Hydrogen

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Hydrogen

Triple point 13.8033 K, 7.042 kPa

Hydrogen (, from : "water" and : "forming") 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 (H₂). With an atomic mass of just 1.00794 g/mol, hydrogen is the lightest element of the universe. It is also the most abundant, constituting roughly 75% of all the universe's elemental matter.Hydrogen in the Universe, NASA Website. URL accessed on 2 June 2006. It is present in water, all organic compounds, and in all living organisms. Hydrogen is able to react chemically with most other elements. Stars in their main sequence are overwhelmingly composed of hydrogen in its plasma state. The element is currently used primarily in fossil fuel upgrading. Other uses include as a lifting gas, as an alternative fuel (see Hydrogen economy), and more recently as a power source in fuel cells.

Different meanings of "hydrogen"

The word "hydrogen" means different things to different people, leading to much confusion. 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 would be a convenient fuel except that it occurs naturally only in trace amounts and must be extracted from other sources, such as fossil fuels; 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.

Thus when one says that "hydrogen is ubiquitous in the universe, but surprisingly difficult to produce in large quantities on the Earth" we mean that H atoms and H₂ occur in interstellar space but H atoms are rare and difficult to generate in concentrated form on Earth, and even H₂ is relatively expensive. The Earth has lots of hydrogen, but it is bound up in molecules such as hydrocarbons and water, from which it requires energy and effort to remove.

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. However, it is very rare in the Earth's atmosphere (1 ppm by volume). Its scarcity is due to the fact that hydrogen is the lightest gas, allowing it to escape Earth's gravity. When compounds are included, though, hydrogen is the tenth most abundant element on Earth. The most common source for this element on Earth is water, which is composed two parts hydrogen to one part oxygen (H₂O). Other sources include most forms of organic matter including coal, natural gas, and other fossil fuels. Methane (CH₄) is an increasingly important source of hydrogen.

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. 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.

Isotopes

Hydrogen is the only element that has different names for its isotopes. (During the early study of radioactivity, various heavy radioactive isotopes were given names, but such names are no longer used, although one element, radon, has a name that originally applied to only one of its isotopes.) The symbols D and T (instead of ²H and ³H) are sometimes used for deuterium and tritium, although IUPAC states that while this use is common it is not "preferred." (The symbol P is already in use for phosphorus and is not available for protium.)

¹H

The most common isotope of hydrogen, this stable isotope has a nucleus consisting of a single proton; hence the descriptive, although rarely used, name protium. The spin of a protium atom is 1/2+.

²H

The other stable isotope is deuterium, with an extra neutron in the nucleus. Deuterium comprises 0.0184%–0.0082% of all hydrogen on Earth (IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water. The spin of a deuterium atom is 1+. Harold C. Urey discovered deuterium in 1931, by spectrographic study of the last residual milliliter after evaporation of 5 liters of cryogenically-produced liquid hydrogen. Urey was also able to concentrate deuterium in water by repeated fractional distillation. For the discovery of deuterium Urey received the Nobel Prize in Chemistry in 1934. In the same year, the discovery of the third isotope, tritium, was announced.

³H

The third naturally occurring hydrogen isotope is the radioactive tritium. The tritium nucleus contains two neutrons in addition to the proton. It decays through beta decay and has a half-life of 12.32 years. Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. Like ordinary hydrogen, tritium reacts with the oxygen in the atmosphere to form T₂O. This radioactive "water" molecule constantly enters the Earth's seas and lakes in the form of slightly radioactive rain, but its half-life is short enough to prevent a buildup of hazardous radioactivity. The spin of a tritium atom is 1/2+.

⁴H

Hydrogen-4 was synthesized by bombarding tritium with fast-moving deuterium nuclei. It decays through neutron emission and has a half-life of 9.93696x10⁻²³ seconds. The spin of a hydrogen-4 atom is 2-.

⁵H

In 2001 scientists detected hydrogen-5 by bombarding a hydrogen target with heavy ions. It decays through neutron emission and has a half-life of 8.01930x10⁻²³ seconds.

⁶H

Hydrogen-6 decays through triple neutron emission and has a half-life of 3.26500x10⁻²² seconds.

⁷H

In 2003 hydrogen-7 was created (article) at the RIKEN laboratory in Japan by colliding a high-energy beam of helium-8 atoms with a cryogenic hydrogen target and detecting tritons—the nuclei of tritium atoms—and neutrons from the breakup of hydrogen-7, the same method used to produce and detect hydrogen-5.

Electronic structure

Hydrogen is the lightest chemical element; its most common isotope comprises just one negatively charged electron, distributed around a positively charged proton (the nucleus of the hydrogen atom — all other atoms have more complex nuclei involving more protons or neutrons). The electron is bound to the proton by the Coulomb force, the electrical force that one stationary electrically charged particle exerts on another. The hydrogen atom has special significance in quantum mechanics as a simple physical system for which there is an exact solution to the Schrödinger equation; from that equation, the experimentally observed frequencies and intensities of hydrogen's spectral lines can be calculated. Spectral lines are dark or bright lines in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies.

