p-block elements (xith)einsteinclasses.com/p-block_xi.pdffigure : similarly in structure between (a)...

CPB – 1

Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road

New Delhi – 110 018, Ph. : 9312629035, 8527112111

p-BLOCK ELEMENTS (XIth)

C1A The Boron Family :

(i) Boron is mainly occurs in orthoboric acid (H3BO

3), borax Na

2B

4O

7·10H

2O and kernite

(Na2B

4O

7 · 4H

2O). Bauxite (Al

2O

3·2H

2O) and cryolite (Na

3AlF

6) are the important minerals of

aluminium.

(ii) Atomic radius of Ga is less than that of Al.

(iii) The observed discontinuity in the ionisation enthalpy values between Al and Ga and between Inand Tl are due to inability of d- and f-electrons, which have low screening effect, to compensatethe increase in nuclear charge.

(iv) Chemical property of Boron family :

(a) The relative stability of +1 oxidation state in the order of Al < Ga < In < Tl

(b) Electron deficient molecule act as a Lewis acids. This behaviour decreases down thegroup as size increases.

(c) Reactivity in air is

)s(OE2)g(O3)s(E2 322

)s(EN2)g(N)s(E2 2

(E - elements)

(d) Boron does not react with acids and bases. Aluminium reacts with acids and bases asfollows :

)g(H3)aq(Cl6)aq(Al2)aq(HCl6)s(Al2 23

)g(H3(aq)][Al(OH)2NaO(l)6H2NaOH(aq)2Al(s) 2II)aluminate(xytetrahydrosodium

42

(e) Reaction with halogen as follows :

)s(EX2)s(X3)s(E2 32

(X = F, Cl, Br, I)

C1B Compound of Boron :

(a) Borax dissolves in water to give an alkaline solution

acidorthoboric332

Borax742 BOH4NaOH2OH7OBNa

(b) When borax is heated it convert into B2O

3 as follows :

anhydrideBoric

322

metaboratesodium

7422742 OBNaBO2OBNaOH10·OBNa

(c) Orthoboric acid can be prepare by :

acidboricortho32742 )OH(B4NaCl2OH5HCl2OBNa

(d) H3BO

3 heated to give B

2O

3

32233 OBHBOBOH

(e) Diborane can be prepared by :

(i) 36243 AlF3LiF3HB2LiAlH3BF4

CPB – 2


New Delhi – 110 018, Ph. : 9312629035, 8527112111

(ii) 26224 HNaI2HBINaBH2

(iii) NaF6HBNaH6BF2 62k450

3

(f) Some of reaction of Boron are :

(i) 232262 H6)aq(OBO3HB

(ii) 23262 H6)aq()OH(B2)(OH6)g(HB l

(iii) 33362 NMe·B2NMe2HB

(iv) CO·BH2CO2HB 362

(v) 2borazine

6334233362 H12HNB2]BH[])NH(BH[3NH6HB3

(g) Borazine is inorganic benzene.

(h) The structure of diborane is :

(i) Borohydrides can be prepare by B2H

6 as :

]BH[M2HBMH2 462 [M = Li or Na]

Detailed explanation of compound of Boron :

Boron Hydrides

The boron hydrides are sometimes called boranes by analogy with the alkanes. They fall into two series :

1. BnH

(n + 4) (called nido-boranes)

2. A less stable series BnH

(n + 6) (called arachno-boranes)

Use of Diborane (B2H

6)

It is used to prepare the higher boranes, and is an important reagent in synthetic organic chemistry. Diboraneis used as a powerful electrophilic reducing agent for certain functional groups.

R — C N RCH2NH

2

R — NO2 RNH

2

R — CHO RCH2OH

Preparation

1.diborane

62heat

104

AcidPhosphoricOrtho

43

boridemagnesium

23 HBHBmainlyboranesofmixturePOHBMg

2. B2O

3 + 3H

2 + 2Al C150,atmosphere750 0

B2H

6 + Al

2O

3

3. NaF6HBNaH6BF2gas

62C180

gas3

0

There are several convenient laboratory preparations :

a. Reducing the etherate complexes of the boron halides with Li[AlH4].

4[Et2O . BF

3] + 3Li[AlH

4] ether

2B2H

6 + 3Li[AlF

4] + 4Et

2O

CPB – 3


New Delhi – 110 018, Ph. : 9312629035, 8527112111

b. Reacting Na[BH4] and iodine in the solvent diglyme. Diglyme is a polyether CH

3OCH

2CH

2OCH

2CH

2OCH

3.

2Na[BH4] + I

2 solutiondiglymein

B2H

6 + H

2 + 2NaI

c. Reducing BF3 with Na[BH

4] diglymein

2B2H

6 + 3Na[BF

4] + 4Et

2O

Method (c) is particularly useful when diborane is required as a reaction intermediary. It is produced in situ,and used without the need to isolate or purify it.

Properties

Diborane is a colourless gas, and must be handled with care as it is highly reactive. It catches firespontaneously in air and explodes with dioxygen. The heat of combustion is very high. In the laboratory itis handled in a vacuum frame. Since it reacts with the grease used to lubricate taps, special taps must beused. it is instantly hydrolysed by water, or aqueous alkali. At red heat the boranes decompose to boron andhydrogen.

B2H

6 + 3O

2 2B

2O

3 + 3H

2O H = –2165 kJ mol–1

B2H

6 + 6H

2O 2H

3BO

3 + 3H

2

Reactions of the Boranes :

Hydroboration

A very important reaction occurs between B2H

6 (or BF

3 + NaBH

4) and alkenes and alkynes.

½B2H

6 + 3RCH = CHR B(CH

2 – CH

2R)

3

½B2H

6 + 3RC CR B(RC = CHR)

3

The reactions are carried out in dry ether under an atmosphere of dinitrogen because B2H

6 and the products

are very reactive. The alkylborane products BR3 are not usually isolated. They may be converted as

follows :

1. to hydrocarbons by treatment with carboxylic acids,

2. to alcohols by reaction with alkaline H2O

2, or

3. to either ketones or carboxylic acids by oxidation with chromic acid (H2CrO

4).

The complete process is called hydroboration, and results in cis-hydrogenation, or cis-hydration. Where theorganic molecule is not symmetrical, the reaction follows the anti-Markovnikov rule, that is B attaches tothe least substituted C atom.

BR3 + 3CH

3COOH 3RH + B(CH

3COO)

3

hydrocarbon

B(CH2 · CH

2R)

3 + H

2O

2 3RCH

2CH

2OH + H

3BO

3

42CrOH

(CH3 · CH

2)

3 – B 42CrOH CH

3COOH

carboxylic acid

(CH3 · CH

2)

3 – B + CO diglyme [(CH

3 · CH

2)

3 – CBO]

2 22OH [CH

3 · CH

2]

3 – COH

Hydroboration is a simple and useful process for two main reasons :

1. The mild condition required for the initial hybride addition.

2. The variety of products which can be produced using different reagents to breakthe B – C bond.

H.C. Brown won the Nobel Prize for Chemistry in 1979 for work on these organoboron compounds.

Reaction with ammonia

All the boranes act as Lewis acids and can accept electron pairs. Thus they react with amines, formingsimple adducts. They also react with ammonia, but the products depend on the conditions :

CPB – 4


New Delhi – 110 018, Ph. : 9312629035, 8527112111

B2H

6 + 2(Me)

3N 2[Me

3N · BH

3]

362

NHexcess

etemperaturlow362 NH2·HBNHHB3

nitrideboron)BN( x

NHexcess

etemperaturhigh

3

borazineHNB 633

HB1:NH2ratio

etemperaturhigher

623

The compound B2H

6 · 2NH

3 is ionic, and comprises [H

3N BH

2 NH

3]+ and [BH

4]– ions. On heating, it

forms borazine.

Boron nitride is a white slippery solid. One B atom and one N atom together have the same number ofvalency electrons as two C atoms. Thus boron nitride has almost the same structure as graphite, with sheetsmade up of hexagonal rings of alternate B and N atoms joined together. The sheets are stacked one on topof the other, giving a layer structure. (see figure)

Figure : Similarly in structure between (a) boron nitride and graphite, (b) borazine and benzene.

Borazine B3N

3H

6 is sometimes called ‘inorganic benzene’ because its structure shows some formal

similarity with benzene, with delocalized electrons and aromatic character. Their physical properties arealso similar.

Borazine and substituted borazines are now made :

633]BH[Na

)Me(HNB

MeMgBr

3333C140

43 HNBClHNBClNH3BCl3 4

3333

0

Borazine forms complexes such as B3N

3H

6 – Cr(CO)

3 with transition metal compounds. Borazine is

considerably more reactive than benzene, and addition reactions occur quite readily :

B3N

3H

6 + 3HCl B

3N

3H

9Cl

3

CPB – 5


New Delhi – 110 018, Ph. : 9312629035, 8527112111

If heated with water, borazine hydrolyses slowly.

B3N

3H

6 + 9H

2O 3NH

3 + 3H

3BO

3 + 3H

2

Some other reactions of boranes

B2H

6 + 6H

2O 2B(OH)

3 + 6H

2

2H3BO

3 + 6H

2

B2H

6 + 6MeOH 2B(OMe)

3 + 6H

2

B2H

6 + 2Et

2S 2[Et

2S BH

3]

B2H

6 + 2LiH 2Li [BH

4]

2B2H

6 + 2Na Na[BH

4] + Na[B

3H

8] (slow)

B2H

6 + HCl B

2H

5Cl + H

2

B2H

6 + 3Cl

2 2BCl

3 + 6HCl

Structures of the Borane

The bonding and structures of the boranes are of great interest. They are different from all other hydrides.There are not enough valency electrons to form conventional two-electron bonds between all of the adja-cent pairs of atoms, and so these compounds are termed electron dificient.

Figure : The structure of diborane

In diborane there are 12 valency electrons, three from each B atom and six from the H atoms.

The two bridging H atoms are in a plane perpendicular to the rest of the molecule and prevent rotationbetween the two B atoms. Specific heat measurement confirm that rotation is hindered. Four of the H atomsare in a different environment from the other two. This is confirmed by Raman spectra and by the fact thatdiborane cannot be methylated beyond Me

4B

2H

2 without breaking the molecule into BMe

3.

