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			Stupid Period 4 Earth Space Science kid in Mrs. Soto's class. Good Luck finding information on hydrogen for your cube project here. I know who you are...ounds.
	{{Two_other_uses\|the chemistry of hydrogen\|the physics of atomic hydrogen\|hydrogen atom\|other meanings \|hydrogen (disambiguation)}}
	{{Infobox hydrogen}}
	'''Hydrogen''' ({{pronEng\|ˈhaɪdrədʒən}}), is a ] represented by the symbol '''H''' and an ] of 1. At ] it is a colorless, odorless, ]lic, tasteless, highly ] ] ] (H<sub>2</sub>).
	==Properties==
	With an ] of ] g/], hydrogen is the lightest element.

	Hydrogen is the most ] of the chemical elements, constituting roughly 75% of the universe's elemental mass.<ref>, NASA Website. URL accessed on 2 June 2006.</ref> ]s in the ] are mainly composed of hydrogen in its ] state. Elemental hydrogen is relatively rare on ], and is industrially produced from ]s such as methane, after which most elemental hydrogen is used "captively" (meaning locally at the production site), with the largest markets about equally divided between fossil fuel upgrading (e.g., ]) and ] production (mostly for the fertilizer market). Hydrogen may be produced from water using the process of ], but this process is presently significantly more expensive commercially than hydrogen production from natural gas.

	The most common naturally occurring ] of hydrogen, known as ], has a single ] and no ]s. In ]s it can take on either a positive charge (becoming a ] composed of a bare proton) or a negative charge (becoming an ] known as a ]). Hydrogen can form compounds with most elements and is present in ] and most ]s. It plays a particularly important role in ], in which many reactions involve the exchange of protons between soluble molecules. As the only neutral atom for which the ] can be solved analytically, study of the energetics and bonding of the hydrogen atom has played a key role in the development of ].

	==Chemistry==
	The ] and ] of hydrogen with various metals are not very important in ] (as many metals can suffer ]) and in developing safe ways to store it for use as a fuel. Hydrogen is highly soluble in many compounds composed of ]s and ]s<ref name="Takeshita"> Takeshita T, Wallace WE, Craig RS. (1974). Hydrogen solubility in 1:5 compounds between yttrium or thorium and nickel or cobalt. ''Inorg Chem'' 13(9):2282.</ref> and can be dissolved in both ] and ] metals.<ref name="Kirchheim1">Kirchheim R, Mutschele T, Kieninger W. (1988). Hydrogen in amorphous and nanocrystalline metals ''Mater. Sci. Eng.'' 99: 457–462.</ref> Hydrogen solubility in metals is influenced by local distortions or impurities in the metal ].<ref name="Kirchheim2">Kirchheim R. (1988). Hydrogen solubility and diffusivity in defective and amorphous metals. ''Prog. Mater. Sci.'' 32(4):262–325.</ref>

	===Combustion===
	] disaster on ] ]]]

	Hydrogen gas is highly flammable and will burn at concentrations as low as 4% H<sub>2</sub> in air. The ] of combustion for hydrogen is – 286 kJ/mol; it combusts according to the following balanced equation.
	:2 H<sub>2</sub>(g) + O<sub>2</sub>(g) → 2 H<sub>2</sub>O(l) + 572 kJ/mol

