Case Study Questions Class 11 Chemistry p-block Elements

Case Study Questions Class 11 Chemistry Chapter 11 p-block  Elements

CBSE Class 11 Case Study Questions Chemistry p-block  elements. Important Case Study Questions for Class 11 Board Exam Students. Here we have arranged some Important Case Base Questions for students who are searching for Paragraph Based Questions p-block  elements.

At Case Study Questions there will given a Paragraph. In where some Important Questions will made on that respective Case Based Study. There will various types of marks will given 1 marks, 2 marks, 3 marks, 4 marks.

CBSE Case Study Questions Class 11 Chemistry p-block Elements

 

Case Study – 1

In p-block elements the last electron enters the outermost p orbital. As we know that the number of p orbitals is three and, therefore, the maximum number of electrons that can be accommodated in a set of p orbitals is six. Consequently there are six groups of p–block elements in the periodic table numbering from 13 to 18. Boron, carbon, nitrogen, oxygen, fluorine and helium head the groups. Their valence shell electronic configuration is ns2np1-6(except for He). The inner core of the electronic configuration may, however, differ. The difference in inner core of elements greatly influences their physical properties (such as atomic and ionic radii, ionisation enthalpy, etc.) as well as chemical properties. The occurrence of oxidation states two unit less than the group oxidation states are sometime attributed to the ‘inert pair effect’.

Group 13 elements: the boron family This group elements show a wide variation in properties. Boron is a typical non-metal, aluminium is a metal but shows many chemical similarities to boron, and gallium, indium, thallium and nihonium are almost exclusively metallic in character. Boron is a fairly rare element, mainly occurs as orthoboric acid, (H3BO3), borax, Na2B4O7·10H2O, and kernite, Na2B4O7·4H2O. In India borax occurs in Puga Valley (Ladakh) and Sambhar Lake (Rajasthan). The abundance of boron in earth crust is less than 0.0001% by mass. There are two isotopic forms of boron 10B (19%) and 11B (81%). Aluminium is the most abundant metal and the third most abundant element in the earth’s crust (8.3% by mass) after oxygen (45.5%) and Si (27.7%). Bauxite, Al2O3. 2H2O and cryolite, Na3AlF6 are the important minerals of aluminium. In India it is found as mica in Madhya Pradesh, Karnataka, Orissa and Jammu. Gallium, indium and thallium are less abundant elements in nature. Nihonium has symbol Nh, atomic number 113, atomic mass 286 g mol-1 and electronic configuration [Rn]5f146d107s27p2. So far it has been prepared in small amount and half life of its most stable isotope is 20 seconds. Due to these reasons its chemistry has not been established. Nihonium is a synthetically prepared radioactive element. Here atomic, physical and chemical properties of elements of this group leaving nihonium are discussed below.

The outer electronic configuration of these elements is ns 2np1 . A close look at the electronic configuration suggests that while boron and aluminium have noble gas core, gallium and indium have noble gas plus 10 d-electrons, and thallium has noble gas plus 14 f- electrons plus 10 d-electron cores. Thus, the electronic structures of these elements are more complex than for the first two groups of elements discussed in unit 10. This difference in electronic structures affects the other properties and consequently the chemistry of all the elements of this group.

Atomic Radii On moving down the group, for each successive member one extra shell of electrons is added and, therefore, atomic radius is expected to increase. However, a deviation can be seen. Atomic radius of Ga is less than that of Al. This can be understood from the variation in the inner core of the electronic configuration. The presence of additional 10 d-electrons offer only poor screening effect (Unit 2) for the outer electrons from the increased nuclear charge in gallium. Consequently, the atomic radius of gallium (135 pm) is less than that of aluminium (143 pm).

Boron is non-metallic in nature. It is extremely hard and black coloured solid. It exists in many allotropic forms. Due to very strong crystalline lattice, boron has unusually high melting point. Rest of the members are soft metals with low melting point and high electrical conductivity. It is worthwhile to note that gallium with unusually low melting point (303K), could exist in liquid state during summer. Its high boiling point (2676 K) makes it a useful material for measuring high temperatures. Density of the elements increases down the group from boron to thallium.

[A] MCQ

1) There are … groups of p–block elements in the periodic table.

a) six

b) seven

c) eight

d) two

Ans- a) six

 

2) Boron is … in nature.

a) metallic

b) non-metallic

c) metalloid

d) All the above

Ans- b) non-metallic

 

3) Boiling point of gallium is …

a) 303K

b) 1345K

c) 2676 K

d) 1854K

Ans- c) 2676 K

 

4) The occurrence of oxidation states two unit less than the group oxidation states are sometime attributed to the …

a) loan pair effect

b) middle pair effect

c) outer pair effect

d) inert pair effect

Ans- d) inert pair effect

 

5) Density of the elements … down the group from boron to thallium.

a) increases

b) decreases

c) remains constant

d) none of above

Ans- a) increases

[B]Short Answers

 

1) Write the physical properties of the boron family elements .

