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EXERCISE # 1

P. JOY Note

ELEMENTAL BORON : Q.1

What are the elements of 3rd group ?

Q.2

What is the general configuration of 13th group element

Q.3

How many valence electrons are present in element of 13 th group?

Q.4

Which element of group 3rd is the only non-metal ?

Q.5

To which part and group indium element belongs ?

Q. 6

How melting point varies in the III-A group

Q.7

What is the correct order of M.P. of group 3 elements ?

Q.8

Boron is the only Non-metal of group - 3 which element of group 3 has highest M.P.

Q.9

Which property in group 13th does not smoothly increase.

Q. 10

Which property in group 13th smoothly increases ?

Q.11

Which element of group 3rd is liquid at room temperature ?

Q.12

How boron looks like ?

Q.13

In which two forms elemental boron exists ?

Q.14

Which form of boron is more stable ?

Q.15

β - Rhombohedron is made up of which units ?

Q.16

What is icosahedral unit and what is its structure ?

Q.17

How can you represent β - Rhombohedron ?

Q.18

What is Ionisation energy ?

Q.19

Which element has the higher I.E. among all elements ?

Q.20

How Ionisation energy gets affected on successive loss of electron?

Q.21

Arrange third group elements in decreasing order of I.E.

Q.22

Why Be has higher I.E1 than B ?

Q. 23

Why I.E. cannot be measured in solid state ?

Q.24

What is inert pair effect ?

Q.25

Give the trend of inert pair effect from top to bottom ?

Q.26

How inert pair effect affects the oxidation state of 13th group element ?

Q.27

Why +1 oxidation state of thallium is stable than +3 state ?

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Note

Q.28

Which ion of Boron present in its aq. state

Q.29

What are the ions formed by the ionisation of TlI3 ?

COMPOUNDS OF BORON BORIC ACID : Q.30

What are the important compounds of boron ?

Q.31

What are the different names of H3BO3 ?

Q.32

What are the various formula by which Boric acid can be shown?

Q.33

Is Boric acid triprotic or monoprotic acid ?

Q.34

Why Boric acid is a monoprotic acid ?

Q.35

Show what happens when boric acid is dissolved in heavy water?

Q.36

Does there a lewis acid - base reaction take place when boric acid is dissolved in water.

Q.37

Why boric acid is a Lewis acid.

Q.38

In which compound H3BO3 may exhibit acidic character ?

Q.39

Why B(OH)3 is not a base like NaOH, Ba(OH)2 ?

Q.40

Write the reactions of preparation of boric acid ?

Q.41

What happens when Boric acid is strongly heated ?

Q.42

Why carbonic acid is unstable ?

Q.43

Two OH group on a carbon atom

OH on silicon atom

C

OH OH

is unstable but two

is stable. Give reason.

Q.44

Although Carbon and Boron both belong to second row and both donot have d-orbitals. Yet two hydroxy group on a Carbon is unstable whereas on Boron there are stable.

Q.45

Why orthoboric acid exist as a slippery solid ?

Q.46

Although Boric acid is a weak acid but it can’t be titrated with sodium hydroxide ............ explain why ?

Q.47

Why diols like ethylene glycol, catechol are added when boric acid is titrated with sodium hydroxide ?

Q.48

Why catechol is added when boric acid is titrated with NaOH ?

Q.49

Why titration of orthoboric acid is considered to be a difficult titration ?

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DIBORANES : Q.50

What are boranes ?

Q.51

How many boranes are known ?

Q.52

Which is simplest borane ?

Q.53

Draw the structure of diborane ?

Q.54

Why structure of diborane is peculiar ?

Q.55

Describe the structure of B2H6 ?

Q.56

What is banana bond ?

Q.57

How B2H6 can be converted into its monomer ?

Q.58

Mention various reaction of B2H6 with NH3 :

Q.59

What is inorganic benzene ? Is it more reactive or less reactive than benzene ?

Q.60

What happens when inorganic benzene is reacted with HCl ?

Q.61

Give methods of preparation of diborane ?

Q.62

Compare borane with ethane ?

P. JOY Note

BORAX : Q.63

What is the formula of Borax ?

Q.64

Draw the structure of Borax ?

Q.65

What is commercial name of Borax ?

Q.66

How many number of B–O bond are present in Borax.

Q.67

How many number of B–O–B bond are present in Borax.

Q.68

How many number of sp3 hybridised Boron are present in Borax.

Q.69

How many number of sp2 hybridised Boron are present in Borax.

Q.70

Write the formula of buffer solution of Borax in water.

Q.71

How many Boron units are connected by an oxygen atom diagonally ?

Q.72

How many Boron atoms in Borax acts as Lewis acid ?

Q.73

Write an equation to show hydrolysis of Borax ?

Q.74

What are hydrolysis product(s) of Borax ?

Q.75

What is the nature of aqueous solution of borax ?

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Q.76

Write an equation to show reaction of Borax with HCl or H2SO4.

Q.77

How many moles of NaOH are consumed when Borax is treated with NaOH

Q.78

What happen's when [B(OH)4]– is reacted with HCl ?

Q.79

What happens when borax is heated ?

Q.80

What are the different types of Borate ?

BORON FAMILY

Note

BORON HALIDE : Q.81

What are the physical state of BF3 , BCl3 , BI3 ?

Q.82

What is the general characteristics of borontrihalide ?

Q.83

Which boron trihalide is most stable ?

Q.84

Which boron trihalide is used in Friedal-Craft Reaction ?

Q.85

Why B – F bond is shorter & stronger than its theoretically expected value ?

Q.86

What is the correct order of Lewis acid character of various boron halide ?

Q.87

Which BX 3 has more efficiency of back-bonding ?

Q.88

Which boron trihalide is used in Vanarkel method ?

Q.89

What is the structure of BF3 ?

Q.90

Pure BBr3 and BI3 are colourless but on exposure to light they become coloured. Explain why ?

Q.91

BF3 has higher ∆Hf than other halides of Boron. Explain why ?

Q.92

Dimer of BH3 is known but not of BCl3 ?

Q.93

How BI3, BBr3, BCl3 can be reduced to pure boron ?

