Calcium hydride is an inorganic compound with the chemical formula CaH2. It appears as a grayish or white crystalline solid and is an important hydride of alkaline earth metals. Calcium hydride was first prepared in the early 19th century by direct reaction of calcium metal with hydrogen gas at elevated temperatures. Its discovery provided a new class of metal hydrides with strong reducing and hydrogen-releasing capabilities.
Calcium hydride is primarily used as a drying agent for solvents and gases because it readily reacts with water to produce calcium hydroxide and hydrogen gas. This property makes it highly effective for removing traces of moisture from organic solvents, particularly those sensitive to water. Due to its relatively high reactivity toward water and alcohols, calcium hydride is often preferred over molecular sieves or other drying agents in specific laboratory and industrial processes.
In addition to its use as a drying agent, calcium hydride serves as a solid-state source of hydrogen. When exposed to water or protic solvents, it liberates hydrogen gas, which can be captured and used for chemical reactions or as a reducing atmosphere. This ability has found applications in small-scale hydrogen generation and in chemical synthesis requiring controlled hydrogen supply.
Calcium hydride is also used as a reducing agent in organic chemistry. It can reduce certain metal salts and organic compounds, although its reducing power is generally milder compared to reagents like lithium aluminum hydride or sodium borohydride.
The compound is typically handled as a powder or granules and must be stored in airtight containers to prevent reaction with moisture in the air. Contact with water or humid air results in vigorous hydrogen gas evolution, posing flammability and explosion hazards. Therefore, safe handling procedures, including working under inert atmospheres or in dry environments, are essential.
Calcium hydride crystallizes in a structure similar to that of lead chloride, where calcium ions are surrounded by hydride ions in a lattice arrangement. Its melting point is approximately 816 °C, and it is insoluble in organic solvents.
The commercial production of calcium hydride involves the direct combination of calcium metal and hydrogen gas at high temperatures, making it a relatively straightforward industrial process. This accessibility has made it a common reagent in laboratories and industries.
In summary, calcium hydride’s discovery and use have provided chemists and engineers with a versatile reagent that functions effectively as a drying agent, hydrogen source, and mild reducing agent. Its applications span from moisture removal in chemical synthesis to controlled hydrogen generation, supporting diverse fields including organic chemistry, materials science, and industrial processing.
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