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What are hydrides?

Hydrides are chemical compounds that result from the reaction of hydrogen with metallic elements (sodium, potassium, calcium, etc.) and non-metals (chlorine, fluorine, oxygen, nitrogen, carbon, etc.). Hydrogen reacts with almost all chemical elements, so hydrides are very diverse substances.

Their general formulas can be the MH n type , where M is the metal and n is its valence (+1, +2, +3), such as aluminum hydride, AlH 3 ; or of the type EH n or H n E, where E is a non-metallic element and n its valence, such as NH 3 or H 2 S.

Hydrides are usually binary compounds, that is, they result from the combination of hydrogen with another chemical element. However, hydrogen can also combine with various chemical elements to form more complex hydrides. All hydrides explained here will be binary, such as AlH 3 and NaH.

There are three types of hydrides depending on the nature of their chemical bond : ionic, molecular and metallic. Ionic or saline hydrides are characterized by the presence of an ionic bond . This bond consists of the electrical attraction between the positive charge of an alkali metal (Na + ) or alkaline-earth metal (Mg 2+ ), and the negative charge of the hydride anion (H  ).

And metal hydrides result from the combination of hydrogen with some transition metals. This type of hydride, unlike the previous ones, has complex structures and will not be explained here.

Properties of hydrides

The properties of hydrides vary depending on the type of hydride: ionic, molecular or metallic, so their properties must be noted separately.

Ionic or saline hydrides

They are crystalline white solids, but due to the presence of impurities they acquire a grayish color. These hydrides are not soluble in common solvents, such as water, alcohol, etc. They also have high boiling and melting points.

They have a high density and in the molten state they can conduct electricity.

Ionic hydrides are considered basic or alkaline in nature, since dissolved in water they form hydroxides and release molecular hydrogen:

The reaction of ionic hydride with water is very vigorous and dangerous: it produces hydrogen in gaseous form, which is a combustible and flammable material.

Ionic hydrides can also react with metal halides, made up of a metal and a non-metal known as halogen (MX, X = F, Cl, Br, I):

4 LiH + AlCl 3 → LiAlH 4        + 3 LiCl

LiH (lithium hydride) is a binary compound; while LiAlH 4 (lithium aluminum hydride) is a ternary compound, that is, it is made up of three chemical elements.

Molecular or covalent hydrides

They are in a liquid or gaseous state, unlike ionic hydrides, which are solid. They have low boiling and melting points, it being observed that as the molecular weight of the hydride increases, the boiling point increases ; With the exception of ammonia (NH 3 ), water (H 2 O) and fluorine hydride (HF).

Covalent hydrides are poor conductors of electricity and many of their properties are due to the formation of hydrogen bonds. Covalent hydrides are considered acids, since dissolved in water they can give rise to acids, for example, hydrochloric acid.

Metal hydrides

They are in a solid state , presenting the dark tone of metals. They are good conductors of electricity and have glossy surfaces. Metal hydrides that are stable in air become reactive when heated, for example uranium hydride (UH 3 ) exhibits this property.

Nomenclature

The way to name the hydrides is simple, and it does not vary much in any of the nomenclatures. If the metal or non-metallic element has more than one valence, this is specified in parentheses and with a Roman numeral according to the stock nomenclature.

For example, FeH 3 is called iron (III) hydride , and CoH 2 cobalt (II) hydride. These same hydrides can also be named using systematic nomenclature, which makes use of the Greek numerator prefixes (di, tri, tetra, etc.). Thus, FeH 3 is called iron trihydride, and CoH 2 cobalt dihydride.

The classical nomenclature is not widely used for hydrides. For example, NaH is better known as sodium hydride, not sodium hydride.

How are hydrides formed?

Ionic or saline hydrides

They are formed by the reaction of hydrogen with a metal belonging to the group of alkali metals or to the group of alkaline earth metals.

In hydrides formed with these metals, hydrogen is in the H  form , known as the hydride anion.