	When mixed with oxygen across a wide range of proportions, hydrogen does nothing upon ignition. Hydrogen burns violently in air. <!--It ignites automatically at a temperature of 500 degrees celsius, and burns up to 2,500 degrees celsius. ... commenting this out pending verification; added by an anon with a history of adding unsourced ignition points; original writing didn't inspire confidence-->
	Pure hydrogen-oxygen flames burn in the ] color range and are nearly invisible to the naked eye, as illustrated by the faintness of flame from the main ] 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. The ] is an infamous case of hydrogen combustion (pictured), although the tragedy was due mainly to combustible materials in the skin of the zeppelin, which were also responsible for the coloring of the flames.<ref>{{cite web \| last = Dziadecki \| first = John \| year = 2005 \| url = http://spot.colorado.edu/~dziadeck/zf/LZ129fire.htm \| title = Hindenburg Hydrogen Fire \| accessdate = 2007-01-16 }}</ref> Another characteristic of hydrogen fires is that the flames tend to ascend rapidly with the gas in air, as illustrated by the Hindenberg flames, causing less damage than hydrocarbon fires. For example, two-thirds of the Hindenburg passengers survived the fire, and many of the deaths which occurred were from falling or from gasoline burns.<ref>{{cite web \| url = http://www.hydropole.ch/Hydropole/Intro/Hindenburg.htm \| title = The Hindenburg Disaster \| publisher = Swiss Hydrogen Association \| accessdate = 2007-01-16 }}</ref>

	===Electron energy levels===
	{{main\|Hydrogen atom}}
	] radius. (Image not to scale)]]

	The ] ] of the electron in a hydrogen atom is -13.6 ], which is equivalent to an ultraviolet ] of roughly 92 ].

	The energy levels of hydrogen can be calculated fairly accurately using the ] of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the sun. However, the ] force attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by ]. Because of the discretization of ] postulated in early ] 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 ] or the equivalent ] ] to calculate the ] of the electron around the proton. Treating the electron as a ] reproduces chemical results such as shape of the hydrogen atom more naturally than the particle-based Bohr model, although the energy and spectral results are the same. Modeling the system fully using the ] of nucleus and electron (as one would do in the ] in celestial mechanics) yields an even better formula for the hydrogen spectra, and also the correct spectral shifts for the isotopes ] and ]. Very small adjustments in energy levels in the hydrogen atom, which correspond to actual spectral effects, may be determined by using a full quantum mechanical theory which corrects for the effects of ] (see ]), and by accounting for quantum effects arising from production of virtual particles in the vacuum and as a result of electric fields (see ]).

	In hydrogen gas, the electronic ] ] is split into ] levels because of ] effects of the quantum mechanical ] 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 ] transition. ]s can detect the radiation produced in this process, which is used to map the distribution of hydrogen in the galaxy.

	H<sub>2</sub> reacts directly with other oxidizing elements. A violent and spontaneous reaction can occur at room temperature with ] and ], forming the corresponding hydrogen halides: ] and ].
	===Elemental molecular forms===
	] at the ].]]

	There are two different types of diatomic hydrogen molecules that differ by the relative ] of their nuclei.<ref>{{cite web \| title= Universal Industrial Gases, Inc. – Hydrogen (H<sub>2</sub>) Applications and Uses \| url=http://www.uigi.com/hydrogen.html \| accessmonthday= September 15 \| accessyear= 2005 }}</ref> In the ] form, the spins of the two protons are parallel and form a triplet state; in the ] 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".<ref name="Tikhonov">Tikhonov VI, Volkov AA. (2002). Separation of water into its ortho and para isomers. ''Science'' 296(5577):2363.</ref> The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but since the ortho form is an ] 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.<ref name="NASA">NASA Glenn Research Center Glenn Safety Manual. CH. 6 - Hydrogen. Document GRC-MQSA.001, March 2006. </ref> The ortho/para distinction also occurs in other hydrogen-containing molecules or functional groups, such as water and ].

	The uncatalyzed interconversion between para and ortho H<sub>2</sub> increases with increasing temperature; thus rapidly condensed H<sub>2</sub> contains large quantities of the high-energy ortho form that convert to the para form very slowly.<ref>Milenko YY, Sibileva RM, Strzhemechny MA. (1997). Natural ortho-para conversion rate in liquid and gaseous hydrogen. ''J Low Temp Phys'' 107(1-2):77–92.</ref> The ortho/para ratio in condensed H<sub>2</sub> is an important consideration in the preparation and storage of liquid hydrogen: the conversion from ortho to para is ] and produces enough heat to evaporate the hydrogen liquid, leading to loss of the liquefied material. ]s for the ortho-para interconversion, such as ] compounds, are used during hydrogen cooling.<ref name="Svadlenak"> Svadlenak RE, Scott AB. (1957). The Conversion of Ortho-to Parahydrogen on Iron Oxide-Zinc Oxide Catalysts. ''J Am Chem Soc'' 79(20); 5385–5388.</ref>