Q.94

Why hydrolysis of BF3 is difficult than hydrolysis of other boron trihalide (BX3) ?

Q.95

Why BBr3 and BCl3 fumes in air ?

Q.96

What happens when any two of halides BF3, BCl3, BBr3 are mixed together ?

Q.97

What happens if BCl3, BF3, BBr3 all are mixed together ?

Q.98

Draw the structure of B4Cl4 ?

Q.99

What happens when BF3 is reacted with RMgX [Grignard reagent]?

Q.100 What is the Vanarkel method of purification of Boron.

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ELEMENTAL ALUMINIUM : Q.101 Aluminium belongs to which block elements ? Q.102 Which type of compounds are formed by Aluminium : Q.103 Which element is the most electropositive in 13th group ? Q.104 Which metal is most abundant in the earth's crust ? Q.105 Why atomic radius of Ga is lesser than that of Al ? Q.106 Write the decreasing order of reduction potential of IIIA elements ? Q.107 Why Al reduces most of the metallic oxides ? Q.108 On heating Al at 800ºC in air, Al 2O3 is formed, which type of reaction is this ? Q.109 Why Aluminium becomes passive in nitric acid ? Q.110 Al is more reactive than Fe but Al is less easily corroded than Fe. Why? Q.111 Why Aluminium metal is corroded in coastal places near to the sea ? Q.112 What are the uses of Aluminium ? Q.113 What is thermite ? Q.114 Which reaction forms the basis of Goldschmidt aluminothermite process? Q.115 What is Alumino-thermite process ? Q.116 Why Aluminothermy used for on the spot welding of large iron structures Q.117 Why Aluminium vessels should not be washed with materials containing washing soda Q.118 Write the reactions of aluminium with (a) O2

(b) N2

(e) Fe2O3

(f) Mn3O4

(c) Cl2

(g) HCl (i) H2SO4 (conc.) (k) NaOH

(d) Cr2O3

(h) H2SO4(dil.) (j) HNO3 (l) AlCl3

HYDROXIDE : Q.119 What are the various hydroxides of group III element and how do their acidic and basic properties vary ?

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Q.120 Which hydroxide of group III show amphoteric behaviour ?

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Note

Q.121 How do you show that Al(OH)3 is amphoteric ? Q.122 What happens when Al(OH)3 is dissolved in sodium hydroxide (NaOH)? Q.123 Al(OH)3 is water soluble/insoluble ? Q.124 How can you dissolve Al(OH)3 ? Q.125 Alumina is water soluble/insoluble ? Q.126 NaAlO2 is water soluble/insoluble ? Q.127 Write the reactions of Al(OH)3 with (a) Acid (b) NaOH (c) Heating Below 450ºC (d) Heating above 450ºC Q.128 What is the nature of oxides of IIIA group elements ? Q.129 Which is the most stable compound of aluminium. Q.130 What are the different forms of alumina. Q.131 Which form of alumina is corundum. Q.132 Which form of alumina is hard abbrasive. Q.133 Which form of alumina is soft. Q.134 Which form of alumina is amphoteric. Q.135 Which form of alumina is acid soluble.

EXTRACTION OF ALUMINIUM : Q.136 Which ore of Aluminium is used for its extraction ? Q.137 What is the formula of Bauxite ? Q.138 What is the major composition of gangue (impurity) in Bauxite ? Q.139 What are the steps involved in the extraction of Al ? Q.140 What are the various processes used for concentration of Bauxite ore ? Q.141 What is Baeyer's process ? Q.142 What is Hall's process ? Q.143 What is Serpeck's process ? Q.144 In which purification process of bauxite ore, ore is mixed with sodium carbonate (Na 2CO 3) ?

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Q.145 Which process is used for purification of bauxite containing high silica content as impurity ?

P. JOY Note

Q.146 What is the by-product obtained in the serpeck’s process ? Q.147 Which substance is used to remove SiO2 in the serpeck’s process of purification of bauxite Q.148 What are the reactions taking place at cathode and anode in electrolytic reduction ? Q.149 Why electrolytic reduction of pure alumina is not possible ? Q.150 What is cryolite ? Q.151 Why fused cryolite is added during electrolytic extraction of aluminium ? Q.152 Which process is used for purification of aluminium by electrolytic refining ?

HALIDES : Q.153 Which halide of IIIA group element exist as a monomer ? Q.154 What is the nature of halide of Thallium ? Q.155 Which IIIA group element form dimer ? Q.156 In which phase and condition aluminium chloride exists in dimeric form ? Q.157 How AlCl3 exist above 800ºC and below 400ºC ? Q.158 How AlCl3 exist in solid state ? Q.159 Are the bonding states of Al2Cl6 in vapour phase and in aqueous phase are different. Q.160 Describe the structure of Al2Cl6 ? Q.161 What is the similarity between structure Al2Cl6 and B2H6 ? Q.162 Unlike Al2Cl6, boron trichloride does not form a dimer. Q.163 What happens when Al2Cl6 dimer is added to water. Q.164 How Al2Cl6 is prepared. Q.165 How Al2O3 can be converted into Al2Cl6 Q.166 What happens when Al2Cl6 is reacted with LiH Q.167 What is the nature of AlCl3 in aqueous state.

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Note

Q.168 Does AlCl3 on heating melts. Q.169 Why is it difficult keep AlCl3 dry Q.170 What happens when AlCl3 reacted with Cl– -ions Q.171 What happens when AlCl3 vapours are passed over fused Al2O3 at 1000ºC. Q.172 Comment on stability of AlCl Q.173 Why AlCl3.3H2O cannot be dehydrated by heating ? Q.174 Why AlCl3 fumes in air ? Q.175 Describe the structure of Al2(CH3)6 ? Q.176 Which of the following B(CH3)3 or Al(CH3)3 form dimer ?

