Calcium hydride
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{{Chembox new
| Name = Calcium hydride
| ImageFile = Calcium hydride.jpg
<!-- | ImageSize = 200px -->
| ImageName = Calcium hydride
| IUPACName = Calcium(II) hydride
| OtherNames = Calcium hydride<br />Calcium dihydride
| Section1 = {{Chembox Identifiers
| CASNo = 7789-78-8
}}
| Section2 = {{Chembox Properties
| Formula = CaH<sub>2</sub>
| MolarMass = 42.094 g/mol
| Appearance = gray (pure: colourless)
| Density = 1.90 g/cm<sup>3</sup>, solid
| Solubility = reacts violently
| MeltingPt = 816 °C
}}
}}
'''Calcium hydride''' is the [[chemical compound]] with the formula CaH<sub>2</sub>. This grey powder (white if pure, which is rare) reacts vigorously with water liberating [[hydrogen]] gas. CaH<sub>2</sub> is thus used as a drying agent, i.e. a [[desiccant]].
CaH<sub>2</sub> 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. CaH<sub>2</sub> crystallizes in the PbCl<sub>2</sub> motif.<ref>Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.</ref>
==Use as a desiccant==
The reaction of CaH<sub>2</sub> with water can be represented as follows:
:CaH<sub>2</sub> + 2 H<sub>2</sub>O → Ca(OH)<sub>2</sub> + 2 H<sub>2</sub>
The two hydrolysis products, H<sub>2</sub>, a gas, and [[calcium hydroxide|Ca(OH)<sub>2</sub>]], 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 [[NaK|sodium-potassium alloy]]. Calcium hydride is widely used as a [[desiccant]] for [[base (chemistry)|basic]] solvents such as [[amine]]s and [[pyridine]].<ref>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}}.</ref> It is also used to pre-dry solvents prior to the use of a more reactive desiccant.
===Drawbacks===
Although CaH<sub>2</sub> 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 [[Lithium aluminium hydride|LiAlH<sub>4</sub>]], thus the speed of its drying action can be slow.
*it is incompatible with some solvents, and can in fact explosively react with [[chlorocarbon]]s (LiAlH<sub>4</sub> also suffers from this disadvantage).
*Because CaH<sub>2</sub> and [[calcium hydroxide|Ca(OH)<sub>2</sub>]] are almost indistinguishable in appearance, the quality of a sample of CaH<sub>2</sub> is not obvious visually.
*Since calcium hydride does not remove dissolved oxygen, it is not useful for deoxygenating solvents.
==References==
<references/>
[[Category:Metal hydrides]]
[[Category:Calcium compounds]]
[[Category:Desiccants]]
{{inorganic-compound-stub}}
[[ar:هيدريد كالسيوم]]
[[cs:Hydrid vápenatý]]
[[fr:Hydrure de calcium]]
[[lv:Kalcija hidrīds]]
[[pt:Hidreto de cálcio]]
[[sr:Калцијум-хидрид]]
[[zh:氢化钙]]