	A molecular form called ], or H<sub>3</sub><sup>+</sup>, is found in the ] (ISM), where it is generated by ionization of molecular Hydrogen from ]s. It has also been observed in the upper atmosphere of the planet ]. This molecule is relatively stable in the environment of outer space due to the low temperature and density. H<sub>3</sub><sup>+</sup> is one of the most abundant ions in the Universe, and it plays a notable role in the chemistry of the interstellar medium.<ref>{{cite web \| url = http://h3plus.uiuc.edu/ \| title = H3+ Resource Center \| publisher = Universities of Illinois and Chicago \| accessdate = 2007-02-09 }}</ref>

	===Compounds===
	{{further\|]}}

	====Covalent and organic compounds====
	While H<sub>2</sub> is not very reactive under standard conditions, it does form compounds with most elements. Millions of ]s are known, but they are not formed by the direct reaction of elementary hydrogen and carbon (although ] production followed by the ] to make hydrocarbons comes close to being an exception, as this begins with coal and the elemental hydrogen is generated in situ). Hydrogen can form compounds with elements that are more ], such as ]s (e.g., F, Cl, Br, I) and ]s (O, S, Se); in these compounds hydrogen takes on a partial positive charge. When bonded to ], ], or ], hydrogen can participate in a form of strong noncovalent bonding called ]ing, which is critical to the stability of many biological molecules. Hydrogen also forms compounds with less electronegative elements, such as the ]s and ]s, in which it takes on a partial negative charge. These compounds are often known as ]s.

	Hydrogen forms a vast array of compounds with ]. Because of their general association with living things, these compounds came to be called ]s; the study of their properties is known as ] and their study in the context of living ]s is known as ]. By some definitions, "organic" compounds are only required to contain carbon (as a classic historical example, ]). However, most of them also contain hydrogen, and since it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry. (This latter definition is not perfect, however, as in this definition urea would ''not'' be included as an organic compound).

	In ], hydrides can also serve as ]s that link two metal centers in a ]. This function is particularly common in ]s, especially in ]s (] hydrides) and ] complexes, as well as in clustered ]s.<ref name="Miessler" />

	====Hydrides====
	Compounds of hydrogen are often called ]s, a term that is used fairly loosely. To chemists, the term "hydride" usually implies that the H atom has acquired a negative or anionic character, denoted H<sup>−</sup>. The existence of the hydride anion, suggested by G.N. Lewis in 1916 for group I and II salt-like hydrides, was demonstrated by Moers in 1920 with the electrolysis of molten ] (LiH), that produced a ] quantity of hydrogen at the anode.<ref name="Moers">K. Moers, (1920). 2. Z. Anorg. Allgem. Chem., 113:191.</ref> For hydrides other than group I and II metals, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group II hydrides is BeH<sub>2</sub>, which is polymeric. In ], the AlH<sub>4</sub><sup>−</sup> anion carries hydridic centers firmly attached to the Al(III). Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, there are over 100 binary borane hydrides known, but only one binary aluminum hydride.<ref name="Downs">Downs AJ, Pulham CR. (1994). The hydrides of aluminium, gallium, indium, and thallium: a re-evaluation. ''Chem Soc Rev'' 23:175–83.</ref> Binary ] hydride has not yet been identified, although larger complexes exist.<ref name="Hibbs"> Hibbs DE, Jones C, Smithies NA. (1999). A remarkably stable indium trihydride complex: synthesis and characterization of . ''Chem Commum'' 185–6.</ref>