ALUM : Q.177 What are alums ? Q.178 What is the general formula of alum. Q.179 Give some examples of alum ? Q.180 What is feather alum or hair salt. Q.181 Which unipositive cations do form alum. Q.182 Which tripositive cations do form alum. Q.183 Which alkali metal cation does not form alum. Q.184 Why Li+ does not form alum ? Q.185 How many water molecules act as ligands about mono and trivalent metal cations in alum. Q.186 How do structures of all alums differ. Q.187 How are alums prepared. Q.188 What happens when alums are heated. Q.189 Alums of which element(s) are sparing soluble ? Q.190 What is the nature of aqueous solution of alums. Q.191 What are pseudo alums. Q.192 Are pseudo alums isomorphous to alums. Q.193 What are the uses of Potash alum ?

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Note Elements

Atomic No.

B Al Ga In Tl

5 13 31 49 81

Relative atomic mass 10.81 26.98 69.72 114.82 204.36

M.P.

Density/kgm

2573 933.52 302.9 429.32 576.7

–3

2390 2698 5967 7310 11850

IONISATION ENERGY / kJ mol–1 1st 800.6 577.4 578.8 558.3 589.3

Elements B Al Ga In Tl

2nd 2427 1816.6 1979 1820.6 1971

Q.1

What are the elements of 3rd group ?

Ans.

B Al Ga

3rd 3860 2744.6 2963 2704 2878

4th 25025 11575 6200 5200 4900

In Tl

Q.2

What is the general configuration of 13th group element

Ans.

ns2 np 1

Q.3

How many valence electrons are present in element of 13 th group?

Ans.

13th group element has 3 valence electrons which reaches to 6 electrons by covalent sharing.

Q.4

Which element of group 3rd is the only non-metal ?

Ans.

Boron is the non metal.

Q.5

To which part and group indium element belongs ?

Ans.

Indium element belongs to III A group (13th group) and p-block of perodic table.

Q. 6

How melting point varies in the III-A group B

Ans.

Tl In

Al

M.P.

Ga B

Al

Ga

In

Tl

Q.7

What is the correct order of M.P. of group 3 elements ?

Ans.

B > Al > Tl > In > Ga

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Q.8

Boron is the only Non-metal of group - 3 which element of group 3 has highest M.P.

Ans.

Boron

Q.9

Which property in group 13th does not smoothly increase.

Ans.

Melting point

Q. 10

Which property in group 13th smoothly increases ?

Ans.

Atomic mass, density, atomic number

Q.11

Which element of group 3rd is liquid at room temperature ?

Ans.

Gallium

Q.12

How boron looks like ?

Ans.

Boron looks like a greyish solid with a shiny surface and has slight metallic lusture although it is non-metallic.

Q.13

In which two forms elemental boron exists ?

Ans.

Elemental boron exists in two from

BORON FAMILY

Note

(1) α - Rhomobohedron (2) β - Rhomobohedron Q.14

Which form of boron is more stable ?

Ans.

β - Rhomobohedron is more stable and naturally occurring form of boron.

Q.15

β - Rhombohedron is made up of which units ?

Ans.

β - Rhombohedron is made up of lcosahedral boron units.

Q.16

What is icosahedral unit and what is its structure ?

Ans.

lcosahedron B 12 unit is an arrangement of 12 atoms with 20 faces. Boron exists in this arrangment Like - C60 , B60

also exists.

Q.17

How can you represent β - Rhombohedron ?

Ans.

β - Rhombohedron is represented by B - 84

Q.18

What is Ionisation energy ?

Ans.

It is the amount of energy needed to remove an outermost e – from neutral gaseous isolated atom. It is expressed in kJ/mol or Kcal / mol. It is evaluated for a single atom. It can be expressed in ev/atom.

Q.19

Which element has the higher I.E. among all elements ?

Ans.

Helium has the highest I.E. among all elements

Q.20

How Ionisation energy gets affected on successive loss of electron?

Ans.

I.E. successively increases on successive loss of e–. I.E1 < I.E2 < I.E3 < I.E4

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Q.21

Arrange third group elements in decreasing order of I.E.

Ans.

B > Tl > Ga > Al > In

Q.22

Why Be has higher I.E1 than B ?

Ans.

P. JOY Note

Be has stable electronic cofiguration 1s2 2s2 due to completely filled orbital so it will not loss e – . & has high value of I.E. whlile B has electronic configuration 1s2 2s2 2p1 . After loosing one e – it achieves stable electronic configuration 1s2 2s2 so it has lesser first ionisation energy value than Be.

Q. 23

Why I.E. cannot be measured in solid state ?

Ans.

I.E. cannot be measured in solid state because lattice energy is also added and in solid state isolated atom is not present.

Q.24

What is inert pair effect ?

Ans.

It is the reductance of the s-electrons of the valence shell to take part in bonding. It arises due to poor or ineffective shielding of the ns2 electrons of the valence shell by the intervening d- and f-electrons.

Q.25

Give the trend of inert pair effect from top to bottom ?

Ans.

Inert pair effect increases down a group and thus the elements present in the lower part of the group show lower oxideation states. In group 13 elements, inert pair effect is more pronounced in Tl.

Q.26

How inert pair effect affects the oxidation state of 13th group element ?

Ans.

B and Al show an oxidation state of +3 only while Ga, In and Tl show oxidation states of both +1 and +3. Since in case of Ga and In, +3 oxidation state is more stable than +1 oxidation state, therefore, both Ga+ and In+ salts undergo disproportionation. +1

0

3+

3GaX(x) → 2Ga(s) + Ga(aq) + 3X– (aq) +1

0

3+

3InX(s) → 2In(s) + In(aq) + 3X– (aq) as we move down the group, the stability of +3 oxidation state decreases while that of +1 oxidation state increases. Q.27

Why +1 oxidation state of thallium is stable than +3 state ?

Ans.

Due to inert pair effect.

Q.28

Which ion of Boron present in its aq. state

Ans.

B+3

Q.29

What are the ions formed by the ionisation of TlI3 ?

Ans.

Tl+, I3 –

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Note

COMPOUNDS OF BORON BORIC ACID : Q.30

What are the important compounds of boron ?

Ans.

(a) Boric acid : H3BO3 (b) Boron Sesquioxide : B2O3 (c) Boranes : B2H6 (d) Boronhalide : BF3, BCl3, BBr3, BI3 (e) Borax : Na2B4O7 (f) Boron nitride : BN (g) Borazole : B3N3H6

Q.31

What are the different names of H3BO3 ?

Ans.

