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Hydrides

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Hydrides - Lesson Summary

Dihydrogen combines with almost all the elements, except noble gases under suitable conditions to form binary compounds called hydrides.

These can be represented as EHx (or) EmHn, where E is the symbol for an element.

Ex:

EHx → MgH2
Where x = number of hydrogen atoms
EmHn → B2H6
Where m = number of atoms of elements E
     and n = number of hydrogen atoms

There are three types of hydrides depending on the behaviour and the nature of the bond formed between the hydrogen atom and the element.

The types of hydrides are ionic (or) salt-like hydrides, covalent (or) molecular hydrides and metallic (or) non-stoichiometric hydrides.

Ionic (or) salt-like hydrides are binary compounds of hydrogen with s - block elements that are highly electropositive in nature. But compounds formed with lithium, beryllium and magnesium exhibit higher covalent character.

Ionic hydrides like sodium hydride and calcium hydride are formed by the transfer of electrons from the metal atom to hydrogen atom.

Ionic hydrides are crystalline in solid state with high melting and boiling points. In the molten state they conduct electricity and on electrolysis liberate hydrogen gas at the anode.

Ionic hydrides react vigorously with water to liberate dihydrogen gas.

Ex: Sodium hydride reacts with water to give sodium hydroxide and hydrogen gas.

Ionic hydrides decompose into highly inflammable dihydrogen and undergo combustion when strongly heated in air.

Ex: Calcium hydride when heated at 675-775K decomposes into calcium and dihydrogen.

Lithium hydride is unreactive at moderate temperature with oxygen and chlorine gas, and hence, is used in the synthesis of other useful hydrides such as lithium aluminium hydride and lithium boron hydride.

Lithium aluminium hydride is a powerful reducing agent and used extensively in organic chemistry.

     8LiH + Al2Cl6 → 2LiAlH4 + 6LiCl

Covalent (or) molecular hydrides are binary compounds of hydrogen with more electronegative elements such as p block elements.

Covalent hydrides are further classified into three types, depending on the number of electrons and bonds present in their Lewis structures.

They are electron-deficient hydrides, electron-precise hydrides and electron-rich hydrides.

The hydrides of group -13 form electron-deficient compounds. They act as Lewis acids (electron acceptors).

Ex: BH3, AlH3

Electron-precise hydrides have the required number of electrons to write their Lewis structure.

These are formed by the elements of group 14.

Ex: CH4, SiH4...etc

Electron-rich hydrides have excess electrons, which are present as lone pairs. These are formed by the elements of groups 15, 16 and 17.

Ex: NH3, H2O, HCl...etc

Due to the presence of excess lone pairs, these hydrides behave as Lewis bases (electron donors).

Electron-rich hydrides are poor conductors of electricity and are soluble in organic solvents.

Metallic hydrides:

These are binary compounds of hydrogen with d- and f- block elements, except for chromium from group 6, the metals of groups 6, 7, 8 and 9 do not form hydrides.

Metallic hydrides are deficient in hydrogen and are almost always non-stoichiometric. The composition of these hydrides does not correspond to simple whole number ratio and also varies with change in temperature and pressure and hence these hydrides are called non stoichiometric hydrides.

  Non-Stoichiometric Hydrides      LaH 287

      YbH 255

      TiH 1.5-1.8

 The property of absorption of hydrogen on transition metals is widely used in catalytic reduction.

Metallic hydrides are conductors of heat and electricity and are harder than the parent metal.

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