Calcium hydride

Calcium hydride
IUPAC name	Calcium hydride
Other names	Calcium(II) hydride Calcium dihydride
Identifiers
CAS number	[7789-78-8]
PubChem	105052
EC number	232-189-2
Properties
Molecular formula	CaH2
Molar mass	42.094 g/mol
Appearance	gray powder (colourless when pure)
Density	1.70 g/cm3, solid
Melting point	816 °C
Solubility in water	reacts violently
Solubility	reacts in alcohol
Structure
Crystal structure	Orthorhombic, oP12
Space group	Pnma, No. 62
Hazards
EU Index	001-004-00-5
EU classification	Flammable (F)
R-phrases	R15
S-phrases	(S2), S7/8, S24/25, S42
Related compounds
Other cations	Sodium hydride, Potassium hydride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

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.^[1]

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 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.^[2] 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" ^[3] 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.' ^[4]

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

1. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
2. ^ 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.
3. ^ http://www.thefreedictionary.com/hydrolith
4. ^ Adlam G.H.J. and Price L.S., A Higher School Certificate Inorganic Chemistry, John Murray, London, 1940