H3BO3 is called as – Orthoboric acid – Boracic acid – Borie – Boric acid

Q.32

What are the various formula by which Boric acid can be shown? H O

Ans.

B

(a)

O

(b) H3BO3

O

H

(c) B(OH)3

H

Q.33

Is Boric acid triprotic or monoprotic acid ?

Ans.

Monoprotic acid.

Q.34

Why Boric acid is a monoprotic acid ?

Ans.

Boric acid when dissolve in water release a proton along with an anion. B(OH)3 + H2O

[B(OH)4]– + H+

OH OH

Q.35

B

Q

OH

Show what happens when boric acid is dissolved in heavy water? H

O

B

Ans. O H

H

H

O

12

OH

+ D2O → O H

Q B

O H

D

O →

Å O

OH OH

B

OH OD

Q + D⊕

D

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Q.36

Does there a lewis acid - base reaction take place when boric acid is dissolved in water.

Sol.

Yes, there is a lewis acid - base reaction in which water acts as lewis base and boric acid acts as lewis acid.

Q.37

Why boric acid is a Lewis acid.

Ans.

H3 BO 3 is a boric acid because it accepts a pair of electron from species like water to complete octet.

P. JOY Note

H3 BO 3 is a monoprotic acid. It could be monoprotic in water, heavy water or alcohols. Q.38

In which compound H3BO3 may exhibit acidic character ?

Ans.

Such compound which can act as a Lewis base example : H2O, CH3OH, C2H5OH

Q.39

Why B(OH)3 is not a base like NaOH, Ba(OH)2 ?

Ans.

Unlike NaOH, Ba(OH) 2 , Al(OH) 3 , it doesn’t release OH– . Thus, it is not a base.

Q.40

Write the reactions of preparation of boric acid ?

Ans.

Preparation of boric acid : H3PO4 1. Mg 3 B2 ¾¾¾¾¾ ® H3 BO 3 HCl

2. B ¾¾¾¾ ® H3 BO 3 H2SO4

H2O 3. BCl3 ¾¾¾¾ ® H3 BO 3

(BX3 : BF3 ,BCl3 ,BBr3 ,BI 3 ) H2O 4. BH3 ¾¾¾¾ ® H3 BO 3

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Note

Q.41

What happens when Boric acid is strongly heated ?

Ans.

Boric acid on heating looses water molecule first intramolecularly then intermolecularly. H3BO3 → HBO2 → B2O3 Metaboric Boron acid sesquioxide

Q.42 Ans.

Why carbonic acid is unstable ? Carbonic acid is H2CO3, it has two OH groups on the same carbon atom and the electrostatic repulsion between the singly bonded oxygen atoms make them unstable causing loss of water. H2CO3 C

Q.43

H2O + CO2

OH OH

C = O + H2O

Two OH group on a carbon atom

OH on silicon atom

C

OH OH

is unstable but two

is stable. Give reason.

Ans.

Carbon has no d-orbital but silicon has vacant d-orbital. Thus a double bond character is generated by a slight back bonding from lone pair into vacant d-orbital of silicon.

Q.44

Although Carbon and Boron both belong to second row and both donot have d-orbitals. Yet two hydroxy group on a Carbon is unstable whereas on Boron there are stable. Because Boron has vacant p-orbital which stabilizes three OH group by back donation.

Ans.

Q.45

Why orthoboric acid exist as a slippery solid ?

Ans.

Orthoboric acid exists as a slippery solids in which borate units are hexagonally clustered by H-bonds into a layer. These layers are separated by large distance about 3.18 Å resulting into easy sliding of layers over one another.

Q.46

Although Boric acid is a weak acid but it can’t be titrated with sodium hydroxide ............ explain why ? There are two reasons : (i) The borate ion produce in the titration produce back boric acid by the hydrolysis. (ii) The pH change on titration curve is not sharp thus end point of titration is not clear.

Ans.

Q.47

Why diols like ethylene glycol, catechol are added when boric acid is titrated with sodium hydroxide ?

Ans.

The tetrahydroxo borate ion produced in the titration dissociates in reversed direction producing back orthoboric acid. When catechol or other diols are added then an esterification take place and tetrahydroxo borate units get bind with diols irreversibly this doesn’t produce back orthoboric acid.

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Q.48

Why catechol is added when boric acid is titrated with NaOH ?

Ans.

H3BO3 is added with vicinal diols like glycol, catechol, sorbitol, mannitol etc. for effective titration which removes BO33– ion from solution and prevent its hydrolysis.

— C — OH — C — OH

Q.49

+

H—O H—O

B

O—H O—H

+

HO — C HO — C

→

—C—O —C—O

B

P. JOY Note

O—C— O—C—

Why titration of orthoboric acid is considered to be a difficult titration ?

Ans.

The pH change of titration is not sharp, thus end point of titration is not clear. BO33– ion produced in titration produces back boric acid by hydrolysis.

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Note

DIBORANES : Q.50

What are boranes ?

Ans.

Boron forms binary compounds with hydrogen known as boranes.

Q.51

How many boranes are known ?

Ans.

About 20 boranes are known.

Q.52

Which is simplest borane ?

Ans.

The simplest borane is diborane.

Q.53

Draw the structure of diborane ?

Ans.

Structure of B2H6

H 122º

H

9Å 1.1

B

3Å 1.2

BORON FAMILY

H

H 97º

B

H

122º

H

1.77Å

Q.54

Why structure of diborane is peculiar ?

Ans.

Structure is peculiar due to multicentre bond.

Q.55

Describe the structure of B2H6 ?

Ans.

(a) In diborane boron atom is sp3 hybridised in which one sp3-orbital is empty in its hybridised state before forming bridge bonds. (b) In diborane the outward 4B — H bonds lie in the same plane making H — B — H bond angle of 122º. (c) There are bridging hydrogens perpendicular to the plane of 4 single B — H bonds. These bridging hydrogen are said to form bridge bonds are banana bonds. (d) Due to bent shape of B — H — B bonds these are called banana bond.

Q.56

What is banana bond ?

Ans.

