 Lithium cation,Li+ Hydrogen anion,H− | |
 __H− __Li+ Structure of lithium hydride. | |
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| Identifiers | |
|---|---|
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.028.623 |
| RTECS number |
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| UNII | |
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| Properties | |
| LiH | |
| Molar mass | 7.95 g·mol−1 |
| Appearance | colorless to gray solid[1] |
| Density | 0.78 g/cm3[1] |
| Melting point | 688.7 °C (1,271.7 °F; 961.9 K)[1] |
| Boiling point | 900–1,000 °C (1,650–1,830 °F; 1,170–1,270 K) (decomposes)[2] |
| reacts | |
| Solubility | slightly soluble indimethylformamide reacts withammonia,diethyl ether,ethanol |
| −4.6·10−6 cm3/mol | |
Refractive index (nD) | 1.9847[3]: 43 |
| Structure | |
| fcc (NaCl-type) | |
a = 0.40834 nm[3]: 56 | |
| 6.0 D[3]: 35 | |
| Thermochemistry | |
| 3.51 J/(g·K) | |
Std molar entropy(S⦵298) | 170.8 J/(mol·K) |
Std enthalpy of formation(ΔfH⦵298) | −90.65 kJ/mol |
Gibbs free energy(ΔfG⦵) | −68.48 kJ/mol |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | extremely strong irritant, highly toxic, highly corrosive |
| GHS labelling: | |
   | |
| Danger | |
| H260,H301,H314 | |
| P223,P231+P232,P260,P264,P270,P280,P301+P316,P301+P330+P331,P302+P335+P334,P302+P361+P354,P304+P340,P305+P354+P338,P316,P321,P330,P363,P370+P378,P402+P404,P405,P501 | |
| NFPA 704 (fire diamond) | |
| 200 °C (392 °F; 473 K) | |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | 77.5 mg/kg (oral, rat)[5] |
LC50 (median concentration) | 22 mg/m3 (rat, 4 h)[6] |
| NIOSH (US health exposure limits): | |
PEL (Permissible) | TWA 0.025 mg/m3[4] |
REL (Recommended) | TWA 0.025 mg/m3[4] |
IDLH (Immediate danger) | 0.5 mg/m3[4] |
| Safety data sheet (SDS) | ICSC 0813 |
| Related compounds | |
Othercations | Sodium hydride Potassium hydride Rubidium hydride Caesium hydride |
Related compounds | Lithium borohydride Lithium aluminium hydride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Lithium hydride is aninorganic compound with the formulaLiH. Thisalkali metalhydride is a colorless solid, although commercial samples are grey. Characteristic of asalt-like (ionic) hydride, it has a high melting point, and it is not soluble but reactive with allproticorganic solvents. It is soluble and nonreactive with certainmolten salts such aslithium fluoride,lithium borohydride, andsodium hydride. With amolar mass of 7.95 g/mol, it is the lightestionic compound.
LiH isdiamagnetic and anionic conductor with anelectric conductivity gradually increasing from2×10−5 Ω−1cm−1 at 443 °C to 0.18 Ω−1cm−1 at 754 °C; there is no discontinuity in this increase through the melting point.[3]: 36 Thedielectric constant of LiH decreases from 13.0 (static, low frequencies) to 3.6 (visible-light frequencies).[3]: 35 LiH is a soft material with aMohs hardness of 3.5.[3]: 42 Itscompressive creep (per 100 hours) rapidly increases from < 1% at 350 °C to > 100% at 475 °C, meaning that LiH cannot provide mechanical support when heated.[3]: 39
Thethermal conductivity of LiH decreases with temperature and depends on morphology: the corresponding values are 0.125 W/(cm·K) for crystals and 0.0695 W/(cm·K) for compacts at 50 °C, and 0.036 W/(cm·K) for crystals and 0.0432 W/(cm·K) for compacts at 500 °C.[3]: 60 The linearthermal expansion coefficient is 4.2×10−5/°C at room temperature.[3]: 49
LiH is produced by treatinglithium metal withhydrogen gas:
This reaction is especially rapid at temperatures above 600 °C. Addition of 0.001–0.003% carbon, and/or increasing temperature/pressure, increases the yield up to 98% at 2-hour residence time.[3]: 147 However, the reaction proceeds at temperatures as low as 29 °C. The yield is 60% at 99 °C and 85% at 125 °C, and the rate depends significantly on the surface condition of LiH.[3]: 5
Less common ways of LiH synthesis includethermal decomposition oflithium aluminium hydride (200 °C),lithium borohydride (300 °C),n-butyllithium (150 °C), orethyllithium (120 °C), as well as several reactions involving lithium compounds of low stability and available hydrogen content.[3]: 144–145
Chemical reactions yield LiH in the form of lumpedpowder, which can be compressed intopellets without abinder. More complex shapes can be produced bycasting from themelt.[3]: 160 ff. Large singlecrystals (about 80 mm long and 16 mm in diameter) can be then grown from molten LiH powder in hydrogen atmosphere by theBridgman–Stockbarger technique. They often have bluish color owing to the presence ofcolloidal Li. This color can be removed by post-growthannealing at lower temperatures (~550 °C) and lower thermal gradients.[3]: 154 Major impurities in these crystals areNa (20–200ppm),O (10–100 ppm),Mg (0.5–6 ppm),Fe (0.5-2 ppm) andCu (0.5-2 ppm).[3]: 155
Bulk cold-pressed LiH parts can be easily machined using standard techniques and tools tomicrometer precision. However,cast LiH isbrittle and easily cracks during processing.[3]: 171
A more energy efficient route to form lithium hydride powder is byball milling lithium metal under high hydrogen pressure. To preventcold welding of lithium metal (due to its highductility), small amounts of lithium hydride powder are added during this process.[7]
