| |||
 | |||
| Names | |||
|---|---|---|---|
| IUPAC name Hydrogen | |||
| Systematic IUPAC name Liquid hydrogen | |||
| Other names Hydrogen (cryogenic liquid), Refrigerated hydrogen; LH2, para-hydrogen | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
| ||
| KEGG |
| ||
| RTECS number |
| ||
| UNII | |||
| UN number | 1966 | ||
| |||
| |||
| Properties | |||
| H2(l) | |||
| Molar mass | 2.016 g·mol−1 | ||
| Appearance | Colorless liquid | ||
| Density | 0.07085 g/cm3 (4.423 lb/cu ft)[1] | ||
| Melting point | −259.14 °C (−434.45 °F; 14.01 K)[2] | ||
| Boiling point | −252.87 °C (−423.17 °F; 20.28 K)[2] | ||
| Hazards | |||
| GHS labelling:[3] | |||
  | |||
| Danger | |||
| H220,H280 | |||
| P210,P377,P381,P403 | |||
| NFPA 704 (fire diamond) | |||
| 571 °C (1,060 °F; 844 K)[2] | |||
| Explosive limits | LEL 4.0%; UEL 74.2% (in air)[2] | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Liquid hydrogen (H2(l)) is theliquid state of the elementhydrogen. Hydrogen is found naturally in themolecular H2 form.[4]
To exist as a liquid, H2 must be cooled below itscritical point of 33 K. However, for it to be in a fully liquid state atatmospheric pressure, H2 needs to be cooled to 20.28 K (−252.87 °C; −423.17 °F).[5] A common method of obtaining liquid hydrogen involves acompressor resembling a jet engine in both appearance and principle. Liquid hydrogen is typically used as a concentrated form ofhydrogen storage. Storing it as liquid takes less space than storing it as a gas at normal temperature and pressure. However, the liquid density is very low compared to other common fuels. Once liquefied, it can be maintained as a liquid for some time in thermally insulated containers.[6]
There are twospin isomers of hydrogen; whereas room temperature hydrogen is mostly orthohydrogen, liquid hydrogen consists of 99.79% parahydrogen and 0.21% orthohydrogen.[5]
Hydrogen requires a theoretical minimum of 3.3 kWh/kg (12 MJ/kg) to liquefy, and 3.9 kWh/kg (14 MJ/kg) including converting the hydrogen to the para isomer, but practically generally takes 10–13 kWh/kg (36–47 MJ/kg) compared to a 33 kWh/kg (119 MJ/kg) heating value of hydrogen.[7]
In 1885,Zygmunt Florenty Wróblewski published hydrogen's critical temperature as 33 K (−240.2 °C; −400.3 °F); critical pressure, 13.3 standard atmospheres (195 psi); and boiling point, 23 K (−250.2 °C; −418.3 °F).
Hydrogen was liquefied byJames Dewar in 1898 by usingregenerative cooling and his invention, thevacuum flask. The first synthesis of the stable isomer form of liquid hydrogen, parahydrogen, was achieved byPaul Harteck andKarl Friedrich Bonhoeffer in 1929.
The two nuclei in a dihydrogen molecule can have two differentspin states.Parahydrogen, in which the twonuclear spins are antiparallel, is more stable than orthohydrogen, in which the two are parallel. At room temperature, gaseous hydrogen is mostly in the ortho isomeric form due to thermal energy, but an ortho-enriched mixture is onlymetastable when liquified at low temperature. It slowly undergoes anexothermic reaction to become the para isomer, with enough energy released as heat to cause some of the liquid to boil.[8] To prevent loss of the liquid during long-term storage, it is therefore intentionally converted to the para isomer as part of the production process, typically using acatalyst such asiron(III) oxide,activated carbon, platinized asbestos, rare earth metals, uranium compounds,chromium(III) oxide, or some nickel compounds.[8]
Liquid hydrogen is a commonliquidrocket fuel forrocketry application and is used byNASA and theU.S. Air Force, which operate a large number of liquid hydrogen tanks with an individual capacity up to 3.8 million liters (1 million U.S. gallons).[9]
In mostrocket engines fueled by liquid hydrogen, it firstcools the nozzle and other parts before being mixed with the oxidizer, usuallyliquid oxygen, and burned to produce water with traces ofozone andhydrogen peroxide. Practical H2–O2 rocket engines run fuel-rich so that the exhaust contains some unburned hydrogen. This reduces combustion chamber and nozzle erosion. It also reduces the molecular weight of the exhaust, which can increasespecific impulse, despite the incomplete combustion.
Liquid hydrogen can be used as the fuel for aninternal combustion engine orfuel cell. Various submarines, including theType 212 submarine,Type 214 submarine, and others, and concepthydrogen vehicles have been built using this form of hydrogen, such as theDeepC,BMW H2R, and others. Due to its similarity, builders can sometimes modify and share equipment with systems designed forliquefied natural gas (LNG). Liquid hydrogen is being investigated as azero carbon fuel foraircraft. Because of the lowervolumetric energy, the hydrogen volumes needed for combustion are large. Unlessdirect injection is used, a severe gas-displacement effect also hampers maximum breathing and increases pumping losses.
Liquid hydrogen is also used to cool neutrons to be used inneutron scattering. Since neutrons and hydrogen nuclei have similar masses, kinetic energy exchange per interaction is maximum (elastic collision). Finally, superheated liquid hydrogen was used in manybubble chamber experiments.
The firstthermonuclear bomb,Ivy Mike, used liquiddeuterium, also known as hydrogen-2, for nuclear fusion.
The product of hydrogen combustion in a pure oxygen environment is solely water vapor. However, the high combustion temperatures and present atmospheric nitrogen can result in the breaking of N≡N bonds, forming toxic NOx if no exhaust scrubbing is done.[10] Since water is often considered harmless to the environment, an engine burning it can be considered "zero emissions". In aviation, however, water vapor emitted in the atmosphere contributes toglobal warming (to a lesser extent than CO2).[11] Liquid hydrogen also has a much higherspecific energy than gasoline, natural gas, or diesel.[12]
The density of liquid hydrogen is only 70.85 kg/m3 (at 20 K), arelative density of just 0.07. Although the specific energy is more than twice that of other fuels, this gives it a remarkably low volumetricenergy density, many fold lower.
Liquid hydrogen requirescryogenic storage technology such as special thermally insulated containers and requires special handling common to allcryogenic fuels. This is similar to, but more severe thanliquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away (typically at a rate of 1% per day[12]). It also shares many of the samesafety issues as other forms of hydrogen, as well as being cold enough to liquefy, or even solidify atmospheric oxygen, which can be an explosion hazard.
Thetriple point of hydrogen is at 13.81 K[5] and 7.042 kPa.[13]
Due to its cold temperatures, liquid hydrogen is a hazard forcold burns. Hydrogen itself is biologically inert and its only human health hazard as a vapor is displacement of oxygen, resulting in asphyxiation, and its very high flammability and ability to detonate when mixed with air. Because of its flammability, liquid hydrogen should be kept away from heat or flame unless ignition is intended. Unlike ambient-temperature gaseous hydrogen, which is lighter than air, hydrogen recently vaporized from liquid is so cold that it is heavier than air and can form flammable heavier-than-air air–hydrogen mixtures.
