Calcium hydride

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Calcium hydride is the chemical compound with the formula CaH2, and is therefore an alkaline earth hydride. 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.[2]


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. CaH2 crystallizes in the PbCl2 (cotunnite) structure.[3]




Contents





  • 1 Preparation


  • 2 Uses

    • 2.1 Reduction of metal oxides


    • 2.2 Hydrogen source


    • 2.3 Desiccant


    • 2.4 Sonar decoy



  • 3 See also


  • 4 References




Preparation


Calcium hydride is prepared from its elements by direct combination of calcium and hydrogen at 300 to 400 °C.[4]



Uses



Reduction of metal oxides


CaH2 is a reducing agent for the production of metal powders from the oxides of Ti, V, Nb, Ta, and U. It is proposed to operate via its decomposition to Ca metal:[4]


TiO2 + 2 CaH2 → Ti + 2 CaO + 2 H2


Hydrogen source



CaH2 has been used for hydrogen production. In the 1940s, it was available under the trade name "Hydrolith"[5] 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.'[6]



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 CaH2.[4]



Desiccant


The reaction of CaH2 with water can be represented as follows:


CaH2 + 2 H2O → Ca(OH)2 + 2 H2

The two hydrolysis products, gaseous H2 and Ca(OH)2, are readily separated from the dried solvent.


Calcium hydride is a relatively mild desiccant and, compared to molecular sieves, probably inefficient.[7] 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.[2]


Despite its convenience, CaH2 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.

  • Because CaH2 and Ca(OH)2 are almost indistinguishable in appearance, the quality of a sample of CaH2 is not obvious visually.


Sonar decoy


During the Battle of the Atlantic, German submarines used calcium hydride as a sonar decoy called bold.[8]



See also


  • Calcium monohydride


References




  1. ^ Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A21. ISBN 978-0-618-94690-7..mw-parser-output cite.citationfont-style:inherit.mw-parser-output .citation qquotes:"""""""'""'".mw-parser-output .citation .cs1-lock-free abackground:url("//upload.wikimedia.org/wikipedia/commons/thumb/6/65/Lock-green.svg/9px-Lock-green.svg.png")no-repeat;background-position:right .1em center.mw-parser-output .citation .cs1-lock-limited a,.mw-parser-output .citation .cs1-lock-registration abackground:url("//upload.wikimedia.org/wikipedia/commons/thumb/d/d6/Lock-gray-alt-2.svg/9px-Lock-gray-alt-2.svg.png")no-repeat;background-position:right .1em center.mw-parser-output .citation .cs1-lock-subscription abackground:url("//upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Lock-red-alt-2.svg/9px-Lock-red-alt-2.svg.png")no-repeat;background-position:right .1em center.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registrationcolor:#555.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration spanborder-bottom:1px dotted;cursor:help.mw-parser-output .cs1-ws-icon abackground:url("//upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Wikisource-logo.svg/12px-Wikisource-logo.svg.png")no-repeat;background-position:right .1em center.mw-parser-output code.cs1-codecolor:inherit;background:inherit;border:inherit;padding:inherit.mw-parser-output .cs1-hidden-errordisplay:none;font-size:100%.mw-parser-output .cs1-visible-errorfont-size:100%.mw-parser-output .cs1-maintdisplay:none;color:#33aa33;margin-left:0.3em.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-formatfont-size:95%.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-leftpadding-left:0.2em.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-rightpadding-right:0.2em


  2. ^ ab Gawley, R. E., Davis, A., "Calcium Hydride," in Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, Ltd. doi:10.1002/047084289X.rc005


  3. ^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press.
    ISBN 0-19-855370-6.


  4. ^ abc Peter Rittmeyer, Ulrich Wietelmann “Hydrides” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a13_199


  5. ^ Hydrolith in thefreedictionary.com


  6. ^ Adlam G.H.J. and Price L.S., A Higher School Certificate Inorganic Chemistry, John Murray, London, 1940


  7. ^ Williams, D. B. G., Lawton, M., "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants", The Journal of Organic Chemistry 2010, vol. 75, 8351. doi: 10.1021/jo101589h


  8. ^ McNeil, Ian (2002-06-01). An Encyclopedia of the History of Technology. ISBN 9781134981649.


Calcium hydride

Calcium hydride
Names

IUPAC name
Calcium hydride

Other names
Calcium(II) hydride
Calcium dihydride
Hydrolith

Identifiers

CAS Number



  • 7789-78-8 ☑Y


3D model (JSmol)


  • Interactive image


ChemSpider


  • 94784 ☑Y


ECHA InfoCard

100.029.263

EC Number
232-189-2


PubChem CID


  • 105052





Properties

Chemical formula

CaH2

Molar mass
42.094 g/mol
Appearance
gray powder (white when pure)

Density
1.70 g/cm3, solid

Melting point
 816 °C (1,501 °F; 1,089 K)

Solubility in water

reacts violently

Solubility
reacts in alcohol
Structure

Crystal structure


Orthorhombic, oP12

Space group

Pnma, No. 62
Thermochemistry


Std molar
entropy (So298)

41.4 J·mol−1·K−1[1]


Std enthalpy of
formation (ΔfHo298)

−181.5 kJ·mol−1


Gibbs free energy (ΔfG˚)

-142.5 kJ/mol
Hazards

GHS pictograms

The exclamation-mark pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)Water-react. 1The environment pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)

GHS signal word
DANGER

GHS hazard statements


H260

NFPA 704



Flammability code 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g., gasolineHealth code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond

3


3


2

W



Related compounds

Other cations


Sodium hydride,
Potassium hydride

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).


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Infobox references









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