	===="Protons" and acids====
	Oxidation of H<sub>2</sub> formally gives the ], H<sup>+</sup>. This species is central to discussion of ]s, though the term proton is used loosely to refer to positively charged or ]ic hydrogen, denoted H<sup>+</sup>. A bare proton H<sup>+</sup> cannot exist in solution because of its strong tendency to attach itself to atoms or molecules with electrons. To avoid the convenient fiction of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain the ] ion (H<sub>3</sub>O<sup>+</sup>) organized into clusters to form H<sub>9</sub>O<sub>4</sub><sup>+</sup>.<ref name="Okumura">Okumura M, Yeh LI, Myers JD, Lee YT. (1990). Infrared spectra of the solvated hydronium ion: vibrational predissociation spectroscopy of mass-selected H3O+•H2On•H2m.</ref> Other ] ions are found when water is in solution with other solvents.<ref name="Perdoncin">Perdoncin G, Scorrano G. (1977). Protonation equilibria in water at several temperatures of alcohols, ethers, acetone, dimethyl sulfide, and dimethyl sulfoxide. 99(21); 6983–6986.</ref>

	Although exotic on earth, one of the most common ions in the universe is the ] ion, known as protonated molecular hydrogen or the triatomic hydrogen cation.<ref name="Carrington">Carrington A, McNab IR. (1989). The infrared predissociation spectrum of triatomic hydrogen cation (H3+). ''Accounts of Chemical Research'' 22:218–22.</ref>

	==Isotopes==
	{{main\|Isotopes of hydrogen}}
	] for discussion of why others do not exist)]]

	Hydrogen has three naturally occurring isotopes, denoted <sup>1</sup>H, ²H, and ³H. Other, highly unstable nuclei (<sup>4</sup>H to <sup>7</sup>H) have been synthesized in the laboratory but not observed in nature.<ref name="Gurov">Gurov YB, Aleshkin DV, Berh MN, Lapushkin SV, Morokhov PV, Pechkurov VA, Poroshin NO, Sandukovsky VG, Tel'kushev MV, Chernyshev BA, Tschurenkova TD. (2004). Spectroscopy of superheavy hydrogen isotopes in stopped-pion absorption by nuclei. ''Physics of Atomic Nuclei'' 68(3):491–497.</ref><ref name="Korsheninnikov">Korsheninnikov AA. et al. (2003). Experimental Evidence for the Existence of 7H and for a Specific Structure of 8He. ''Phys Rev Lett'' 90, 082501.</ref>
	* '''<sup>1</sup>H''' is the most common hydrogen isotope with an abundance of more than 99.98%. Because the ] of this isotope consists of only a single ], it is given the descriptive but rarely used formal name ''protium''.
	* '''²H''', the other stable hydrogen isotope, is known as '']'' and contains one proton and one ] in its nucleus. Deuterium comprises 0.0026 – 0.0184% (by mole-fraction or atom-fraction) of hydrogen samples on Earth, with the lower number tending to be found in samples of hydrogen gas and the higher enrichments (0.015% or 150 ppm) typical of ocean water. Deuterium is not radioactive, and does not represent a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called ]. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for <sup>1</sup>H-]. Heavy water is used as a ] and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial ].
	* '''³H''' is known as '']'' and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into ] through ] with a ] of 12.32 ].<ref name="Miessler" /> Small amounts of tritium occur naturally because of the interaction of cosmic rays with atmospheric gases; tritium has also been released during ]. It is used in nuclear fusion reactions, as a tracer in ], and specialized in ] devices. Tritium was once routinely used in chemical and biological labeling experiments as a ] (this has become less common).

	Hydrogen is the only element that has different names for its isotopes in common use today. (During the early study of radioactivity, various heavy radioactive isotopes were given names, but such names are no longer used). The symbols D and T (instead of ²H and ³H) are sometimes used for deuterium and tritium, but the corresponding symbol P is already in use for ] and thus is not available for protium. ] states that while this use is common it is not preferred.



