Calcium sulfide
Encyclopedia
Calcium sulfide is the chemical compound
with the formula CaS. This white material crystallizes in cubes like rock salt. CaS has been studied as a component in a process that would recycle gypsum
, a product of flue gas desulfurization
. Like many salts containing sulfide
ions, CaS typically has an odour of H2S
, which results from small amount of this gas formed by hydrolysis of the salt.
In terms of its atomic structure, CaS crystallizes in the same motif as sodium chloride
indicating that the bonding in this material is highly ionic
. The high melting point is also consistent with its description as an ionic solid. In the crystal, each S2−
ion is surrounded by an octahedron
of six Ca2+ ions, and complementarily, each Ca2+ ion surrounded by six S2− ions.
, which entails the conversion of carbon, usually as charcoal, to carbon dioxide
:
and can react further:
In the second reaction the sulfate
(+6 oxidation state
) oxidizes the sulfide
(-2 oxidation state) to sulfur dioxide
(+4 oxidation state), while it is being reduced to sulfur dioxide itself (+4 oxidation state).
CaS is also a byproduct in the Leblanc process
.
Milk of lime, Ca(OH)2, reacts with elemental sulfur to give a "lime-sulfur
", which has been used as an insecticide
. The active ingredient is probably a calcium polysulfide
, not CaS.
It reacts with acid
s such as hydrochloric acid
to release toxic hydrogen sulfide
gas.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula CaS. This white material crystallizes in cubes like rock salt. CaS has been studied as a component in a process that would recycle gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
, a product of flue gas desulfurization
Flue gas desulfurization
Sulfur dioxide is one of the elements forming acid rain. Tall flue-gas stacks disperse emissions by diluting the pollutants in ambient air and transporting them to other regions....
. Like many salts containing sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
ions, CaS typically has an odour of H2S
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, which results from small amount of this gas formed by hydrolysis of the salt.
In terms of its atomic structure, CaS crystallizes in the same motif as sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
indicating that the bonding in this material is highly ionic
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
. The high melting point is also consistent with its description as an ionic solid. In the crystal, each S2
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
ion is surrounded by an octahedron
Octahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
of six Ca2+ ions, and complementarily, each Ca2+ ion surrounded by six S2
Production
CaS is produced by "carbothermic reduction" of calcium sulfateCalcium sulfate
Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...
, which entails the conversion of carbon, usually as charcoal, to carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
:
- CaSO4 + 2 C → CaS + 2 CO2
and can react further:
- 3 CaSO4 + CaS → 4 CaO + 4 SO2Sulfur dioxideSulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
In the second reaction the sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
(+6 oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
) oxidizes the sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
(-2 oxidation state) to sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
(+4 oxidation state), while it is being reduced to sulfur dioxide itself (+4 oxidation state).
CaS is also a byproduct in the Leblanc process
Leblanc process
The Leblanc process was the industrial process for the production of soda ash used throughout the 19th century, named after its inventor, Nicolas Leblanc. It involved two stages: Production of sodium sulfate from sodium chloride, followed by reaction of the sodium sulfate with coal and calcium...
.
Reactivity and uses
Calcium sulfide decomposes upon contact with water, including moist air, giving a mixture of Ca(SH)2, Ca(OH)2, and Ca(SH)(OH).- CaS + H2O → Ca(SH)(OH)
- Ca(SH)(OH) + H2O → Ca(OH)2 + H2S
Milk of lime, Ca(OH)2, reacts with elemental sulfur to give a "lime-sulfur
Lime sulfur
In horticulture, lime sulfur is a mixture of calcium polysulfides formed by reacting calcium hydroxide with sulfur, used in pest control. It can be prepared by boiling calcium hydroxide and sulfur together with a small amount of surfactant...
", which has been used as an insecticide
Insecticide
An insecticide is a pesticide used against insects. They include ovicides and larvicides used against the eggs and larvae of insects respectively. Insecticides are used in agriculture, medicine, industry and the household. The use of insecticides is believed to be one of the major factors behind...
. The active ingredient is probably a calcium polysulfide
Polysulfide
Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: anions and organic polysulfides. Anions have the general formula Sn2−. These anions are the conjugate bases of the hydrogen polysulfides H2nSn...
, not CaS.
It reacts with acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s such as hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
to release toxic hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
gas.
- CaS + 2 HCl → CaCl2 + H2S