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Calcium hydride
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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.
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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 motif.
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 the solvent by distillation, filtration, or decantation.
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. 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 (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.
- Since calcium hydride does not remove dissolved oxygen, it is not useful for deoxygenating solvents.
Use for hydrogen production
Calcium hydride was, in the 1940s, 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.'
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.
Looking to the future, Calcium hydride is a possible source of hydrogen for fuel cells but the use of sodium borohydride in the Direct borohydride fuel cell may be preferable.
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