The terminal B — H distances are the same as the bond lengths measured in non-electron-deficientcompounds. These are assumed to be normal covalent bonds, with two electrons shared between twoatoms. We can describe these bonds as two-centre two-electron bonds (2c-2e).

Thus the electron deficiency must be associated with the bridge groups. The nature of the bonds in thehydrogen bridges is now well established. Obviously they are abnormal bonds as the two bridges involveonly one electron from each boron atom and one from each hydrogen atom, making a total of four electrons.An sp3 hybrid orbital from each boron atom overlaps with the 1s orbital of the hydrogen. This gives adelocalized molecular orbital covering all three nuclei, containing one pair of electrons and making up oneof the bridges. This is a three-centre two-electron bond (3c-2e). A second three-centre bond is also formed.

CPB – 6


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Figure : Overlap of approximately sp2 hybrid orbitals from B with an s orbital from H to give a ‘banana-shaped’ three-centre two-electron bond.

ORGANOMETALLIC COMPOUNDS

All the Group 13 trihalides will react with Grignard reagents and organolithium reagents, forming trialkylor triaryl compounds.

BF3 + 3C

2H

5MgI B(C

2H

5)

3

AlCl3 + 3CH

3MgI Al(CH

3)

3

The aluminium compounds are unusual because they have dimeric structures, and appear to havethree-centre bonds involving sp3 hybrid orbitals on Al and C in Al–C–Al bridges (see in figure).

Another important route to organoaluminium compounds is from aluminium metal and H2. The two

elements do not react directly to give AlH3. However, aluminium does take up hydrogen in the presence of

aluminium alkyl catalyst (Ziegler catalysts).

Figure : Structure of aluminium trimethyl dimer

Al + 3/2H2 + 2Et

3Al 3Et

2AlH

Alkenes may be added to Al—H bonds.

Et2AlH + H

2C = CH

2 Et

2Al — CH

2 — CH

3i.e. Et

3Al

ethene

Aluminium alkyls catalyse the dimerization of propene in the formation of isoprene :

2

CH|

223AlR

2propene

3 CHC.CH.CH.CHCHCH.CH2

3

3

42

CH|

2crack CHCHC.CHCH

isoprene3

Boric Acid

Preparation

1. Na2B

4O

7 + H

2SO

4 + 5H

2O Na

2SO

4 + 4H

3BO

3

2. Na2B

4O

7 + 2HCl + 5H

2O 2NaCl + 4H

3BO

3

CPB – 7


New Delhi – 110 018, Ph. : 9312629035, 8527112111

3. Mainly from native calcium borate, colemanite. Sulphur dioxide is passed through powdred colemanitesuspension in boiling water; calcium sulphite and basic acid are formed - the former dissolves in excess ofsulphur dioxide forming calcium bisulphite and the boric acid crystallises out on cooling.

2CaO.3B2O

3 + 2SO

2 + 9H

2O 2CaSO

3 + 6H

3BO

3

2CaSO3 + 2H

2O + 2SO

2 2Ca (HSO

3)

2

Properties :

1. B2O

3 is made conveniently by dehydrating boric acid :

esesquioxidboron

32heatred

acidmetaboric

2C100

acidorthoboric

33 OBHBOBOH0

B2O

3 is a typical non-metallic oxide and is acidic in its properties. It is the anhydride of orthoboric acid, and

it reacts with basic (metallic) oxides, forming salts called borates or metaborates, for example :CoO + B

2O

3 Co(BO

2)

2

cobalt metaborate (blue colour).

However, it is possible to formce B2O

3 to behave as a basic oxide by reacting with very strongly scidic

compounds. Thus with P2O

5 boron phosphate is formed.

B2O

3 + P

2O

5 2BPO

4

2. Orthoboric acid H3BO

3 is soluble in water, and behaves as a weak monobasic acid. It does not donate

protons like most acids, but rather it accepts OH—. It is therefore a Lewis acid, and is better written asB(OH)

3.

B(OH)3 + 2H

2O H

3O+ + [B(OH)

4]— pK = 9.25

[H3BO

3]

Polymeric metaborate species are formed at higher concentrations, for example :

3B(OH)3 H

3O+ + [B

3O

3(OH)

4]— + H

2O pK = 6.84

[3H3BO

3]

3. Acidic properties of H3BO

3 or B(OH)

3

Since B(OH)3 only partially reacts with water to form H

3O+ and [B(OH)

4]—, it behaves as a weak acid. Thus

H3BO

3 or (B(OH)

3) cannot be titrated satisfactory with NaOH, as a sharp end point is not obtained. If

certain organic polyhydroxy compounds such as glycerol, mannitol or sugars are added to the titrationmixture, then B(OH)

3 behaves as a strong monobasic acid. It can now be titrated with NaOH, and the end

point is detected using phenolphtalien as indicator (indicator changes at pH 8.3-10.0)

CPB – 8


New Delhi – 110 018, Ph. : 9312629035, 8527112111

B(OH)3 + NaOH Na[B(OH)

4]

NaBO2 + 2H

2O

sodium metaborate

The added compound must be a cis-diol, to enhance the acidic properties in this way. (This mean that it hasOH group on adjacent carbon atoms in the cis configuration). The cis-diol forms very stable complexeswith the [B(OH)

4]— formed by the forward reaction above, thus effectively removing it from solution. The

reaction is reversible. Thus removal of one of the product at the right hand side of the equation upsets thebalance, and the reaction proceeds completely to the right. Thus all the B(OH)

3 reacts with NaOH: in effect

it acts as a strong acid in the presence of the cis-diol.

Structures of boric acid :

Thus orthoboric acid contains triangular BO33— units. In the solid the B(OH)

3 units are hydrogen bonded

together into two-dimensional sheets with almost hexagonal symmetry. The layers are quite a large distanceapart (3.18 Å) and thus the crystal breaks quite easily into very fine particles.

Figure : Hydrogen bonded structure of orthoboric acid

Borax

The most common metaborate is borax Na2[B

4O

5(OH)

4] · 8H

2O. Borax is usually written as Na

2B

4O

7 ·

10H2O. It is actually made from two tetrahedra and two triangular units joined as shown in figure and

should be written Na2[B

4O

5(OH)

4] · 8H

2O.

CPB – 9


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Preparation :

1. Orthoboric acid, H3BO

3 on neutralization with Na

2CO

3 gives borax.

Na2CO

3 + 4H

3BO

3 Na

2B

4O

7 + 6H

2O + CO

2

2. Colemanite, Ca2B

6O

11 is converted into borax by boiling it with concentrated solution of Na

2CO

3.

metaboratesodium

23borax

742321162 NaBO2CaCO2OBNaCONa2OBCa

Properties :

1. The solution of borax is alkaline in nature and hence it is a useful primary standard for titrating againstacids.

(Na2[B

4O

5(OH)

4] · 8H

2O) + 2HCl 2NaCl + 4H

3BO

3 + 5H

2O

One of the products H3BO

3 is itself a weak acid. Thus the indicator used to detect the end point of this

reaction must be one that is unaffected by H3BO

3. Methyl orange is normally used, which changes in the pH

range 3.1–4.4.

One mole of borax reacts with two moles of acid. This is because when borax is dissolved in water bothB(OH)

3 and [B(OH)

4]— are formed, but only the [B(OH)

4]— reacts with HCl.

[B4O

5(OH)

4]2— + 5H

2O 2B(OH)

3 + 2[B(OH)

4]—

2[B(OH)4]— + 2H

3O+ 2B(OH)

3 + 4H

2O

Borax is also used as a buffer since its aqueous solution contains equal amount of weak acid and its salt.

2. Borax Bead Test

In the borax bead test, B2O

3 or borax Na

2[B

4O

5(OH)

4] . 8H

2O is heated in a Bunsen burner flame with metal

oxides on a loop of platinum wire. The mixture fuses to give a glass like metaborate bead. Metaboratebeads of many transition metals have characteristic colours, and so this reaction provides a means ofidentifying the metal. This simple test provided the first proof that vitamin B

12 contained cobalt.

A cupric salt forms blue cupric metaborate in the oxidising flame :

Na2B

4O

7 + CuO Cu(BO)

2 + 2NaBO

2

In the reducing flame. (i.e. in presence of carbon) the coloured salt is reduced to colourless cuprousmetaborate :

2Cu(BO2)

2 + 2NaBO

2 + C 2CuBO

2 + Na

2B

4O

7 + CO. and to metallic copper and hence bead becomes

dull red and opaque.

2Cu(BO2) + 4NaBO

2 + 2C 2Cu + 2Na

2B

4O

7 + 2CO

Compounds of Colour of borax bead

Oxidising flame Reducing flame

Chromium Green Green

Maganese Amethyst Colourless

Iron Yellow (cold) Bottle green

CPB – 10


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Cobalt Deep blue Deep blue

Nickel Browinish (cold) Grey

Copper Green(hot), blue (cold) Colourless or red

Sodium peroxoborate

Preparation :

1. Electrolysis of a solution of sodium borate (containing some Na2CO

3).

2. By oxidizing boric acid or sodium metaborate with hydrogen peroxide.

2NaBO2 + 2H

2O

2 + 6H

2O Na

2[(OH)

2B(O – O)

2B(OH)

2] · 6H

2O

sodium metaborate sodium peroxoborate

Sodium peroxoborate is used as a brightener in washing powders. In very hot water (over 800C) theperoxide linkages O—O break down to give H

2O

2.

Isopolyacids of B, Si and P

Other elements form polymeric compounds similar to the borates; notably Si forms silicates and P formsphosphates. These polymeric compounds are called isopolyacids.

There is a certain difference between the structure of borates and phosphates (silicates). The structure ofborates often based on triangular BO

3 units and some times based on tetrahedral BO

4 units. But the

structure of phosphates and silicates are always based on tetrahedral PO4 and SiO

4 units.

The structure of phosphates and silicates are more stable than borates and do not break-up in solution.