	==Natural occurrence==
	], a giant ] in the ]]]

	Hydrogen is the most ] element in the universe, making up 75% of ] by ] and over 90% by number of atoms.<ref>{{cite web \| title= Jefferson Lab – Hydrogen \| url=http://education.jlab.org/itselemental/ele001.html \| accessmonthday= September 15 \| accessyear= 2005 }}</ref> This element is found in great abundance in ]s and ] planets. ]s of H<sub>2</sub> are associated with ]. Hydrogen plays a vital role in powering ] through ] ].

	Throughout the universe, hydrogen is mostly found in the ] and ] states whose properties are quite different from molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the ] and other ]). The charged particles are highly influenced by magnetic and electric fields. For example, in the ] they interact with the Earth's ] giving rise to ]s and the ]. Hydrogen is found in the neutral atomic state in the ]. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the ] up to ] ''z''=4.<ref>{{cite web \| title= Surveys for z > 3 Damped Lyα Absorption Systems: The Evolution of Neutral Gas \| url=http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v543n2/51444/51444.html \| accessmonthday= October 13 \| accessyear= 2006 }}</ref>

	Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H<sub>2</sub> (for data see table). However, hydrogen gas is very rare in the Earth's atmosphere (1 ] by volume) because of its light weight, which enables it to ] more easily than heavier gases. Although H atoms and H<sub>2</sub> molecules are abundant in interstellar space, they are difficult to generate, concentrate, and purify on Earth. Still, hydrogen is the third most abundant element on the Earth's surface.<ref name="ArgonneBasic">"Basic Research Needs for the Hydrogen Economy." Argonne National Laboratory, U.S. Department of Energy, Office of Science Laboratory. 15 May 2003. </ref> Most of the Earth's hydrogen is in the form of ]s such as ]s and ].<ref name="Miessler">Miessler GL, Tarr DA. (2004). ''Inorganic Chemistry'' 3rd ed. Pearson Prentice Hall: Upper Saddle River, NJ, USA</ref> Hydrogen gas is produced by some ] and ] and is a natural component of ]. ] is a hydrogen source of increasing importance.


	==History==
	===Discovery of H<sub>2</sub>===
	Hydrogen gas, H<sub>2</sub>, was first artificially produced and formally described by T. Von Hohenheim (also known as ], ] – ]) via the mixing of ]s with ]s. He was unaware that the flammable ] produced by this ] was a new ]. In 1671, ] rediscovered and described the reaction between ] filings and dilute ]s, which results in the production of hydrogen gas.<ref>{{cite web \| title= Webelements – Hydrogen historical information \| url=http://www.webelements.com/webelements/elements/text/H/hist.html \| accessmonthday= September 15 \| accessyear= 2005 }}</ref> In ], ] was the first to recognize hydrogen gas as a discrete substance, by identifying the gas from a ] as "inflammable air" and further finding that the gas produces ] when burned. Cavendish had stumbled on hydrogen when experimenting with acids and ]. Although he wrongly assumed that hydrogen was a liberated component of the mercury rather than the ], he was still able to accurately describe several key properties of hydrogen. He is usually given credit for its discovery as an element. In 1783, ] gave the element the name of hydrogen when he (with ]) reproduced Cavendish's finding that water is produced when hydrogen is burned. Lavoisier's name for the gas won out.

	One of the first uses of H<sub>2</sub> was for ]s, and later ]. The H<sub>2</sub> was obtained by reacting ] and metallic ]. Infamously, H<sub>2</sub> was used in the ] airship that was destroyed in a midair fire. The highly flammable hydrogen (H<sub>2</sub>) was later replaced for airships and most balloons by the unreactive ] (He).

