Hydrides: Properties, Reactions & Uses - Britannica

Hydrides are those compounds that contain hydrogen bonded to some other more electropositive element. Based on the nature of the bond between hydrogen and another element, they may be classified into three principal categories: ionic or saline, covalent or molecular, and metallic hydrides. Covalent hydrides result whenever hydrogen bonds with non-metals to form compounds that share electron pairs. On the other hand, metallic hydrides occur when hydrogen reacts with metals to gain a lattice structure whereby hydrogen atoms occupy the interstices within the metal lattice.

This Story also Contains

1. Properties
2. Covalent/Molecular Hydrides
3. Metallic/Molecular Hydrides
4. Applications and Relevance
5. Some Solved Examples
6. Summary

The saline hydrides are compounds of hydrogen with a strongly electropositive metal, i.e., alkali and alkaline earth metals which can transfer electrons easily to hydrogen atoms. However, significant covalent character is found in the hydrides of Li, Be, and Mg due to the high polarising power of the smaller-sized cations. It is to be mentioned that the hydrides of Be and Mg are polymeric in nature.
These hydrides are generally prepared by heating the metal with hydrogen under pressure at temperatures between 150⁰C to 600^oC.

Properties

• They are colorless or greyish crystalline solids.
• They have high melting and boiling points.
• They conduct electricity in a molten state liberating hydrogen at anode.
• Ionic hydrides can undergo an oxidation-reduction reaction with water to produce hydrogen and a basic solution.LiH(s)+H2O(l)→H2( g)+LiOH(aq)
• They have high heats of formation.
• The density of ionic hydrides is higher than those of metals from which they are formed.
• The stability of the hydrides decreases as the size of the cation increases.
  LiH > NaH > KH > RbH > CsH and CaH₂ > SrH₂ > BaH₂
• LiH is rather unreactive with O2 or Cl₂ under moderate temperatures and is therefore used in the preparation of other useful hydrides

8LiH+Al₂Cl₆→2LiAlH₄+6LiCl₂LiH+B₂H₆→2LiBH₄

Covalent/Molecular Hydrides

Most typical covalent hydrides are non-metal elements—essentially carbon, nitrogen, oxygen, and the halogens: methane, CH₄; ammonia, NH₃; and water, H₂O. These hydride covalent bonds are those in which hydrogen has shared electrons with the non-metal atom. The hydrides of covalent bonds vary considerably in their properties. For instance, under similar conditions of temperature and pressure, methane is a gas, used as fuel, whereas water is used as the basis of life since it is liquid. Some are constituents of fertilizers; an example includes ammonia. The strength and nature of the covalent bonds in these hydrides determine their reactivity, physical state, and uses. This makes covalent hydrides quite an indispensable component in most chemical processes and industries.

Covalent hydrides are molecular compounds in which hydrogen is covalently bonded to another element. For example, some covalent hydrides are NH₃, H₂O, H₂O_2, and HF. These hydrides are formed by all the true non-metals (except zero group elements) and the elements like Al, Ga, Sn, Pb, Sb, Bi, Po, etc., which are normally metallic in nature, i.e., this class includes the hydrides of p-block elements. Except for third group elements, each other element forms a simple mononuclear hydride of the formula, MH_8-x where x is the number of electrons present in the outermost orbit of the element M. The simplest hydride of B and Ga are dimeric materials, B₂H₆(diborane) and Ga₂H₆ respectively and the hydride of aluminium is polymeric in nature, (AlH₃)_n. In addition to mononuclear hydrides, the elements like Si, Ge, N, P, O, S, B, etc., form polynuclear hydrides.
Molecular hydrides are further classified according to their relative numbers of electrons and bonds in their Lewis structures.

• Electron-deficient molecular hydrides: These have too few electrons for writing its conventional Lewis structure. Diborane(B₂H₆) is an example of this type.
• Electron precise molecular hydrides: These are formed by elements of group 14. The molecules are tetrahedral. Methane CH₄ is an example of this type.
• Electron-rich molecular hydrides: The excess electrons are present as lone pairs. Examples are NH₃, H₂O, HF, etc. The hydrides NH₃, H₂O, and HF due to the presence of highly electronegative atoms possess hydrogen bonding also. These hydrides can be obtained by a direct combination of elements or by hydrolysis of compounds such as borides, silicides, phosphides, sulfides, carbides, etc., or by use of LiAlH₄.

