Hydrogen an Overview
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PDF generated at: Wed, 17 Nov 2010 02:58:54 UTC Contents Articles Overview 1 Hydrogen 1 Antihydrogen 18 Hydrogen atom 20 Hydrogen-like atom 26 Hydrogen spectral series 29 Liquid hydrogen 34 Solid hydrogen 38 Metallic hydrogen 39 Nascent hydrogen 43 Isotopes 45 Isotopes of hydrogen 45 Deuterium 49 Tritium 59 Hydrogen-4 69 Hydrogen-5 70 Spin isomers of hydrogen 71 Reactions 74 Bosch reaction 74 Hydrogen cycle 75 Hydrogenation 76 Dehydrogenation 84 Transfer hydrogenation 85 Hydrogenolysis 89 Hydron 90 Sabatier reaction 91 Risks 93 Hydrogen damage 93 Hydrogen embrittlement 96 Hydrogen leak testing 99 Hydrogen safety 100 Fuel 103 Timeline of hydrogen technologies 103 Biohydrogen 107 Hydrogen production 113 Hydrogen infrastructure 118 Hydrogen line 119 Hydrogen purity 124 References Article Sources and Contributors 125 Image Sources, Licenses and Contributors 128 Article Licenses License 130 1 Overview Hydrogen Hydrogen Appearance Colorless gas with purple glow in its plasma state Spectral lines of Hydrogen General properties Name, symbol, number hydrogen, H, 1 [1] Pronunciation /ˈhaɪdrɵdʒɪn/ HYE-dro-jin Element category nonmetal Group, period, block 1, 1, s −1 Standard atomic weight 1.00794(7) g·mol Electron configuration 1s1 Electrons per shell 1 (Image) Physical properties Color colorless Phase gas Density (0 °C, 101.325 kPa) 0.08988 g/L [2] −3 Liquid density at m.p. 0.07 (0.0763 solid) g·cm Melting point 14.01 K,-259.14 °C,-434.45 °F Boiling point 20.28 K,-252.87 °C,-423.17 °F Triple point 13.8033 K (-259°C), 7.042 kPa Critical point 32.97 K, 1.293 MPa Heat of fusion (H ) 0.117 kJ·mol−1 2 Heat of vaporization (H ) 0.904 kJ·mol−1 2 Specific heat capacity (25 °C) (H ) 28.836 J·mol−1·K−1 2 Vapor pressure Hydrogen 2 P/Pa 1 10 100 1 k 10 k 100 k at T/K 15 20 Atomic properties Oxidation states 1, -1 (amphoteric oxide) Electronegativity 2.20 (Pauling scale) Ionization energies 1st: 1312.0 kJ·mol−1 Covalent radius 31±5 pm Van der Waals radius 120 pm Miscellanea Crystal structure hexagonal [3] Magnetic ordering diamagnetic Thermal conductivity (300 K) 0.1805 W·m−1·K−1 Speed of sound (gas, 27 °C) 1310 m/s CAS registry number 1333-74-0 Most stable isotopes Main article: Isotopes of hydrogen iso NA half-life DM DE (MeV) DP 1H 99.985% 1H is stable with 0 neutron 2H 0.015% 2H is stable with 1 neutron 3H trace 12.32 y β− 0.01861 3He [4] Hydrogen ( /ˈhaɪdrɵdʒɪn/ HYE-dro-jin) is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of 1.00794 u (1.007825 u for Hydrogen-1), hydrogen is the lightest and most abundant chemical element, constituting roughly 75 % of the Universe's elemental mass.[5] Stars in the main sequence are mainly composed of hydrogen in its plasma state. Naturally occurring elemental hydrogen is relatively rare on Earth. The most common isotope of hydrogen is protium (name rarely used, symbol 1H) with a single proton and no neutrons. In ionic compounds it can take a negative charge (an anion known as a hydride and written as H−), or as a positively charged species H+. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species. Hydrogen forms compounds with most elements and is present in water and most organic compounds. It plays a particularly important role in acid-base chemistry with many reactions exchanging protons between soluble molecules. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanics. Hydrogen gas (now known to be H ) was first artificially produced in the early 16th century, via the mixing of 2 metals with strong acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance,[6] and that it produces water when burned, a property which later gave it its name, which in Greek means Hydrogen 3 "water-former." At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustible diatomic gas with the molecular formula H . 2 Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of water.[7] Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle many metals,[8] complicating the design of pipelines and storage tanks.[9] Properties Combustion Hydrogen gas (dihydrogen or molecular hydrogen)[10] is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.[11] The enthalpy of combustion for hydrogen is −286 kJ/mol:[12] 2 H (g) + O (g) → 2 H O(l) + 572 kJ (286 kJ/mol)[13] 2 2 2 Hydrogen gas forms explosive mixtures with air in the concentration range 4–74% (volume per cent of hydrogen in air) and with chlorine in the range 5–95%. The mixtures spontaneously detonate by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).[14] Pure hydrogen-oxygen flames emit ultraviolet light The Space Shuttle Main Engine and are nearly invisible to the naked eye, as illustrated by the faint plume of the burns hydrogen with oxygen, Space Shuttle main engine compared to the highly visible plume of a Space producing a nearly invisible flame at full thrust. Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible flames were the result of combustible materials in the ship's skin.[15] Because hydrogen is buoyant in air, hydrogen flames tend to ascend rapidly and cause less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived the fire, and many deaths were instead the result of falls or burning diesel fuel.[16] H reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with 2 chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerous acids.[17] Hydrogen 4 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 wavelength.[18] 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, the electromagnetic force attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by gravity. Because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, Depiction of a hydrogen atom showing the and therefore only certain allowed energies.[19] diameter as about twice the Bohr model radius (image not to scale). A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the Schrödinger equation or the equivalent Feynman path integral formulation to calculate the probability density of the electron around the proton.[20] Elemental molecular forms There exist two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei.[21] In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 (½+½); in the parahydrogen form the spins are antiparallel and form a singlet with a molecular spin quantum number of 0 (½-½). 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".[22] The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an excited state and has a higher energy than the para form, it is unstable and cannot be purified. At very low temperatures, the equilibrium state is composed almost exclusively of the para form. The liquid and gas phase thermal properties of pure parahydrogen differ significantly from those of the normal form because of differences in rotational heat capacities, as discussed more fully in Spin isomers of hydrogen.[23] The ortho/para First tracks observed in liquid hydrogen bubble distinction also occurs in other hydrogen-containing molecules or chamber at the Bevatron functional groups, such as water and methylene, but is of little significance for their thermal properties.[24] The uncatalyzed interconversion between para and ortho H increases with increasing temperature; thus rapidly 2 condensed H contains large quantities of the high-energy ortho form that converts to the para form very slowly.[25] 2 The ortho/para ratio in condensed H is an important consideration in the preparation and storage of liquid hydrogen: 2 the conversion from ortho to para is exothermic and produces enough heat to evaporate some of the hydrogen liquid, leading to loss of liquefied material.