GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

UNIT 2 Lithium Hydride, LiH

Preparation
Hydrides
It is prepared by passing H2 gas over heated lithium at
The binary compounds of hydrogen are termed as 700 – 800 oC.
hydrides.

Classification of Hydrides
Properties
Hydrides may be classified according to the general

properties and bond types. In the year 1941, Gibb gave the 1. It is a hard white solid.

following classification:
2. It reacts with H2O evolving H2.

1. Ionic or Salt like hydrides

2. Covalent or molecular hydrides
3. Metallic or interstitial or alloy type hydrides
4. Complex hydrides 3. When an etheral solution of LiH is treated with anhydrous
5. Borderline hydrides AlCl3, lithium aluminium hydride (LiAlH4) is obtained.

Ionic or Salt like hydrides

These hydrides are given by the elements of groups I A

(Fr is an exception), II A (Be and Mg are exceptions) and some 4. When an etheral solution of LiH is treated with B2H6, lithium
highly electropositive lanthanides and actinides. borohydride (LiBH4) is obtained.

Ex: LiH, NaH, CaH2

5. Fused LiH, on electrolysis, gives Li at the cathode and H2 at the

anode.

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

Uses Properties

It is used 1. It is a colourless crystalline substance.

(i) as a strong reducing agent in synthetic organic 2. It reacts with H2O evolving H2.

chemistry

(ii) in the preparation of LiAlH4 which is a valuable

reducing agent 3. It absorbs CO2 forming sodium formate, HCOONa.

(iii) for making borohydrides like LiBH4 which are used

in jet propulsion.
4. When an etheral solution of NaH is treated with B2H6, sodium
Structure borohydride (NaBH4) is obtained.

Its crystal has face-centred cubic structure.

Sodium Hydride, NaH

5. Fused NaH, on electrolysis, gives Na at the cathode and H2 at

Preparation the anode.

It is prepared by passing pure and dry H2 over metallic Uses

sodium kept in a nickel boat and heated in a glass tube at 365 oC.
(i) NaH is used to prepare NaBH4, which is a valuable reducing

agent.

(ii) NaH itself is also used as a reducing agent in organic

chemistry.

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

Structure Pentasilane - Si5H12

NaH has a face-centred cubic lattice. Hexasilane - Si6H14

Heptasilane - Si7H16
Covalent or molecular hydrides
Preparation
These hydrides are given by most of p-block elements.

When 20% HCl acts on Mg2Si in an atmosphere of H2, a mixture of

Preparation
silanes is obtained.
By the direct union of the free element with H2 at

elevated temperature.

Properties

1. Silanes are analogous to alkanes. These are colourless and

volatile covalent hydrides.

Silanes 2. Their melting point increases with the increase of their

molecular weights.
Hydrides of silicon are called silanes.

3. Thermal stability of silanes decreases with the increase of

These are represented by the general formula SinH2n+2
their molecular weights.

Various silanes are given below,

4. The decomposition of higher silanes gives a mixture of simple
Monosilane or silane - SiH4 gaseous silanes.

Disilane - Si2H6

Trisilane - Si3H8

Tetrasilane - Si4H10

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

5. Silanes are very reactive and burn spontaneously in air or 3. Action of heat (Decomposition)

oxygen. When heated to 400 oC it breaks down into its elements.

6. With water in the presence of trace of a base silanes form

4. Precipitation reactions
silicates with the liberation of H2.
SiH4 gives a precipitate of copper silicide, Cu2Si when

passed in solution of copper salt like CuSO4. Similarly silver

metal is obtained from AgNO3 solution.

Monosilane, SiH4

Preparation

SiH4 can be prepared by passing dry H2 over hot silicon

in an electric arc.
5. Action of alkalies

SiH4 reacts with alkalies to form silicates with the

Properties liberation of H2.

1. It is a colourless gas has faint odour.

2. Action of air
Uses
In the presence of H2 and other hydrides of silicon, it
(i) SiH4 is used as a source for the preparation of silicon
spontaneously burns in air with a bright flame to form SiO2 and
of high purity.
H2O.

(ii) It is also used as a reducing agent.

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

Disilane, Si2H6

Preparation

It is prepared by the reaction of Si2Cl6 with LiAlH4 in ether 5. Reducing properties

solution.
It reduces KMnO4, K2Cr2O7, FeCl3, AgNO3, HgCl2 etc.

Properties
Uses
1. It is a colourless liquid with more repulsive smell than SiH4.
(i) Si2H6 is used as a source for the preparation of silicon
2. Decomposition of high purity.

It decomposes into its elements at 200 oC in absence of (ii) It is also used as a reducing agent.
air.
Differences between Alkanes and Silanes

1. In the hydrocarbon series, the number of C-atoms in a chain

have any value but in case of silanes the maximum number of

3. Reaction with alkalies
Si-atoms is seven. This is because of the fact that relatively
It reacts with alkalies to form silicates with the weak Si – Si bond makes unstable silanes linked with large
liberation of H2. number of Si-atoms.

2. SiH4 gets hydrolysed while CH4 does not. This is because

maximum covalency of carbon is four which is attained in CH4.

