Dihydrogen combines with a number of elements to form binary compounds called hydrides.
The general formula being MHx where M represents the element and x is the number of hydrogen atoms.
The various elements which form hydrides are :
1) All main group elements except those of noble gases and probably Indium and thallium.
2)All lanthanides and actinides
3)Transition metals – Sc, Y , La, Ac, Tc, Zr, Hf, Pd
Depending upon the physical and chemical properties, the hydrides have been divided into the following three categories:
1) Ionic or salt like or saline hydrides
2) Metallic or interstitial hydrides
3) Covalent hydrides
These are formed by those metal whose electronegativity values are lower than that of hydrogen.
All elements of group 1 and group 2 on heating at high temperature form ionic hydrides.
Properties of Ionic Hydrides
1) These are formed by transfer of electrons from the metal to hydrogen atoms and thus contains hydride ion.
2) These are white crystalline solids and their crystal structure consists of ions.
3) Alkali metal hydrides (LiH, NaH, KH, RbH) have rock salt structure. Their thermal stability decreases from LiH to CsH due to the reason that lattice energies of these hydride decreases progressively as the size of the metal cation increases from Li+ to Cs+ .
The stability of hydrides of alkaline earth metal decreases in the order :
CaH2 >SrH2 >BaH2
4) The density of these hydrides is higher than those of the metals from which they are formed. This is because the hydride Ions occupy holes in the lattice of the metal without distorting the metal lattice.
5) They have high melting and boiling points and conduct electricity in the fused state, liberating dihydrogen at the anode.
At anode : 2H‾ ————> H2 (g) + 2e‾
At cathode Na+ (melt) + e‾ ———> Na(l)
6) They have high heat of formation and are always stoichiometric.
7) Except LiH, they burn in air on strong heating due to their decomposition into hydrogen which is inflammable.
8) They react violently with water to form corresponding metal hydroxide with the liberation of dihydrogen.Thus they act as a strong base.
NaH (s) + H2O(l) ——-> NaOH (aq) + H2 (g)
CaH2 (s) + 2H2O(l) ——-> Ca(OH)2 (aq) + 2H2 (g)
Because of exothermic nature of the reaction, the evolved dihydrogen catches fire.The fire so produced cannot be extinguished by carbon dioxide because it get reduced by hot metal hydride. However sand is useful since it is highly stable solid.
9) They are powerful reducing agent ,especially at high temperature.
2CO + NaH ——–> HCOONa + C heat
CO2 + NaH ——–> HCOONa heat
2 BF3 + 6 NaH ———> B2H6 + 6NaF 450 K
SiCl4 + 4 NaH ———> SiH4 + 4NaCl heat
Their reactivity towards water limits their usefulness as reducing agents in aqueous solution.
10) Lithium hydride is unstable at moderate temperature.It is therefore, used in the synthesis of other complex metal hydrides such as Lithium aluminium hydride and lithium borohydride.
LiH + Al2Cl6-——-> 2LiAlH4 + 6LiCl
2LiH + B2H6 ————> 2LiBH4
2NaH + B2H6 ——-> 2NaBH4
These complex metal hydrides are widely used as reducing agent in organic synthesis,
Uses of Ionic Hydrides
Ionic hydrides and their complexes are used as reducing agent.
On heating ionic hydrides decompose to evolve dihydrogen which ignites spontaneously therefore they are used as solid fuels.
Metallic or interstitial Hydrides
d- group elements like 3, 4 ,5 ,10, 11 12 ,and f block elements on heating with dihydrogen under pressure form hydrides.
In group 6 , chromium alone forms the hydride, CrH.
The metals of group 7, 8 and 9 do not form hydrides. The region of the periodic table from 7 to 9 which does not form hydride is referred to as hydride gap.
Hydrogen atom being small in size occupy some in the metallic lattice producing distortion without any change in its type. That is why these hydrides are called as interstitial hydrides.
Properties of Metallic or interstitial Hydrides
1) They are hard ,have a metallic lustre, conduct electricity and have magnetic properties.
2) The density of these hydrides is lower than those of metals from which they are formed since the crystal lattice expands due to inculsion of dihydrogen.
3) These hydrides are often non stoichiometric i.e. in these hydrides law of constant composition does not hold good. The ratio of hydrogen atoms to the metal atoms in these hydride is not fixed but varies with the temperature and pressure.
Uses of Metallic or interstitial Hydrides
Due to interstitial hydride formation ,these metal adsorb large volume of hydrogen on their surface. This property of adsorption of a gas by a metal is called as occlusion.