The alkali metals (lithium, sodium, potassium, rubidium, and cesium) react with hydrogen to form their corresponding hydrides:

2 M (l) + H 2 (g) → 2MH (s)

The metal M when reacting is in the liquid state, and the hydrogen in the gaseous state , so the reaction requires high temperatures and high pressures. Consider the following example:

2 Li (l) + H 2 (g) → 2 LiH (s)

The reaction of hydrogen with the alkaline earth metals (beryllium, magnesium, calcium, strontium and barium), for the formation of hydrides, is similar to that observed with the alkali metals.

Magnesium and beryllium form hydrides, but their bonds with hydrogen are of the covalent type; that is, non-ionic, unlike the other alkaline earth metals:

Ca (l) + H 2 (g) → CaH 2 (s)

Molecular hydrides

Hydrogen can react with nonmetals, corresponding to groups 14, 15, 16 and 17 of the Periodic Table, to form molecular hydrides. Consider, for example, the formation of fluorine hydride by direct blending or mixing of hydrogen with fluorine:

2 (g) + F 2 (g) → 2 HF (g)

HF is better known as hydrogen fluoride, but it is also called fluorine hydride.

Likewise, hydrogen can react with oxygen (group 16 of the Periodic Table) following the same scheme:

2 H 2 (g) + O 2 (g) → 2 H 2 O (g)

The reaction requires a supply of heat to produce hydrogen oxide (water) or oxygen hydride.

Likewise, hydrogen can react with nitrogen, belonging to group 15 of the Periodic Table to form nitrogen hydride, better known as ammonia (NH 3 ).

2 (g) + 3 H 2 (g) → 2 NH 3 (g)

The reaction requires the supply of heat, high pressure and the presence of catalysts (they accelerate chemical reactions).

Metal hydrides

They are formed by the reaction of transition metals (groups 3, 4, 5, 6, 10, 11 and 12 of the Periodic Table) with hydrogen. However, the transition metals of groups 7, 8 and 9 do not form compounds with hydrogen, a phenomenon known as a hydride gap.

A unique feature of transitional metal hydrides is that they do not always have a fixed ratio between the number of hydrogen atoms and the number of atoms present in the hydride. That is, its formulas and structures are not simple to represent.

Uses / applications of hydrides

Hydrides are very present in one of the many types of batteries: such as NiMH. Source: Multicherry, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Hydrides are present in nickel metal hydride (NiMH) batteries, used in conjunction with rare earth metals, cobalt, or manganese. Various metal hydrides have been examined for use in electric cars powered by hydrogen-related fuel cells.

Reducing agents

Lithium hydride and sodium borohydride (NaBH 4 ) are used as chemical reducing agents in organic synthesis reactions.

Hydrogen storage and other related uses

There are pilot tests of the use of hydrogen as a fuel in motor vehicles, but due to its explosiveness its use as a fuel has been limited. The use of hydrides as a form of hydrogen storage has been proposed to control its danger.

In addition, hydrides have been used for the purification and separation of hydrogen and in temperature sensors.

Desiccants

Calcium hydride is used to remove the water present in organic solvents.

Acids originating from molecular hydrides

For example: hydrochloric acid is formed by the dissolution in an aqueous medium of chlorine hydride (HCl), using hydrochloric acid in the elimination of the oxides that cover the iron, the acidification of oil wells, the production of chloride of calcium and in mineral treatment.

Fertilizers

Ammonia (NH 3 ) is a hydride that is used as a liquid fertilizer and is used for the synthesis of compounds with ammonium nitrate and ammonium sulfate, which are used as fertilizers.

Water

It is the most important hydride. It is used in many industries with different uses and applications. Water is the main responsible for the existence of living beings.

Examples of hydrides

Finally, other examples of hydrides will be listed with their respective formulas and names:

SnH4: tin hydride

PH3: phosphorus hydride (phosphine) or phosphorous trihydride

KH: lead hydride

MgH2: magnesium hydride or magnesium dihydride

KH: potassium hydride

CH4: carbon hydride or carbon tetrahydride (methane)

CaH2: calcium hydride

FeH2: iron (II) hydride

NaH: sodium hydride

ZnH2: zinc hydride

LiH: lithium hydride

BaH2: barium hydride

CsH: cesium hydride

BH3: boron hydride (borane)

BeH2: beryllium hydride or beryllium dihydride

SiH4: silicon hydride (silane)

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