4. Reaction with CHCl3 and CCl4
Thus CH4 is resistant to the attack of electron donating
SiCl4 is formed in each case. molecules. The maximum covalency of silicon atom is six. In

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

SiH4 molecule, only four covalent bonds are present. It is, Properties
therefore, possible for SiH4 to accept lone pair of electrons from
1. The densities of metallic hydrides are usually lower than
the donor groups such as water.
those of the parent metal.

Metallic or Interstitial or alloy type hydrides

2. Their electronegativity values are fairly close to those of I A

Many d-block elements, lanthanides and actinides at elevated (alkali metals) and II A (alkaline earth metals) groups.

temperature absorb hydrogen into the holes or interstices

3. Since these hydrides possess metallic properties such as
existing between the atoms comprising the metallic lattice
hardness, lusture and electrical conductivity, these are also
without changing the original crystal structure of the metal
called metallic hydrides.
and thus give hydrides which are called metallic hydrides.
4. The absorbed hydrogen can be expelled from the interstices
e.g. TiH1-73, VH0-6, ZrH1-92 etc.
simply by heating the hydrides and increasing the gas pressure.

Thus these hydrides are interstitial combinations of Thus these hydrides are good reducing agents.

hydrogen and the metal and it is for this reason that these are
Complex Hydrides
called interstitial hydrides.

Some of the hydrides contain borohydride (BH4-) and

Preparation
aluminium hydride (AlH4-) ions. These hydrides are called

When phenol magnesium bromide solution is mixed complex hydrides.

with metal (II) chloride and H2 is passed through, black

e.g. lithium borohydride (LiBH4), sodium borohydride
precipitates of NiH2, CoH2 and FeH2 are formed.
(NaBH4), lithium aluminium hydride (LiAlH4)

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

Sodium borohydride (NaBH4) Uses

Preparation It is used a reducing agent in organic synthesis.

When an etheral solution of NaH is treated with B2H6, Structure

sodium borohydride (NaBH4) is obtained.
 The crystal of NaBH4 has an orthorhombic lattice.

 Each Na+ ion is associated with four tetrahedral BH4 ions.

Properties

1. It is soluble in polar solvents like H2O.

2. Sodium borohydride reacts with phosphoric acid gives

diborane.
Lithium aluminium hydride (LiAlH4)

Preparation

3. It reacts with iodine and gives diborane. It is prepared by the action of LiH on an ether solution

of AlCl3.

4. It reacts with HCl and BF3 to produce B2H6

Properties

1. It is decomposed by H2O, liberating H2 gas.

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 1

2. When heated, it gets decomposed according to the equation. Structure

It is an ionic complex hydride containing the complex

anion i.e., alumino hydride anion, [AlH4]- ion which is ionically

attached with Li+ cation.

3. It reacts with diborane to form lithium borohydride and

aluminium borohydride. [AlH4]- is formed by the coordination of the hydride ion

to AlH3 molecule.

4. It converts the chlorides of Si, Sn etc., into their hydrides.

5. BCl3 reacts with lithium aluminium hydride and gives

diborane.

Uses

(i) It is used for reducing compounds containing a

carbonyl group like ketones, carboxylic acids and their

derivatives to alcohols.

(ii) It is also used for the preparation of the hydrides of

Si, Sn, B, Al etc., from their chlorides.

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 2

Carbides 3. Explosive nature

Carbides of U, Cu, Hg etc., are explosive substances.

The binary compounds of carbon with the elements

which are more electropositive than carbon are called carbides. 4. Reducing property

e.g. BeC2, Ag2C2etc. CaC2 reduces MgO and MgCl2 in to its metals.

Preparation

1. By heating metal with carbon

Many metals like Be, Ag, Mn, Li etc., combine with carbon

at high temperature to form carbides. 5. Hydrolysis

Ionic carbides can easily be hydrolysed by water or

dil.acids with the formation of different types of hydrocarbons.

Classification
2. By passing CO over metal at high temperature.
Carbides are classified into four types depending on the

nature of bonding between carbon atoms and other elements.

These four types are:

Properties
1. Ionic or salt-like carbides
1. Generally carbides are transparent crystalline solids. In the
2. Interstitial or metallic carbides
solid state they are non-conductors of electricity.
3. Iron type or borderline carbides
4. Covalent carbides.
2. Softness and Hardness

Alkali metal carbides are soft while others are usually

hard.

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GENERAL CHEMISTRY – II 12UCH02 UNIT 2 PART 2

Technical applications of Carbides

1. In general, carbides are very hard and are used as efficient

abrasives, drilling tools and cutting tools.

2. Silicon carbide being used as an abrasive and a deoxidant in

the metallurgy and as resistors for electric furnaces.

3. Carbides have been used as reducing agents in metallurgy

and in electro-thermal work.

4. Aluminium carbide is used for the preparation of methane

whereas calcium carbide for acetylene.

5. Tungsten carbide is used for the manufacture of high speed

tools.

6. Boron carbide is being used for

(i) cutting diamonds

(ii) for drilling holes in rocks

(iii) for making electrodes of electric furnaces and

(iv) for making lamp filaments.

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