At standard temperature and pressure, hydrogen exists as the diatomic gas, H₂, with a boiling point of 20.27 K, and a melting point of 14.02 K. Under extreme pressures, such as those at the center of gas giants, the molecules lose their identity and the hydrogen becomes a metal (metallic hydrogen). Under the extremely low pressure in space — virtually a vacuum — the element tends to exist as individual atoms, simply because it is statistically unlikely for them to combine. However, clouds of H₂ and possibly single hydrogen atoms are said to form in H I and H II regions and are associated with star formation. Hydrogen plays a vital role in powering stars through the proton–proton and carbon–nitrogen cycle. These are nuclear fusion processes, which release huge amounts of energy in stars and other hot celestial bodies as hydrogen atoms combine into helium atoms.

At high temperatures, hydrogen gas can exist as a mixture of atoms, protons, and negatively charged hydride ions. This mixture has a high emissivity and absorptivity in the visible light range, and such emanations give rise to the light from the sun and other stars.

Electron energy levels

See also: hydrogen atom.

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.

With the Bohr Model the energy levels of hydrogen can be calculated fairly accurately. This is done by modeling the electron as revolving around the proton, much like the earth revolving around the sun, except that the sun holds earth in orbit with the force of gravity, but the proton holds the electron in orbit with the force of electromagnetism. Another difference between the Earth-Sun system and the electron-proton system is that, in this model, due to quantum mechanics the electron is allowed to only to be at specific distances from the proton. Today, the hydrogen atom is most accurately described by the use of a pure quantum mechanical model. This model uses Schrodinger's wave equation to calculate the probability density of the electron around the nucleus in a H-atom. Since the electron is treated not as a particle but as a matter wave, its dual properties agree very well with supposedly paradoxical results obtained by experiments which are unexplainable under Bohr Model. The wave features of electron are again well explained by de Broglie's equation hν = mV. This equation relates ν (frequency of the electron matter wave) to mV (the momentum of the material electron). In this way, electrons are treated at different times as either matter and as a wave, depending on the needs of the model. Modeling the hydrogen atom in this fashion yields the correct energy levels and spectrum. As an added feature, modeling the system fully using the reduced mass of nucleus and electron (as one would do in the two-body problem in celestial mechanics) yields an even better formula for the hydrogen spectra, and also the correct spectral shifts for the isotopes deuterium and tritium, which are induced by changes only in this parameter.

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 5.9 x 10^-6 eV higher than when they are not aligned. The transition from the upper to lower levels can occur through emission of a photon through a magnetic dipole transition. A photon of this energy has a frequency of 1420.4 MHz and a wavelength of 21.1 cm. Astronomers observe this radiation with radio telescopes in order to map the distribution of hydrogen in the Galaxy.

Elemental molecular forms

Under normal conditions, hydrogen gas is a mixture of two different kinds of molecules which differ from one another by the relative spin of the nuclei. These two forms are known as ortho- and para-hydrogen (this is different from isotopes, see below). In ortho-hydrogen the nuclear spins are parallel and form a triplet, whereas in the para form. the spins are antiparallel, giving rise to a singlet. At standard conditions hydrogen is composed of about 25% of the para form and 75% of the ortho form (the so-called "normal" form). The equilibrium ratio of these two forms depends on temperature, but since the ortho form has higher energy (is an excited state), it cannot be stable in its pure form. At low temperatures (around boiling point), the equilibrium state is comprised almost entirely of the para form.

The uncatalized interconversion between para and ortho H₂ is slow enough that rapidly condensed H₂ contains large quantities of the high-energy ortho form. The ortho/para ratio is important in the preparation and storage of liquid H₂, since the ortho-para conversion produces more heat than the heat of its evaporation, which can cause much hydrogen to be lost by evaporation in this way for several days after liquefying. Therefore, catalysts for the ortho-para interconversion process (such as iron filings) 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.

Elemental hydrogen can exist in over 50 different forms, arising from either ionized species such H⁺, H⁻, H²⁺…H^1- , or from the three isotopes: H-1, H-2(D), H-3(T), and their corresponding ions which also include H with different nuclear spin isomers.

History

Discovery of H₂

Hydrogen gas, H₂, was first produced and formally described by Theophrastus Bombastus von Hohenheim (1493–1541)—also known as Paracelsus— in a method which mixed 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, and not of 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 (with Laplace) reported that pure water is produced by burning hydrogen and oxygen. This was essentially a confirmation of Cavendish's finding (and also some earlier work by Joseph Priestley), but it was Lavoisier's name for the gas which won out.

