Calcium Hydride, CaH₂

By heating lime with magnesium in an atmosphere of hydrogen, Winkler obtained an earthy substance which he regarded as a mixture of magnesium oxide with calcium hydride, CaH₂, to which he gave the formula CaH. Probably unattacked calcium still remained. Calcium hydride, CaH₂, is formed by the direct combination of calcium with hydrogen, the rate of reaction depending on the physical state of the metal as well as on the temperature. The most favourable temperatures, apparently, are between 150° C. and 300° C., and above 600° C. A superficial layer of hydride facilitates further absorption.

It may also be prepared by fusing lime electrically in a current of hydrogen and cooling the product in hydrogen.

Calcium hydride is a white crystalline body of density 1.7, insoluble in the usual solvents, and capable of being heated up to 600° C. in vacuo without decomposition. It does not conduct electricity, indicating that it is not to be regarded as an alloy, since hydrogen does not function as a metal.

Calcium hydride is more stable than either strontium or barium hydrides.

Recent observations seem to show that calcium hydride and calcium form a complete series of solid solutions, and also that calcium will dissolve hydrogen without combining with it. The latter fact probably explains Sieverts' observations, to which reference has already been made. The absorption of hydrogen at low temperatures is no doubt due to simple solution which ceases at 300° C., whilst hydride formation does not begin until 600° C. is reached.

These different solution phenomena make it difficult to determine the dissociation pressure of the hydride because the system is divariant. At 810° C. the dissociation pressure of the pure hydride is 290 mm., but it is lowered by the presence of the metal up to a concentration of 48 milligram-molecules of calcium per gram of hydride. It is, therefore, evident that hydride formation may take place at a temperature much higher than the dissociation temperature of the pure hydride.

From his study of the dissociation pressures of the hydride between 641° C. and 747° C., Bronsted calculated the heat of formation at 18° C. to be 43-930 Cal. as against the observed value 45.100 Cal.

It is a valuable reducing agent, especially for organic substances, and can be kept more under control than metallic calcium. Methane may be synthesised by the action of carbon, carbon monoxide, or carbon dioxide, on calcium hydride.

At 100° C. calcium hydride, in common with the other alkaline earth hydrides, reacts with acetylene, producing an acetylenic acetylide, CaC₂.C₂H₂, and liberating hydrogen. If this compound be warmed gently in vacuo the carbide may be obtained at a low temperature.

Calcium hydride under the name of " hydrolith," as a portable form of hydrogen, has been made the subject of patents issued by Jaubert. It contains 90 per cent, pure calcium hydride mixed with some oxide and nitride. When acted upon by water, 1 kgm. produces 1 cub. metre of hydrogen. It has been employed in the French army for the filling of dirigibles. " Hydrogenite " is a mixture similar in its behaviour to " thermite," and contains the hydride, a certain amount of a metal or alloy, and water in the form of a hydrated body such as slaked lime. When heated at one point reaction proceeds vigorously throughout the mass and yields hydrogen.

When a solution of calcium hydride in a fused eutectic mixture of potassium and lithium chloride is electrolysed, the hydrogen behaves as a halogen and is liberated at the anode in the proportion required by Faraday's law.

From vapour-pressure measurements Molden-hauer and Roll-Hansen concluded that a lower hydride of calcium, CaH, exists, but their results were probably due to inequalities of temperature in the tube in which the hydride was heated.

Hoffmeister supposed that he had discovered a gaseous hydride of calcium, but this result has received no further confirmation.