4 + 3 SiO
2 + 3 H
fayalite + water → magnetite + quartz + hydrogen
This reaction closely resembles the
Schikorr reaction observed in the
anaerobic oxidation of the ferrous hydroxide in contact with water.
Formation in transformers
From all the fault gases formed in power transformers, hydrogen is the
most common and is generated under most fault conditions; thus,
formation of hydrogen is an early indication of serious problems in
the transformer's life cycle.
Consumption in processes
Large quantities of H
2 are needed in the petroleum and chemical industries. The largest
application of H
2 is for the processing ("upgrading") of fossil fuels, and in the
production of ammonia. The key consumers of H
2 in the petrochemical plant include hydrodealkylation,
hydrodesulfurization, and hydrocracking. H
2 has several other important uses. H
2 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. H
2 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
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's high solubility is a metallurgical
problem, contributing to the embrittlement of many metals,
complicating the design of pipelines and storage tanks.
Apart from its use as a reactant, H
2 has wide applications in physics and engineering. It is used as a
shielding gas in welding methods such as atomic hydrogen
welding. H2 is used as the rotor coolant in electrical
generators at power stations, because it has the highest thermal
conductivity of any gas. Liquid H2 is used in cryogenic research,
including superconductivity studies. Because H
2 is lighter than air, having a little more than 1⁄14 of the
density of air, it was once widely used as a lifting gas in balloons
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.
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
Main article: Hydrogen-cooled turbo generator
Hydrogen is commonly used in power stations as a coolant in generators
due to a number of favorable properties that are a direct result of
its light diatomic molecules. These include low density, low
viscosity, and the highest specific heat and thermal conductivity of
Hydrogen economy and
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 requires more energy to
make 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
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
2 sequestration followed by carbon capture and storage could be
conducted at the point of H
2 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. Fuel cells can convert hydrogen and oxygen directly
to electricity more efficiently than internal combustion engines.
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, LaAlO3, SiO2, Al2O3, ZrSiO4, HfSiO4,
Biohydrogen and Biological hydrogen production
H2 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 H2 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. The natural cycle of hydrogen production and consumption
by organisms is called the hydrogen cycle.
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 H2 gas by specialized hydrogenases in the chloroplast.
Efforts have been undertaken to genetically modify cyanobacterial
hydrogenases to efficiently synthesize H2 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 asphyxiant in
its pure, oxygen-free form. In addition, liquid hydrogen is a
cryogen and presents dangers (such as frostbite) associated with very
Hydrogen dissolves in many metals, and, in addition
to leaking out, may have adverse effects on them, such as hydrogen
embrittlement, leading to cracks and explosions. 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.
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ISBN 1-55963-703-X. CS1 maint: Multiple names: authors list
Scerri, Eric (2007). The Periodic System, Its Story and Its
Significance. New York: Oxford University Press.
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