Lithium hydride

Description

Lithium hydride is an off-white to grayish,translucent, odorless solid or white powder that darkens rapidly on exposure to light. Molecular weight = 7.95; Specificgravity (H2O:1)=0.78; Boiling point = 850℃ (decomposesbelow BP); Freezing/Melting point = 689℃; Autoignitiontemperature = 200℃. Hazard Identification (based onNFPA-704 M Rating System): Health 3, Flammability 4,Reactivity 2. A combustible solid that can form airbornedust clouds which may explode on contact with flame, heat,or oxidizers.

Chemical Properties

Lithium hydride (LiH) is a crystalline salt substance(face-centered cubic) that is white in its pure form, As an engineering material, it has properties of interest in many technologies. For example,the high hydrogen content and light weight of LiH make it useful for neutron shields and moderators in nuclear power plants. In addition, the high heat of fusion combined with light weight make LiH appropriate for heat storage media for solar power plants on satellites and may be used as a heat sink for different applications. Typically, processes for production of LiH involve handling of LiH at temperatures above its meltingpoint (688 DC). Type 304L stainless steel is utilized for many process components handling molten LiH.

Lithium hydride is a typical ionic hydride with lithium cations and hydride anions. Electrolysis of molten material results in formation of lithium metal at the cathode and hydrogen at the anode. The lithium hydride-water reaction, which results in the release of hydrogen gas, is also indicative of a negatively charged hydrogen.

Chemical Properties

Lithium hydride is an off-white to grayish, translucent, odorless solid or white powder that darkens rapidly on exposure to light. Pure lithium hydride forms colorless, cubic crystals. The commercial product contains traces of impurities, e.g., unreacted lithium metal, and is consequently light gray or blue. Lithium hydride is thermally very stable, being the only ionic hydride that melts without decomposition at atmospheric pressure (mp 688 ℃). In contrast to the other alkali metal hydrides, lithium hydride is slightly soluble in inert polar organic solvents such as ethers. It forms eutectic mixtures with a large number of salts. Lithium hydride is stable in dry air but ignites at increased temperature. In moist air it is hydrolyzed exothermically; finely divided material can ignite spontaneously. At elevated temperature, it reacts with oxygen to form lithium oxide, with nitrogen to form lithium nitride and hydrogen, and with carbon dioxide to form lithium formate.

Physical properties

White crystalline solid; cubic crystals; density 0.82 g/cm³; melts at 686.4°C; decomposes in water; soluble in acids.

Uses

Lithium hydride is a bluish-white crystal that is flammable in moisture. Used as a source of hydrogen gas that is liberated when LiH becomes wet. LiH is an excellent desiccant and reducing agent as well as a shield that protects from radiation created by nuclear reactions.

Uses

Lithium hydride is used in the manufactureof lithium aluminum hydride and silane, as apowerful reducing agent, as a condensationagent in organic synthesis, as a portablesource of hydrogen, and as a lightweight nuclear shielding material. It is now beingused for storing thermal energy for spacepower systems (Morris et al. 1988).

Uses

Preparation

Lithium hydride is prepared by heating lithium metal with hydrogen above 440°C. The reaction is exothermic and can be controlled once it is initiated, without external heating. The heat of formation is greater than that of sodium hydride: 2Li + H2 → 2LiH

Definition

lithium hydride: A white solid,LiH; cubic; r.d. 0.82; m.p. 680°C; decomposesat about 850°C. It is producedby direct combination of theelements at temperatures above500°C. The bonding in lithium hydrideis believed to be largely ionic;i.e. Li⁺H^- as supported by the factthat hydrogen is released from theanode on electrolysis of the moltensalt. The compound reacts violentlyand exothermically with water toyield hydrogen and lithium hydroxide.It is used as a reducing agent toprepare other hydrides and the 2Hisotopic compound, lithiumdeuteride, is particularly valuable fordeuterating a range of organic compounds.Lithium hydride has alsobeen used as a shielding material forthermal neutrons.

Reactions

Lithium hydride reacts vigorously with water, forming lithium hydroxide with the evolution of hydrogen:
LiH + H2O → LiOH + H2
The hydride also reacts with ammonia forming lithium amide and evolving hydrogen:
LiH + NH3 → LiNH2 + H2
Lithium hydride is a strong reducing agent and would, therefore, react with compounds that contain oxygen. Even many highly stable oxides of metals and nonmetals can be reduced. It reduces metal oxides to metals and carbon dioxide to carbon:
Fe3O4 + 4LiH → 3Fe + 4NaOH
2LiH + CO2 → Li2O + C + H2O
It undergoes violent reactions with oxidizing agents.
Lithium hydride reacts with aluminum hydride forming lithium aluminum hydride, a powerful reducing agent: LiH + AlH3 → LiAlH4
Lithium hydride consisting of Li+ and H– ions exhibits properties of an ionic salt, both cationic and anionic; such as a strong electrolyte. Thus, when electrolyzed at temperatures slightly below its melting point, it dissociates to Li+ and H¯ ions. Hydrogen gas is liberated at the anode.
The hydride ion, H:¯ being a strong base, would react with alcohols, forming alkoxides and liberating hydrogen:
CH3CH2OH + LiH → CH3CH2OLi + H2
(ethanol) (lithium ethoxide)
(CH3)3COH + LiH → (CH3)3COLi + H2
(tert-butanol) (lithium tert-butoxide)

General Description

A white or translucent crystalline mass or powder. The commercial product is light bluish-gray lumps due to the presence of minute amounts of colloidally dispersed lithium.