Qualitative analysis of boron compounds

When borates are treated with HF (or with concentrated H2SO

4 and CaF

2) the volatile compound BF

3 is

formed. If the BF3 gas produced is introduced into a flame (for example a Bunsen flame) the flame gives a

characteristic green coloration.

conc. H2SO

4 + CaF

2 2HF + CaSO

4

H3BO

3 + 3HF 2BF

3 + 3H

2O

An alternative test is to make the ester methyl borate B(OCH3)

3. The suspected borate sample is mixed with

concentrated H2SO

4 to form H

3BO

3, and warmed with methyl alcohol in a small evaporating basin.

B(OH)3 + 3CH

3OH B(OCH

3)

3 + 3H

2O

The concentrated H2SO

4 removes the water formed. The mixture is then set on fire. Methyl borate is

volatile, and colours the flame green.

Fluoboric acid

H3BO

3 dissolves in aqueous HF, forming fluoboric acid HBF

4.

H3BO

3 + 4HF H+ + [BF

4]— + 3H

2O

Fluoboric acid is a strong acid. The [BF4]— ion is tetrahedral, and fluoborates resemble perchlorates ClO

4—

and sulphates in crystal structure and solubility (KClO4 and KBF

4 are both not very soluble in water).

Trihalides of Boron

The boron halides are covalent. BF3 is gaseous, BCl

3 liquid and BI

3 is solid. BF

3 is covalant but AlF

3, GaF

3,

InF3, TlF

3 are ionic. The other halides are largely covalent of Al, Ga, In, Tl when anhydrous.

Preparation :

1. B2O

3 + 3CaF

2 + conc. 3H

2SO

4 heat

2BF3 + 3CaSO

4 + 3H

2O

2. B2O

3 + 6NH

4BF

4 heat

8BF3 + 6NH

3 + 3H

2O

CPB – 11


New Delhi – 110 018, Ph. : 9312629035, 8527112111

3. B2O

3 + 6HF + 3H

2SO

4 2BF

3 + 3H

2SO

4 · H

2O

4. OHNaHO2BF4])BF(ONa[HF12])OH(OB[Na 243SOH2

432OH

4542422

Properties :

1. BF3 is very useful for promoting certain organic reactions, e.g. :

Friedel – Crafts reactions such as alkylations and acylations. In these the BF3 is used up in the reaction, and

so is not strictly catalytic.

C6H

6 + C

2H

5F + BF

3 C

6H

5 · C

2H

5 + H+ + [BF

4]—

2. The boron halides are all hydrolysed by water. BF3 hydrolyses incompletely and forms fluoborates. This is

because the HF first formed reacts with the H3BO

3.

4BF3 + 12H

2O 4H

3BO

3 + 12HF

12HF + 3H3BO

3 3H+ + 3[BF

4]— + 9H

2O

4BF3 + 3H

2O H

3BO

3 + 3H+ + 3[BF

4]—

The other halides hydrolyse completely, giving boric acid.

BCl3 + 3H

2O H

3BO

3 + 3HCl

3. BX3 is electron-deficient (octet of B incomplete) and thus behaves as a Lewis acid.

Structure of BF3

The shape of the BF3 molecule is a planar triangle with bond angles of 1200. The bond lengths in BF

3 are

1.30 Å. The bond energy is very high, which is higher than for any single bond. The shortness and strengthof the bonds is interpreted in terms of a p – p interaction, that is the bonds possess some double bondcharacter.

C1C Compounds of Aluminium :

Alumina (Aluminium oxide)

Alumina Al2O

3 exists principally in two crystalline forms called -Al

2O

3 or corundum, and -Al

2O

3, and in

addition there is a fibrous form.

Preparation :

1. -Al2O

3 is made by dehydrating Al(OH)

3 below 4500C,

32C1000,

32C450

3 OAlOAl)OH(Al00

2. In the lab, it is prepared by igniting aluminium hydroxide, aluminium sulphate or ammonium alum.

2Al(OH)3 Al

2O

3 + 3H

2O

Al2(SO

4)

3 Al

2O

3 + 3SO

3

(NH4)

2SO

4.Al

2(SO

4)

3.24H

2O 2NH

3 + Al

2O

3 + 4SO

3 + 25H

2O

3. Aluminium has a very strong affinity for oxygen. Al may be used in the thermite reduction of less stablemetal oxides. 3Mn

3O

4 + 8Al 4Al

2O

3 + 9Mn

Properties :

1. Corundum is unaffected by acids. The crystal structure of corundum is hexagonally close-packed oxygenatoms, with two thirds of the octahedral holes filled by Al3+ ions.

2. In contrast to -Al2O

3, -Al

2O

3 dissolves in acids, absorbs water, and is used for chromatography. It is

amphoteric in nature.

CPB – 12


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Al2O

3 + 6HCl 2AlCl

3 + 3H

2O

Al2O

3 + 2NaOH 2NaAlO

2 + H

2O

3. Alumina is white, but it can be coloured by the addition of Cr2O

3 or Fe

2O

3.

Aluminium Chloride

Preparation :

2Al + 6HCl 2AlCl3 + 3H

2

Properties :

1. Anhydrous AlCl3 (and to a lesser extent AlBr

3) is used as the ‘catalyst’ in a variety of Friedel-Crafts type of

reactions for alkylations and acylations.

C6H

5 · H + CH

3CH

2Cl + AlCl

3 C

6H

5 · CH

2CH

3 + H+ + [AlCl

4]—

This is not true ‘catalytic’ action, as the AlCl3 is used up, and the formation of [AlCl

4]— or [AlBr

4]— is an

essential part of the reaction. Acylations are similar :

C6H

5 · H + RCOCl + AlCl

3 RCOC

6H

5 + H+ + [AlCl

4]—

AlCl3 is also used to catalyse the reaction to make ethyl bromide (which is used to make the petrol additive

PbEt4).

CH2 = CH

2 + HBr C

2H

5Br

2. AlCl3 exists as dimer (Al

2Cl

6) in inert (non-polar) solvent as well as in vapour state.

However, when the halides dissolve in water, the high enthalpy of hydration is sufficient to break thecovalent dimer into [M · 6H

2O]3+ and 3X— ions. At low temperatures AlCl

3 exists as a close packed lattice

of Cl— with Al3+ occupying octahedral holes. On heating Al2Cl

6 species are formed.

3. Aqueous solution of AlCl3 is acidic due to hydrolysis :

AlCl3 + 3H

2O Al(OH)

3 + 3HCl

4. Al forms monohalides in the gas phase at elevated temperatures, e.g.

AlCl3AI2AlClhigh

etemperatur3

This compound is not very stable, and is covalent.

Alums

Alums are the double sulphates having general formula : X2SO

4.M

2(SO

4)

3.24H

2O

X = monovalent cation such as Na+, K+, NH4+ etc.

M = trivalent cation such as Al3+, Cr3+, Fe3+ etc.

when alum contains aluminium as trivalent cation then it is named after monovalent cation.

e.g. K2SO

4.Al

2(SO

4)

3.24H

2O potash alum

Na2SO

4.Al

2(SO

4)

3.24H

2O Soda alum.

When trivalent cation is not aluminium then alum is named after both, monovalent as well as trivalentcation.

(NH4)

2SO

4.Fe

2(SO

4)

3.24H

2O - ferric ammonium alum.

Preparation of Potash alum

It is prepared by boiling powdered alum stone, K2SO

4.Al

2(SO

4)

3.4Al(OH)

3 with dil sulphuric acid and

filtered. The filtrate is mixed with a requisite quantity of potassium sulphate and crystallized.

K2SO

4.Al

2(SO

4)

3.4Al(OH)

3 + 6H

2SO

4 K

2SO

4 + 3Al

2(SO

4)

3 + 12H

2O

K2SO

4 + Al

2(SO

4)

3 + 24H

2O 2[KAl(SO

4)

2 . 12H

2O]

CPB – 13


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Qualitative analysis of aluminium

In qualitative analysis Al(OH)3 is precipitated as a white gelatinous substance when NH

4OH is added to the

solution (after previously removing acid-insoluble sulphides with H2S). Fe(OH)

3, Cr(OH)

3 grey-green or

grey-blue. Zn(OH)2 is white, like Al(OH)

3, but it is not gelatinous. Zn(OH)

2 dissolves in excess NH

4OH,

whereas Al(OH)3 does not. A confirmatory test for aluminium is the formation of a red precipitate from

Al(OH)3 and the dye aluminon.

Amphoteric behaviour – aluminates

Al(OH)3 is amphoteric. It reacts principally as a base. It reacts with acids to form salts that contain the

[Al(H2O)

6]3+ ion. However, Al(OH)

3 shows some acidic properties when it dissolves in NaOH, forming

sodium aluminate.

Al(OH)3 NaOHexcess

NaAl(OH)4

sodium

NaAlO2 · 2H

2O aluminate

the structure of the aluminate ion changes with both pH and concentration :

1. Between pH 8 and 12 the ions polymerize using OH bridges and each aluminium is octahedrallycoordinated.

2. In dilute solutions above pH values of 13, a tetrahedral [Al(OH)4]— ion exists.

3. In concentrated solutions above 1.5 M and at pH values greater than 13 the ion exists as a dimer :[(HO)

3Al — O — Al(OH)

3]2–

Difference between Boron and the other Elements

Boron differs significantly from the other elements in Group 13, mainly because the atoms are very small.It is always covalent, and it is non-metallic. In addition, boron shows a diagonal relationship with sliliconin Group 14.

1. B2O

3 is an acidic oxide, like SiO

2. This is in contrast to Al

2O

3, which is amphoteric.

2. H3BO

3, which may be written B(OH)

3, is acidic, whilst Al(OH)

3 is amphoteric.

3. Simple borates and silicate ions can polymerize, forming isopolyacids. Aluminium forms no analogouscompounds.

4. The hydrides of B are gaseous, readily hydrolysed and spontaneously inflammable. In contrast aluminiumhydride is a polymeric solid (AlH

3)n. SiH

4 is gaseous, readily hydrolysed and inflammable.

5. Apart from BF3, the halides of B and Si hydrolyse readily and vigoruosly. The aluminium halides are only

partly hydrolysed in water.

Practice Problems :

1. Boron does not usually form a cation.

2. The polarity of B — X bonds is in the order : B — F > B — Cl > B — Br but Lewis acidicity shows thesequence : BF

3 < BCl

3 < BBr

3.