Early uses

One of the first uses of H₂ was for balloons. The H₂ was obtained by reacting sulfuric acid and metallic iron.

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 absobed 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-1920's.

Interestingly, one of the first quantum effects to be explicitly noticed (but not understood at the time) was Maxwell's observation, half a century before full quantum mechanical theory arrived, that the specific heat capacity of H₂ unaccountably resembles that of a monatomic gas below room temperature. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H₂ due to its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gasses composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.

Production

H₂ is produced in the laboratory (often not intentionally); in industry for the hydrogenation of unsaturated substrates; and in nature as a means of expelling reducing equivalents in biochemical reactions.

Laboratory routes to H₂

In the laboratory, H₂ is usually prepared by the reaction of acids on metals such as zinc.

Zn + 2 H⁺ → Zn²⁺ + H₂

Al produces H₂ upon treatment with acids but also with base:

2 Al + 6 H₂O → 2 Al(OH)₃ + 3 H₂

The electrolysis of water is a simple but expensive method of producing hydrogen. Typically the cathode electrode is made from platinum.

Industrial routes to H₂

Hydrogen can be prepared in several different ways but the economically 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 (700–1100 °C), steam (water vapor) reacts with methane to yield carbon monoxide and H₂.

CH₄ + H₂O → CO + 3 H₂

This reaction is favored at low pressures but is nonetheless conducted at high pressures (20 atm) since high pressure H₂ is the most marketable product. One of the many complications to this highly optimized technology is the formation of coke or carbon:

CH₄ → C + 2 H₂

Consequently, steam reforming typically employs an excess of H₂O.

Additional hydrogen from steam reforming can be recovered from the carbon monoxide through the Water gas shift reaction:

CO + H₂O → CO₂ + H₂

Other important methods for H₂ production include partial oxidation of hydrocarbons:

CH₄ + 0.5 O₂ → CO + 2 H₂

and water electrolysis.

Note: hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the Haber process for the production of ammonia and the world's fifth most produced industrial compound, hydrogen is generated in situ from natural gas.

Biological routes to H₂

H₂ is produced by several microorganisms, usually via reactions catalyzed by enzymes called hydrogenases. These Fe and sometimes Ni-containing catalyst transfer reducing equivalents from fermentaton to water.Cammack, R.; Frey, M.; Robson, R. Hydrogen as a Fuel: Learning from Nature; Taylor & Francis: London, 2001. Some of these organisms will split water, via operation of both an O₂ and H₂ generating cycles, which operate in the light and the dark, respectively.

Nature employs various minor but mechanistically interesting routes to H₂. Nitrogenase produces approximately one equivalent of H₂ for each equivalent of N₂ reduced to ammonia. Some phosphatases reduced phosphite to H₂.

Chemical and physical properties

H₂ is less soluble in water, alcohol, or ether than oxygen is. Its solubility and adsorption characteristics with various metals are very important in metallurgy (as many metals can suffer hydrogen embrittlement) and in developing safe ways to store it for use as a fuel.

Combustion

Hydrogen gas has the widest % range ignition mix with air of any known flammable agent, and will burn at concentrations as low as 4% H₂ in air. 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 (as opposed to the easily visible flames from the shuttle boosters). Thus it is difficult to visually detect if a hydrogen leak is burning and for similar reasons, it is easy to walk into a pure hydrogen fire inadvertently. (It should be noted that the flames from the burning Hindenburg, seen at right, are from the covering skin, which contained carbon and pyrophoric aluminium powder). Another characteristic of hydrogen fires, is that they tend to rapidly lift off the ground, causing less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived, partly for this reason.

The enthalpy of combustion for hydrogen is -286 kJ/mol; it combusts according to the following balanced equation.

2 H₂(g) + O₂(g) → 2 H₂O(g) + 572 kJ

H₂ reacts directly with other oxidizing elements. A violent reaction can occur with chlorine and fluorine, forming the corresponding hydrogen halides, HCl and HF.

Compounds

See also Hydrogen compounds.

Covalents and organics

Hydrogen forms compounds with most other elements, although interestingly H₂ does not directly react with most common elements. For example, millions of hydrocarbons are known, but none arise from direct reactions of hydrogen and carbon. Hydrogen with an electronegativity of 2.2 (Pauling's scale) forms compounds with elements that are both more electronegative such as halogens (F, Cl, Br, I) and chalcogens (O, S, Se). It also forms compounds with elements that are less electronegative, such as the metals and metalloids.

Hydrogen forms a vast array of compounds with carbon. Because of their association with living things, these compounds are called organic compounds, and their study is called organic chemistry. (By some definitions "organic" compounds are only required to contain carbon; however most of them also contain hydrogen, and it is addition of hydrogen which gives them their particular chemical characteristics). The chemistry of hydrogen and carbon in combination is the basis for biochemistry.

