Calcium hydride

Calcium hydride
IUPAC name Calcium hydride
Other names Calcium(II) hydride Calcium dihydride
Identifiers
CAS number	7789-78-8 Y
PubChem	105052
ChemSpider	94784 Y
EC number	232-189-2
Jmol-3D images	{{#if:[H-].[H-].[Ca+2]|Image 1
SMILES [H-].[H-].[Ca+2]
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
Properties
Molecular formula	CaH2
Molar mass	42.094 g/mol
Appearance	gray powder (white when pure)
Density	1.70 g/cm3, solid
Melting point	816 °C
Solubility in water	reacts violently
Solubility	reacts in alcohol
Structure
Crystal structure	Orthorhombic, oP12
Space group	Pnma, No. 62
Hazards[1]
GHS pictograms
GHS signal word	DANGER
GHS hazard statements	H260
EU Index	001-004-00-5
EU classification	Flammable (F)
R-phrases	R15
S-phrases	(S2), S7/8, S24/25, S43
Related compounds
Other cations	Sodium hydride, Potassium hydride
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Calcium hydride is the chemical compound with the formula CaH₂. 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.

CaH₂ is a saline hydride, meaning that its structure is salt-like. The alkali metals and the alkaline earth metals 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 because they have extended structures. CaH₂ crystallizes in the PbCl₂ (cotunnite) structure.^[2]

Preparation

Calcium hydride may be prepared from its elements by direct combination of calcium and hydrogen at 300 to 400°C. It also can be made by heating calcium chloride with hydrogen in the presence of sodium metal:

CaCl₂ + H₂ + 2 Na → CaH₂ + 2 NaCl

Alternatively, calcium hydride may be prepared by the reduction of calcium oxide with magnesium in the presence of hydrogen:

CaO + Mg + H₂ → CaH₂ + MgO

Use as a desiccant

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

CaH₂ + 2 H₂O → Ca(OH)₂ + 2 H₂

The two hydrolysis products, H₂, a gas, and Ca(OH)₂, an aqueous mixture, are readily separated from each other.

As calcium hydride is a relatively mild desiccant, it is safe compared with 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.^[3] It is also used to pre-dry solvents prior to the use of a more reactive desiccant.

Drawbacks

Although CaH₂ is indeed convenient and often the drying agent of choice, it 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.
• it is incompatible with some solvents, and can in fact explosively react with chlorocarbons ^{[citation needed]}(LiAlH₄ also suffers from this disadvantage).
• Because CaH₂ and Ca(OH)₂ are almost indistinguishable in appearance, the quality of a sample of CaH₂ is not obvious visually.

Use for hydrogen production

Calcium hydride was, in the 1940s, available under the trade name "Hydrolith"^[4] 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.'^[5]

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.

References