Calcium hydride
{{chembox
| Watchedfields = changed
| verifiedrevid = 451655643
| Name = Calcium hydride
| ImageFile = Calcium Hydride (CaH2).jpg
| ImageName = Calcium hydride
| IUPACName = Calcium hydride
| OtherNames = Calcium(II) hydride
Calcium dihydride
Hydrolith
| SystematicName =
| Section1 = {{Chembox Identifiers
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChemSpiderID = 94784
| InChI = 1/Ca.2H/q+2;2*-1
| SMILES = [H-].[H-].[Ca+2]
| InChIKey = UUGAXJGDKREHIO-UHFFFAOYAG
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/Ca.2H/q+2;2*-1
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = UUGAXJGDKREHIO-UHFFFAOYSA-N
| CASNo = 7789-78-8
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = WY779SQ0XW
| PubChem = 105052
| EINECS = 232-189-2
}}
| Section2 = {{Chembox Properties
| Formula = {{chem2|CaH2}}
| MolarMass = 42.094 g/mol
| Appearance = gray powder (white when pure)
| Density = 1.70 g/cm{{sup|3}}, solid
| Solubility = reacts violently
| SolubleOther = reacts in alcohol
| MeltingPtC = 816
| MeltingPt_notes =
}}
| Section3 = {{Chembox Structure
| CrystalStruct = Orthorhombic, oP12
| SpaceGroup = Pnma, No. 62
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = −181.5 kJ/mol
| DeltaGf = −142.5 kJ/mol
| Entropy = 41.4 J/(mol·K){{cite book |last=Zumdahl |first=Steven S. |title=Chemical Principles |edition=6th |publisher=Houghton Mifflin Company|year=2009 |isbn=978-0-618-94690-7 |page=A21}}
}}
| Section5 =
| Section6 =
| Section7 = {{Chembox Hazards
| ExternalSDS =
| GHSPictograms = {{GHS07}}{{GHS05}}{{GHS02|Water-react. 1}}{{GHS09}}
| GHSSignalWord = DANGER
| HPhrases = {{H-phrases|260}}
| NFPA-H = 3
| NFPA-F = 3
| NFPA-R = 2
| NFPA-S = W
}}
| Section8 = {{Chembox Related
| OtherAnions =
| OtherCations = {{unbulleted list | Sodium hydride | Potassium hydride | Magnesium hydride}}
}}
}}
Calcium hydride is the chemical compound with the formula {{chem2|CaH2}}, an alkaline earth hydride. This grey powder (white if pure, which is rare) reacts vigorously with water, liberating hydrogen gas. {{chem2|CaH2}} is thus used as a drying agent, i.e. a desiccant.{{Cite book |doi = 10.1002/047084289X.rc005 |isbn = 0471936235 |chapter = Calcium Hydride |title = Encyclopedia of Reagents for Organic Synthesis |year = 2001 |last1 = Gawley |first1 = Robert E. |last2 = Davis |first2 = Arnold}}
{{chem2|CaH2}} is a saline hydride, meaning that its structure is salt-like. The alkali metals and the alkaline earth metals heavier than beryllium 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. {{chem2|CaH2}} crystallizes in the {{chem2|PbCl2}} (cotunnite) structure.Wells, A. F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. {{ISBN|0-19-855370-6}}.
Preparation
Calcium hydride is prepared from its elements by direct combination of calcium and hydrogen at 300 to 400 °C.{{Ullmann|first1=Peter|last1=Rittmeyer|first2=Ulrich|last2=Wietelmann|title=Hydrides|year=2000|doi=10.1002/14356007.a13_199}}{{cite book|author=P. Ehrlich|chapter=Calcium Strontium and Barium Hydrides|title=Handbook of Preparative Inorganic Chemistry, 2nd Ed. |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY, NY|volume=1|pages=929}}
Uses
=Reduction of metal oxides=
=Hydrogen source=
{{chem2|CaH2}} has been used for hydrogen production. In the 1940s, it was available under the trade name "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. The moisture content of diesel fuel is estimated by the hydrogen evolved upon treatment with CaH{{sub|2}}.
=Desiccant=
The reaction of {{chem2|CaH2}} with water can be represented as follows:
:{{chem2|CaH2 + 2 H2O → Ca(OH)2 + 2 H2}}
The two hydrolysis products, gaseous {{chem2|H2}} and {{chem2|Ca(OH)2}}, are readily separated from the dried solvent.
Calcium hydride is a relatively mild desiccant and, compared to molecular sieves, probably inefficient.{{Cite journal |doi = 10.1021/jo101589h|pmid = 20945830|title = Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants|year = 2010|last1 = Williams|first1 = D. Bradley G.|last2 = Lawton|first2 = Michelle|journal = The Journal of Organic Chemistry|volume = 75|issue = 24|pages = 8351–8354| s2cid=17801540 }} Its use is safer than 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. It is also used to dry alcohols.
Despite its convenience, {{chem2|CaH2}} has a few drawbacks:
- It is insoluble in all solvents with which it does not react vigorously, in contrast to {{chem2|LiAlH4}}, thus the speed of its drying action can be slow.
- Because {{chem2|CaH2}} and {{chem2|Ca(OH)2}} are almost indistinguishable in appearance, the quality of a sample of {{chem2|CaH2}} is not obvious visually.
=History=
During the Battle of the Atlantic, German submarines used calcium hydride as a sonar decoy called bold.{{cite book|url=https://books.google.com/books?id=-sXFBQAAQBAJ|title=An Encyclopedia of the History of Technology|isbn=9781134981649|last1=McNeil|first1=Ian|date=2002-06-01|publisher=Routledge }}
Other calcium hydrides
Although the term calcium hydride almost always refers to CaH{{sub|2}}, a number of molecular hydrides of calcium are known. One example is (Ca(μ-H)(thf)(nacnac)){{sub|2}}.{{cite journal |doi=10.1002/anie.201801869|title=Hydrido Complexes of Calcium: A New Family of Molecular Alkaline-Earth-Metal Compounds |year=2018 |last1=Mukherjee |first1=Debabrata |last2=Schuhknecht |first2=Danny |last3=Okuda |first3=Jun |journal=Angewandte Chemie International Edition |volume=57 |issue=31 |pages=9590–9602 |pmid=29575506 |s2cid=4355887 }}
See also
References
{{reflist|30em}}
{{Calcium compounds}}
{{Hydrides by group}}