Encyclopedia
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Triple point || 13.8033 K, 7.042 kPa
Hydrogen is a
chemical element in the periodic table that has the symbol
H and atomic number 1. At standard temperature and pressure it is a colorless, odorless, nonmetallic, univalent, tasteless, highly flammable
diatomic gas . With an atomic mass of 1.00794 g/mol, hydrogen is the lightest element. It is also the most
abundant, constituting roughly 75% of the universe's elemental matter.
Stars in their
main sequence are overwhelmingly composed of hydrogen in its
plasma state. Elemental hydrogen is industrially produced from
hydrocarbons and is used primarily in fossil fuel upgrading but has a variety of other applications in both the energy and other sectors of the world's economy.
The most common naturally occurring isotope of hydrogen contains one
electron and an atomic nucleus of one
proton. In ionic compounds it can take on either a positive charge or a negative charge . Hydrogen can form compounds with most elements and is present in
water and all
organic compounds. It plays a particularly important role in acid-base chemistry, in which many reactions involve the exchange of protons between soluble molecules. As the only element for which the 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.
Different meanings of "hydrogen"
The word "hydrogen" has several different meanings that are important to distinguish. Possible uses:
- Hydrogen is the name of an element.
- Hydrogen is an atom, sometimes called "H dot" that is abundant in space but essentially absent on earth, because it dimerizes.
- Hydrogen is a diatomic molecule that occurs naturally in trace amounts in the Earth's atmosphere; chemists increasingly refer to H2 as dihydrogen to distinguish this molecule from atomic hydrogen and hydrogen found in other compounds.
- Hydrogen is atomic constituent within all organic compounds, water, and many other chemical compounds.
It is especially important not to confuse
elemental forms of hydrogen with hydrogen as it appears in chemical compounds.
History
Discovery of H2
Hydrogen gas, H
2, was first artificially produced and formally described by Theophrastus Bombastus von Hohenheim —also known as
Paracelsus—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", and further finding that the gas produces water when burned in air. Cavendish had stumbled on hydrogen when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a liberated component of the mercury rather than the acid, he was still able to accurately describe several key properties of hydrogen, including the fact that it produced water when burned. In 1783
Antoine Lavoisier gave the element its name and reported that pure water is produced by burning hydrogen and
oxygen.
One of the first uses of H
2 was for
balloons. The H
2 was obtained by reacting
sulfuric acid and metallic
iron. Infamously, H
2 was used in the Hindenburg airship that was destroyed in a midair fire.
Role in history of 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 H
2+ 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 was Maxwell's observation, half a century before full quantum mechanical theory arrived, 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 rotational energy levels, which are particularly wide-spaced in H
2 due to 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 nuclear fusion.
Throughout the universe, hydrogen is mostly found in the
plasma state 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 . 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.
Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H
2 . However, hydrogen gas is very rare in the Earth's atmosphere due to its light weight, which enables it to escape from Earth's gravity more easily than heavier gases. Although H atoms and H
2 molecules are abundant in interstellar space, they are difficult to generate, concentrate, and purify on Earth. Most of the Earth's hydrogen is in the form of chemical compounds such as
hydrocarbons and
water – mostly water, in fact. Hydrogen gas is produced by some
bacteria and
algae and is a natural component of flatus.
Methane is a hydrogen source of increasing importance.
The hydrogen atom
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.
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, electrons and protons are attracted to one another by the electromagnetic force, while planets and celestial objects are attracted to each other by
gravity. Due to the discretization of energy inherent in
quantum mechanics, the electron in the Bohr model can only occupy certain allowed distances from the proton. A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation to calculate the probability density of the electron around the proton. Treating the electron as a matter wave reproduces experimental results such as the energy levels and
hydrogen spectrum more accurately than the particle-based Bohr model. Finally, modeling the system fully using the reduced mass of nucleus and electron yields an even better formula for the hydrogen spectra, and also the correct spectral shifts for the isotopes
deuterium and
tritium.
The electronic ground state energy level is split into hyperfine structure levels because of
magnetic effects due to the quantum mechanical spin of the electron and proton. The energy of the atom when the proton and electron spins are aligned is higher than when they are not aligned. The transition between these two states can occur through emission of a photon through a
magnetic dipole transition.
