HYDRIDES
 
  Binary compounds of hydrogen are known as hydrides.
 
TYPES OF HYDRIDES
 
There are six types of hydrides.
  Ionic hydrides
Covalent Hydrides
Polymeric Hydrides
Metallic Hydrides
Border line Hydrides
Complex Hydrides
 
IONIC HYDRIDES
 
  Elements of group I-A and II-A except Be and Mg form ionic hydrides. In these hydrides, bond between metal atom and hydrogen atom is ionic. Ionic hydrides are also known as Saline hydrides or Salt like hydrides because their properties are identical to salts.
In ionic hydrides,hydrogen acts as H-
 
GENERAL FORMULA
 
  For the hydrides of group I-A
 
                     MH
 
Where M represents Li, Na, K, Rb, Cs and Fs
 
EXAMPLES
 
LiH, NaH, KH, RbH, etc.
For the hydrides of group II-A
                     MH2
Where M represents Ca, Sr and Ba
 
EXAMPLES
 
  CaH2, SrH2 , BaH2
 
METHODS OF PREPARATION
 
  Ionic hydrides can be prepared directly on passing hydrogen gas over hot alkali metals or alkaline earth metals.
 
GENERAL EXPRESSION
OF PREPARATION
 
  For the hydrides of group I-A               2M + H2è 2MH
  For the hydrides of group II-A                M + H2 è MH2
 
EXAMPLES
 
  For the hydrides of group I-A
                2Li +H2 è 2LiH
              2K + H2 è 2KH
              2Rb + H2 è 2RbH
              2Na + H2 è 2NaH
  For the hydrides of group II-A
                              Ca + H2 è CaH2
                            Sr + H2 è SrH2
                            Ba + H2 è BaH2
 
PHYSICAL PROPERTIES
OF IONIC HYDRIDES
 
  Ionic hydrides are solids.
They are thermally very stable.
They have high melting and boiling point.
They are soluble in water.
They are insoluble in organic compounds.
They are conductors (electrolytes) of electricity.
 
USES OF IONIC HYDRIDES
 
  To produce hydrogen gas.
Reducing agent in metallurgical process.
Dehydrating agent for organic solvents.
 
CHEMICAL PROPERTIES
OF IONIC HYDRIDES
 
 

With Water

For I-A

NaH + H2O è NaOH + H2

LiH + H2O è LiOH + H2

KH + H2O è KOH + H2

For II-A

CaH2 + 2H2O è Ca(OH)2 + 2H2

BaH2 + 2H2O è Ba(OH)2 + 2H2

SrH2 + 2H2O è Sr(OH)2 + 2H2

 

With HCl:

For I-A

NaH + HCl è NaCl + H2

LiH + HCl è LiCl +H2

KH + HCl è KCl + H2

For II-A

CaH2 + 2HCl è CaCl2 + 2H2

BaH2 + 2HCl è BaCl2 + 2H2

SrH2 + 2HCl è SrCl2 + 2H2

  With Ethyl Alcohol:
  C2H5OH + NaH è C2H5ONa + H2
2C2H5OH + CaH2
è (C2H5O)2Ca + 2H2
 
COVALENT HYDRIDES
 
  Elements of group III-A to VII-A form covalent hydrides. They possess covalent bond in their structure. For example:
BH3, AlH3, CH4, NH3, H2S, HCl
  Method of preparation
Covalent hydrides can be prepared by two methods.

Direct method

N2 + 3H2 è 2NH3
H2 + S è H2S
H2 + Cl2 è 2HCl
Indirect method
By the action of water on a suitable compound we can prepare different covalent hydrides.
i) By Al4C3
Al4C3 + 6H2O è 2Al2O3 + 3CH4
ii) By Mg3N2
Mg3N2 + 6H2O è 3Mg(OH)2 + 2NH3
iii) By Ca3P2
Ca3P2 + 6H2O è 3Ca(OH)2 +2 PH3 (phosphene)
iv) By PCl3
2PCl3 +6H2O è 2H3PO3 +6HCl
Properties of covalent hydrides
1) Covalent hydrides are gases or liquids.
e.g.
Gases : CH4, NH3, H2S,
Liquid : H2O, HCl
2) Liquids are volatile.
3) They have low melting and boiling points.
4) They are colorless compounds.
Nature:
1) Hydrides of III-A and IV-A are neutral.
2) Hydrides of V-A are basic.
3) Hydrides of VI-A and VII-A are acidic.
 
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