	===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 ]ic structure. Furthermore, the corresponding simplicity of the hydrogen molecule and the corresponding cation H<sub>2</sub><sup>+</sup> allowed fuller understanding of the nature of the ], 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 ] arrived. Maxwell observed that the ] of H<sub>2</sub> unaccountably departs from that of a diatomic gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H<sub>2</sub> 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.<ref name="Berman">Berman R, Cooke AH, Hill RW. ''Cryogenics'', Ann. Rev. Phys. Chem. 7 (1956). 1–20.</ref>

	==Applications==
	Large quantities of H<sub>2</sub> are needed in the petroleum and chemical industries. The largest application of H<sub>2</sub> is for the processing ("upgrading") of fossil fuels, and in the production of ]. The key consumers of H<sub>2</sub> in the petrochemical plant include ], ], and ].<ref>{{cite web \| title= Los Alamos National Laboratory – Hydrogen \| url=http://periodic.lanl.gov/elements/1.html \| accessmonthday= September 15 \| accessyear= 2005 }}</ref> H<sub>2</sub> has several other important uses. H<sub>2</sub> is used as a hydrogenating agent, particularly in increasing the level of saturation of unsaturated ]s and ]s (found in items such as ]), and in the production of ]. It is similarly the source of hydrogen in the manufacture of ]. H<sub>2</sub> is also used as a ] of metallic ]s.

	Apart from its use as a reactant, H<sub>2</sub> has wide applications in physics and engineering. It is used as a ] in ] methods such as ]. H<sub>2</sub> is used as the rotor coolant in ]s at ]s, because it has the highest ] of any gas. Liquid H<sub>2</sub> is used in ] research, including ] studies. Since H<sub>2</sub> is ], having a little more than 1/15th of the density of air, it was once widely used as a lifting agent in ]s and ]s. However, this use was curtailed after the ] convinced the public that the gas was too dangerous for this purpose. Hydrogen is still regularly used for the inflation of ]s.

	In more recent application Hydrogen is used pure or mixed with Nitrogen (sometime called Forming Gas) as a tracer gas for minute leak detection. Applications can be found in automotive, aircraft, consumer goods, medical device and chemical industry. Hydrogen is a food autorized additive ] that allows food package leak testing among other anti-oxyding properties.

	Hydrogen's rarer isotopes also each have specific applications. ] (hydrogen-2) is used in ] as a ] to slow ]s, and in ] reactions. Deuterium compounds have applications in ] and ] in studies of reaction ]s. ] (hydrogen-3), produced in ]s, is used in the production of ]s, as an isotopic label in the biosciences, and as a ] source in luminous paints.

	The ] temperature of equilibrium hydrogen is a defining fixed point on the ] temperature scale.

	===Energy carrier===
	:{{main\|Hydrogen economy}}
	Hydrogen is not an energy source, except in the hypothetical context of commercial ] power plants using ] or ], a technology presently far from development. The sun's energy comes from nuclear fusion of hydrogen but this process is difficult to achieve on earth. Elemental hydrogen from solar, biological, or electrical sources costs more in energy to make than is obtained by burning it. Hydrogen may be obtained from fossil sources (such as methane) for less energy than required to make it, but these sources are unsustainable, and are also themselves direct energy sources (and are rightly regarded as the basic source of the energy in the hydrogen obtained from them).

	Molecular hydrogen has been widely discussed in the context of energy, as a possible carrier of energy on an economy-wide scale. A theoretical advantage of using H<sub>2</sub> as an energy carrier is the localization and concentration of environmentally unwelcome aspects of hydrogen manufacture from fossil fuel energy sources. For example, CO<sub>2</sub> ] followed by ] could be conducted at the point of H<sub>2</sub> production from ]. Hydrogen used in transportation would burn cleanly, without carbon emissions. However, the infrastructure costs associated with full conversion to a hydrogen economy would be substantial.<ref>See {{cite book\|last=Romm\|first=Joseph\|year=2004\|title=The Hype about Hydrogen, Fact and Fiction in the Race to Save the Climate\|location=New York\|publisher=Island Press}} (ISBN 1-55963-704-8)</ref> In addition, the ] of both liquid hydrogen and hydrogen gas at any practicable pressure is significantly less than that of traditional fuel sources.

	==Production==
	H<sub>2</sub> is produced in chemistry and biology laboratories, often as a by-product of other reactions; in industry for the ] of ] substrates; and in nature as a means of expelling ] equivalents in biochemical reactions.

