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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]

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

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
InChI InChI=1S/Ca.2H/q+2;2*-1 Y Key: UUGAXJGDKREHIO-UHFFFAOYSA-N Y InChI=1/Ca.2H/q+2;2*-1 Key: UUGAXJGDKREHIO-UHFFFAOYAG
SMILES [H-].[H-].[Ca+2]
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
GHS signal word	DANGER
GHS hazard statements	H260
NFPA 704
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