Metallic/Molecular Hydrides

Metallic hydrides are, in fact, interstitial hydrides. Hydrogen atoms occupy the interstitial spaces in a metal lattice. Some common examples include palladium hydride, PdHₓ, and titanium hydride, TiH₂. These hydrides exhibit metallic bonding characteristics and quite frequently have high electrical conductivity. Metallic hydrides have applications in hydrogen storage materials and hydrogen fuel-cell technologies. Such hydrides have eventual applications associated with energy storage solutions due to their potential to absorb and release hydrogen. This special structure also conveys special properties, hence allowing them to act as catalysts during chemical reactions.

These are formed by many d-block and f-block elements. However, the metals of groups 7, 8, and 9 do not form hydride. Even from group 6, only chromium forms CrH. These hydrides conduct heat and electricity though not as efficiently as their parent metals do. Unlike saline hydrides, they are almost always non-stoichiometric, being deficient in hydrogen. For example, LaH_2.87, YbH_2.55, TiH_1.5–1.8, ZrH_1.3–1.75, VH_0.56, NiH_0.6–0.7, PdH_0.6–0.8 etc. In such hydrides, the law of constant composition does not hold good.
Earlier it was thought that in these hydrides, hydrogen occupies interstices in the metal lattice producing distortion without any change in its type. Consequently, they were termed interstitial hydrides. However, recent studies have shown that except for hydrides of Ni, Pd, Ce, and Ac, other hydrides of this class have lattices different from those of the parent metal. The property of absorption of hydrogen on transition metals is widely used in catalytic reduction/hydrogenation reactions for the preparation of a large number of compounds. Some of the metals (e.g., Pd, Pt) can accommodate a very large volume of hydrogen and, therefore, can be used as storage media. This property has a high potential for hydrogen storage and as a source of energy.

Applications and Relevance

Hydrides play an important part in everyday life and complex scientific research; in energy, metallic hydrides—hydrogen storage materials required by developing clean energy technologies like fuel cells. As ammonia, covalent hydrides are huge fertilizers in agriculture. Hence, in that sense, they pretty directly influence agriculture. Since hydride is a key material for renewable energy storage in battery technology in electronics, they find extensive application here. In the academic domain, studies of hydrides help broaden our understanding of chemical bonding, reactivity, and material science maneuvers. Therefore, new hydride materials research has taken the upper hand in the continuous crackdown of boundaries into fields that will lead to innovation in energy storage, pharmaceuticals, and other related areas.

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Some Solved Examples

Example 1
The nature of hydrides formed depends primarily on which factor?

1. Bond dissociation energy
2. Electronegativity of other atoms (Correct)
3. Hydration enthalpy of the compound formed
4. The melting point of the compound formed

Solution:
The type of hydrides formed primarily depends on the electronegativity of other atoms. Therefore, the correct answer is option (2).

Example 2
NaH is an example of:

1. Electron-rich hydride.
2. Saline hydride. (Correct)
3. Metallic hydride.
4. Molecular hydride.

Solution:
NaH is an example of an ionic hydride, which is also known as a saline hydride. Hence, the correct answer is option (2).

Example 3
Which statement is false regarding ionic hydrides?

1. Alkali hydrides generally have a rock salt structure.
2. They have high heat of formation.
3. Their density is higher than the metals they are formed of.
4. They don't undergo an oxidation-reduction reaction with water. (Correct)

Solution:
Ionic hydrides undergo oxidation-reduction reactions with water to produce hydrogen.
For example:
LiH(s)+H2O(l)→H2(g)+LiOH(aq)

Therefore, the correct answer is option (4).

Summary

Hydrides represent an important class of compounds and reagents with considerable implications in various applications and scientific domains. The covalent hydrides, formed by non-metals, and metallic hydrides involving metals have their own special characteristics and applications. From domestic fuels and fertilizers to new energy storage devices and electronic components, hydrides have played their roles in most technologies and industries. Knowing their properties and uses just enriches our arsenal in chemistry, which at the same time will facilitate innovation and progress in various industries.