Calcium hydride 4623969 222739480 2008-06-30T21:38:21Z 88.167.60.194 +[[fr:Hydrure de calcium]] {{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]]