LiH powder reacts rapidly withair of lowhumidity, formingLiOH,Li2O andLi2CO3. In moist air the powder ignites spontaneously, forming a mixture of products including some nitrogenous compounds. The lump material reacts with humid air, forming a superficial coating, which is a viscous fluid. This inhibits further reaction, although the appearance of a film of "tarnish" is quite evident. Little or nonitride is formed on exposure to humid air. The lump material, contained in a metal dish, may be heated in air to slightly below 200 °C without igniting, although it ignites readily when touched by an open flame. The surface condition of LiH, presence of oxides on the metal dish, etc., have a considerable effect on the ignition temperature. Dryoxygen does not react with crystalline LiH unless heated strongly, when an almost explosive combustion occurs.[3]: 6
LiH is highly reactive towardswater and otherprotic reagents:[3]: 7
LiH is less reactive with water than Li and thus is a much less powerful reducing agent for water,alcohols, and other media containing reduciblesolutes. This is true for all the binarysaline hydrides.[3]: 22
LiH pellets slowly expand in moist air, formingLiOH; however, the expansion rate is below 10% within 24 hours in a pressure of 2 Torr of water vapor.[3]: 7 If moist air containscarbon dioxide, then the product islithium carbonate.[3]: 8 LiH reacts withammonia, slowly at room temperature, but the reaction accelerates significantly above 300 °C.[3]: 10 LiH reacts slowly with higheralcohols andphenols, but vigorously with lower alcohols.[3]: 14
LiH reacts withsulfur dioxide to give thedithionite:
though above 50 °C the product islithium sulfide instead.[3]: 9
LiH reacts withacetylene to formlithium carbide andhydrogen. With anhydrousorganic acids, phenols andacid anhydrides, LiH reacts slowly, producing hydrogen gas and the lithium salt of the acid. With water-containing acids, LiH reacts faster than with water.[3]: 8 Many reactions of LiH with oxygen-containing species yield LiOH, which in turn irreversibly reacts with LiH at temperatures above 300 °C:[3]: 10
Lithium hydride is rather unreactive at moderate temperatures withO2 orCl2. It is, therefore, used in the synthesis of other useful hydrides,[8] e.g.,
With a hydrogen content in proportion to its mass 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 thwarted by its stability to decomposition. Thus removal ofH2 requires temperatures above the 700 °C used for its synthesis, such temperatures are expensive to create and maintain. The compound was once tested as a fuel component in a model rocket.[9][10]
LiH is not usually a hydride-reducing agent, except in the synthesis of hydrides of certain metalloids. For example,silane is produced in the reaction of lithium hydride andsilicon tetrachloride by the Sundermeyer process:
Lithium hydride is used in the production of a variety of reagents fororganic synthesis, such aslithium aluminium hydride (Li[AlH4]) andlithium borohydride (Li[BH4]).Triethylborane reacts to givesuperhydride (Li[BH(CH2CH3)3]).[11]
Lithium hydride (LiH) is sometimes a desirable material for the shielding ofnuclear reactors, with the isotopelithium-6 (Li-6), and it can be fabricated by casting.[12][13]
Lithium deuteride, in the form oflithium-7 deuteride (7Li2H or7LiD), is a goodmoderator fornuclear reactors, becausedeuterium (2H or D) has a lowerneutron absorptioncross-section than ordinary hydrogen orprotium (1H) does, and the cross-section for7Li is also low, decreasing the absorption of neutrons in a reactor.7Li is preferred for a moderator because it has a lower neutron capture cross-section, and it also forms lesstritium (3H or T) under bombardment with neutrons.[14]
The correspondinglithium-6deuteride (6Li2H or6LiD) is the primaryfusion fuel inthermonuclear weapons.[citation needed] In hydrogen warheads of theTeller–Ulam design, anuclear fission trigger explodes to heat and compress the lithium-6 deuteride, and to bombard the6LiD withneutrons to produce tritium in anexothermic reaction:
The deuterium and tritium then fuse to producehelium, one neutron, and 17.59 MeV of free energy in the form ofgamma rays,kinetic energy, etc. Tritium has a favorable reactioncross section. The helium is an inert byproduct.[citation needed]
Before theCastle Bravonuclear weapons test in 1954, it was thought that only the less common isotope6Li would breed tritium when struck with fast neutrons. The Castle Bravo test showed (accidentally) that the more plentiful7Li also does so under extreme conditions, albeit by anendothermic reaction.
LiH reacts violently with water to give hydrogen gas and LiOH, which is caustic. Consequently, LiH dust can explode in humid air, or even in dry air due to static electricity. At concentrations of5–55 mg/m3 in air the dust is extremely irritating to the mucous membranes and skin and may cause an allergic reaction. Because of the irritation, LiH is normally rejected rather than accumulated by the body.[3]: 157, 182
Some lithium salts, which can be produced in LiH reactions, are toxic. LiH fire should not be extinguished using carbon dioxide, carbon tetrachloride, or aqueous fire extinguishers; it should be smothered by covering with a metal object or graphite ordolomite powder. Sand is less suitable, as it can explode when mixed with burning LiH, especially if not dry. LiH is normally transported in oil, using containers made of ceramic, certain plastics or steel, and is handled in an atmosphere of dry argon or helium.[3]: 156 Whilst nitrogen can be used, it will react with lithium at elevated temperatures.[3]: 157 LiH normally contains some metallic lithium, which corrodes steel orsilica containers at elevated temperatures.[3]: 173–174, 179
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