Hydrides - Practice Questions & MCQ

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 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 150^oC to 600^oC.

Properties

• They are colourless or greyish crystalline solids.
• They have high melting and boiling points.
• They conduct electricity in molten state liberating hydrogen at anode. 
• Ionic hydrides can undergo an oxidation-reduction reaction with water to produce hydrogen and a basic solution.
  
• 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  or  under moderate temperatures and is therefore used in the preparation of other useful hydrides​​​​​​

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Covalent hydrides are molecular compounds in which hydrogen is covalently bonded to another element. For example some covalent hydrides are NH₃, H₂O, H₂O₂ 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 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. The 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 direct combination of elements or by hydrolysis of compounds such as borides, silicides, phosphides, sulphides, carbides, etc. or by use of LiAlH₄.

These are formed by many d-block and f-block elements. However, the metals of group 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 as interstitial hydrides. However, recent studies have shown that except for hydrides of Ni, Pd, Ce and Ac, other hydrides of this class have lattice different from that 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 its storage media. This property has a high potential for hydrogen storage and as a source of energy.