Lithium Hydride
Introduction
Lithium Hydride (LiH) is a binary inorganic compound consisting of lithium and hydrogen. It is a colorless solid, although commercial samples are often gray due to impurities. This compound is known for its high reactivity and is widely used in various industrial applications, particularly in the field of hydrogen storage and as a reducing agent in organic synthesis.
Chemical Properties
Lithium hydride is an ionic compound, characterized by the presence of lithium cations (Li⁺) and hydride anions (H⁻). The ionic nature of LiH contributes to its high melting point of 688 °C and its significant thermal stability. The compound is highly reactive with water, producing lithium hydroxide (LiOH) and hydrogen gas (H₂):
\[ \text{LiH} + \text{H}_2\text{O} \rightarrow \text{LiOH} + \text{H}_2 \]
Due to the presence of the hydride ion, LiH is a strong reducing agent and can react with a variety of organic and inorganic compounds. It is also highly reactive with oxygen and carbon dioxide, forming lithium oxide (Li₂O) and lithium carbonate (Li₂CO₃), respectively.
Physical Properties
Lithium hydride is a crystalline solid with a density of 0.78 g/cm³. It has a face-centered cubic (fcc) crystal structure, similar to that of sodium chloride (NaCl). The compound is highly hygroscopic, meaning it readily absorbs moisture from the air, which can lead to hydrolysis and the release of hydrogen gas.
Synthesis
Lithium hydride can be synthesized by the direct reaction of lithium metal with hydrogen gas at elevated temperatures:
\[ 2\text{Li} + \text{H}_2 \rightarrow 2\text{LiH} \]
This reaction is highly exothermic and typically occurs at temperatures between 500 and 700 °C. The process requires an inert atmosphere, such as argon or nitrogen, to prevent the lithium from reacting with oxygen or moisture in the air.
Applications
Hydrogen Storage
One of the most significant applications of lithium hydride is in hydrogen storage. Due to its high hydrogen content (12.7% by weight), LiH is an effective material for storing hydrogen in a compact and stable form. The compound can release hydrogen gas upon heating, making it useful in portable hydrogen fuel cells and other energy storage systems.
Reducing Agent
Lithium hydride is widely used as a reducing agent in organic synthesis. It can reduce various functional groups, including carbonyl compounds, nitriles, and esters, to their corresponding alcohols or amines. The reactivity of LiH is often enhanced by using it in combination with other reagents, such as lithium aluminum hydride (LiAlH₄).
Nuclear Industry
In the nuclear industry, lithium hydride is used as a neutron moderator and shield. Its ability to absorb neutrons makes it valuable in controlling nuclear reactions and protecting against radiation. Additionally, LiH is used in the production of tritium, a radioactive isotope of hydrogen, through neutron irradiation.
Safety and Handling
Lithium hydride is highly reactive and must be handled with care. It is flammable and can ignite spontaneously in air, especially when finely divided. The compound should be stored in a dry, inert atmosphere to prevent hydrolysis and the release of hydrogen gas. Personal protective equipment, such as gloves and goggles, should be worn when handling LiH to avoid contact with skin and eyes.