Radio telescopes can detect the radiation produced in this process, which is used to map the distribution of hydrogen in the galaxy.
Isotopes
Hydrogen has three naturally occurring isotopes, denoted
1H,
2H, and
3H. Other, highly unstable nuclei 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 protium.
- 2H, the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in its nucleus. Deuterium comprises 0.0026–0.0184% of all hydrogen on Earth. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium is used in nuclear fusion reactions, as a radiolabel in biochemistry, and as a compound, in solvents for 1H-NMR spectroscopy. Heavy water is used as a neutron moderator and coolant for nuclear reactors.
- 3H is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decays through beta decay with a half-life of 12.32 years In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state; in the parahydrogen form the spins are antiparallel and form a singlet. 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 physical properties of pure parahydrogen differ slightly from those of the normal form. The ortho/para distinction also occurs in other hydrogen-containing molecules or functional groups, such as water and methylene.
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 convert 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, since the ortho-para conversion is exothermic and produces enough heat to evaporate the hydrogen liquid, which causes hydrogen loss after liquefying.
Catalysts for the ortho-para interconversion, such as
iron compounds, are used during hydrogen cooling.
Chemical and physical properties
The solubility and
adsorption characteristics of hydrogen with various metals are very important in metallurgy and in developing safe ways to store it for use as a fuel. Hydrogen is highly soluble in many compounds composed of
rare earth metals and
transition metals and can be dissolved in both
crystalline and
amorphous metals. Hydrogen solubility in metals is influenced by local distortions or impurities in the metal
crystal lattice.
Combustion
Hydrogen gas is highly flammable and will burn at concentrations as low as 4% H
2 in air. The enthalpy of combustion for hydrogen is -286 kJ/mol; it combusts according to the following balanced equation.
- 2 H2 + O2 ? 2 H2O + 572 kJ
When mixed with oxygen across a wide range of proportions, hydrogen explodes upon ignition. Uniquely, hydrogen-oxygen flames are nearly invisible to the naked eye, as illustrated by the faintness of flame from the main
Space Shuttle engines . Thus it is difficult to visually detect if a hydrogen leak is burning. Although it is widely believed that the Hindenburg zeppelin burned due to the hydrogen gas it contained, the flames seen at right are actually from the covering skin of the blimp that contained carbon and pyrophoric aluminium powder. Another characteristic of hydrogen fires is that the flames tend to ascend rapidly with the gas in air, causing less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived, partly for this reason.
H
2 reacts directly with other oxidizing elements. A violent reaction can occur with chlorine and
fluorine, forming the corresponding hydrogen halides, HCl and HF.
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 and chalcogens ; 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 association with living things, these compounds 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. .
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 and
aluminum complexes, as well as in clustered
carboranes.
Other rarer but mechanistically interesting routes to H
2 production also exist in nature. Nitrogenase produces approximately one equivalent of H
2 for each equivalent of N
2 reduced to ammonia. Some phosphatases reduce phosphite to H
2.
Applications
Large quantities of H
2 are needed in the petroleum and chemical industries. The largest applications of H
2 is for the processing 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 , 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.
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. 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 H
2 is lighter than air, having a little more than 1/15th of the density of air, it was once widely used as a lifting agent in
balloons and
airships. However, this use was curtailed after the
Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
Hydrogen's rarer isotopes also each have specific applications.
Deuterium 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 , 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.
Hydrogen as an energy carrier
Having been used as an ingredient in some rocket fuels for several decades, hydrogen, or more specifically H
2, is now widely discussed in the context of energy. Hydrogen is not an energy
source, since it is not an abundant natural resource and more energy is used to produce it than can be ultimately extracted from it. However, it could become useful as a
carrier of energy, as elucidated in the
United States Department of Energy's 2003 report, “Among the various alternative energy strategies, building an energy infrastructure that uses hydrogen — the third most abundant element on the earth’s surface — as the primary
carrier that connects a host of energy sources to diverse end uses may enable a secure and clean energy future for the Nation.” One theoretical advantage of using H
2 as a carrier, is the localization and concentration of environmentally unwelcome aspects of hydrogen manufacture. For example, CO
2 sequestration could be conducted at the point of H
2 production.
See also
References
Further reading
External links
