Calcium hydride

Chembox new
Name = Calcium hydride
ImageFile = Calcium hydride.jpg
ImageName = Calcium hydride
IUPACName = Calcium(II) hydride
OtherNames = Calcium hydride
Calcium dihydride
Section1 = Chembox Identifiers
CASNo = 7789-78-8
Section2 = Chembox Properties
Formula = CaH₂
MolarMass = 42.094 g/mol
Appearance = gray (pure: colourless)
Density = 1.90 g/cm³, solid
Solubility = reacts violently
MeltingPt = 816 °C

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 motif. [Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.]

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, a solid, are readily separated from the solvent by distillation, filtration, or decantation.

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. [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.] It is also used to pre-dry solvents prior to the use of a more reactive desiccant.

Drawbacks

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.
*Since calcium hydride does not remove dissolved oxygen, it is not useful for deoxygenating solvents.

Use for hydrogen production

Calcium hydride was, in the 1940s, available under the trade name "Hydrolith" [http://www.thefreedictionary.com/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.' [Adlam G.H.J. and Price L.S., "A Higher School Certificate Inorganic Chemistry", John Murray, London, 1940]

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.

Looking to the future, Calcium hydride is a possible source of hydrogen for fuel cells but the use of sodium borohydride in the Direct borohydride fuel cell may be preferable.

References