Lithium hydride

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Lithium hydride
Space-filling model of part of the crystal structure of lithium hydride
Identifiers
CAS number 7580-67-8 YesY
PubChem 62714
ChemSpider 56460 YesY
RTECS number OJ6300000
Jmol-3D images Image 1
Properties
Molecular formula LiH
Molar mass 7.95 g/mol
Appearance colorless to gray solid[1]
Density 0.78 g/cm3[1]
Melting point 688.7 °C[1]
Boiling point decomposes at 900–1000 °C[2]
Solubility in water reacts
Refractive index (nD) 1.9847[3]:43
Structure
Lattice constant a = 0.40834 nm[3]:56
Dipole moment 6.0 D[3]:35
Thermochemistry
Specific
heat capacity
C
3.51 J/(g·K)
Std enthalpy of
formation
ΔfHo298
-11.39 kJ/g
Hazards
MSDS ICSC 0813
EU Index Not listed
NFPA 704
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g., diesel fuel Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Autoignition temperature 200 °C
Related compounds
Other cations Sodium hydride
Potassium hydride
Rubidium hydride
Caesium hydride
Related compounds Lithium borohydride
Lithium aluminium hydride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Lithium hydride is the inorganic compound with the formula LiH. It is a colorless 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. With a molecular mass of slightly less than 8, it is the lightest ionic compound.

Physical properties[edit]

LiH is a diamagnetic and an ionic conductor with a conductivity gradually increasing from 2×10−5 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 The dielectric constant of LiH decreases from 13.0 (static, low frequencies) to 3.6 (visible light frequencies).[3]:35 LiH is a soft material with a Mohs hardness of 3.5.[3]:42 Its compressive creep (per 100 hours) rapidly increases from < 1% at 350 °C to > 100% at 475 °C meaning that LiH can't provide mechanical support when heated.[3]:39

The thermal 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 linear thermal expansion coefficient is 4.2×10−5/°C at room temperature.[3]:49

Synthesis and processing[edit]

LiH is produced by treating lithium metal with hydrogen gas:

2 Li + H2 → 2 LiH

This reaction is especially rapid at temperatures above 600 °C. Addition of 0.001–0.003% carbon, or/and increasing temperature or/and 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 include thermal decomposition of LiAlH4 (200 °C), LiBH4 (300 °C), C4H9Li (150 °C), or ethyl lithium (120 °C), as well as several reactions involving lithium compounds of low stability and hydrogen.[3]:144–145

Chemical reactions yield LiH in the form of lumped powder, which can be compressed into pellets without a binder. More complex shapes can be produced by casting from the melt.[3]:160 ff. Large single crystals (about 80 mm long and 16 mm in diameter) can be then grown from molten LiH powder in hydrogen atmosphere by the Bridgman–Stockbarger technique. They often have bluish color owing to the presence of colloidal Li. This color can be removed by post-growth annealing at lower temperatures (~550 °C) and lower thermal gradients.[3]:154 Major impurities in these crystals are Na (20–200 parts per million, ppm), O (10–100 ppm), Mg (0.5–6 ppm), Fe (0.5-2 ppm) and Cu (0.5-2 ppm).[3]:155

Cracking in cast LiH after machining with a fly cutter. Scale is in inches.

Bulk cold-pressed LiH parts can be easily machined using standard techniques and tools to micrometer precision. However, cast LiH is brittle and easily cracks during processing.[3]:171

Reactions[edit]

LiH powder reacts rapidly with air of low humidity, forming LiOH, Li
2
O
and Li
2
CO
3
. 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 no nitride 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. Dry oxygen does not react with crystalline LiH unless heated strongly, when an almost explosive combustion occurs.[3]:6

LiH is highly reactive toward water and other protic reagents:[3]:7

LiH + H2O → LiOH + H2

LiH is less reactive with water than Li and thus is a much less powerful reducing agent for water, alcohols, and other media containing reducible solutes. This is true of all the binary saline hydrides.[3]:22

LiH pellets slowly expand in moist air forming LiOH; however, the expansion rate is below 10% within 24 hours in a pressure of 2 mm of water vapor.[3]:7 If most air contains carbon dioxide then the product is lithium carbonate.[3]:8 LiH reacts with ammonia, slowly at room temperature, but the reaction accelerates significantly above 300 °C.[3]:10 LiH reacts slowly with higher alcohols and phenols but vigorously with lower alcohols.[3]:14

LiH reacts with sulfur dioxide:

2 LiH + 2 SO2 → Li2S2O4 + H2

though above 50 °C the product is lithium sulfide.[3]:9

LiH reacts with acetylene to form lithium carbide and hydrogen. With anhydrous organic acids, phenols and acid 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

LiH + LiOH → Li2O + H2

Applications[edit]

Hydrogen storage and fuel[edit]

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 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.[4][5]

Precursor to complex metal hydrides[edit]

LiH 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

Lithium hydride is used in the production of a variety of reagents for organic synthesis, such as lithium aluminium hydride (LiAlH4) and lithium borohydride (LiBH4). Triethylborane reacts to give superhydride (LiBHEt3).[6]

In nuclear chemistry and physics[edit]

LiH is a desirable material for shielding nuclear reactors and can be fabricated by casting.[7][8]

Lithium deuteride[edit]

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[edit]

As discussed above, LiH reacts explosively 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 of 5–55 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]:182[3]:157

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; they should be smothered by covering with a metal object or graphite or dolomite 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 Nitrogen can be used, but not at elevated temperatures as it reacts with lithium.[3]:157 LiH normally contains some metallic Li, which corrodes steel or silica containers at elevated temperatures.[3]:173–174, 179

References[edit]

  1. ^ a b c Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. p. 4.70. ISBN 0-8493-0486-5. 
  2. ^ David Arthur Johnson; Open University (12 August 2002). Metals and chemical change. Royal Society of Chemistry. pp. 167–. ISBN 978-0-85404-665-2. Retrieved 1 November 2011. 
  3. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae Smith, R. L.; Miser, J. W. (1963). Compilation of the properties of lithium hydride. NASA. 
  4. ^ Lex. Astronautix.com (1964-04-25). Retrieved on 2011-11-01.
  5. ^ Empirical laws for hybrid combustion of lithium hydride with fluorine in small rocket engines. Ntrs.nasa.gov. Retrieved on 2011-11-01.
  6. ^ Peter Rittmeyer, Ulrich Wietelmann "Hydrides" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a13_199
  7. ^ Peter J. Turchi (1998). Propulsion techniques: action and reaction. AIAA. pp. 339–. ISBN 978-1-56347-115-5. Retrieved 2 November 2011. 
  8. ^ Welch, Frank H. (February 1974). "Lithium hydride: A space age shielding material". Nuclear Engineering and Design 26 (3): 440–460. doi:10.1016/0029-5493(74)90082-X. 

External links[edit]