Ionic or salt-like hydrides are moderately reactive and foams strong bonds with many other elements. We as well studied that it forms ionic and covalent hydrides with metals or non metals respectively. In this unit we will be studying the formation of hydrides, their types and characteristics of the hydrides.
Ionic or salt - like hydrides:
Such are formed through metals with low electronegativity values and are more electropositive with respect to hydrogen. These hydrides are structured via transfer of an electron from the metal to the hydrogen atom. Hydride ion is a peculiar chemical species or in contrast to proton that has small size, it is uncommonly large. It is superior to any of the negative ions except iodide.
The cause for this apparent paradox lies in the lack of control through a single nuclear proton over 2 naturally repelling electrons. Alkali or Alkaline-earth metal of groups 1 and 2 are adequately electropositive and force the hydrogen atom to accept an electron to form the hydride ion, if example Lithium hydride Li H- and calcium hydride
Ca2+ (H-) 2.
Ionic hydrides are formed via warming metals in hydrogen at 973k. Ionic hydrides are white crystalline solids. They contain high melting points or conduct electricity in liquid state, liberating hydrogen at the anode. Their density is higher than that of the metal. They are powerful falling agents especially at high temperatures for example:-
2 CO + NaH → HCOO Na + C
SiCl4 + 4 NaH → SiH4 + 4NaCl
PbSO4 + 2CaCH2 → PbS + 2 Ca (OH)2
Li+ H- and Na+ H- are used in making precious reducing managers similar to lithium aluminum hydride (LiAIH4) and sodium-boro-hydride (NaBH4). The complex hydrides are frequently utilized in the reduction of aldehydes, ketones, acids and their derivatives to provide alcohols.
2 - COO H → (LiAIH4/Na BH4) → R - CH2 OH
Organic acid Primary alcohol
>C = O → (LiAIH4/Na BH4) → >CH - OH
Ketone/aldehyde Secondary alcohol
Such bonds are formed via elements of comparatively higher electronegativity these as the P- block elements and Be and Mg. The links formed in this class of hydrides are frequently covalent in character but in some cases, for instance, in HF, the connection may be partially ionic.
The covalent hydrides can be arranged either through direct reaction of non-metals with hydrogen beneath suitable conditions or via the reaction of H2O and acids or nitrides, carbides, bonides, silicides, stanides of alkali and alkaline earth metals or through the reduction of halides. These are demonstrated via the given reactions.
N2 + 3H2 → (Catalyst, 750K) → 2NH3
Mg3N2+6 H2O → (600 -1000atm) → 3Mg (OH)2 + 2NH3
CaC2 + 2H2O → Ca (OH)2 + C2H2
As Cl3 + 6H → (Zn + HCl) → 4As H3 + 3 HCl
4 PCl3 + 3 LiAlH4 → (Ether) → 4PH3 + 3LiCl + 3Al C13
Such hydrides contain molecular lattice made up of individual saturated covalent molecules, with only weak Vander Waals forces or in several cases along with hydrogen bonds. This accounts for their softness, low melting or boiling points, their volatility and be short of conductivity. Several covalent hydrides are unstable in the presence of air, for example stannane, SnH4.
Several covalent hydride hydrides of groups 2 or 13 are electron deficient. Such have structures between ionic and covalent hydrides. These are either dimeric, example boron hydride (B2H6), or polymeric, for example beryllium hydride (Be H2) n, magnesium hydride (MgH2)n and aluminium hydride (AIH3 )n.
When heated, hydrogen reacts with many transition metals (lantharindes and actinides) to form metallic hydrides. Most of these contain metallic appearance and are less dense than the parent metal. They all conduct heat and electricity although not also as the parent metal. They are approximately always non-stoichiometric, being deficient in hydrogen. For instance, Ti H5.8 VH0.56, Cr H17 Ni H 0.6 0.7 TaH22 . 76, La H28 YbH etc.
Most of such hydrides are steady to water up to 375K but are quantitatively decomposed via acids or show several reducing properties. Previously such hydrides were formed as interstitial compounds in that hydrogen was throughout to be accommodated in the interstices in the metal lattice producing distortion although no change in its type. But recent studies contain shown that except for hydrides of nickel actinium, cerium and palladium, other hydrides of this class contain lattice of a kind different from that of the parent metal. For instance, the hexagonal close packed lattice of several lanthanides is changed to a face-centered cubic lattice in their dihydrides. Since pointed out previous, such interstitial hydrides are poorer conductors of electricity, exhibit less Para magnetism or are more brittle than the parent metal. Such characteristics suggest that hydrogen is present in the metal lattice because hydrogen atoms rather than as hydrogen molecules. The single electron of hydrogen is paired with an electron of the metal, therefore reducing the extent of metallic bonding. Breaking of the H-H bond is in agreement with the fact that there metals catalyse reactions of hydrogen.
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