In banana bond 3 atoms {B — H --- B} i.e. 3 centre 2 electrons only. Thus these are called 2e — 3C bond. {Normally a covalent bond is 2e — 2C bond}. Due to banana bonds B2H6 is said to be an electron deficient compound.

Q.57

How B2H6 can be converted into its monomer ?

Ans.

B2H6 can be converted into its monomer by various complexing ligand like CO, NH3, Amines (RNH2), THF (Tetrahydrofurane)etc. B2H6 dimer

16

2BH3 monomer

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Note

Examples :

H H

H H

B

:O H

.. B + NH3 H

NH3

C O

BH3 + CO

Q.58 Ans.

Mention various reaction of B2H6 with NH3 :

Q.59

What is inorganic benzene ? Is it more reactive or less reactive than benzene ? Inorganic benzene is more reactive. H

Ans.

H

H

P. JOY

B NÅ Q ÅN Q B Å QB N

H

H

H

Q.60 Ans.

What happens when inorganic benzene is reacted with HCl ? B3N3H6 + 3HCl → B3N3H9Cl3 H

H H

N

B

N

H

H 3HCl

H

B

N H

B

¾¾¾¾ ®

H

H H

Cl B

N B

N B

N

Cl H

H H H Cl

H

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Q.61

Give methods of preparation of diborane ?

Ans.

I2 + 2NaBH4 → B2H6 + 2NaI + H2

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Note

2(NaHBH3)

Q.62 Ans.

Compare borane with ethane ? ETHANE

BORANE

(i) Molecular formula - C2H6,

B2H6

(ii) Does not show combustion spontaneously

Instant combustion

(iii) Has no monomer

Has monomer BH3

(iv) Ethane on combustion produces CO2 + H2O

Borane on combustion produced B2O3 + H2O

Note : Known number of alkanes are very large as compared to known number of boranes because carbon has property of catenation due to which large number of alkanes are formed.

18

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BORAX : Q.63

What is the formula of Borax ?

Ans.

Na2B4O7.10H2O

Q.64

Draw the structure of Borax ?

OH Na

O H—O—B

Na

2—

B

+

Ans.

P. JOY

+

O O

O

B — OH O

≡

B OH

Q.65

What is commercial name of Borax ?

Ans.

Tinnacle

Q.66

How many number of B–O bond are present in Borax.

Ans.

14 bonds

Q.67

How many number of B–O–B bond are present in Borax.

Ans.

5 bonds

Q.68

How many number of sp3 hybridised Boron are present in Borax.

Ans.

2

Q.69

How many number of sp2 hybridised Boron are present in Borax.

Ans.

2

Q.70

Write the formula of buffer solution of Borax in water.

Ans.

2H3BO3 + 2NaB(OH)4

Q.71

How many Boron units are connected by an oxygen atom diagonally ?

Ans.

Two

Q.72

How many Boron atoms in Borax acts as Lewis acid ?

Ans.

Two

Q.73

Write an equation to show hydrolysis of Borax ?

Ans.

[Na2B4O7.10H2O]

E D U C A T I O N S

H2O

2H3BO O3 + 2[B(OH)4]– + 2H+

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Q.74

What are hydrolysis product(s) of Borax ?

Ans.

Hydrolysis of borax results into formation of orthoboric acid and tetrahydroxoborate ion. OH Na

2—

B

+

O H—O—B

Na

BORON FAMILY

O O

B — OH

O

+

H2O

O

2B(OH)3 + 2 NaB(OH)4

B OH

Q.75 Ans.

What is the nature of aqueous solution of borax ? Aqueous solution of borax acts as a buffer solution. (where pH does not change on addition of small amount of acid or base). It contains H3BO3 (WA) & its conjugate base NaB(OH4)

Q.76

Write an equation to show reaction of Borax with HCl or H2SO4. OH Na

O H—O—B

Ans. Na

+

2—

B

+

O O

B — OH

O

O

+ HCl → 2H3BO3 + 2[B(OH)4]–

B

H

+

OH

2H3BO3 Q.77 Ans.

How many moles of NaOH are consumed when Borax is treated with NaOH 2 moles of NaOH is consumed by 2 moles of [B(OH)4]–

Q.78 Ans.

What happen's when [B(OH)4]– is reacted with HCl ? 2[B(OH)4]– + HCl → 2H3BO3

Q.79 Ans.

What happens when borax is heated ? Action of heat on borax. D

D

® Na2B4O7.5H2O ¾¾¾ ® Na2B4O7 → NaBO Na2B4O7.10H2O ¾¾¾ O2 + B2O3 (Jeweller’s Borax)

Q.80 Ans.

20

Grassy solid and used for borax bead test

What are the different types of Borate ? There are three types of borate

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BORON HALIDE : Q.81

What are the physical state of BF3 , BCl3 , BI3 ?

Ans.

BF3 - gas

P. JOY Note

BCl3 - liquid BI3 - solid Q.82

What is the general characteristics of borontrihalide ?

Ans.

Lewis acid.

Q.83

Which boron trihalide is most stable ?

Ans.

BF3

Q.84

Which boron trihalide is used in Friedal-Craft Reaction ?

Ans.

BF3

Q.85

Why B – F bond is shorter & stronger than its theoretically expected value ?

Ans.

Due to Back bonding between empty p-orbital of boron and 2p-orbital of fluorine.

Q.86

What is the correct order of Lewis acid character of various boron halide ?

Ans.

BI3 > BBr3 > BCl3 > BF3

Q.87

Which BX 3 has more efficiency of back-bonding ?

Ans.

BF3

Q.88

Which boron trihalide is used in Vanarkel method ?

Ans.

BI3

Q.89

What is the structure of BF3 ?

Ans.

Trigonal planar.

Q.90

Pure BBr3 and BI3 are colourless but on exposure to light they become coloured. Explain why ?

Ans.

BBr3 and BI3 trihalides decompose on exposure to light and turns into the respective halogen i.e. Br2 and I2. Br2 and I2 are already coloured. BBr3 and BI3 undergoes partial photolysis in presence of sunlight which gives Br2 and I2 which are responsible for imparting colour. Down the group, the colour of halogens increases.

Q.91

BF3 has higher ∆Hf than other halides of Boron. Explain why ?

Ans.