Hydrides

Many compounds of hydrogen are called hydrides, but the term is used fairly loosely. To chemists, the term "hydride" usually implies that the H atom has acquired a negative charge, H⁻-like. The hydride ion itself, H⁻, exists only in few compounds such as alkali metals hydrides. In fact in 1920, K. Moers demonstrated that electrolysis of molten lithium hydride LiH (m.p. 692 °C) produced the stoichiometric quantity of hydrogen at the anode. Well known hydrides include NaH, an ionic solid, and lithium aluminum hydride, a salt containing the AlH₄⁻ complex anion. Palladium hydride contains insterstitial hydrogen atoms, i.e. the H atoms are bonded to multiple Pd atoms without perturbing the overall Pd framework. Hydrogen forms hydrides with all main group elements with the exception of the noble gases and indium and thallium.

"Protons" and acids

Oxidation of H₂ formally gives the proton, H⁺. The proton is central to discussions of acids and the term proton is loosely used to refer to hydrogen with H⁺-like character. Being a bare nucleus, H⁺ cannot exist in solution; it would have a strong tendency to attach itself to atoms or molecules with electrons. In acknowledgement of the non-existence of H⁺, chemists sometimes discuss acidic aqueous solutions in the context of hydronium (H₃O⁺). Even the hydronium ion is a poor representation of the "solvated proton"; H₉O₄⁺ is a better description.

Although exotic on earth, one of the most common ions in the universe is the H₃⁺ ion.

H₂ reacts with oxygen to form water, H₂O. Considerable energy is released in this process. At room temperature no reaction occurs between H₂ and O₂ in the absence of a catalyst.

Applications

Large quantities of H₂ are needed in the petroleum and chemical industries. By far the largest application of H₂ is for the processing ("upgrading") of fossil fuels. The key consumers of H₂ in the petrochemical plant include hydrodealkylation, hydrodesulfurization, and hydrocracking. H₂ has several other important uses.

used in the hydrogenation of fats and oils (found in items such as margarine), and in the production of methanol.
H₂ is used in the manufacture of hydrochloric acid
H₂ is used in certain welding methods
H₂ is used in the reduction of metallic ores.
H₂ is an ingredient in some rocket fuels.
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.
The triple point temperature of equilibrium hydrogen is a defining fixed point on the ITS-90 temperature scale.
Since H₂ 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.
Deuterium, an isotope of hydrogen (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.

Hydrogen as an energy source

Hydrogen is not a pre-existing source of energy like fossil fuels, but a carrier, much like a battery. There are no "hydrogen wells" or "hydrogen mines" on Earth, so H₂ cannot be considered a primary energy source such as sunlight or uranium. Since H₂ is so light, any amount present on earth will float up into the atmosphere and out into space. H₂ can however be burned in internal combustion engines, an approach advocated by BMW's experimental hydrogen car. There are several methods of storing hydrogen for transport applications, the most commonly-used being gaseous storage in gas cylinders similar to those used for the storage of any pressurised gas. Alternatives include storage as metal or chemical hydrides, cryogenic storage of liquid hydrogen (as in BMW's hydrogen internal combustion engine car) and research points to nanomaterials that will be able to store hydrogen more efficiently than any of the methods above.

Hydrogen fuel cells are being investigated as mobile power sources with lower emissions than hydrogen-burning internal combustion engines. The low emissions of hydrogen in internal combustion engines and fuel cells are currently offset by the pollution created by hydrogen production. This may change if the substantial amounts of electricity required for water electrolysis can be generated primarily from low pollution sources such as solar energy or wind. Research is being conducted on H₂ as a replacement for fossil fuels. It could become the link between a range of energy sources, carriers and storage. H₂ can be converted to and from electricity (solving the electricity storage and transport issues), from biofuels, and from and into natural gas and diesel fuel. All of this can theoretically be achieved with zero emissions of CO₂ and toxic pollutants. ''

The extraction of H₂ from water or hydrocarbons requires energy; these are endothermic processes. H₂ cannot be produced from water or hydrocarbons without the expenditure of energy, and this problem is the central quandry confronting hydrogen production. The one possibly sustainable method for production of H₂ entails photochemical water "splitting," where the input energy comes from our sun. This approach avoids production of greenhouse-gases, which are associated with fossil fuels. Certain species of green algae utilize this method under very special conditions. A second possible hydrogen production route is via the sulfur-iodine cycle using a heat source such as nuclear energy, but there are arguments about whether or not nuclear energy should be considered sustainable energy. Stripping H₂ from biomass or even purified organic compounds such as glucose or sorbitol also generates CO₂, but this process, like woodburning, is not a net greenhouse producer because the CO₂ is derived from the atmosphere to begin with.

Cited references

General references

Author interview at Global Public Media.

External links

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