	===Laboratory syntheses===
	In the ], H<sub>2</sub> is usually prepared by the reaction of acids on metals such as ].
	:] + 2 H<sup>+</sup> → Zn<sup>2+</sup> + H<sub>2</sub>

	] produces H<sub>2</sub> upon treatment with acids but also with base:
	:2 Al + 6 H<sub>2</sub>O → 2 Al(OH)<sub>3</sub> + 3 H<sub>2</sub>

	The ] of water is a simple method of producing hydrogen, although the resulting hydrogen necessarily has less energy content than was required to produce it. A low voltage current is run through the water, and gaseous oxygen forms at the ] while gaseous hydrogen forms at the ]. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, however, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is between 80 – 94%.
	:2H<sub>2</sub>O(aq) → 2H<sub>2</sub>(g) + O<sub>2</sub>(g)

	In 2007, it was discovered that an alloy of ] and ] in pellet form added to water could be used to generate hydrogen.<ref>{{cite web \|url=http://www.physorg.com/news98556080.html \| title=New process generates hydrogen from aluminum alloy to run engines, fuel cells}}</ref> The process creates also creates ], but the expensive gallium, which prevents to formation of an oxide skin on the pellets, can be re-used. This potentially has important implications for a hydrogen economy, since hydrogen can be produced on-site and does not need to be transported.

	===Industrial syntheses===
	<!-- Image with unknown copyright status removed: ] -->
	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 ] of ].<ref name="Oxtoby">Oxtoby DW, Gillis HP, Nachtrieb NH. (2002). ''Principles of Modern Chemistry'' 5th ed. Thomson Brooks/Cole</ref> At high temperatures (700 – 1100 °C; 1,300 – 2,000 °F), steam (water vapor) reacts with methane to yield ] and H<sub>2</sub>.
	:] + ] → ] + 3 H<sub>2</sub>

	This reaction is favored at low pressures but is nonetheless conducted at high pressures (20 atm; 600 ]) since high pressure H<sub>2</sub> is the most marketable product. The product mixture is known as "]" because it is often used directly for the production of ] and related compounds. ]s other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbon:
	:] → C + 2 H<sub>2</sub>
	Consequently, steam reforming typically employs an excess of H<sub>2</sub>O.

	Additional hydrogen from steam reforming can be recovered from the carbon monoxide through the ], especially with an ] catalyst. This reaction is also a common industrial source of ]:<ref name="Oxtoby" /> :] + ] → ] + H<sub>2</sub>

	Other important methods for H<sub>2</sub> production include partial oxidation of hydrocarbons:
	:] + 0.5 ] → ] + 2 H<sub>2</sub>

	and the coal reaction, which can serve as a prelude to the shift reaction above:<ref name="Oxtoby" /> :] + ] → ] + H<sub>2</sub>

	Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the ] for the production of ] (the world's fifth most produced industrial compound), hydrogen is generated from natural gas.

	Hydrogen is also produced in usable quantities as a co-product of the major petrochemical processes of ] and ]. ] of ] to yield ] also produces hydrogen as a co-product.

	===Biological syntheses===
	H<sub>2</sub> is a product of some types of ] and is produced by several ]s, usually via reactions ] by ]- or ]-containing ]s called ]s. These enzymes catalyze the reversible ] reaction between H<sub>2</sub> and its component two protons and two electrons. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during ] ] to water.<ref>Cammack, R.; Frey, M.; Robson, R. Hydrogen as a Fuel: Learning from Nature; Taylor & Francis: London, 2001</ref>