BF3 has highest heat of formation due to following reason. (a) Small size and highest electronegativity of halogen atom results into a strong sigma bond formation making ∆Hf a large value. (b) Due to back bonding there is further strengthening of bond and resulting more loss of heat as ∆Hf. Note : One should remember that ∆Hf depends on bond strength. Stronger the bond formed, larger the ∆Hf.

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Q.92

Dimer of BX3 is known but not of BCl3 ?

Ans.

BH3 has no back bonding from hydrogen to boron. Thus boron remains

highly electron dificient and it undergoes dimerization to complete its octet by multicentre (2e – 3C) bonds. In BCl3 the back bonding reduces electron difficiency of B and B does not afford formation of dimer.

Q.93

How BI3, BBr3, BCl3 can be reduced to pure boron ?

Ans.

By reduction boron trihalide can be reduced into pure boron by using various reducing agents. Ex. : The decomposition of BI3 at tungsten filament results into deposition of pure boron.

Q.94

Why hydrolysis of BF3 is difficult than hydrolysis of other boron trihalide (BX3) ?

Ans.

F F

B—F +

H

.. O:

H

F F

OH F

B — OH

B — OH

OH OH

B — OH

In BF3 an efficient backbonding takes place between fluorine and boron which reduces electron deficient lewis acid behaviour of boron. This does not allow an appreciable lewis acid-base interaction between water and BF3. Thus BF3 is difficult to hydrolysis. Q.95

Why BBr3 and BCl3 fumes in air ?

Ans.

Air contains molecule which causes hydrolysis of BCl3 and BBr3 resulting into formation of HCl and HBr gases. These gases when released appears as fumes. But hydrolysis of BF3 occurs in a poor extent and does not release sufficient quantity of HF to appear as a fume.

Q.96

What happens when any two of halides BF3, BCl3, BBr3 are mixed together ?

Ans.

Redistribution of halogen takes place and an equilibrium is established.

Q.97

What happens if BCl3, BF3, BBr all are mixed together ?

Ans.

Cl Cl

Cl Cl

22

B

B

Cl + Br

Br +

I

B

B

Br Br

I Cl

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P. JOY Note

Draw the structure of B4Cl4 ?

Cl Ans.

Cl — B

B B

B — Cl

Cl

Q.99

What happens when BF3 is reacted with RMgX [Grignard reagent]? F

Ans.

B R

R

RMgX

F

B R

R

RMgX

F

B R

R

Q.100 What is the Vanarkel method of purification of Boron. Ans.

Re d hot W or Ta filament 2BI3 ¾¾¾¾¾¾¾¾¾¾¾¾¾ ® 2B + 3I3

(Pure form)

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Q.101 Aluminium belongs to which block elements ? Ans.

BORON FAMILY

p-block elements

Q.102 Which type of compounds are formed by Aluminium : Ans.

Electrovalent and covalent compounds both

Q.103 Which element is the most electropositive in 13th group ? Ans.

Al

Q.104 Which metal is most abundant in the earth's crust ? Ans.

Al

Q.105 Why atomic radius of Ga is lesser than that of Al ? Ans.

Due to Poor shielding power of d-electrons of Ga atoms.

Q.106 Write the decreasing order of reduction potential of IIIA elements ? Ans.

Al > Ga > In > Tl

Q.107 Why Al reduces most of the metallic oxides ? Ans.

Due to its greater affinity for Oxygen.

Q.108 On heating Al at 800ºC in air, Al2O3 is formed, which type of reaction is this ? Ans.

An exothermic reaction

Q.109 Why Aluminium becomes passive in nitric acid ? Ans.

Because Al forms a thin film of oxide (Al2O3) around it & become inert towards nitric acid.

Q.110 Al is more reactive than Fe but Al is less easily corroded than Fe. Why? Ans.

Because Al forms a protective oxide layer around it.

Q.111 Why Aluminium metal is corroded in coastal places near to the sea ? Ans.

Because protective oxide film (Al2O3) is attacked by salt present in sea water.

Q.112 What are the uses of Aluminium ? Ans.

Aluminium is used in thermite welding, for making utensils, in sillvery paints.

Q.113 What is thermite ? Ans.

It is a mixture of 1 part of powdered Al and 3 parts of Fe2O3

Q.114 Which reaction forms the basis of Goldschmidt aluminothermite process ? Ans.

24

2Al + Fe 2O3 → Al2O3 + 2Fe.

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Q.115 What is Alumino-thermite process ? Ans.

P. JOY Note

It is an exothermic reduction of metal oxides by heating with Al.

Q.116 Why Aluminothermy used for on the spot welding of large iron structures Ans.

As compared to iron, aluminium has greater affinity for oxygen & releases more amount of energy.

Q.117 Why Aluminium vessels should not be washed with materials containing washing soda Ans.

Because washing soda reacts with Al to form soluble aluminate

Q.118 Write the reactions of aluminium with

Ans.

(a) O2

(b) N2

(c) Cl2

(d) Cr2O3

(e) Fe2O3

(f) Mn3O4

(g) HCl

(h) H2SO4(dil.)

(i) H2SO4 (conc.)

(j) HNO3

(k) NaOH

(l) AlCl3

(a) 2Al + 3O2 → Al2O3 (b) 2Al + N2 → 2AlN (c) 2Al + 3Cl2 → 2AlCl3 (d) Al + Cr2O3 → Al2O3 + Cr (e) Al + Fe 2O3 → Al2O3 + 2Fe (f) Al + Mn3O4 → Al2O3 + Mn (g) Al + HCl → AlCl3 + H2 ↓H2O Al(OH)3 + 3HCl (h) 2Al + 3H2SO4(dil) → Al2(SO4)3 + 6H2 (i) 2Al + H2SO4(conc.) → Al2(SO4)3 + SO2 + H2O (j) Al + HNO3 → No reaction (k) Al + NaOH → Al(OH)3 + 3/2H2 High Temp.

→ 3AlCl (l) 2Al + AlCl3  

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HYDROXIDE : Q.119 What are the various hydroxides of group III element and how do their acidic and basic properties vary ? Ans.

