Lithium hydride
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| Identifiers | |
|---|---|
| ECHA InfoCard | 100.028.623 |
PubChem CID
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| RTECS number |
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CompTox Dashboard (EPA)
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| Properties | |
| LiH | |
| Molar mass | 7.95 g/mol |
| Appearance | colorless to gray solid |
| Density | 0.82 g/cm3,[1] solid |
| Melting point | 692 °C[2] |
| reacts | |
| Thermochemistry | |
Heat capacity (C)
|
3.51 J/(g.K) |
Std enthalpy of
formation (ΔfH⦵298) |
-11.39 kJ/g |
| Hazards | |
| NFPA 704 (fire diamond) | |
| Related compounds | |
Other cations
|
Sodium hydride Potassium hydride Rubidium hydride Cesium hydride |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Lithium hydride is the inorganic compound with the formula LiH. It is a colourless solid, although commercial samples are gray. Characteristic of a salt-like, or ionic, hydride, it has a high melting point and is not soluble in any solvent with which it does not react. It has a standard heat capacity of 29.73 J/mol.K with thermal conductivity that varies with composition and pressure (from at least 10 to 5 W/m.K at 400 K) and decreases with temperature. With a molecular mass of slightly less than 8, it is the lightest ionic compound.
Synthesis and reactions
LiH is produced by treating lithium metal with hydrogen gas at high temperatures:[3]
- 2 Li + H2 → 2 LiH
It is highly reactive toward water and other protic reagents:
- LiH + H2O → LiOH + H2
It is not usually a hydride reducing agent except in the synthesis of hydrides of certain metalloids. For example, silane is produced by the reaction of lithium hydride and silicon tetrachloride via the Sundermeyer process:
- 4 LiH + SiCl4 → 4 LiCl + SiH4
With a hydrogen content three times that of NaH, LiH has the highest hydrogen content of any hydride. LiH is periodically of interest for hydrogen storage, but applications have been precluded by the high stability of this material. Thus removal of H2 requires high temperatures, well above the 700 °C used for its synthesis. The compound was once tested as a fuel component in a model rocket.[1][2][dead link]
Applications
Precursor to complex metal hydrides
Lithium hydride is used in the production of a variety of reagents useful in organic synthesis, such as lithium aluminium hydride (LiAlH4) and lithium borohydride (LiBH4). Triethylborane reacts to give Super hydride (LiBHEt3).[4]
Applications in nuclear chemistry and physics
LiH has been used as both a coolant and shielding in nuclear reactors.
Lithium deuteride
The corresponding lithium-6 deuteride, formula 6Li2H or 6LiD, is the fusion fuel in thermonuclear weapons. In warheads of the Teller-Ulam design, a fission trigger explosion heats, compresses and bombards 6LiD with neutrons to produce tritium in an exothermic reaction. The deuterium and tritium (both isotopes of hydrogen) then fuse to produce helium-4, a neutron and 17.59 MeV of energy.
Before the Castle Bravo nuclear test it was thought that only the less common lithium-6 isotope would breed tritium when struck with fast neutrons. The test showed that the more plentiful lithium-7 also does so, albeit by an endothermic reaction. The result was a yield three times the expected value.
Safety
As discussed above, LiH reacts explosively with water to give hydrogen gas and LiOH, which is corrosive.
References
- ^ Sigma-Aldrich website
- ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 65. ISBN 978-0-08-037941-8.
- ^ Dr. Floyd Beckford. "University of Lyon course online (powerpoint) slideshow". Archived from the original on November 4, 2005. Retrieved 2008-07-27.
definitions:Slides 8-10 (Chapter 14)
- ^ Peter Rittmeyer, Ulrich Wietelmann “Hydrides” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. {{|10.1002/14356007.a13_199}}
External links
