Calcium hydride (\(\text{CaH}_2\)) is an inorganic compound composed of the alkaline earth metal calcium and hydrogen. It is classified chemically as a saline hydride. Saline hydrides are characterized by a salt-like structure where the hydride ion (\(\text{H}^-\)) is paired with a metal cation, in this case, the calcium ion (\(\text{Ca}^{2+}\)).
Physical Characteristics and Chemical Identity
Physical Properties
In its purest form, calcium hydride is a white crystalline solid, though commercial samples typically appear as a grayish powder due to slight impurities or partial decomposition. This compound possesses an orthorhombic crystal structure and is relatively dense, registering a value of approximately 1.70 grams per cubic centimeter. It is insoluble in common solvents, a characteristic shared among many compounds with a salt-like structure.
Hydrolysis Reaction
The defining chemical property of calcium hydride is its vigorous reaction with water, known as hydrolysis. This reaction happens quickly and exothermically, releasing heat into the surroundings. The chemical equation is \(\text{CaH}_2 + 2\text{H}_2\text{O} \rightarrow \text{Ca}(\text{OH})_2 + 2\text{H}_2\).
The reaction generates two main products: calcium hydroxide (a solid or aqueous slurry) and flammable hydrogen gas. The rapid liberation of hydrogen gas can potentially ignite, especially when the reaction occurs in a confined space. This vigorous reactivity underscores why the compound is never mixed with water for disposal.
Calcium hydride also exhibits similar reactivity when exposed to other protic solvents, such as low molecular weight alcohols. This strong chemical identity is the basis for its utility in applications requiring the complete absence of moisture. The resulting solid calcium hydroxide is easily separated, making the process effective for purification.
Key Applications in Industry and Research
Desiccant Use
The affinity of calcium hydride for water makes it an excellent and widely used drying agent, or desiccant, in chemical research and industry. It is effective for removing trace amounts of moisture from organic solvents and gases that are sensitive to water. For instance, it is commonly employed to dry basic solvents like pyridine and various amines, as it is safer than more reactive alternatives such as pure sodium metal.
The moisture is chemically consumed by the hydride, converting it into inert hydrogen gas and calcium hydroxide. This process creates anhydrous, or water-free, conditions necessary for many synthesis reactions. Its convenience and ease of use in the laboratory setting keep it in frequent rotation, despite being a relatively mild desiccant compared to some other reagents.
Reducing Agent and Hydrogen Source
Calcium hydride functions as a reducing agent in both organic synthesis and metallurgy. In powder metallurgy, it is used to reduce the oxides of metals, such as titanium, uranium, and vanadium, to their pure metallic forms at high temperatures. This process relies on the hydride’s ability to strip oxygen from the metal oxide compounds.
Historically, calcium hydride was a portable source for generating hydrogen gas on demand, commercially known as “Hydrolith.” By simply adding water, the compound could quickly generate hydrogen to inflate devices like weather balloons and life rafts. This application highlights its compact nature as a solid-state hydrogen carrier.
Safe Handling and Storage Requirements
Due to its high reactivity with moisture, storing calcium hydride requires strict protocols to ensure safety and maintain the integrity of the compound. It must always be kept in tightly closed, airtight containers within a dry and well-ventilated storage area. Any contact with humid air or water must be avoided, as this will trigger the exothermic reaction and the release of flammable hydrogen gas.
The storage environment must be kept away from heat, open flames, and any sources of ignition that could interact with the released hydrogen gas. Calcium hydride should not be stored near acids or alcohols, as these substances can also react with it.
When handling the powder, laboratory personnel should use appropriate personal protective equipment, including safety goggles and nitrile or neoprene gloves. It is recommended to handle the material under an inert atmosphere, such as argon or nitrogen gas, to prevent any reaction with atmospheric moisture. Proper ventilation is necessary to prevent the accumulation of hydrogen gas should any accidental exposure to moisture occur.