Hydrogen

Hydrogen (Latin: hydrogenium, from Greek: hydro: water, genes: 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. Hydrogen is the lightest and most abundant element in the universe. It is present in water, nearly 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. Here is a glossary:

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 that these two species are rare or expensive to generate in pure form on earth. Obviously, the earth has lots of hydrogen, but it is all bound up in molecules such as hydrocarbons and water. In the laboratory, H₂ is prepared by the reaction of acids on metals such as zinc. The electrolysis of water is a simple but expensive method of producing hydrogen. Large-scale production is usually by the process called steam reforming of natural gas.

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. Stripping H₂ from biomass or even purified organic compounds such as glucose or sorbitol also generate greenhouse gases, the precursors are after all only hydrocarbons. Furthermore all such processes require catalysts.

Basic features

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 nanoparticle 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.^{[citation needed]} 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.

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

It reacts violently with chlorine and fluorine, forming hydrohalic acids, which can damage the lungs and other tissues. In air, it is highly flammable, burning at concentrations as low as 4% H₂. When mixed with oxygen, hydrogen explodes upon ignition. A unique property of hydrogen is that its flame is nearly invisible in air. This makes it difficult to tell if a leak is burning, and carries the added risk that it is easy to walk into a hydrogen fire inadvertently.

See also: hydrogen atom.

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^[1]. 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 14.5 times lighter than 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.

There are no "hydrogen wells" or "hydrogen mines" on Earth, so H₂ cannot be considered a primary energy source such as fossil fuels 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. However, it is currently difficult and dangerous to store and handle in sufficient quantity for motor fuel use. 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. (See also Hydrogen economy.)

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.

History

Discovery of H₂

H₂ was first produced by Theophrastus Bombastus von Hohenheim (1493–1541)—also known as Paracelsus—by mixing metals with acids. He was unaware that the inflammable gas produced by this chemical reaction was H₂. In 1671, Robert Boyle described the reaction between iron filings and dilute acids, which results in the production of H₂.^[2] In 1766, Henry Cavendish was the first to recognize H₂ as a discrete substance, by identifying the gas from this reaction as "inflammable" and finding that the gas produces water when burned in air. Cavendish stumbled on H₂ when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a compound of mercury—and not of the acid—he was still able to accurately describe several key properties of hydrogen.

Antoine Lavoisier gave the element its name and proved that water is composed of hydrogen and oxygen. One of the first uses of H₂ was for balloons. The H₂ was obtained by reacting sulfuric acid and metallic iron.

Because of its relatively simple atomic structure, consisting only of a proton and an electron, the hydrogen atom has been central to the development of the theory of atomic structure.

Isotopes of hydrogen

In 1931, Harold C. Urey discovered deuterium, an isotope of hydrogen, 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.

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.

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 be at very specific distances from the proton. Modeling the hydrogen atom in this fashion yields the correct energy levels and spectrum. As a 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 fine 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 $5.9\times 10^{-6}$ 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.

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. ^[3] 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.

Production

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 very 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.

In the laboratory, H₂ can be generated by treatment of many metals with acids or base.

Zn + 2 H⁺ → Zn²⁺ + H₂

2 Al + 6 H₂O → 2 Al(OH)₃ + 3 H₂

Compounds

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. Hydrogen with an electronegativity of 2.2 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.

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^-, does not exist in any real sense. Well known hydrides include NaH, a non-molecular solid, and lithium aluminum hydride, a salts composed of the AlH₄^- anion. Palladium hydride contains insterstitial hydrogen atoms, i.e. the H atoms are bonded to multiple Pd atoms without perturbing the overall Pd framework. All so-called hydrides are covalent, because the H^- ion would be so extraordinarily basic.

Protons

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. No reaction occurs between H₂ and O₂ in the absence of a catalyst or a flame. Deuterium oxide, or D₂O, is commonly referred to as heavy water. Hydrogen also 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.

See also hydrogen compounds.

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.^[4] 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 interconversion between para and orther H₂ is slow. Rapidly condense if H₂ contains large quantities of the 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, and a lot of hydrogen can be lost by evaporation in this way during several days after liquefying. Therefore, some catalysts for the ortho-para interconversion process 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.

Isotopes

Main Article: Isotopes of hydrogen

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 this is not officially sanctioned. (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+. ^[5]

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+.

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.

Biology

Scientists from the University of Colorado at Boulder discovered in 2005 that microbes living in the hot waters of Yellowstone National Park gain their sustenance from molecular hydrogen.

References

^ "Los Alamos National Laboratory – Hydrogen". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
^ "Webelements – Hydrogen historical information". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
^ "Jefferson Lab – Hydrogen". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
^ "Universal Industrial Gases, Inc. – Hydrogen (H₂) Applications and Uses". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
^ "Lawrence Berkeley National Laboratory – Hydrogen isotopes". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

RIKEN Beam Science Laboratory, Japan - Heavy hydrogen research
Nuclides and Isotopes Fourteenth Edition: Chart of the Nuclides, General Electric Company, 1989

Book references

Stwertka, Albert (2002). A Guide to the Elements. New York, NY: Oxford University Press. ISBN 0195150279.
Krebs, Robert E. (1998). The History and Use of Our Earth's Chemical Elements : A Reference Guide. Westport, Conn.: Greenwood Press. ISBN 0313301239.
Newton, David E. (1994). The Chemical Elements. New York, NY: Franklin Watts. ISBN 0531125017.
Rigden, John S. (2002). Hydrogen : The Essential Element. Cambridge, MA: Harvard University Press. ISBN 0531125017.

Other references

"New Trends in Reforming Technologies: from Hydrogen Industrial Plants to Multifuel Microreformers" P. Ferreira-Aparicio, M. J. Benito, J. L. Sanz Catalysis Reviews, volume 47, pages 491-588, 2005.

External links

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[1] "Los Alamos National Laboratory – Hydrogen". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

[2] "Webelements – Hydrogen historical information". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

[3] "Jefferson Lab – Hydrogen". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

[4] "Universal Industrial Gases, Inc. – Hydrogen (H₂) Applications and Uses". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

[5] "Lawrence Berkeley National Laboratory – Hydrogen isotopes". Retrieved September 15. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

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