The amount of hydrogen occluded depends upon the nature and physical state of the metal i.e. colloidal palladium > palladium > platinum> gold> nickel
If red hot Pd is cooled in H2, it adsorbs about 935 times its own volume of H2 gas. This may be used to separate H2 or D2 from He other gases. On strong heating, the occluded hydrogen is liberated. Thus metallic hydride can be used as hydrogen storage media. This property has high potential for hydrogen storage and a source of energy.
Metals such Ni, Pd, Pt which can adsorb large volume of hydrogen are widely used in catalytic reduction or hydrogenation reaction for preparation of large number of compounds.
Molecular or Covalent Hydrides
These are mainly formed by p block elements and some s block elements primarily due to the reason that the electronegativity difference between these elements and hydrogen atom is quite small.
General formula is XHn or XH8-n where n is the number of electrons in valence shell. These hydrides usually consist of discrete covalent molecules which are held together by weak van der waal forces of attraction and hence are called covalent or molecular hydrides.
Preparation of Molecular or Covalent Hydrides
1) By direct combination of elements with dihydrogen
N2 (g) + 3H2(g) ——-> 2NH3 673 K, 200 atm, Fe, Mo
2H2 (g) + O2 (g) ——–> 2H2O electric discharge
H2 (g) + S (l) ——> H2S(g)
H2 (g) + F2 ( g ) ———> 2HF dark, little moisture, 23 K
2) By reduction of a suitable halide with LiAlH4 in dry ether
4BCl3 + LiAlH4 . ———> 2B2H6 + 3AlCl3 + 3LiCl
SiCl4 + LiAlH4 . ———-> SiH4 + LiCl + AlCl3
SnCl4 + LiAlH4 . ———-> SnH4 + LiCl + AlCl3
GeCl4 + LiAlH4 . ———-> GeH4 + LiCl + AlCl3
3) By hydrolysis of metal borides, carbides, nitrides, phosphide
CaC2 (s) + 2 H2O (l) ——-> Ca(OH)2 (aq) + HC≡CH
Ca3N2 (s) + 6 H2O (l) ——-> 3 Ca(OH)2 (aq) + 2NH3 (g)
Ca3P2 (s) +6 H2O (l) ———> 3Ca (OH)2 (aq) + 2PH3 (g)
4) By action on suitable binary compounds
2Mg3B2 + 4 H3PO4 ———-> B4H10 + 2 Mg3 (PO4 )2 + H2
Al4C3 + 12 HCl ——> 3CH4 + 4AlCl3
FeS + H2SO4 ——–> H2S + FeSO4
Ca3P2 + 3H2SO4————> 2PH3 + 3 CaSO4
5) By action of an oxo acids with NaBH4 in aqueous solution
4H3AsO3 + 3 NaBH4 ——–> 4AsH3 + 3 H3BO3 + 3NaOH
Depending upon the number of electrons and bonds present in their Lewis structure, molecular hydrides have been classified as electron deficient, electron exact or electron precise and electron rich hydrides.
Properties of Molecular or Covalent Hydrides
1) Covalent hydrides are usually volatile compounds having low melting and boiling point and also do not conduct electricity.
2)Hydrides of group 13 ( BH3, AlH3 )do not have sufficient number of electrons to form normal covalent bond and hence are called electron deficient hydrides. They exist in polymeric forms such as B2H6, B4H10 , (AlH3)n
3) Hydrides of group 14 ( CH4, SiH4, SnH4, PbH4 ) have exact number of electrons to form normal covalent bond and hence are called electron exact or electron precise hydrides. Their Bond length increases from CH4 to PbH4 as the size of the element increases from C to Pb.
4) Hydrides of group 15 ,16 and 17( NH3 , PH3 , H2O , H2S, HF, HCl ) have more electrons than required to form normal covalent bond and hence are called electron rich hydrides.
The excess electrons in these hydrides are present as lone pair of electrons.
Group 15 hydrides have one lone pair ,group 16 hydrides have two lone pair ,group 17 hydride have 3 lone pair of electrons.
The presence of lone pair of electrons on the highly electronegative oxygen ,nitrogen and fluorine atoms results in the formation of intramolecular hydrogen bonding.As a result of intermolecular hydrogen bond these hydrides exist as associated molecules.
The hydrides of first elements of group 15 ,16 and 17 have abnormally high boiling point as compared to boiling point of the hydrides of second element of each group. The boiling point of the hydrides of the rest of the elements of each group increases as the atomic number or the molecular mass of the hydride increases down the group.
5) The lighter elements of group 14 ,15 and 16 form polynuclear hydrides in which two or more atoms of same elements are linked together this property of self linking of atoms is called catenation and is maximum for Karbonn than Nitrogen Oxygen and sulphur
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