CALCIUM HYDRIDE

Description

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.

Chemical properties

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.

Physical properties

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

The Uses of CALCIUM HYDRIDE

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.

The Uses of CALCIUM HYDRIDE

Calcium hydride is a relatively mild desiccant. It is safer than the more reactive agents such as sodium metal. Calcium hydride is widely used as a desiccant for basic solvents such as amines and pyridine in organic syntheses. It is also used to pre-dry solvents prior to the use of a more reactive desiccant. 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 Uses of CALCIUM HYDRIDE

Calcium hydride is used as an efficient drying agent for aprotic base-stable solvents like ethers and tertiary amines. It is a useful dehydrating agent in a synthesis of aldehyde enamines in high yield and purity.

What are the applications of Application

Calcium hydride is a drying agent

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 + 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.

Production Methods

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.

Reactions

Once ignited, it burns with a strongly exothermic reaction:
CaH2+O2→CaO+H2O
The reaction of calcium hydride with water is also very exothermic. Heating is necessary to initiate the reaction with alcohols to form alcoholates. The reaction with ammonia takes place at 700 ℃ with formation of calcium amide. At high temperature (600 – 1000℃) calcium hydride is a powerful reducing agent.

General Description

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

Air & Water Reactions

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].

Reactivity Profile

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).

Hazard

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

Health Hazard

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.

Fire Hazard

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.

Flammability and Explosibility

Not classified

Potential Exposure

Calcium hydride is used as a dryingand reducing agent and a cleaner for blocked up oil wells.

First aid

If this chemical gets into the eyes, remove anycontact lenses at once and irrigate immediately for atleast 30 min, occasionally lifting upper and lower lids. Seekmedical attention immediately. If this chemical contacts theskin, remove contaminated clothing and wash immediatelywith soap and water. Seek medical attention immediately. Ifthis chemical has been inhaled, remove from exposure, beginrescue breathing (using universal precautions, includingresuscitation mask) if breathing has stopped and CPR if heartaction has stopped. Transfer promptly to a medical facility.When this chemical has been swallowed, get medical attention. Give large quantities of water and induce vomiting. Donot make an unconscious person vomit. Medical observationis recommended for 24-48 h after breathing overexposure,as pulmonary edema may be delayed. As first aid for pulmonary edema, a doctor or authorized paramedic may consideradministering a corticosteroid spray.

storage

Color Code—Red Stripe: Flammability Hazard:Do not store in the same area as other flammable materials.Calcium hydride must be stored to avoid contact with wateror steam since violent reactions occur and flammablehydrogen gas is produced. Store in tightly closed containersin a cool, well-ventilated area.

Shipping

Calcium hydride should carry a “DANGEROUSWHEN WET” label. It falls in Hazard Class 4.3 andPacking Group I.

Incompatibilities

Reacts with water, moist air, and steam,releasing flammable hydrogen gas and may self-ignite inair.
Incompatible with metal halogenates, silver fluoride,and tetrahydrofuran.