	], in which water is decomposed into its component protons, electrons, and oxygen, occurs in the ]s in all ] organisms. Some such organisms — including the ] '']'' and ] — have evolved a second step in the ]s in which protons and electrons are reduced to form H<sub>2</sub> gas by specialized hydrogenases in the ].<ref>Kruse O, Rupprecht J, Bader KP, Thomas-Hall S, Schenk PM, Finazzi G, Hankamer B. (2005). Improved photobiological H2 production in engineered green algal cells. ''J Biol Chem'' 280(40):34170–7.</ref> Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H<sub>2</sub> gas even in the presence of oxygen.<ref>United States Department of Energy FY2005 Progress Report. IV.E.6 Hydrogen from Water in a Novel Recombinant Oxygen-Tolerant Cyanobacteria System. HO Smith, Xu Q. http://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf Accessed 16 August 2006.</ref>

	Other rarer but mechanistically interesting routes to H<sub>2</sub> production also exist in nature. ] produces approximately one equivalent of H<sub>2</sub> for each equivalent of N<sub>2</sub> reduced to ammonia. Some phosphatases reduce ] to H<sub>2</sub>.



	==Precautions==
	Hydrogen can act as an ].

	==Etymology==
	'''Hydrogen''', {{lang-la\|''hydrogenium''}}, is from ] ] (''hydor''): "water" and (''genes''): "forming". ] ] (''geinomai''): "to beget or sire")<ref>], "of the father ''to beget'', rarely of the mother ''to give birth''.</ref>

	The word "hydrogen" has several different meanings;
	# the ''name of an element''.
	# an ''atom'', sometimes called "H dot", that is abundant in space but essentially absent on Earth, because it ]izes.
	# a '']'' that occurs naturally in trace amounts in the ]; ]s increasingly refer to H<sub>2</sub> as ''dihydrogen'',<ref>Kubas, G. J., Metal Dihydrogen and σ-Bond Complexes, Kluwer Academic/Plenum Publishers: New York, 2001</ref> or ''hydrogen molecule'', to distinguish this ] from ] and hydrogen found in other compounds.
	# the atomic ''constituent'' within all organic compounds, water, and many other ]s.

	The ''elemental'' forms of hydrogen should not be confused with hydrogen as it appears in chemical compounds.

	==See also==		==See also==

Revision as of 00:49, 21 November 2007

Stupid Period 4 Earth Space Science kid in Mrs. Soto's class. Good Luck finding information on hydrogen for your cube project here. I know who you are...ounds.

v t e Diatomic chemical elements
Common	H₂ N₂ O₂ F₂ Cl₂ Br₂ I₂
Other	He₂ Ar₂ C₂ P₂ S₂ Li₂ Rb₂

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v t e E numbers
Colors (E100–199) Preservatives (E200–299) Antioxidants & acidity regulators (E300–399) Thickeners, stabilisers & emulsifiers (E400–499) pH regulators & anticaking agents (E500–599) Flavour enhancers (E600–699) Miscellaneous (E900–999) Additional chemicals (E1100–1599) Waxes (E900–909) Synthetic glazes (E910–919) Improving agents (E920–929) Packaging gases (E930–949) Sweeteners (E950–969) Foaming agents (E990–999) Calcium peroxide (E930) Argon (E938) Helium (E939) Dichlorodifluoromethane (E940) Nitrogen (E941) Nitrous oxide (E942) Butane (E943a) Isobutane (E943b) Propane (E944) Oxygen (E948) Hydrogen (E949)

E numbers

Colors (E100–199)
Preservatives (E200–299)
Antioxidants & acidity regulators (E300–399)
Thickeners, stabilisers & emulsifiers (E400–499)
pH regulators & anticaking agents (E500–599)
Flavour enhancers (E600–699)
Miscellaneous (E900–999)
Additional chemicals (E1100–1599)

Waxes (E900–909)
Synthetic glazes (E910–919)
Improving agents (E920–929)
Packaging gases (E930–949)
Sweeteners (E950–969)
Foaming agents (E990–999)

Calcium peroxide (E930)
Argon (E938)
Helium (E939)
Dichlorodifluoromethane (E940)
Nitrogen (E941)
Nitrous oxide (E942)
Butane (E943a)
Isobutane (E943b)
Propane (E944)
Oxygen (E948)
Hydrogen (E949)

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