Magnesium silicide
Encyclopedia
Magnesium silicide, Mg2Si, is an inorganic compound
consisting of magnesium
and silicon
. As a powder magnesium silicide is dark blue or slightly purple in color. Silicon dioxide
, SiO2, found in sand
and glass
, when heated with magnesium
forms magnesium oxide, and, if an excess of magnesium is used, magnesium silicide is formed. The first product in this reaction is silicon
, which then reacts further with magnesium to produce Mg2Si.
The stoichiometry of these reactions are such that with a 2:1 Mg:SiO2 molar ratio, MgO is formed:
If an excess of Mg is present, Mg2Si is formed from the reaction of the remaining magnesium with the silicon via:
Hence, the overall reaction for the formation of magnesium silicide from a 4:1 Mg:SiO2 molar ratio may be represented as:
Theses reactions proceed violently, producing a great amount of heat.
Magnesium silicide can also be formed by the reaction between magnesium hydride
and silicon
with the evolution of hydrogen
gas at temperatures above 250°C:
This reaction releases 5 wt.% hydrogen and has been considered for hydrogen storage
. However, its reversibility has yet to be demonstrated.
Magnesium silicide is used to create aluminium alloys of the 6xxx group, containing up to approximately 1.5% Mg2Si. An alloy of this group can be age-hardened to form Guinier-Preston zones and a very fine precipitate, both resulting in increased strength of the alloy.
When magnesium silicide is placed into hydrochloric acid
, HCl(aq), the gas silane
, SiH4, is produced. This gas is the silicon analogue of methane
, CH4, but is more reactive. Silane is pyrophoric
, that is, due to the presence of oxygen
, it spontaneously combusts in air:
These reactions are typical of a Group 2
silicide
. Mg2Si reacts similarly with sulfuric acid
. Group 1
silicides are even more reactive. For example, sodium silicide, Na2Si, reacts rapidly with water to yield sodium silicate
, Na2SiO3, and hydrogen
gas.
lattice. It possesses the antifluorite structure with Si4- ions occupying the corners and face-centered positions of the unit cell and Mg2+ ions occupying eight tetrahedral sites in the interior of the unit cell. There are also four equivalent interstitial sites. Hence, the structure contains isolated Si4- ions. This feature makes the Si4 - ion of Mg2Si a good Brønsted–Lowry base (proton acceptor).
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
consisting of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
. As a powder magnesium silicide is dark blue or slightly purple in color. Silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
, SiO2, found in sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, when heated with magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
forms magnesium oxide, and, if an excess of magnesium is used, magnesium silicide is formed. The first product in this reaction is silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, which then reacts further with magnesium to produce Mg2Si.
The stoichiometry of these reactions are such that with a 2:1 Mg:SiO2 molar ratio, MgO is formed:
- 2 Mg + SiO2 → 2 MgOMagnesium oxideMagnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
+ Si
If an excess of Mg is present, Mg2Si is formed from the reaction of the remaining magnesium with the silicon via:
- 2 Mg + Si → Mg2Si
Hence, the overall reaction for the formation of magnesium silicide from a 4:1 Mg:SiO2 molar ratio may be represented as:
- 4 Mg + SiO2 → 2 MgO + Mg2Si
Theses reactions proceed violently, producing a great amount of heat.
Magnesium silicide can also be formed by the reaction between magnesium hydride
Magnesium hydride
Magnesium hydride is the chemical compound MgH2. It contains 7.66% by weight of hydrogen and has been studied as a potential hydrogen storage medium.- Preparation :...
and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
with the evolution of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas at temperatures above 250°C:
- 2 MgH2 + Si → Mg2Si + 2 H2
This reaction releases 5 wt.% hydrogen and has been considered for hydrogen storage
Hydrogen storage
Hydrogen storage describes the methods for storing H2 for subsequent use. The methods span many approaches, including high pressures, cryogenics, and chemical compounds that reversibly release H2 upon heating...
. However, its reversibility has yet to be demonstrated.
Magnesium silicide is used to create aluminium alloys of the 6xxx group, containing up to approximately 1.5% Mg2Si. An alloy of this group can be age-hardened to form Guinier-Preston zones and a very fine precipitate, both resulting in increased strength of the alloy.
When magnesium silicide is placed into 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....
, HCl(aq), the gas silane
Silane
Silane is a toxic, extremely flammable chemical compound with chemical formula SiH4. In 1857, the German chemists and Friedrich Woehler discovered silane among the products formed by the action of hydrochloric acid on aluminum silicide, which they had previously prepared...
, SiH4, is produced. This gas is the silicon analogue of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, CH4, but is more reactive. Silane is pyrophoric
Pyrophoricity
A pyrophoric substance is a substance that will ignite spontaneously in air. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thin. Pyrophoric materials are often water-reactive as well and will ignite when they contact water or humid air...
, that is, due to the presence of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, it spontaneously combusts in air:
- Mg2Si(s) + 4 HCl(aq) → SiH4(g) + 2 MgCl2(s)
- SiH4 + 2 O2 → SiO2 + 2 H2O
These reactions are typical of a Group 2
Alkaline earth metal
The alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...
silicide
Silicide
A silicide is a compound that has silicon with more electropositive elements.Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides....
. Mg2Si reacts similarly with sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
. Group 1
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
silicides are even more reactive. For example, sodium silicide, Na2Si, reacts rapidly with water to yield sodium silicate
Sodium silicate
Sodium silicate is the common name for a compound sodium metasilicate, Na2SiO3, also known as water glass or liquid glass. It is available in aqueous solution and in solid form and is used in cements, passive fire protection, refractories, textile and lumber processing, and automobiles...
, Na2SiO3, and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas.
Crystal Structure
Mg2Si crystallizes in a face-centered cubicCubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
lattice. It possesses the antifluorite structure with Si4- ions occupying the corners and face-centered positions of the unit cell and Mg2+ ions occupying eight tetrahedral sites in the interior of the unit cell. There are also four equivalent interstitial sites. Hence, the structure contains isolated Si4- ions. This feature makes the Si4 - ion of Mg2Si a good Brønsted–Lowry base (proton acceptor).