3. Discuss the pattern of variation in the oxidation states of (i) B to Tl (ii) C to Pb.

4. How can you explain higher stability of BCl3 as compared to TlCl

3 ?

5. Why does boron trifluoride bahave as a Lewis acid ?

6. Consider the compounds, BCl3 and CCl

4. How will they behave with water ? Justify.

7. Is boric acid a protic acid ? Explain.

8. Explain, what happens when boric acid is heated.

9. Describe the shapes of BF3 and [BF

4]–. Assign the hybridisation of boron in these species.

10. Write reactions to justify amphoteric nature of aluminium.

11. What are electron deficient compounds ? Are BCl3 and SiCl

4 electron deficient species ? Explain.

12. Suggest a reason why the B–F bond length in BF3 (130 pm) and BF

4– (143 pm) differ.

13. If B–Cl bond has a dipole moment, explain why BCl3 molecule has zero dipole moment.

CPB – 14


New Delhi – 110 018, Ph. : 9312629035, 8527112111

14. Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminiumtrifluoride precipitates out of the resulting solution when gaseous BF

3 is bubbled through. Give

reasons.

15. What happens when

(a) Borax is heated strongly, (b) Boric acid is added to water,

(c) Aluminium is treated with dilute NaOH, (d) BF3 is reacted with ammonia ?

16. Explain the following reactions

(a) Silicon is heated with methyl chloride at high temperature in the presence of copper.

(b) Silicon dioxide is treated with hydrogen fluoride,

(c) CO is heated with zinc oxide,

(d) Hydrated alumina is treated with aqueous NaOH solution.

17. Give reasons :

(i) Conc. HNO3 can be transported in aluminium container.

(ii) A mixture of dilute NaOH and aluminium pieces is used to open drain.

(iii) Graphite is used as a lubricant.

(iv) Diamond is used as an obrasive.

(v) Aluminium alloys are used to make aircraft body.

(vi) Aluminium utensils should not be kept in water overnight.

(vii) Aluminium wire is used to make transmission cables.

18. A certain salt X gives the following results.

(i) Its aqueous solution is alkaline to litmus.

(ii) It swells up to a glassy material Y on strong heating.

(iii) When conc. H2SO

4 is added to a hot solution X of white crystal of an acid Z separates out.

Write equations for all the above reactions and identify X, Y and Z.

19. Write balanced equations for :

(i) LiHBF3 (ii) OHHB 262

(iii) 62HBNaH (iv)

33BOH

(v) NaOHAl (vi) 362 NHHB

20. Boric acid is polymeric due to

(a) its acidic nature (b) the presence of hydrogen bonds

(c) its monobasic nature (d) its geometry

21. The type of hybridisation of boron in diborane is

(a) sp (b) sp2 (c) sp3 (d) dsp2

22. If the starting material for the manufacture of silicones is RSiCl3, write the structure of product

formed.

[Answers : (1) B has (IE)1 = 801 kJ mol–1, (IE)

2 = 2419 kJ mol–1, (IE)

3 = 3646 kJ mol–1. Thus the total

energy required to give B3+ ions is far more than that which would be compensated by latticeenergies of ionic compounds or hydration of such ions in solution. Thus formation of cation (like B3+)is not possible (2) With increase in polarity of B — X bond, acidity also increases and should be thusin the order : BF

3 > BCl

3 > BBr

3. But Lewis acidity is in reverse order : BF

3 < BCl

3 < BBr

3. There is

lateral overlap of the vacant 2p orbital of B with one completely filled orbital of F leading to p – pbonds between B and F. This B — F bond thus acquires double bond character. This also leads tocompensate electron deficiency of boron and thus Lewis acid character of BF

3 is reduced. This

p - p bonding decreases going from BF3 to BBr

3 and thus Lewis acidic nature increases in the

CPB – 15


New Delhi – 110 018, Ph. : 9312629035, 8527112111

order : BF3 < BCl

3 < BBr

3

(3) (i) The first two elements e.g., B and Al exhibit an oxidation state of +3 because of the presence oftwo electrons in s- and one electron in the p-orbital of valence shell. On the other hand, all the otherelements from Ga and Tl contain only d-and f-electrons and hence, exhibit oxidation states of +1 and+3 due to inert pair effect. (ii) The first two elements e.g., C and Si exhibit an oxidation state of +4because of the presence of two electrons in s- and two electrons in the p-orbital of valence shell. Onthe other hand, all the other elements from Ge to Pb contain d- or d- and f-electrons and henceexhibit two oxidation states +2 and +4 due to inert pair effect. (4) Because of the absence of d- andf- electrons in B, all the three valence electrons e.g., two 2s and one 2p, take part in the bondformation, showing an oxidation state of +3 in the formation of BCl

3. On the other hand, because of

the poor shielding of the s- electrons of the valence shell by the 3d-, 4d- and 4f- electrons, inert paireffect is maximum in Tl. As a result, only 6p1 electron take part in the bond formation and hence, themost stable state of Tl is +1 rather than +3. Thus, BCl

3 is more stable than TlCl

3. (5) Boron trifluoride

is an electron deficient species (6) BCl3(s) + 3H

2O(l) H

3BO

3(s) + 3HCl(l). On the other hand,

carbon tetrachloride is an electron rich species, hence, it cannot accept any lone pair of electronsfrom H

2O and hence, does not undergo hydrolysis. (7) No, boric acid is not a protic acid, as it does

not ionise water to give a proton (H+). On the other hand, boric acid accepts a lone pair of electronto

act as Lewis acid : (8) Boric acid, when

heated, loses water at three different stages at different temperature to tive borontrioxide.

(9) sp2 and sp3 respectively

(10) It reacts, both with acids and alkalines to evolve dihydrogen, hence, it is amphoteric in nature.

)(OH3)aq()SO(Al)aq(SOH3)s(Al2 234242 l ,

(III)minatehydroxoalutetraSodium242 O(g)3H(aq)][Al(OH)2NaO(l)6H2NaOH(aq)2Al(s) (11) Species in which the

central atom either does not have eight electrons in the valency shell or those which have eightelectrons in the valency shell but can expand their covalency beyond four due to the presence ofd-orbitals are called electron deficient compound. Example : (i) In BCl

3, the central boron atom has

only six electrons, therefore, it is an electron deficient compound. (ii) In SiCl4, the central silicon

atom has 8 electrons it can expand its covalency beyond 4 due to the presence of vacant d-orbitals.Therefore, according to the definition, SiCl

4 should also be taken as electron deficient compound

but, in fact, it does not accept two more Cl– ions to form [SiCl6]2–. Hence, it is not an electron deficient

molecule. (12) In BF3, boron is sp2-hybridised and hence it is a polar molecule. It has an empty

2p- orbital. F-atom also have lone pair of electrons in 2p-orbitals. Because of similar sizes, p-pback bonding takes place in which free lone pair of fluorine is transferred to boron. As a result ofthis back bonding, B – F bond acquires some double bond character. On the other hand in [BF

4]– ion,

B is sp3 hybridized and does not have empty p-orbital to accept the electron donated by F atom.Consequently in [BF

4]–, B – F is purely single bond. We know that double bonds are shorter than

single bonds, and hence, bond length in BF3 is shorter (130 pm) than, B – F bond length (143 pm) in

[BF4]–. (14) (a) Aluminium trifluoride is insoluble in anhydrous HF, because HF is strongly H-bonded

covalent compound and it does not give F– ions to dissolve AlF3. On the other hand, KF is an ionic

compound, gives F– ions which combines with AlF3 to give soluble complex.

CPB – 16


New Delhi – 110 018, Ph. : 9312629035, 8527112111

(III)aluminatehexafluorosodiumSoluble

633 ][AlFNaAlF3NaF (b) Because of smaller size and higher electro-negativity of B,

it has much higher tendency to form complexes than aluminium, therefore, when BF3 is bubbled

through soluble complex of aluminium, AlF3 gets precipitate with the formation of soluble complex

of boron. (s)AlF]3Na[BF3BF][AlFNa 3

(III)oboratetetrafluorsodium.Soluble

4363 . (15) (a) When borax is heated strongly,,

a transparent bead which consists of sodium metaborate and boric anhydride is formed.

beadglassyttransparen

rideBoricanhydmetaborate.Sod3222742

Borax2742 OBNaBO2OH10OBNaOH10.OBNa

(b) Boric acid acts as Lewis

acid by accepting hydroxyl ion of water and releasing a proton into the solution. HOH + B(OH)3

[B(OH)4]– + H+ (c) Sodium tetrahydroxoaluminate (III) is formed alongwith the evolution of

dihydrogen. 2Al(s) + 2NaOH(aq) + 6H2O(l) 2Na+[Al(OH)

4]–(aq) + 3H

2(g) (d) BF

3 being a Lewis

acid accpets a pair of electrons from NH3 to form a complex. 3

Complexbase'LewisacidLewis333 NHBFNH:BF .

(16) (a) When silicon is heated with methyl chloride at high temperature in the presence of copper,

dimethylchlorosilane is formed : lorosilaneDimethylch

223K570

powderCu3 SiCl)CH(SiClCH2 (b) When SiO

2 reacts with

HF, silicon tetrafluoride is formed. This dissolves in excess of HF to give hydrofluorosilicic acid.

acidosilicicHydrofluor624242 SiFHHF2SiF,OH2SiFHF4SiO (c) When C is heated with ZnO,

ZnO is reduced to zinc metal. ZnO + C Zn + CO. (d) When hydrated alumina is treated with

aqueous NaOH solution, soluble metaaluminate is formed.

O3H2NaAlO2NaOH(aq)O(s).2HOAl 2atemetaaluminSod.

2heat

232 or

(III)exoaluminattetrahydroSod.4

heat2232 ](aq)2Na[Al(OH)O(l)H2NaOH(aq)O(l).2HOAl (17) (i) Conc. HNO

3 reacts

with aluminium to give a very thin film of aluminium oxide, which protects it form further action.

