Chemistry:Calcium hydride

From HandWiki
Calcium hydride
Calcium hydride
Names
IUPAC name
Calcium hydride
Other names
Calcium(II) hydride
Calcium dihydride
Hydrolith
Identifiers
3D model (JSmol)
ChemSpider
EC Number
  • 232-189-2
UNII
Properties
CaH
2
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)
reacts violently
Solubility reacts in alcohol
Structure
Orthorhombic, oP12
Pnma, No. 62
Thermochemistry
41.4 J·mol−1·K−1[1]
−181.5 kJ·mol−1
-142.5 kJ/mol
Hazards
GHS pictograms GHS07: HarmfulGHS05: CorrosiveWater-react. 1GHS09: Environmental hazard
GHS Signal word DANGER
H260
NFPA 704 (fire diamond)
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. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acidNFPA 704 four-colored diamond
3
3
2
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).
☑Y verify (what is ☑Y☒N ?)
Infobox references

Calcium hydride is the chemical compound with the formula CaH
2
, and is therefore an alkaline earth hydride. This grey powder (white if pure, which is rare) reacts vigorously with water liberating hydrogen gas. CaH
2
is thus used as a drying agent, i.e. a desiccant.[2]

CaH
2
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. CaH
2
crystallizes in the PbCl
2
(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

CaH
2
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]

TiO
2
+ 2 CaH
2
→ Ti + 2 CaO + 2 H
2

Hydrogen source

CaH
2
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 CaH
2
with water can be represented as follows:

CaH
2
+ 2 H
2
O → Ca(OH)
2
+ 2 H
2

The two hydrolysis products, gaseous H
2
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, CaH
2
has a few drawbacks:

  • It is insoluble in all solvents with which it does not react vigorously, in contrast to LiAlH
    4
    , thus the speed of its drying action can be slow.
  • Because CaH
    2
    and Ca(OH)
    2
    are almost indistinguishable in appearance, the quality of a sample of CaH
    2
    is not obvious visually.

History

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

See also

References

  1. Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A21. ISBN 978-0-618-94690-7. 
  2. 2.0 2.1 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. 4.0 4.1 4.2 Rittmeyer, Peter; Wietelmann, Ulrich (2000). "Ullmann's Encyclopedia of Industrial Chemistry". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a13_199. 
  5. P. Ehrlich (1963). "Calcium Strontium and Barium Hydrides". in G. Brauer. Handbook of Preparative Inorganic Chemistry, 2nd Ed.. 1. NY,NY: Academic Press. pp. 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. 
  8. McNeil, Ian (2002-06-01). An Encyclopedia of the History of Technology. ISBN 9781134981649. https://books.google.com/books?id=-sXFBQAAQBAJ.