Calcium hydride is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the formula , and is therefore an alkaline earth hydride. This grey powder (white if pure, which is rare) reacts vigorously with water liberating

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

gas. is thus used as a drying agent, i.e. a

desiccant A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids that absorb water. Desiccants ...

. 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 Sodium hydride is the chemical compound with the empirical formula Na H. 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 ...

, which crystallizes in the NaCl motif. These species are insoluble in all solvents with which they do not react. crystallizes in the (

cotunnite Cotunnite is the natural mineral form of lead(II) chloride with formula PbCl2. It was first described in 1825 from an occurrence on Mount Vesuvius, Naples Province, Campania, Italy. It was named for Domenico Cotugno (Cotunnius) (1736–1822), I ...

)

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such a ...

Preparation

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

Uses

Reduction of metal oxides

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: :

Hydrogen source

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.'

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.

Desiccant

The reaction of with water can be represented as follows: : The two hydrolysis products, gaseous and , are readily separated from the dried solvent. Calcium hydride is a relatively mild desiccant and, compared to

molecular sieve A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecu ...

s, probably inefficient. Its use is safer than more reactive agents such as

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

metal or sodium-potassium alloy. Calcium hydride is widely used as a

for

basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...

solvents such as

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent ...

s and

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...

. It is also used to dry alcohols. Despite its convenience, has a few drawbacks: *It is insoluble in all solvents with which it does not react vigorously, in contrast to , thus the speed of its drying action can be slow. *Because and are almost indistinguishable in appearance, the quality of a sample of is not obvious visually.

History

During the

Battle of the Atlantic The Battle of the Atlantic, the longest continuous military campaign in World War II, ran from 1939 to the defeat of Nazi Germany in 1945, covering a major part of the naval history of World War II. At its core was the Allies of World War II, ...

, German submarines used calcium hydride as a

sonar decoy A sonar decoy is a device for decoying sonar. Most are released from submarines to act as a false target. Submarine decoys The first submarine decoys were the German Bold fitted to U-boats of World War II. These were a pellet of calcium hydrid ...

called

bold In typography, emphasis is the strengthening of words in a text with a font in a different style from the rest of the text, to highlight them. It is the equivalent of prosody stress in speech. Methods and use The most common methods in W ...

Other calcium hydrides

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

References

{{Hydrides by group Metal hydrides Calcium compounds Desiccants Hydrogen storage