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| Calcium hydride | |
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| Image:Calcium hydride.jpg | |
| IUPAC name | Calcium(II) hydride |
| Other names | Calcium hydride Calcium dihydride |
| Identifiers | |
| CAS number | [7789-78-8] |
| Properties | |
| Molecular formula | CaH2 |
| Molar mass | 42.094 g/mol |
| Appearance | gray (pure: colourless) |
| Density | 1.90 g/cm3, solid |
| Melting point |
816 °C |
| Solubility in water | reacts violently |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references |
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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, 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.[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 an insoluble in all solvents, 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.
References
- ^ 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.
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