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| Names | |
|---|---|
| IUPAC name Calcium hydride | |
| Other names Calcium(II) hydride Calcium dihydride Hydrolith | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.029.263 |
| EC Number |
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| UNII | |
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| Properties | |
| CaH2 | |
| Molar mass | 42.094 g/mol |
| Appearance | gray powder (white when pure) |
| Density | 1.70 g/cm3, solid |
| Melting point | 816 °C (1,501 °F; 1,089 K) |
| reacts violently | |
| Solubility | reacts in alcohol |
| Structure | |
| Orthorhombic,oP12 | |
| Pnma, No. 62 | |
| Thermochemistry | |
Std molar entropy(S⦵298) | 41.4 J/(mol·K)[1] |
Std enthalpy of formation(ΔfH⦵298) | −181.5 kJ/mol |
Gibbs free energy(ΔfG⦵) | −142.5 kJ/mol |
| Hazards | |
| GHS labelling: | |
    | |
| Danger | |
| H260 | |
| NFPA 704 (fire diamond) | |
| Related compounds | |
Othercations | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Calcium hydride is thechemical compound with the formulaCaH2, analkaline earth hydride. This grey powder (white if pure, which is rare) reacts vigorously with water, liberatinghydrogen gas.CaH2 is thus used as a drying agent, i.e. adesiccant.[2]
CaH2 is a saline hydride, meaning that its structure is salt-like. The alkali metals and the alkaline earth metals heavier than beryllium all form saline hydrides. A well-known example issodium hydride, which crystallizes in the NaCl motif. These species are insoluble in all solvents with which they do not react.CaH2 crystallizes in thePbCl2 (cotunnite)structure.[3]
Calcium hydride is prepared from its elements by direct combination of calcium and hydrogen at 300 to 400 °C.[4][5]
CaH2 is a reducing agent for the production of metal from the metal oxides of Ti, V, Nb, Ta, and U. It is proposed to operate via its decomposition to Ca metal:[4]
CaH2 has been used for hydrogen production. In the 1940s, it was available under the trade name "Hydrolith" as a source of hydrogen:
The trade name for this compound is "hydrolith"; in cases of emergency, it can be used as a portable source of hydrogen, for filling airships. It is rather expensive for this use.[6]
The reference to "emergency" probably refers to wartime use. The compound has, however, been widely used for decades as a safe and convenient means to inflate weather balloons. Likewise, it is regularly used in laboratories to produce small quantities of highly pure hydrogen for experiments. The moisture content of diesel fuel is estimated by the hydrogen evolved upon treatment with CaH2.[4]
The reaction ofCaH2 with water can be represented as follows:
The two hydrolysis products, gaseousH2 andCa(OH)2, are readily separated from the dried solvent.
Calcium hydride is a relatively mild desiccant and, compared tomolecular sieves, probably inefficient.[7] Its use is safer than more reactive agents such assodium metal orsodium-potassium alloy. Calcium hydride is widely used as adesiccant forbasic solvents such asamines andpyridine. It is also used to dry alcohols.[2]
Despite its convenience,CaH2 has a few drawbacks:
During theBattle of the Atlantic, German submarines used calcium hydride as asonar decoy calledbold.[8]
Although the term calcium hydride almost always refers to CaH2, a number of molecular hydrides of calcium are known. One example is (Ca(μ-H)(thf)(nacnac))2.[9]Dimeric calcium hydrides arise through the reaction of a calcium amide withphenylsilane.[10] Subsequent studies have expanded the library of stabilizing ligands, but all aremultidentate ligands that coordinate through nitrogen sites.[11]
