Calcium hydride is an alkaline earth metal hydride. Although it is less stable than lithium hydride, it is more stable than other alkali metal borohydrides. Its chemical formula is CaH₂, and its molecular weight is 42.10. It occurs as white monoclinic crystals or lumps, while the industrial product is gray. When exposed to moist air, it releases hydrogen and forms calcium hydroxide. Its density is 1.9. The decomposition temperature is about 600 °C, and its melting point is 816 °C under hydrogen.
Calcium hydride decomposes upon contact with water, carboxylic acids, or lower alcohols, releasing hydrogen. It begins to decompose at about 600 °C. At room temperature, it does not react with dry oxygen, nitrogen, or chlorine, but it can react with them at high temperatures, forming CaO, Ca₃N₂, and CaCl₂, respectively. At room temperature, it reacts with water to produce calcium hydroxide and hydrogen.
Calcium hydride is a strong reducing agent and can reduce metal oxides and metal chlorides to the corresponding metals. For example:
2CaH₂ + MO₂ → 2CaO + 2H₂ + M (metal)
Calcium hydride is commonly used as a strong reducing agent and can also be used to generate hydrogen in the field.
Preparation: Calcium metal is placed in a reaction vessel and reacted with hydrogen at approximately 300 °C using electric or oil heating:
Ca + H₂ → CaH₂ + 214 kJ·mol⁻¹
Alternatively, calcium hydride can be prepared by reacting calcium oxide with magnesium in a stream of hydrogen. However, because the separation of calcium oxide is difficult, high-purity calcium hydride products cannot be readily obtained:
CaO + Mg + H₂ → CaH₂ + MgO
Purpose: When heated to 600–1000 °C, calcium hydride can reduce the oxides of zirconium, niobium, uranium, and chromium to produce powdered metals. Therefore, it can be used in powder metallurgy. Calcium hydride is insoluble in ether and reacts with ethanol to produce hydrogen and calcium ethoxide. Hydrogen can also be generated by reacting calcium hydride with water. One gram of calcium hydride can release approximately one liter of hydrogen, making it useful as a portable source of hydrogen.
In addition, calcium hydride is used for the dehydration of organic compounds, hydrogenation, and condensation reactions. It can also be used as a drying agent, with a drying effect superior to that of sodium and phosphorus pentoxide.
The above information was compiled by Wang Xiaodong for ChemicalBook.
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.
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.
Grayish orthorhombic crystal or powder; stable at ambient temperature; density 1.70 g/cm3; melts at 816°C; reacts with water and alcohol.
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.
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.
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.
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.
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.
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.
Grayish-colored lump or crystalline solid. Irritating to skin and eyes. Used to make other chemicals.
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].
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).
Evolves highly flammable hydrogen when
wet; solid product is slaked lime. Irritating to skin.
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.
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
Calcium hydride is used as a dryingand reducing agent and a cleaner for blocked up oil wells.
If this chemical gets into the eyes, remove any contact lenses at once and irrigate immediately for at least 30 min, occasionally lifting the upper and lower lids. Seek medical attention immediately.
If this chemical contacts the skin, remove contaminated clothing and wash immediately with soap and water. Seek medical attention immediately.
If this chemical has been inhaled, remove the person from exposure. Begin rescue breathing (using universal precautions, including a resuscitation mask) if breathing has stopped, and perform CPR if cardiac activity has stopped. Transfer the person promptly to a medical facility.
If this chemical has been swallowed, seek medical attention. Give large quantities of water and induce vomiting. Do not make an unconscious person vomit.
Medical observation is 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 consider administering a corticosteroid spray.
Store cool and dry in airtight containers away from heat sources.
Reacts with water, moist air, and steam,releasing flammable hydrogen gas and may self-ignite inair. Incompatible with metal halogenates, silver fluoride,and tetrahydrofuran.