)O(3H(g)6NO(s)OAl(conc.)6HNO2Al(s) 22oxideAluminium

323 l . Thus, aluminium becomes passive

and hence aluminium container can be used to transport conc. HNO3. (ii) Dilute NaOH reacts with

aluminium pieces to give dihydrogen. The dihydrogen has a high pressure which can be used to

open clogged drains. )g(H3)aq(NaAlO2)(OH22NaOH(aq)2Al(s) 222 l . (iii) Graphite has

a layered structure in which the layers are held together by weak van der Waal’s forces and hencecan made to slip one over another. Therefore, graphite can be used as a lubricant. (iv) Diamond isthe hardest material known, hence can be used as abrasive. (v) Aluminium alloys such as magneliumis light, tough and resitant to corrosion and hence are being used to make aircraft body. (vi) A thinfilm of Al

2O

3 is produced by Al in presence of water and oxygen. A small part of this film dissolve in

water to give Al3+ ions, which are injurious to health. (vii) On weight to weight basis, aluminiumconducts twice as copper. Therefore, it is used in transmission cables. (18) (ii)

materialglassy)y(322

2742heat

)x(2742

OBNaBO2

OH10OBNaOH10.OBNa

heat

(iii) OH5SONaBOH4SOHOH10.OBNa 242)Z(

3342)X(

2742 (19) (i) Diborane

62 LiF6HB

CPB – 17


New Delhi – 110 018, Ph. : 9312629035, 8527112111

(ii) acidOrthoboric

233 H6BOH2 (iii) eborohydridSodium

4 ]BH[Na2 (iv) acidMetaboric

22K370

33 OHHBOBOH

(v)

(III)minatehydroxoalutetraSodium

24 H3])OH(Al[Na2 (vi) 2BH3NH

3 (20) (b). Hint : Boric acid is polymeric due to the presence

of hydrogen bonds (21) c

(22) ]

C2A Carbon Family :

(i) Ionization energy slightly increases from Sn to Pb because the poor shieldering effect ofintervening d- and -f orbitals and increase in size of the atom.

(ii) Chemical property of boron family

(a) Tendency to show +2 oxidation state increases in the sequence Ge < Sn < Pb.

(b) The monooxides CO – Neutral, GeO – acidic, SnO, PbO – amphoteric

(c) The dioxides, CO2, SiO

2, GeO

2 – a acidic, SnO

2, PbO

2 – amphoteric

(d) Carbon silicon and germinium and lead are not affected by water, whereas tin forms

dioxide 222 H2SnOOH2Sn

(e) PbI4 does not exist because the reaction does not release enought energy to unpair 6s2

electrons.

(f) Because of thermal and chemical stability, GeX4 is more stable than GeX

2 whereas

PbX2 is more stable than PbX

4.

(g) CCl4 not hydrolysed because of unavailibity of d-orbitals.

(iii) Anomalous Behaviour of carbon :

(a) Anomalous behaviour of carbon due to its smaller size, higher electronegativity, higherionisation enthalpy and unavailability of d-orbitals.

(b) Carbon can form p-p multiple bonds with itself and with other atom of small size andhigh electronegativity for e.g. C = C, C = S, C = N.

(c) The tendency of carbon to link with one another through covalent bonds known ascatenation.

(d) The order of catenation is C > > Si > Ge Sn.

(iv) Allotropes of carbon :

(a) Carbon forms mainly 3 allotropes i.e. Diamond, Graphite and fullerenes.

(b) Diamond has a crystalline lattice and undergoes sp3 hybridisation. It is a hardestsubstance on earth and use to sharping hard tools, in making dyes etc.

(c) Graphite has layered structure and it undergoes sp2 hybridisation because electrons aremobile, therefore it conduct electricity. It is used as a dry lubricant in machine runningat high temperature.

CPB – 18


New Delhi – 110 018, Ph. : 9312629035, 8527112111

(d) Fullerene are made by the heating of graphite in an electric are in the presence of inertgases such as helium or argon. Fullerences are the only pure form of carbon becausethey have smooth structure without having ‘dangling’ bonds. All the carbon atoms areequal and they undergo sp2 hybridisation. It has ball type structured with 60 vertices.

(e) Thermodynamically graphite is most stable, therefore fH of graphite is taken as zero.

(v) Uses of carbon :

The composites are used in products such as tennis rackets, fishing rods etc. Being goodconductor, graphite is used for electrodes in batteries and industrial electrolysis. Carbon black isused as block pigment in black ink and as filler in automobile tyres. Diamond is a precious stoneand used in jewellery.

C2B Compounds of Carbon :

(a) Water gas or Synthesis gas : gaswater

2K1273473

2 )g(H)g(CO)g(OH)s(C

(b) Producer gas : gasproducer

2K1273

22 )g(N4)g(CO2)g(N4)g(O)s(C2

(c) CO is powerful reducing agent and reduces almost all metal oxides other than those of the alkaliand alkaline earth metals, aluminium and a few transition metals. For e.g.

)g(CO3)s(Fe2)g(CO3)s(OFe 232

)g(CO)s(Zn)g(CO)s(ZnO 2

(d) The highly poisonous nature of CO arises because of its ability to form a complex withhaemoglobin.

(e) CO2 prepared by :

)g(OH2)g(CO)g(O2)g(CH 2224

)(OH)g(CO)aq(CaCl)aq(HCl2)s(CaCO 2223 l

(f) CO2 is removed by photo-synthesis as :

OH6O6OHCOH12CO6 226126chlophyll

hv22

(g) CO2 can be obtained as a solid in the form of dry ice, used as a refrigerant for ice-cream and

frozen food.

(h) The resonance structures for CO2 are : :OCO::OCO::OCO:

..

..

......

..

Detailed explanation of compounds of carbon :

Carbides

Compounds of carbon and a less electronegative element are called carbides. This excludes compoundswith N, P, O, S and the halogens from this section.

Carbides are of three main types :

1. ionic or salt-like

2. interstitial or metallic

3. covalent

Salt-like carbides

It is covenient to group these depending on whether the structure contains C, C2 or C

3 anions. Aluminium

carbide Al4C

3 is a pale yellow solid formed by heating the elements in an electric furnace, the structure of

Al4C

3 is complex. It is misleading to formulate the structure as 4Al3+ and 3C4– as such a high charge

separation is unlikely. Both Be2C and Al

4C

3 are called methanides because they react with H

2O, yielding

methane.

CPB – 19


New Delhi – 110 018, Ph. : 9312629035, 8527112111

4Al + 3C Al4C

3, Al

4C

3 12H

2O 4Al(OH)

3 + 3CH

4

Carbides with a C2 unit are well known. By far the most important compound is CaC

2. This is made

commercially by strong heating lime and coke :

CaO + 3C CaC2 + CO H = +466 kJ mol–1

These carbides react exothermically with water, liberating ethyne (formerly called acetylene)., so they arecalled acetylides.

CaC2 + 2H

2O Ca(OH)

2 + HC CH

CaC2 is an important chemical intermediate and is used as an industrial scale to produce calcium

cyanamide. Cyanamide is used as a nitrogenous fertilizer, and to make urea and melamine.

CaC2 + N

2 C11000

Ca (NCN) + C

The acetylides have a NaCl type of lattice, with Ca2+ replacing Na+ and C22– replacing Cl—In CaC

2, SrC

2 and

BaC2 the elongated shape of the (C C)2– ions causes tetragonal distortion of the unit cell, that is it

elongates the unit cell in one direction.

Two carbides of magnesium Mg2C

3 contains a C

3 unit, and on hydrolysis with water it yields propyne

CH3 – C CH.

Mg2C

3+4H

2O 2Mg(OH)

2 + CH

3 – C CH

Interstitial Carbides

These are formed mostly by transition elements, and some of the lanthanides and actinides.

In these compounds, C atoms occupy octahedral holes in the close-packed metal lattice. Provided that thesize of the metal is greater than 1.35 Å, the octahedral holes are large enough to accomodate C atomswithout distorting the metal lattice. If all the octahedral holes are occupied the formula is MC. Interstitialcarbides are generally unreactive. They do not react with H

2O like ionic carbides. Most react slowly with

concentrated HF or HNO3.

Some metals, including Cr, Mn, Fe, Co and Ni, have radii below 1.35 Å: hence the metal lattice is distorted.Cementite Fe

3C is an important constituent of steel. These carbides are more reactive, and are hydrolysed

by dilute acids, and in some cases by water, giving a mixture of hydrocarbons and H2.

Fe3C + 2H

2O CH

4 + Fe

3O

2

Covalent carbides

SiC and B4C are the most important. Silicon carbide is hard infusible and chemically inert. It is widely used

as an abrasive called carborundum and produced annually by heating quartz or sand with an excess of cokein an electric furnace at 2000-25000C.

SiO2 + 2C Si + 2CO

Si + C SiC

SiC is very unreactive. It is unaffected by acids (except H3PO

4), but it does react with NaOH and air, and

with Cl2 at 1000C.

SiC + 2NaOH + 2O2 Na

2SiO

3 + CO

2 + H

2O

SiC + 2Cl2 SiCl

4

SiC is orten dark purple, black or dark green due to traces of Fe and other impurities, but pure sample arepale yellow to colourless SiC has a diamond like structure (hence also called artificial diamond). SiC has athree-dimensional structure of Si and C atoms, each atom tetrahedrally surrounded by four of the other kindof atoms.

Oxygen Compounds of Cabron

Carbon forms more oxides than the other elements, and these oxides differ from those of the other elementsbecause they contain p – p multiple bonds between C and O. Two of these oxides, CO and CO

2, are

extremely stable and important. Three are less stable : C3O

2, C

5O

2 and C

12O

9. Others which are even less

stable include graphite oxide, C2O and C

2O

3.

Carbon monoxide CO

CO is a colourless, odourless, poisonous gas. It is formed when C is burned in a limited amount of air. In thelaboratory it is prepared by dehydrating forming acid with concentrated H

2SO

4.

CPB – 20


New Delhi – 110 018, Ph. : 9312629035, 8527112111

H . COOH + H2SO

4 CO + H

2O

CO can be detected because it burns with a blue flame. It also reduces an aqueous PbCl2 solution to metallic

Pd, and when passed through a solution of I2O

5 it liberates I

2, i.e. it reduces I

2O

5 to I

2. The latter reaction is

used to estimate CO quantitatively. The I2 is titrated with Na

2S

2O

3.

