Calcium hydride

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Calcium hydride
Calcium Hydride (CaH2).jpg
Names
IUPAC name
Calcium hydride
Other names
Calcium(II) hydride
Calcium dihydride
Hydrolith
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.029.263 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 232-189-2
PubChem CID
UNII
  • InChI=1S/Ca.2H/q+2;2*-1 Yes check.svgY
    Key: UUGAXJGDKREHIO-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/Ca.2H/q+2;2*-1
    Key: UUGAXJGDKREHIO-UHFFFAOYAG
  • [H-].[H-].[Ca+2]
Properties
CaH2
Molar mass 42.094 g/mol
Appearancegray powder (white when pure)
Density 1.70 g/cm3, solid
Melting point 816 °C (1,501 °F; 1,089 K)
reacts violently
Solubility reacts in alcohol
Structure
Orthorhombic, oP12
Pnma, No. 62
Thermochemistry
Std molar
entropy
(S298)
41.4 J·mol−1·K−1 [1]
−181.5 kJ·mol−1
-142.5 kJ/mol
Hazards
GHS labelling:
GHS-pictogram-exclam.svg GHS-pictogram-acid.svg GHS-pictogram-flamme.svg GHS-pictogram-pollu.svg
Danger
H260
NFPA 704 (fire diamond)
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 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. gasolineInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
3
3
2
W
Related compounds
Other cations
Sodium hydride,
Potassium hydride
Magnesium hydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Calcium hydride is the chemical compound with the formula CaH2, 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]

Contents

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]

Preparation

Calcium hydride is prepared from its elements by direct combination of calcium and hydrogen at 300 to 400 °C. [4] [5]

Uses

Reduction of metal oxides

CaH2 is a reducing agent for the production of metal from the metal 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" 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:

History

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

Other calcium hydrides

Although the term calcium hydride almost always refers to CaH2, a number of molecular hydrides of calcium are known. One example is (Ca(μ-H)(thf)(nacnac)2. [9]

See also

Related Research Articles

<span class="mw-page-title-main">Alkaline earth metal</span> Group of chemical elements

The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The elements have very similar properties: they are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure.

<span class="mw-page-title-main">Sodium hydroxide</span> Chemical compound with formula NaOH

Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na+ and hydroxide anions OH.

<span class="mw-page-title-main">Hydride</span> Molecule with a hydrogen bound to a more electropositive element or group

In chemistry, a hydride is formally the anion of hydrogen (H), a hydrogen atom with two electrons. The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of nitrogen, etc. For inorganic chemists, hydrides refer to compounds and ions in which hydrogen is covalently attached to a less electronegative element. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.

<span class="mw-page-title-main">Potassium hydroxide</span> Inorganic compound (KOH)

Potassium hydroxide is an inorganic compound with the formula KOH, and is commonly called caustic potash.

<span class="mw-page-title-main">Lithium aluminium hydride</span> Chemical compound

Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li[AlH4] or LiAlH4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing gaseous hydrogen (H2). Some related derivatives have been discussed for hydrogen storage.

<span class="mw-page-title-main">Sodium hydride</span> Chemical compound

Sodium hydride is the chemical compound with the empirical formula NaH. This alkali metal hydride is primarily used as a strong yet combustible base in organic synthesis. NaH is a saline (salt-like) hydride, composed of Na+ and H ions, in contrast to molecular hydrides such as borane, silane, germane, ammonia, and methane. It is an ionic material that is insoluble in all solvents (other than molten sodium metal), consistent with the fact that H ions do not exist in solution.

<span class="mw-page-title-main">Sodium borohydride</span> Chemical compound

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula NaBH4. It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodium borohydride is a reducing agent that finds application in papermaking and dye industries. It is also used as a reagent in organic synthesis.

<i>tert</i>-Butyl alcohol Chemical compound

tert-Butyl alcohol is the simplest tertiary alcohol, with a formula of (CH3)3COH (sometimes represented as t-BuOH). Its isomers are 1-butanol, isobutanol, and butan-2-ol. tert-Butyl alcohol is a colorless solid, which melts near room temperature and has a camphor-like odor. It is miscible with water, ethanol and diethyl ether.

<span class="mw-page-title-main">Lithium hydride</span> Chemical compound

Lithium hydride is an inorganic compound with the formula LiH. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not soluble but reactive with all protic organic solvents. It is soluble and nonreactive with certain molten salts such as lithium fluoride, lithium borohydride, and sodium hydride. With a molar mass of 7.95 g/mol, it is the lightest ionic compound.

<span class="mw-page-title-main">Potassium hydride</span> Chemical compound

Potassium hydride, KH, is the inorganic compound of potassium and hydrogen. It is an alkali metal hydride. It is a white solid, although commercial samples appear gray. It is a powerful superbase that is useful in organic synthesis. It is sold commercially as a slurry (~35%) in mineral oil or sometimes paraffin wax to facilitate dispensing.