Acidic character decreases from top to bottom. Basic character increases from top to bottom B(OH)3 Al(OH)3 Ga(OH)3 In(OH)3 Tl(OH)3

Q.120 Which hydroxide of group III show amphoteric behaviour ? Ans.

Al(OH)3 and Ga(OH)3

Q.121 How do you show that Al(OH)3 is amphoteric ? Ans.

Since it behaves as lewis acid as well as lewis base. HCl

Al(OH)3 + 3HCl ® 3H2O + AlCl3 (Base)

Al(OH)3 NaOH

Al(OH)3+ NaOH ® NaAl(OH)4 (Acid)

Q.122 What happens when Al(OH)3 is dissolved in sodium hydroxide (NaOH)? Ans.

Meta aluminate (NaAlO2) is formed.

Q.123 Al(OH)3 is water soluble/insoluble ? Ans.

Insoluble

Q.124 How can you dissolve Al(OH)3 ? Ans.

Further increasing pH or by adding NaOH.

Q.125 Alumina is water soluble/insoluble ? Ans.

Insoluble.

Q.126 NaAlO2 is water soluble/insoluble ? Ans.

Soluble

Q.127 Write the reactions of Al(OH)3 with (a) Acid

(b) NaOH

(c) Heating Below 450ºC Ans.

+

(a) Al(OH)3 + 3H → Al

+3

(d) Heating above 450ºC + 3H2O

acid (b) Al(OH)3 + NaOH → Na[Al(OH)4] or NaAlO2.2H2O base

sodium meta aluminate

Below 450ºC (c) Al(OH)3     → Al2O3 (α-form)

Above 450ºC (d) Al(OH)3   → Al2O3 (γ-form)

26

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Q.128 What is the nature of oxides of IIIA group elements ? Ans.

P. JOY Note

B2O3 - Acidic Al2O3, Ca2O3, In2O3 - Amphoteric Tl2O - Basic

Q.129 Which is the most stable compound of aluminium. Ans.

Al2O3

Q.130 What are the different forms of alumina. Ans.

α-Al2O3, β-Al2O3

Q.131 Which form of alumina is corundum. Ans.

α-form

Q.132 Which form of alumina is hard abbrasive. Ans.

α-form

Q.133 Which form of alumina is soft. Ans.

γ-form

Q.134 Which form of alumina is amphoteric. Ans.

γ-form

Q.135 Which form of alumina is acid soluble. Ans.

γ-Al 2 O 3

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EXTRACTION OF ALUMINIUM : Q.136 Which ore of Aluminium is used for its extraction ? Ans.

Bauxite.

Q.137 What is the formula of Bauxite ? Ans.

Al2O3.2H2O

Q.138 What is the major composition of gangue (impurity) in Bauxite ? Ans.

Silica (SiO2) or sand

Q.139 What are the steps involved in the extraction of Al ? Ans.

(a) Dressing or concentration (removal of gangue) (b) Reduction (c) Refining

Q.140 What are the various processes used for concentration of Bauxite ore ? Ans. (a) Baeyer's process (b) Hall’s process (c) Serpeck's process Q.141 What is Baeyer's process ? Ans.

Baeyer's Process D

HCl

NaOH NaOH ® Al(OH)3 ¾¾¾ ® Al2O3 Al2O3 ¾¾¾¾® Al(OH)3 ¾¾¾¾® NaAlO2 ¾¾¾¾

Q.142 What is Hall's process ? Ans.

Hall's process

Q.143 What is Serpeck's process ? Ans.

Serpeck's process

Al2O 3

C + N2 D

AIN

HOH

Al(OH)3 + NH3

NaOH

NaAlO2

CO2

Al(OH)3

D

Al2O3

electric arc

Q.144 In which purification process of bauxite ore, ore is mixed with sodium carbonate (Na 2CO 3) ? Ans.

Hall’s process

Q.145 Which process is used for purification of bauxite containing high silica content as impurity ? Ans.

Serpeck’s process

Q.146 What is the by-product obtained in the serpeck’s process ? Ans.

28

Ammonia

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Q.147 Which substance is used to remove SiO2 in the serpeck’s process of purification of bauxite Ans.

P. JOY Note

Coke

Q.148 What are the reactions taking place at cathode and anode in electrolytic reduction ? Ans.

Electrolytic reduction

At cathode :

® 2Al3+ + 6F– 2AlF3 ¾¾¾

® 2Al 2Al3+ + 6e– ¾¾¾

® 2AlF3 + 3O2– 6F– + Al2O3 ¾¾¾ At anode :

® 3CO + 6e– 3O2– + 3C ¾¾¾ Q.149 Why electrolytic reduction of pure alumina is not possible ? Ans.

Because it melts at very high temperature

Q.150 What is cryolite ? Ans.

Na 3AlF6

Q.151 Why fused cryolite is added during electrolytic extraction of aluminium ? Ans.

Alumina has high melting point, addition of cryolite lowers the melting point as well as increases the conductance of fused mass.

Q.152 Which process is used for purification of aluminium by electrolytic refining ? Ans.

Hoope’s process.

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HALIDES : Q.153 Which halide of IIIA group element exist as a monomer ? Ans.

BCl3

Q.154 What is the nature of halide of Thallium ? Ans.

Ionic

Q.155 Which IIIA group element form dimer ? Ans.

Al, Ga & In form M2x6 dimers

Q.156 In which phase and condition aluminium chloride exists in dimeric form ? Ans.

From the molecular weight determinations it follows that aluminium chloride exists in dimeric form in vapour state as well as in inert organic solvents.

Q.157 How AlCl3 exist above 800ºC and below 400ºC ? Ans.

Above 800ºC aluminium chloride (AlCl3) exists as monomer in which Al atom is sp2 hybridised. Below 400ºC it is clearly dimeric and each Al atom is sp3 hybridised.

Q.158 How AlCl3 exist in solid state ? Ans.

In solid state it is monomer.

Q.159 Are the bonding states of Al2Cl6 in vapour phase and in aqueous phase are different. Ans.

Yes

Q.160 Describe the structure of Al2Cl6 ? Ans.

In the dimeric structure, each aluminium atom has completed its octet by accepting a lone pair of electrons from the chlorine atom of another aluminium chloride molecule. This type of structure has been confirmed by X-ray studies.