PdCl2 + CO + H

2O Pd + CO

2 + 2HCl

5CO + I2O

5 5CO

2 + I

2

CO is toxic because it forms a complex with haemoglobin in the blood, and this complex is more stable thanoxy-haemoglobin. This prevents the haemoglobin in the red blood corpuscles from carrying oxygen roundthe body. This causes an oxygen deficiency, leading to unconsciousness and then death. CO is sparinglysoluble in water and is a neutral oxide. CO is an important fuel, because it evolves a considerable amountof heat when it burns in air.

2CO + O2 2CO

2H0 = –565kJ mol–1

The following are all important industrial fuels :

1. Water gas : an equimolecular mixture of CO and H2.

2. Producer gas : a mixture of CO and N2.

3. Coal gas : a mixture of CO, H2, CH

4 and CO

2 produced while distilling. Coal gas is used for cooking and

heating. It was known as town gas.

Water gas is made by blowing steam through red or white hot coke.

C + H2O heatred

CO + H2

(water gas) H0 = +131 kJ mol–1

S0 = +134 kJ mol–1

The water gas reaction is strongly endothermic (G = H – TS). Thus the coke cools down, and atintervals the steam must by turned off and air blown through to reheat the coke. It is particularly good fuel,i.e., it has a high calorific value, because both CO and H

2 burn and evolve heat.

CO is a good reducing agent, it can reduce many metal oxides to the metal.

Fe2O

3 + 3CO furnaceblast

2Fe + 3CO2

CuO + CO Cu + CO2

CO is an important ligand. It can donate an electron pair to many transition metals, forming carbonylcompounds.

Ni + 4CO C280 Ni(CO)

4

Fe + 5CO pressureunderC2000 Fe (CO)

5

2Fe(CO)5 photolysis Fe

2(CO)

9 + CO

CrCl6 + 3Fe(CO)

5 heat Cr(CO)

6 + 3FeCl

2 + 9CO

The bonding in CO may be represented as three electron pairs shared between the two atoms :C O:

It is better represented using the molecular orbital theory

0z

0y0

x2x2

z

2y2222

p2*

p2*,p2*,p2

,p2

,p2s2*,s2,s1*,s1

increasing energy

The carbon-metal bond in carbonyls may be represented as the donation of an electron pair from carbon tothe metal M C O. This original bond is weak. A stronger second bond is formed by back bonding,sometimes called dative bonding. This arises from sideways overlap of a full d

xy orbital on the metal with

the empty antibonding *2py orbital of the carbon, thus forming a M C bond. The total bonding is thus

M = C = O. The filling, or partial filling, of the antibonding orbital on C reduces the bond order of theC — O bond from the triple bond in CO towards a double bond. This is shown by the increase in C — Obond length from 1.128 Å in CO to about 1.15 Å in many carbonyls.

CPB – 21


New Delhi – 110 018, Ph. : 9312629035, 8527112111

CO is the most studied organometallic ligand. Because of the back bonding it is sometimes called a acceptor ligand. The drift of electron density from M to C makes the ligand more negative, which in turnenhances in donating power. Thus CO forms weak bonds to Lewis acids (electron pair acceptors) such asBF

3 as only bonding is involved. In contrast CO forms strong bonds to transition metals where both and

bonding can occur. Other acceptor ligands include CN—, RNC, and NO+. Comparing these ligands, thestrengths of the bonds are in the order CN— > RNC > CO > NO+, whilst their acceptor properties are inthe reverse order.

CO is a very versatile ligand. It may act as a bridging group between the two metal atoms, for example indi-iron ennea carbonyl Fe

2(CO)

9. CO may stabilize metal clusters by the C forming a multi-centre bond

with three metal atoms, and the * orbitals in CO may be involved in bonding to other metal atoms.

Carbon monoxide is quite reactive, and combines readily with O, S and the halogens, F,Cl and Br.

CO + ½O2 CO

2

CO + S COS carbonyl sulphide

CO + Cl2 COCl

2carbonyl chloride (phosgene)

The carbonyl halides are readily hydrolysed by water, and react with ammonia to form urea.

COCl2 + H

2O 2HCl + CO

2, HCl2OC)NH(NH2COCl 22

phasegas32

Carbonyl chloride is extremely toxic, and used as a poisonous gas in World War I.Nowadays it is produced in quite large quantities to make tolylene diisocyanate which is an intermediate inthe manufacture of polyurethane plastics.

Carbon dioxide CO2

CO2 is a colourless, odourless gas. The main industrial source is as a by-product from the manufacture of

hydrogen for making ammonia.

CO + H2O CO

2 + H

2

CH4 + 2H

2O CO

2 + 4H

2

CPB – 22


New Delhi – 110 018, Ph. : 9312629035, 8527112111

It is also recovered from fermentation processes in breweries, from the gases evolved from calcininglimestone in lime kilns and from the flue gases from coal-burning electric power stations. The CO

2 is

recovered by absorbing it in either aqueous Na2CO

3 or ethanolamine.

252

underyeast

conditionsanacrobic6126 CO2OHHC2OHC

CaCO3 heatstrong

CaO + CO2

It is obtained in small amounts by the action of dilute acids on carbonates. It can also be made by burningcarbon in excess of air.

CaCO3 + 2HCl CaCl

2 + CO

2 + H

2O

C + O2 CO

2

Recovery of CO2

Na2CO

3 + CO

2 + H

2O

cool

hot 2NaHCO

3

Girbotol process

eminethanola222 NHCHHOCH2 + CO

2 + H

2O

C6030

0

0

C150100

(HOCH

2CH

2NH

3)

2CO

3

CO2 gas can be qualified under pressure between –570C and +310C. About 80% is sold in liquid form, and

20% as solid. The solid is produced as while snow by expanding the gas from cylinders. (Expansion causescooling). This is compacted into blocks and sold. Solid CO

2 sublimes directly to the vapour state (without

going through the liquid state) at –780C under atmospheric pressure. Over half the CO2 produced is used as

a refrigerant. Solid CO2 is called ‘dry ice’ or ‘cardice’, and is used to freeze meat, frozen foods and ice

cream, and in the laboratory as a coolant. Over a quarter is used to carbonate drinks (Coca-Cola, lemonade,beer etc.). Other uses include the manufacture of urea, as an inert atmosphere, and for neutralizing alkalis.(Urea is the most widely used nitrogeneous fertilizer and is also for making formaldehyde urea resins.)

OH)NH(CONHCONHNH2CO 2urea

22ammonium

224

C180

pressure32

carbamate

0

Small scale uses of CO2 include use in fire extinguishers, blasting in coal mines, as an aerosol propellant,

and for inflating life-rafts.

CO2 gas is detected by its action on lime water Ca(OH)

2 or baryta water Ba(OH)

2, as a white insoluble

precipitate of CaCO3 or BaCO

3 is formed. If more CO

2 is passed through the mixture, the cloudiness

disappears as the soluble bicarbonate is formed.

OHCaCOCO)OH(Ca 2white

322

eprecipitat

lelubso23223 )HCO(CaOHCOCaCO

CO2 is an acidic oxide, and reacts with bases, forming salts. It dissolves in water but it is only slightly

hydrated to carbonic acid H2CO

3, and the solution contains few carbonate or bicarbonate ions. A hydrate

CO2 . 8H

2O can be crystallized at 00C under a pressure of 50 atmospheres CO

2.

CO2 + H

2O H

2CO

3

Carbonic acid has never been isolated, but it gives rise to two series of salts, hydrogencarbonates(otherwise called bicarbonates), and carbonates.

CPB – 23


New Delhi – 110 018, Ph. : 9312629035, 8527112111

CO2 can also act as a ligand, and it form a few complexes such as [Rh(CO

2)Cl(PR

3)

3] and [Co(CO

2)(PPh

3)

3].

In the first complex the C atom in CO2 is bonded to the metal. In the second complex the CO

2 acts as a

bidentate ligand with one C atom and one O atom bonded to the metal and the CO2 molecule is bent.

The structure of CO2 is linear O — C — O, Both C — O bonds are the same length. In addition to bonds

between C and O, there is a three centre four-electron bond covering all three atoms. This adds twobonds to the structure in addition to the two bonds. Thus the C — O bond order is two.

Biologically, carbon dioxide is important in the process of photosynthesis, where the green parts of plantsmanufacture glucose sugar. Ultimately all animal and plant life depends on this process.

2ecosglu

6126sunlight

22 O6OHCOH6CO6

The reverse reaction occurs during the process of respiration, where animals and plants release energy.

C6H

12O

6 + 6O

2 6CO

2 + 6H

2O + energy

C2C Compounds of Silocon :

(a) Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom iscovalency bounded in a tetrahedral manner to four oxygen atoms.

(b) Some reactions of SiO2 are :

OHSiONaNaOH2SiO 2322

OH2SiFHF4SiO 242

(c) Silicons are a group of organosilicon polymers which have (–R2SiO-) as a repeating uint.

(d) The chain length of polymer can be controlled by adding (CH3)

3SiCl which blocks the ends.

(e) Silicons are used as sealant, greases, electrical insulators and for water proofing of fabrices.

(f) The basic structure unit of silicates is SiO44–. Two man-made silicates are glass and cement.

(g) If aluminium atoms replace few silicon atoms in three dimensional network of silicon dioxide,overall structure known as aluminosilicates aquired a negative charge. Cations such as Na+, K+

or Ca2+ balance the negative charge. This type of structured known as zealites, which used inpetrochemical industries for cracking of hydrocarbons.

Detailed explanation of compounds of silicon :

Silicates

About 95% of the earth’s crust is composed of silicate minerals, aluminosilicate clays, or silica.

Soluble silicates

Silicates can be prepared by fusing an alkali metal carbonate with sand in an electric furnace at about14000C.

n3244SiO

22C1400

32 )SiONa(,SiONaONaCOCONa 20

and others

The product is a soluble glass of sodium or potassium silicate. It is dissolved in hot water under pressure,and is filtered from any insoluble material, this is known as water glass. They are used in liquid detergentpreparation to keep the pH high. Soluble silicates must not be used if the water is hard, or they will reactwith Ca2+ to form insoluble calcium silicate.

