He magnesium hydride (MgH 2 of molecular formula), is a chemical compound with content in weight of hydrogen of 7.66%, and found in nature as a white crystalline solid. It is mainly used to prepare other chemical substances, although it has also been studied as a potential storage medium for hydrogen.
It belongs to the family of saline (or ionic) hydrides, those defined by an H-ion negatively charged. These hydrides are considered those that are formed from the alkali metals and alkaline earth metals, but in the case of magnesium (and beryllium) have covalent bonds, in addition to those ionics that characterize this family of hydrides.
Unit cell model of magnesium hydride, MgH2.
Index
- 1 Preparation and formula
- 2 Chemical structure
- 3 Properties
- 4 Applications
- 5 Risks
- 5.1 Reaction with water
- 5.2 It is pyrophoric
- 6 References
Preparation and formula
Magnesium hydride is formed through the direct hydrogenation of magnesium (Mg) metal under conditions of high pressure and temperature (200 atmospheres, 500 ºC) with an MgI catalyst 2 . Its reaction is equivalent to:
Mg + H 2 → MgH 2
The production of MgH has also been investigated 2 at lower temperatures with the use of nanocrystalline magnesium produced in ball mills.
There are also other methods of preparation, but they represent more complex chemical reactions (hydrogenation of magnesium-anthracene, the reaction between diethylmagnesium with lithium-aluminum hydride, and as a product of an MgH complex 2 ).
Chemical structure
This atom has a structure of rutile at room temperature, with a tetragonal crystalline structure. It has at least four different forms under high pressure conditions, and in addition a non-stoichiometric structure with hydrogen deficiencies has been observed; the latter only occurs in very small amounts of particles when formed.
As mentioned above, the bonds that exist in the rutile structure have partially covalent properties instead of being purely ionic, like other saline hydrides.
This causes the magnesium atom to have a spherical shape, totally ionized, but its hydride ion has an elongated structure.
Properties
This chemical compound has a molecular weight of 26,321 g / mol, a density of 1.45 g / cm3 and has a melting point of 327 ° C.
It is important to indicate that this compound can not be brought to liquid state, and when it is carried or its melting point or introduced into water, it decomposes. This hydride is insoluble in ether.
It is a highly reactive and highly flammable substance, and is also pyrophoric, that is, it can ignite spontaneously in the air. These three conditions represent security risks that will be mentioned in the last section of this article.
Applications
Magnesium hydride reacts easily with water to form hydrogen gas, through the following chemical reaction:
MgH 2 + 2H 2 O → 2H 2 + Mg (OH) 2
In addition, this substance decomposes at the temperature of 287 ° C and pressure of 1 bar, as follows:
MgH 2 → Mg + H 2
Therefore, the use of magnesium hydride as a hydrogen storage medium for its use and transport has been proposed.
The hydrogenation and dehydrogenation of a quantity of metallic magnesium is proposed as a way of transporting quantities of gaseous hydrogen, thus ensuring that there is no leakage in its transport and representing a safer and more practical way than with the use of high pressure vessels. .
Although the decomposition temperature of magnesium hydride represents a limiting factor for its use, methods have been proposed to improve the kinetics of hydrogenation and dehydrogenation reactions. One of these is with the reduction of the size of the magnesium particle with the use of ball mills.
In addition, a system that produces a magnesium hydride in the form of mud (more manageable and safe than that in powder or other solid particles), which would be reacted with water to obtain the desired hydrogen, has been proposed.
It is estimated that the previously named sludge would be formed by a finely ground hydride, protected with a protective layer of oils and suspended in dispersing agents to ensure that it maintains its consistency without loss of material, and that it does not absorb moisture from the environment.
This sludge has the advantage that it can be pumped through any common diesel, gasoline or water pump, making this economic proposal as well as efficient.
Finally, magnesium hydride can be implemented in the production of advanced fuel cells, and also in the creation of batteries and energy storage.
Risks
Reaction with water
As already mentioned, magnesium hydride is a substance that reacts very easily and violently with water, presenting the ability to explode in higher concentrations.
This occurs because its exothermic reaction generates enough heat to ignite the hydrogen gas released in the decomposition reaction, leading to a rather dangerous chain reaction.
Is pyrophoric
Magnesium hydride is also pyrophoric, which means that it can ignite spontaneously in the presence of moist air, forming magnesium oxide and water.
Its inhalation is not recommended in solid state or in contact with its vapors: the substance in its natural state and its products of decomposition can cause serious injuries or even death.
It can generate corrosive solutions in contact with water and contamination of it. Contact with the skin and eyes is not recommended, and also generates irritation in the mucous membranes.
It has not been demonstrated that magnesium hydride can generate chronic health effects, such as cancer, reproductive defects or other physical or mental consequences, but the use of protective equipment is recommended when handling it (especially respirators or masks, for its fine dust character).
When working with this substance, the humidity of the air must be kept at low levels, extinguish all sources of ignition and transport it in drums or other container containers.
Always work with large concentrations of this substance when it can be avoided, since the possibility of an explosion decreases significantly.
If a spill of magnesium hydride occurs, the work area should be isolated and the dust collected with a vacuum cleaner. You should never use the dry sweeping method; increases the chances of a reaction with the hydride.
References
- Zumdahl, S. S. (1998). Encyclopedia Britannica . Taken from britannica.com.
- PubChem. (2005). PubChem Open Chemistry Database . Taken from pubchem.ncbi.nlm.nih.gov.
- Safe Hydrogen, L. (2006). Green Car Congress . Taken from greencarcongress.com.
- Chemicals, C. (n.d.). Cameo Chemicals . Taken from cameochemicals.noaa.gov.
- Services, N. J. (1987). New Jersey Department of Health and Senior Services . Taken from nj.gov.