Q.161 What is the similarity between structure Al2Cl6 and B2H6 ? Ans.

The structures of the halide dimers are almost exactly like that of diborane with Al – Cl – Al bridge bonds are three-centre two electron bonds.

Q.162 Unlike Al2Cl6, boron trichloride does not form a dimer. Ans.

The reason is due to smaller size of boron atom than aluminium atom and therefore the chloride bridges will not be stable.

Q.163 What happens when Al2Cl6 dimer is added to water. Ans.

The high enthalpy of hydration is sufficient to break the covalent dimer into [M(H2O)6]3+ and 3X– ions.

Q.164 How Al2Cl6 is prepared. Ans.

30

It is prepared by passing dry chlorine over aluminium powder.

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Q.165 How Al2O3 can be converted into Al2Cl6 Ans.

P. JOY Note

Al2O3 + 3C + 3Cl2 → 2AlCl3 + 3CO

Q.166 What happens when Al2Cl6 is reacted with LiH Ans. Al2Cl6 + LiH (excess) → Li[AlH4] Q.167 What is the nature of AlCl3 in aqueous state. Ans. Acidic, AlCl3 + 3H2O → Al(OH)3 + 3HCl Q.168 Does AlCl3 on heating melts. Ans. Yes Q.169 Why is it difficult keep AlCl3 dry Ans. It is hydroscopic in nature. Q.170 What happens when AlCl3 reacted with Cl– -ions Ans. AlCl3 + Cl– → [AlCl4]– Q.171 What happens when AlCl3 vapours are passed over fused Al2O3 at 1000ºC. Ans. 2Al2O3 + 2AlCl3 → 6AlCl + 3O2 Q.172 Comment on stability of AlCl Ans. AlCl is unstable below 800ºC. It disproportionate below 800ºC 3AlCl → AlCl3 + 2Al Q.173 Why AlCl3.3H2O cannot be dehydrated by heating ? Ans. AlCl3 get hydrolysed to form Al(OH)3 which undergoes intramolecular dehydration and forms alumina. Q.174 Why AlCl3 fumes in air ? Ans. Due to hydrolysis of AlCl3 by moisture, HCl is formed which fumes is air. Q.175 Describe the structure of Al2(CH3)6 ? Ans. Aluminium trimethyl, Al2(CH3)6 is a dimer and has a structure similar to that of the dimeric aluminium chloride with methyl groups replacing the Cl atoms. The structural parameters are given in fig. Trialkyls containing branched chains such as iso-propyl group do not dimerize.

In the dimer of trimethyl aluminium the CH3 group will act as a bridge between two aluminium atoms. The electron cloud which involved in the bridge is distributed on two aluminium atoms and one carbon atom. Even then it should not be considered that the carbon is pentavalent. The carbon will have only eight electrons one shared by two aluminium atoms. Q.176 Which of the following B(CH3)3 or Al(CH3)3 form dimer ? Ans.

Trimethyl boron B(CH3)3 is gaseous monomer. It will not form associated molecules with itself even though the central atom is having only six electrons. But trimethyl aluminium exists as dimer, an ordinary temperatures.

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ALUM : Q.177 What are alums ? Ans.

Alums are double salts

Q.178 What is the general formula of alum. Ans.

M(I) M'(III) (SO4)2.12H2O where MI is a univalent cation such as NH4+, Na+, K+, Rb+ or Cs+ and MIII is a trivalent metal ion, e.g. Al3+, Cr3+ and Fe3+.

Q.179 Give some examples of alum ? Ans.

Some common examples are : potash alum, KAl(SO4)2.12H2O; chrome alum, KCr(SO4)2.12H2O; ferric alum, NH4Fe(SO4)2.12H2O; ammonium alum, NH4Al(SO4)2.12H2O; Thallous alum Tl2SO4.Al2(SO4)2.24H2O

Q.180 What is feather alum or hair salt. Ans.

Al2SO4.18H2O

Q.181 Which unipositive cations do form alum. Ans.

Almost any unipositive cations (except Li+) most common alums have K+, NH 4 +

Q.182 Which tripositive cations do form alum. Ans.

Al+3, Ti+3, V3+, Cr3+, Fe3+, Mn3+, Co3+, Ga3+, Re3+, Ir3+

Q.183 Which alkali metal cation does not form alum. Ans.

Li+

Q.184 Why Li+ does not form alum ? Ans.

The fact that Li+ does not form an alum is noteworthy. This is probably because of the very small size of the Li+ ion so that it cannot become 6-coordinate.

Q.185 How many water molecules act as ligands about mono and trivalent metal cations in alum. Ans.

6 water molecule act as ligands about mono valent metal cations and 6 water molecule act as ligands about trivalent metal cations.

Q.186 How do structures of all alums differ. Ans.

Due to different type of ions.

Q.187 How are alums prepared. Ans.

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Alums may be prepared by mixing saturated solutions containing equimolecular proportions of the component sulphates and allowing to crystallize.

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Q.188 What happens when alums are heated. Ans.

P. JOY Note

On heating strongly, all the water of crystallisation of an alum is eliminated and a swollen mass of oxides is formed. This mixture of oxides is known as slaked alum. D

® K2O + Al2O3 + 4SO3↑ + 24H2O↑ K2SO4.Al2(SO4)3.24H2O ¾¾¾ Q.189 Alums of which element(s) are sparing soluble ? Ans.

Alums of rubidium and cesium are sparingly soluble.

Q.190 What is the nature of aqueous solution of alums. Ans.

Acidic due to cationic hydrolysis of trivalent cation.

Q.191 What are pseudo alums. Ans.

When monovalent cation of alum is replaced by divalent cation, the double salt is then known as pseudo alum. For example MgSO4.Al2(SO4)3.24H2O, FeSO4.Al2(SO4)3.24H2O etc.

Q.192 Are pseudo alums isomorphous to alums. Ans.

No

Q.193 What are the uses of Potash alum ? Ans.

Potash alum finds important uses in : (i) Mordant in dyeing (ii) Arresting bleeding (iii) Calico printing (iv) Purification of water (v) Tanning of leather (vi) Extinguishing the fire

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