CPB – 24


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Principles of silicate structures

The majority of silicate minerals are very insoluble, because they have an infinite ionic structure andbecause of the great strength of the Si—O bond. This made it difficult to study their structures, and physicalproperties such as cleavage and the hardness of rocks were originally studied.

1. The electronegativity difference between O and Si, 3.5 – 1.8 = 1.7, suggests that the bonds are almost 50%ionic and 50% covalent.

2. The structure may therefore be considered theoretically by both ionic and covalent methods. The radiusratio Si4+ : O2— is 0.29, which suggests that Si is four-coordinate, and is surrounded by four O atoms at thecorners of a tetrahedron. This can also be predicted from the use of the 3s and three 3p orbitals by Si forbonding. Thus silicates are based on (SiO

4)4— tetrahedral units.

3. The SiO4 tetrahedra may exist as discrete units, or may polymerize into larger units by sharing corners, that

is by sharing O atoms.

4. The O atoms are often close-packed, or nearly close-packed. Close packed structure have tetrahedral andoctahedral holes, and metal ions may occupy either octahedral or tetrahedral sites depending on their size.Most metal ions are the right size to fit one type of hole, through Al3+ can fit into either. Thus Al can replaceeither a metal in one of the holes, or a silicon atom in the lattice. This is particularly important in thealuminosilicates.

Classification of Silicates

1. Orthosilicates

A wide variety of minerals contain discrete (SiO4)4—, tetrahedra, that is they share no corners, for example

ZrSiO4.

Figure : Structure of orthosilicates.

Zircon ZrSiO4 is used as a gemstone as it can be cut to look like a diamond, but is much cheaper. Zircon is

much softer than diamond. Ziron has a coordination number of 8. The structure is not close-packed.

2. Pyrosilicates

Two tetrahedral units are joined by sharing the O at one corner, thus giving the unit (Si2O

7)6—.Example :

Thortveitite Sc2[Si

2O

7], Hemimorphite Zn

4(OH)

2[Si

2O

7] . H

2O

Figure : Structure of pyrosilicates Si2O

76–.

3. Cyclic silicates

If two oxygen atoms per tetrahedron are shared, ring structure may be formed of general formula (SiO3)

n2n–

. Rings containing three, four, six and eight tetrahedral units are known,but those with three and six are themost common. The cyclic ion Si

3O

96– occurs in wollastonite Ca

3[Si

3O

9] and in benitoite BaTi[Si

3O

9]. The

Si6O

1812— unit occurs in beryl Be

3Al

2[Si

6O

18].

CPB – 25


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Figure : Structure of cyclic silicates Si3O

96— and Si

6O

1812—.

4. Chain silicates

Simple chain silicates or pyroxenes are formed by the sharing of the O atoms on two corners of eachtetrahedron with other tetrahedra. This gives the formula (SiO

3)

n2n–, e.g. in spodumene LiAl[(SiO

3)

2]. Diopsite

CaMg [(SiO3)

2].

Figure : Structure of pyroxenes (SiO3)

n2n–.

Double chains can be formed when two simple chains are joined together by shared oxygens. These miner-als are called amphiboles, and they are will known. There are several ways of forming double chains,giving formulae (Si

2O

5)

n2n–, (Si

4O

11)

n6n–, (Si

6O

17)

n10– and others. The most numerous and best known

amphiboles are the asbestos minerals. Amphiboles always contain hydroxyl groups, which are attached tothe metal ions.

Figure : Structure of amphiboles (Si4O

11)

n6n–.

CPB – 26


New Delhi – 110 018, Ph. : 9312629035, 8527112111

The Si—O bonds in the chains are strong and directional. Adjacent chain are held together by the metal tothe chains. Thus pyroxenes and amphiboles cleave readily parallel to the chains, forming fibres. For thisreason they are called fibrous minerals. The cleavage angle for pyroxenes is 890, and for amphiboles 560.This is used as a means of identifying the minerals. These angles are related to the size of thecross-sectional trapezium of the chains and the way in which they are packed together.

Asbestos minerals come from two different groups of silicates :

1. The amphiboles.

2. The sheet silicates.

The amphiboles include crocidolite Na2Fe

3IIFe

2III [Si

8O

22] (OH)

2, which is called blue asbestos and others

derived from it by isomorphous replacement, for example amosite or brown asbestos (Mg, FeII)7[Si

8O

22](OH)

2.

The mineral chrysotile Mg3(OH)

4[Si

2O

5] is called white asbestos, and this is derived from serpentine, and

is a sheet silicate.

Sheet silicates (phyllo-silicates)

When SiO4 units share three corners the structure formed is an infinite two-dimensional sheet of empirical

formula (Si2O

5)

n2n–.

Structures with simple planar sheets are rare. A large number of sheet silicates are made up of either two orthree layers joined together. These include :

1. Clay minerals (kaolinite, pyrophyllite, talc)

2. White asbestos (chrysotile, biotite)

3. Micas (muscovite and margarite)

4. Montmorillonites (Fullers earth, bentonite and vermiculite)

Figure : Structure of sheet silicates (Si2O

5)

n2n–.

Three-dimensional Silicates

Sharing of all of the four corners of a SiO4 tetrahedron results in a three dimensional lattice of the formula

SiO2 (quartz, tridymite etc). These contain no metal ions. Replacement of one quarter or one half of the Si

atoms are quite common. Such replacements resuls in three groups of minerals.

1. Feldspars

2. Zeolites

3. Ultramarines.

Feldspars are the most important rock forming materials e.g. granite is made up of feldspar with some micaand quartz.

Zeolites are used as ion-exchange materials. This is the reason that sodium zeolites Na2[Al

2Si

3O

10]2H

2O

are used to soften the hard water. By passing the hard water through a column of zeolite or hydrated sodiumaluminium silicate called permutit, NaAlSiO

4 . 3H

2O. Due to ion-exchange, sodium in the silicate / zerolite

gets exchanged for Ca2+/Mg2+ ions and the water gets softened.

CPB – 27


New Delhi – 110 018, Ph. : 9312629035, 8527112111

2NaAlSiO4 + Ca2+ Ca(AlSiO

4)

2 + 2Na+, 2NaAlSiO

4 + Mg2+ Mg (AlSiO)

4 + 2Na+

The zeolite/permutit can be regenerated by treatment with sodium chloride solution.

Organosilicon Compounds and the Silicones

Si — C bonds are almost as strong as C — C bonds. Thus silicon carbide SiC is extremely hard and stable.Many thousands of organosilicon compounds containing Si — C bonds have been made. However, the vastrange of organic compounds is not replicated by silicon for three main reasons.

1. Silicon has little tendency to bond to itself (catenate) whilst carbon has a strong tendency to do so. Thelargest chains formed by Si are contained in Si

16F

34 and Si

8H

18, but these compounds are exceptional. This

is related to the weakness of Si — Si bonds in contrast to the strength of C — C bonds.

2. Silicon does not form p — p double bonds, whilst carbon does not readily.

3. Silicon forms a number of compounds containing p – d double bonds in which the silicon atom uses dorbitals.

Preparation of organosilicon compounds

There are several ways of forming Si — C bonds :

1. By a Grignard reaction

SiCl4 + CH

3MgCl CH

3SiCl

3 + MgCl

2

CH3SiCl

3 + CH

3MgCl (CH

3)

2SiCl

2 + MgCl

2

(CH3)

2SiCl

2 + CH

3MgCl (CH

3)

3SiCl + MgCl

2

(CH3)

3SiCl + CH

3MgCl (CH

3)

4Si + MgCl

2

2. Using an organolithium compound

4LiR + SiCl4 SiR

4 + 4LiCl

R may be alkyl or aryl.

3. Rochow ‘Direct Process’. Alkyl or aryl halides react directly with a fluidized bed of silicon in the presence

of large amounts (10%) of a copper catalyst. Si + 2CH3Cl C300280catalystCu 0

(CH3)

2SiCl

2

Silicones

The silicones are a group of organosilicon polymers.

The complete hydrolysis of SiCl4 yields SiO

2, which has a very stable three-dimensional structure. The

fundamental research of F. S. Kipping on the hydrolysis of alkyl substituted chlorosilanes led, not to theexpected silicon compound analogous to a ketone but to long-chain polymers called silicones.

CPB – 28


New Delhi – 110 018, Ph. : 9312629035, 8527112111

The starting materials for the manufacture of silicons are alkyl or aryl substituted chlorosilanes. Methylcompounds are mainly used, though some phenyl derivatives are used as well. Hydrolysis ofdimethyldichlorosilane (CH

3)

2SiCl

2 gives rise to straight chain polymers and, as an active OH group is left

at each end of the chain, polymerization continues and the chain increases in length. (CH3)

2SiCl

2 is

therefore a chain building unit. Normally, high polymers are obtained.

Hydrolysis under carefully controlled conditions can produce cyclic structures, with rings containing three,four, five or six Si atoms :

Hydrolysis of trimethylmonochlorosilane (CH3)

3SiCl yields (CH

3)

3SiOH trimethylsilanol as a volatile liq-

uid, which can condense, giving hexamethyldisiloxane. Since this compound has no OH groups, it cannotpolymerize any further.

hexamethyldisiloxane

If Some (CH3)

3SiCl is mixed with (CH

3)

2SiCl

2 and hydrolysed, the (CH

3)

3SiCl will block the end of the

straight chain produced by (CH3)

2SiCl

2. Since there is no longer a functional OH group at this end of the

chain, it cannot grow any more at this end. Eventually the other end will be blocked in a similar way. Thus(CH

3)

3SiCl is a chain stopping unit, and the ratio of (CH

3)

3SiCl and (CH

3)

2SiCl

2 in the starting mixture will

determine the average chain size.

The hydrolysis of methyl trichlorosilane RSiCl3 gives a very complex cross-linked polymer.

CPB – 29


New Delhi – 110 018, Ph. : 9312629035, 8527112111

In a similar way addition of a small amount of CH3SiCl

3 to the hydrolysis mixture produces a few

cross-links, or provides a site for attaching other molecule. By controlled mixing of the reactants, any giventype of polymer can be produced.

