Hydrides

Hydrides

Dihydrogen combines with a number of elements to form binary compounds called hydrides.

The general formula being MH_x 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

Ionic 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 + Al₂Cl_6-——-> 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 XH_n or XH_8-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 + LiAlH_{4 .} ———> 2B₂H₆ + 3AlCl₃ + 3LiCl

SiCl₄ + LiAlH_{4 .} ———-> SiH₄ + LiCl + AlCl₃

SnCl₄ +  LiAlH_{4 .} ———-> SnH₄ + LiCl + AlCl₃

GeCl₄  + LiAlH_{4 .} ———-> 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