Air & Water Reactions

Burns readily in air, particularly if powdered. May ignite spontaneously in moist air. Reacts rapidly with water to form caustic lithium hydroxide and hydrogen [Bretherick 1979 p. 107].

Reactivity Profile

Lithium hydride is a strong reducing agent. May decompose violently in contact with most oxidizing materials. Reacts exothermically with water to form caustic lithium hydroxide and hydrogen gas; the hydrogen may ignite. May ignite spontaneously in moist air. Mixtures with liquid oxygen are explosive. Ignites on contact with dinitrogen oxide [Mellor, 1967, vol. 8, suppl. 2.2, p. 214].

Health Hazard

The health hazard due to lithium hydride maybe attributed to the following properties: (1)corrosivity of the hydride, (2) its hydrolysisto strongly basic lithium hydroxide, and (3)toxicity of the lithium metal. However, thelatter property, which may arise becauseof the formation of lithium resulting fromthe decomposition of lithium hydride andthe metabolic role of lithium, is not yetestablished.
This compound is highly corrosive to skin.Contact with eyes can produce severe irritationand possible injury. It can hydrolyzewith body fluid, forming lithium hydroxide,which is also corrosive to the skin andharmful to the eyes. Animal tests indicatedthat exposure to its dust or vapor at a levelexceeding 10 mg/m³ eroded the body fur andskin, caused severe inflammation of the eyes,and led to the destruction of external nasalseptum (ACGIH 1986). No chronic effectswere observed.

Fire Hazard

In a fire, irritating alkali fumes may form. Lithium hydride can form airborne dust clouds which may explode on contact with flame, heat, or oxidizing materials. Additionally, spontaneous ignition occurs when nitrous oxide and Lithium hydride are mixed. Lithium hydride also forms explosive mixtures with liquid oxygen. Contact with heat, moisture or acid causes exothermic reaction and evolution of hydrogen as well as lithium hydroxide. Incompatible with air and moisture, nitrous oxide, strong oxidizers, and liquid oxygen. Lithium hydride may ignite spontaneously in air and should be maintained and handled out of contact with air and moisture. Any contact with nitrous oxide; airborne powders may ignite upon reaching moisture.

Safety Profile

Poison by inhalation. A severe eye, skin, and mucous membrane irritant. Upon contact with moisture, lithium hydroxide is formed. The LiOH formed is very caustic and therefore highly toxic, particularly to lungs and respiratory tract, skin, and mucous membranes. The powder ignttes spontaneously in air. The solid can ignite spontaneously in moist air. Mixtures of the powder with liquid oxygen are explosive. Ignttes on contact with dinitrogen oxide, oxygen + moisture. To fight fire, use special mixtures of dry chemical. See also LITHIUM COMPOUNDS and HYDRIDES.

Potential Exposure

Lithium hydride is used in preparation of lithium aluminum hydride; as a desiccant; it is used in hydrogen generators and in organic synthesis as a reducing agent and condensing agent with ketones and acid esters; it is reportedly used in thermonuclear weapons.

First aid

If this chemical gets into the eyes, remove anycontact lenses at once and irrigate immediately for at least15 min, occasionally lifting upper and lower lids. Seek medical attention immediately. If this chemical contacts theskin, remove contaminated clothing and wash immediatelywith soap and water. Seek medical attention immediately. Ifthis chemical has been inhaled, remove from exposure,begin rescue breathing (using universal precautions, including resuscitation mask) if breathing has stopped and CPR ifheart action has stopped. Transfer promptly to a medicalfacility. When this chemical has been swallowed, get medical attention. If victim is conscious, administer water ormilk. Do not induce vomiting. Medical observation isrecommended for 24-48 h after breathing overexposure, aspulmonary edema may be delayed. As first aid for pulmonary edema, a doctor or authorized paramedic may consideradministering a corticosteroid spray.

storage

The product should be handled under an inert atmosphere to avoid contamination and a fire. Powdered lithium hydride burns readily when exposed to the air. However, large pieces of the material are less flammable. Lithium hydride, like other strong bases, is harmful to the skin and should be handled with caution.

Shipping

UN1414 Lithium, Hazard Class: 4.3; Labels: 4.3-Dangerous when wet material. UN2805 Lithium hydride, fused solid, Hazard Class: 4.3; Labels: 4.3- Dangerous when wet material

Purification Methods

It should be a white powder; otherwise replace it. It darkens rapidly on exposure to air and is decomposed by H2O to give H2 and LiOH, and reacts with lower alcohols. One gram in H2O liberates 2.8L of H2 (could be explosive). [D.nges in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol I p 987 1963.]

Incompatibilities

A Strong reducing agent. Incompatible with oxidizers, halogenated hydrocarbons; acids can cause fire and explosion. Reacts with water, forming caustic lithium hydroxide and flammable hydrogen gas; reaction may cause ignition. May ignite spontaneously in moist air and may reignite after fire is extinguished. Dangerous when wet. Reacts with water to form hydrogen and lithium hydroxide. Powdered form and liquid oxygen form an explosive compound. Decomposes exothermically on contact with acids and upon heating to about 500C, producing flammable hydrogen gas. Reacts with carboxylic acids, lower alcohols; chlorine, and ammonia (at 400C), forming explosive hydrogen gas.

Waste Disposal

Lithium hydride may be mixed with sand, sprayed with butanol and then with water, neutralized and flushed to a sewer with water