<span class="mw-page-title-main">Hexafluorosilicic acid</span> Octahedric silicon compound

Hexafluorosilicic acid is an inorganic compound with the chemical formula H
2
SiF
6
. Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless.

Magnesium compounds are compounds formed by the element magnesium (Mg). These compounds are important to industry and biology, including magnesium carbonate, magnesium chloride, magnesium citrate, magnesium hydroxide, magnesium oxide, magnesium sulfate, and magnesium sulfate heptahydrate.

<span class="mw-page-title-main">Aluminium hydride</span> Chemical compound

Aluminium hydride is an inorganic compound with the formula AlH3. Alane and its derivatives are part of a family of common reducing reagents in organic synthesis based around group 13 hydrides. In solution—typically in etherial solvents such tetrahydrofuran or diethyl ether—aluminium hydride forms complexes with Lewis bases, and reacts selectively with particular organic functional groups, and although it is not a reagent of choice, it can react with carbon-carbon multiple bonds. Given its density, and with hydrogen content on the order of 10% by weight, some forms of alane are, as of 2016, active candidates for storing hydrogen and so for power generation in fuel cell applications, including electric vehicles. As of 2006 it was noted that further research was required to identify an efficient, economical way to reverse the process, regenerating alane from spent aluminium product.

<span class="mw-page-title-main">Sodium aluminium hydride</span> Chemical compound

Sodium aluminium hydride or sodium alumanuide is an inorganic compound with the chemical formula NaAlH4. It is a white pyrophoric solid that dissolves in tetrahydrofuran (THF), but not in diethyl ether or hydrocarbons. It has been evaluated as an agent for the reversible storage of hydrogen and it is used as a reagent for the chemical synthesis of organic compounds. Similar to lithium aluminium hydride, it is a salt consisting of separated sodium cations and tetrahedral AlH
4
anions.

Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loosely: some of them are acidic (e.g., H2Fe(CO)4), whereas some others are hydridic, having H-like character (e.g., ZnH2).

<span class="mw-page-title-main">Beryllium hydride</span> Chemical compound

Beryllium hydride is an inorganic compound with the chemical formula n. This alkaline earth hydride is a colourless solid that is insoluble in solvents that do not decompose it. Unlike the ionically bonded hydrides of the heavier Group 2 elements, beryllium hydride is covalently bonded.

<span class="mw-page-title-main">Metal bis(trimethylsilyl)amides</span>

Metal bis(trimethylsilyl)amides are coordination complexes composed of a cationic metal M with anionic bis(trimethylsilyl)amide ligands (the N 2 monovalent anion, or −N 2 monovalent group, and are part of a broader category of metal amides.

Polysilicon hydrides are polymers containing only silicon and hydrogen. They have the formula where 0.2 ≤ n ≤ 2.5 and x is the number of monomer units. The polysilicon hydrides are generally colorless or pale-yellow/ocher powders that are easily hydrolyzed and ignite readily in air. The surfaces of silicon prepared by MOCVD using silane (SiH4) consist of a polysilicon hydride.

<span class="mw-page-title-main">Metal halides</span>

Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.

Barium hydride is a chemical compound with the chemical formula BaH2.

References

  1. Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A21. ISBN   978-0-618-94690-7.
  2. 1 2 Gawley, Robert E.; Davis, Arnold (2001). "Calcium Hydride". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rc005. ISBN   0471936235.
  3. Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN   0-19-855370-6.
  4. 1 2 3 Rittmeyer, Peter; Wietelmann, Ulrich (2000). "Hydrides". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a13_199. ISBN   978-3527306732.
  5. P. Ehrlich (1963). "Calcium Strontium and Barium Hydrides". In G. Brauer (ed.). Handbook of Preparative Inorganic Chemistry, 2nd Ed. Vol. 1. NY, NY: Academic Press. p. 929.
  6. Adlam G.H.J. and Price L.S., A Higher School Certificate Inorganic Chemistry, John Murray, London, 1940
  7. Williams, D. Bradley G.; Lawton, Michelle (2010). "Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants". The Journal of Organic Chemistry. 75 (24): 8351–8354. doi:10.1021/jo101589h. PMID   20945830. S2CID   17801540.
  8. McNeil, Ian (2002-06-01). An Encyclopedia of the History of Technology. Routledge. ISBN   9781134981649.
  9. Mukherjee, Debabrata; Schuhknecht, Danny; Okuda, Jun (2018). "Hydrido Complexes of Calcium: A New Family of Molecular Alkaline-Earth-Metal Compounds". Angewandte Chemie International Edition. 57 (31): 9590–9602. doi:10.1002/anie.201801869. PMID   29575506. S2CID   4355887.