Гидрид кальция

Описание

Calcium hydride is a gray powder (white if pure, which is rare). It reacts vigorously with water liberating H2 gas. CaH2 is thus used as a drying agent, i.e. a desiccant. It is prepared directly from the metal or by reacting CaCO3 with hydrogen at elevated temperatures. The overall reaction is shown as follows:
CaCO3＋heat＋H2→CaH2＋H2O＋CO2
CaH2 is a saline hydride, meaning that its structure is salt-like. The alkali metals and the alkaline earth metals all form saline hydrides. These species are insoluble in all solvents with which they do not react because they have extended structures. CaH2 crystallizes in the PbCl2 structural pattern.
The reaction of CaH2 with water can be represented as follows:
CaH2＋2H2O0Ca(OH)2＋2H2
The two hydrolysis products, H2, a gas, and Ca(OH)2, an aqueous mixture of solid plus liquid (i.e. a slurry), are readily separated from the solvent by distillation, filtration, or decantation.

Химические свойства

Pure calcium hydride crystallizes in the form of colorless, hexagonal prisms, but the industrial product contains some calcium metal and is consequently gray. On heating, calcium hydride decomposes without melting. In keeping with its salt-like character it is insoluble in inert solvents but dissolves in molten LiCl – KCl eutectic mixtures (352℃). Calcium hydride can be safely handled in air, but is slowly attacked and crumbles to powder if the air is moist. It reacts with dry oxygen or nitrogen if heated above 500℃, forming the oxide or nitride, respectively.

Физические свойства

Grayish orthorhombic crystal or powder; stable at ambient temperature; density 1.70 g/cm³; melts at 816°C; reacts with water and alcohol.

Использование

To prepare rare metals by reduction of their oxides; as a drying agent for liquids and gases; to generate hydrogen: 1 g of calcium hydride in water liberates 1 liter of hydrogen at STP; in organic syntheses.

Подготовка

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 + 2Na → CaH₂ + 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.

Методы производства

Calcium hydride ignites in air on heating and can explode violently if mixed and rubbed with a strong oxidizing agent such as perchlorate or bromate. Contact with water produces hydrogen which can create a fire hazard in a confined space.

Общее описание

Grayish-colored lump or crystalline solid. Irritating to skin and eyes. Used to make other chemicals.

Реакции воздуха и воды

Ignites in air or reacts violently, sometimes explosively, with air of high humidity [Bretherick 1979 p. 107]. Reacts exothermically with water to generate flammable hydrogen gas and calcium hydroxide, a base. [Merck, 11th ed. 1989].

Профиль реактивности

When silver fluoride is ground with CALCIUM HYDRIDE the mass becomes incandescent [Mellor 3:389 1946-47]. Heating the hydride strongly with chlorine, bromine, or iodine leads to incandescence. Mixtures of the hydride with various bromates, i.e. barium bromate; chlorates, i.e. barium chlorate, and perchlorates, i.e. potassium perchlorate; explode on grinding, [Mellor, 1946, vol. 3, 651]. CaH2 reacts incandescently with AgF if subject to friction. (Mellor, 1941, Vol. 3, 389, 651).

Опасность

Evolves highly flammable hydrogen when wet; solid product is slaked lime. Irritating to skin.

Угроза здоровью

Inhalation or contact with vapors, substance or decomposition products may cause severe injury or death. May produce corrosive solutions on contact with water. Fire will produce irritating, corrosive and/or toxic gases. Runoff from fire control may cause pollution.

Пожароопасность

Produce flammable gases on contact with water. May ignite on contact with water or moist air. Some react vigorously or explosively on contact with water. May be ignited by heat, sparks or flames. May re-ignite after fire is extinguished. Some are transported in highly flammable liquids. Runoff may create fire or explosion hazard.