|Jmol-3D images||Image 1|
|Molar mass||42.094 g/mol|
|Appearance||gray powder (white when pure)|
|Density||1.70 g/cm3, solid|
|Melting point||816 °C|
|Solubility in water||reacts violently|
|Solubility||reacts in alcohol|
|Crystal structure||Orthorhombic, oP12|
|Space group||Pnma, No. 62|
|GHS signal word||DANGER|
|GHS hazard statements||H260|
|EU classification||Flammable (F)|
|S-phrases||(S2), S7/8, S24/25, S43|
|Other cations||Sodium hydride,
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Calcium hydride is the chemical compound with the formula CaH2. This grey powder (white if pure, which is rare) reacts vigorously with water liberating hydrogen gas. CaH2 is thus used as a drying agent, i.e. a desiccant.
CaH2 is a saline hydride, meaning that its structure is salt-like. The alkali metals and the alkaline earth metals all form saline hydrides. A well-known example is sodium hydride, which crystallizes in the NaCl motif. These species are insoluble in all solvents with which they do not react because they have extended structures. CaH2 crystallizes in the PbCl2 (cotunnite) structure.
Calcium hydride may be prepared from its elements by direct combination of calcium and hydrogen at 300 to 400°C. It also can be made by heating calcium chloride with hydrogen in the presence of sodium metal:
- CaCl2 + H2 + 2 Na → CaH2 + 2 NaCl
- CaO + Mg + H2 → CaH2 + MgO
Use as a desiccant
The reaction of CaH2 with water can be represented as follows:
- CaH2 + 2 H2O → Ca(OH)2 + 2 H2
The two hydrolysis products, H2, a gas, and Ca(OH)2, an aqueous mixture, are readily separated from each other.
As calcium hydride is a relatively mild desiccant, it is safe compared with more reactive agents such as sodium metal or sodium-potassium alloy. Calcium hydride is widely used as a desiccant for basic solvents such as amines and pyridine. It is also used to pre-dry solvents prior to the use of a more reactive desiccant.
Although CaH2 is indeed convenient and often the drying agent of choice, it has a few drawbacks:
- it is insoluble in all solvents with which it does not react vigorously, in contrast to LiAlH4, thus the speed of its drying action can be slow.
- it is incompatible with some solvents, and can in fact explosively react with chlorocarbons(LiAlH4 also suffers from this disadvantage).
- Because CaH2 and Ca(OH)2 are almost indistinguishable in appearance, the quality of a sample of CaH2 is not obvious visually.
Use for hydrogen production
Calcium hydride was, in the 1940s, 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.'
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.
- Index no. 001-002-00-4 of Annex VI, Part 3, to Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006. OJEU L353, 31.12.2008, pp 1–1355 at p 340.
- Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
- Gawley, R. E.; Davis, A. "Calcium Hydride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.rc005.
- Hydrolith in thefreedictionary.com
- Adlam G.H.J. and Price L.S., A Higher School Certificate Inorganic Chemistry, John Murray, London, 1940