Properties and Uses

Silicones are strongly water repellent, good electrical insulators have non-stick properties and anti-foamingproperties. Their strength and inertness are related to two factors :

1. Their stable silica-like skeleton of Si—O—Si—O—Si. The Si—O bond energy is very high(502 kJ mol–1).

2. The high strength of the Si—C bond.

Their water repellency arises because a silicone chain is surrounded by organic side grous, and looks likean alkane from the outside.

Silanes

Silicon forms a limited number of saturated hydrides, SinH

2n + 2, called the silanes. Monosilane SiH

4 is the

only silicon hybride of importance.

Preparation

1. 2Mg + Si airabsenceinheatMg

2Si

Mg2Si + H

2SO

4 SiH

4(40%)

2. More recently monosilane has been prepared by reducing SiCl4 with Li[AlH

4], LiH or NaH in ether

solution at low temperatures.

SiCl4 + Li[AlH

4] SiH

4 + AlCl

3 + LiCl

Si2Cl

6 + 6LiH Si

2H

6 + 6LiCl

Si3Cl

8 + 8NaH Si

3H

8 + 8NaCl

3. Silanes may also be prepared by direct reaction by heating Si or ferrosilicon with anhydrous HX or RX inthe presence of a copper catalyst.

Si + 2HCl SiH2Cl

2

Si + 3HCl SiHCl3 + H

2

Si + 2CH3Cl CH

3SiHCl

2 + C + H

2

Halides of Silicon

Preparation :

The silicon halides can be prepared by heating either Si or SiC with the appropriate halogen.

Properties :

1. SiF4 is readily hyrolysed by alkali.

SiF4 + 8OH— SiO

44– + 4F– + 4H

2O

[In marked contrast to the inertness of CF4, CCl

4 and the Freons, SiF

4 is readily hydrolysed]

2. In the case of the tetrafluoride, a secondary reaction occurs between the resultant HF and the unchangedSiF

4, forming the hexafluorosilicate ion [SiF

6]2–.

CPB – 30


New Delhi – 110 018, Ph. : 9312629035, 8527112111

SiF4 + 2HF 2H+ + [SiF

6]2–

The [SiF6]2– ion is usually formed from SiO

2 and aqueous HF

SiO2 + 6HF 2H+ + [SiF

6]2– + 2H

2O

The [SiF6]2– complex is stable in water and alkali, but the others in the group are less stable. [SnF

6]2– are

hydrolysed by alkali, and [PbF6]2– is hydrolysed by both alkali and water.

[SiF6]2– gives an octahedral structure (sp3d2 hybridization)

3. The silocon halides are repidly hydrolysed by water to give silicic acid.

SiCl4 + 4H

2O Si(OH)

4 + 4HCl

Large quantity of SiCl4 are hydrolysed at a high temperature (in an oxy-hydrogen flame) giving very finely

powdered SiO2 rather than Si(OH)

4.

Practice Problems :

1. Write the resonance structures of CO32– and HCO

3–.

2. What is the state of hybridisation of carbon in (a) CO32– (b) diamond (c) graphite ?

3. Explain the difference in properties of diamond and graphite on the basis of their structures.

4. Rationalise the given statements and give chemical reactions :

(i) Lead (II) chloride reacts with Cl2 to give PbCl

4

(ii) Lead (IV) chloride is highly unstable towards heat.

(iii) Lead is known not to form an iodide, PbI4.

5. Suggest a reason as to why CO is poisonous ?

6. How is excessive content of CO2 responsible for global warming ?

7. Explain the structures of diborane and boric acid.

8. Explain, why is there a phenomental decrease in ionization enthalpy from carbon to silicon ?

9. How would you explain the lower atomic radius of Ga as compared to Al ?

10. What are allotropes ? Sketch the structure of two allotropes of carbon, namely, diamond andgraphite. What is the impact of structure of physical properties of two allotropes ?

11. Classify following oxides as neutral, acidic, basic or amphoteric : CO, B2O

3, SiO

2, CO

2, Al

2O

3, PbO

2,

Tl2O

3.

12. When metal (X) is treated with sodium hydroxide, a white precipitate (A) is obtained, which is solublein excess of NaOH to give soluble complex (B). Compound (A) is soluble in dilute HCl to formcompound (C). The compound (A) when heated strongly gives (D), which is used to extract metal.Identify (X), (A), (B), (C) and (D). Write suitable equations to support their identities.

13. What do you inderstand by :

(a) inert pair effect

(b) allotropy

(c) catenation ?

14. Give one method for industrial preparation and one for laboratory preparation of CO and CO2

each.

15. An aqueous solution of borax is

(a) neutral (b) amphoteric (c) basic (d) acidic

[Answers : (2) (a)CO32– is sp2 hybridised. (b) diamond is sp3 hybridised. (c) graphite is sp2 hybridized

(3) (i) Diamond is an extremely hard substance, whereas graphite is soft. Explanation : The compactthree dimensional structure in which carbon atoms are tetrahedrally bonded (sp3 hybridisation) tothe other carbon atoms by strong single covalent bond accounts for the hardness of diamond.

CPB – 31


New Delhi – 110 018, Ph. : 9312629035, 8527112111

. Whereas graphite has a sheet of layered structure (sp2 hybridisation)

in which each layer a carbon atom is bonded to only three neighbouring carbon atom by singlecovalent bonds. This result in a hexagonal network of carbon and accounts for its softness.

(ii) Diamond is a non-conductor of electricity while graphite is a good conductor..

Explanation : All the four electrons of each carbon atom in diamond are utilised in the formation offour single covalent bonds and thus does not have free electrons and hence act as non-conductor,whereas in graphite each carbon atom is bonded to only three neighbouring carbon atoms by singlecovalent bonds and one electron being free makes it good conductor of electricity. (iii) Graphite isless dense than diamond. Explanation : In graphite, the distance between two alternate layers isquite large ( 3.40 Å), whereas in diamond the distance between the two adjacent carbon atoms issmall ( 1.54 Å). (iv) Diamond has high melting point, while melting point of graphite iscomparatively much lower. Explanation : Diamond has sp3 hybridisation having compact structure,whereas graphite has sp2 hybridisation having layered structure. Hence, the explanation.(4) (i) Because of inert pair effect, Lead (II) is more stable in +2 than in +4 oxidation state. Hence,Lead (II) chloride does not react with Cl

2 to give Lead (IV) chloride. (ii) This is also because of inert

pair effect. (iii) PbI4 does not exist. This happens because of oxidising power of Pb4+ ion and reducing

power of I– ion. (5) Oxygen combines loosely and reversibly with the haemoglobin present in redblood cells to produce oxyhaemoglobin. This oxyhaemoglobin formed in lungs then travel to thedifferent parts of the body through blood steam and supply O

2 to them. However, CO if present,

combines with haemoglobin to give stable carboxyhaemoglobin, which destroys the oxygen carryingcapacity of haemoglobin and hence acts as poisonous gas. (6) CO

2 is produced through combustion

and other industrial process. It is utilised in photosynthesis process by plant and CO2 is released in

the atmosphere. Through CO2 cycle, a constant percentage of oxygen (about 21%) is maintained in

the atmosphere. However, if the concentration of CO2 increased beyond a limit, some of it will

remain unutilised. This excess CO2 absorbs heat radiated by the earth. Some of it is dissipated into

the atmosphere while the remaining part is radiated back to the earth and other bodies present onthe earth. As a result, temperature of the earth and other bodies on the earth increases. This is calledgreenhouse effect. As a result of greenhouse effect, global warming occurs which has seriousconsequences. (7) (a) Structure of diborane is shown below (a) and (b)

CPB – 32


New Delhi – 110 018, Ph. : 9312629035, 8527112111

Nature of bonding in B2H

6 : Each B atom uses sp3 hybrids for bonding. Out of the four sp3 hybrids

on each B atom, one is without an electron shown in broken lines. The terminal B-H bonds arenormal 2c-2e bonds but the two bridge bonds are 3c-2e bonds. The 3c-2e bridge bonds are alsoreferred to an banana bonds. (b) The structure of boric acid is shown in figure :

Structure of boric acid; the dotted lines represent hydrogen bonds.

(8) This effect is due to increase in atomic size and screening effect. Because of this the force ofattraction of the nucleus for the valence electron decreases in silicon as compared to carbon. As aresult, there is a phenomenal decrease in ionization enthalpy from carbon to silicon. (9) Due to poorshielding effect of the valence electrons of Ga by the inner 3d-electrons, the effective nuclear chargeof Ga is higher in magnitude than that of Al. As a result, the electrons in Ga experience a larger forceof attraction by the nucleus than Al and hence atomic size of Ga (135 pm) is slightly less than that ofAl (143 pm). (10) The phenomenon of existence of an element in two or more forms which havedifferent physical properties but similar chemical properties is called allotropy and the differentforms are called allotropes. (i) Diamond (ii) Graphite. (11) Neutral oxides : CO, Acidic oxides : B

2O

3,

SiO2, CO

2, Amphoteric oxide : Al

2O

3, PbO

2, Basic oxide : Tl

2O

3. (12) Since metal (X) when treated

with sodium hydroxide, gives a white ppt. (A), which dissolves in excess of NaOH to give solublecomplex (B), therefore (X) seems to be metal aluminium, ppt. (A) seems to be Al(OH)

3 and complex

(B) seems to be sodium tetra hydroxoaluminate (III).

)B(4

)A(33

)X(])OH(Al[NaNaOH)OH(Al,Na3)OH(AlNaOH3Al2 . Since, (A) is

amphoteric in nature, it seems to react with dil. HCl to give (C) which may be AlCl3.

OH3AlClHCl3)OH(Al 2)C(

3)A(

3 . Since (A) on heating gives (D) which is used for metal

extraction, therefore, (D) is alumina (Al2O

3) : OH3OAl)OH(Al2 2

(D)alumina

32)A(

3 (13) Catenation : It

may be defined as the ability of lime atoms to link with one another through covalent bonds. This isdue to smaller size and higher electronegativity of carbon atom and unique strength of C – C bonds.(15) (c). Hint : Borax is salt made up of weak acid. (H

3BO

3) and strong base (NaOH). Therefore, it is

basic in nature]

p-block elements (xith)einsteinclasses.com/p-block_xi.pdffigure : similarly in structure between (a)...

Documents