Ans- Boron is non-metallic in nature. It is extremely hard and black coloured solid. It exists in many allotropic forms. Due to very strong crystalline lattice, boron has unusually high melting point. Rest of the members are soft metals with low melting point and high electrical conductivity. It is worthwhile to note that gallium with unusually low melting point (303K), could exist in liquid state during summer. Its high boiling point (2676 K) makes it a useful material for measuring high temperatures. Density of the elements increases down the group from boron to thallium.

 

2) Give the electronic configuration of boron family elements .

Ans- The outer electronic configuration of these elements is ns 2np1 . A close look at the electronic configuration suggests that while boron and aluminium have noble gas core, gallium and indium have noble gas plus 10 d-electrons, and thallium has noble gas plus 14 f- electrons plus 10 d-electron cores. Thus, the electronic structures of these elements are more complex than for the first two groups of elements discussed in unit 10. This difference in electronic structures affects the other properties and consequently the chemistry of all the elements of this group.

 

3) Explain the trend in atomic radii of boron family elements .

Ans- Atomic Radii On moving down the group, for each successive member one extra shell of electrons is added and, therefore, atomic radius is expected to increase. However, a deviation can be seen. Atomic radius of Ga is less than that of Al. This can be understood from the variation in the inner core of the electronic configuration. The presence of additional 10 d-electrons offer only poor screening effect (Unit 2) for the outer electrons from the increased nuclear charge in gallium. Consequently, the atomic radius of gallium (135 pm) is less than that of aluminium (143 pm).

 

[C]Long Answers

 

1) Write short note on boron family elements.

Ans- The boron family This group elements show a wide variation in properties. Boron is a typical non-metal, aluminium is a metal but shows many chemical similarities to boron, and gallium, indium, thallium and nihonium are almost exclusively metallic in character. Boron is a fairly rare element, mainly occurs as orthoboric acid, (H3BO3), borax, Na2B4O7·10H2O, and kernite, Na2B4O7·4H2O. In India borax occurs in Puga Valley (Ladakh) and Sambhar Lake (Rajasthan). The abundance of boron in earth crust is less than 0.0001% by mass. There are two isotopic forms of boron 10B (19%) and 11B (81%). Aluminium is the most abundant metal and the third most abundant element in the earth’s crust (8.3% by mass) after oxygen (45.5%) and Si (27.7%). Bauxite, Al2O3. 2H2O and cryolite, Na3AlF6 are the important minerals of aluminium. In India it is found as mica in Madhya Pradesh, Karnataka, Orissa and Jammu. Gallium, indium and thallium are less abundant elements in nature. Nihonium has symbol Nh, atomic number 113, atomic mass 286 g mol-1 and electronic configuration [Rn]5f146d107s27p2 . So far it has been prepared in small amount and half life of its most stable isotope is 20 seconds. Due to these reasons its chemistry has not been established. Nihonium is a synthetically prepared radioactive element

 

2) What are p-block elements ?

Ans- In p-block elements the last electron enters the outermost p orbital. As we know that the number of p orbitals is three and, therefore, the maximum number of electrons that can be accommodated in a set of p orbitals is six. Consequently there are six groups of p–block elements in the periodic table numbering from 13 to 18. Boron, carbon, nitrogen, oxygen, fluorine and helium head the groups. Their valence shell electronic configuration is ns2np1-6(except for He). The inner core of the electronic configuration may, however, differ. The difference in inner core of elements greatly influences their physical properties (such as atomic and ionic radii, ionisation enthalpy, etc.) as well as chemical properties. The occurrence of oxidation states two unit less than the group oxidation states are sometime attributed to the ‘inert pair effect’.

 

Case Study – 2

Oxidation state and trends in chemical  Reactivity  Due to small size of boron, the sum of its first  Three ionization enthalpies is very high. This  Prevents it to form +3 ions and forces it to form  Only covalent compounds. But as we move from  B to Al, the sum of the first three ionisation  Enthalpies of Al considerably decreases, and  Is therefore able to form Al3+ ions. In fact,  Aluminium is a highly electropositive metal.  However, down the group, due to poor  Shielding effect of intervening d and f orbitals,  The increased effective nuclear charge holds ns  Electrons tightly (responsible for inert pair  Effect) and thereby, restricting their  Participation in bonding. As a result of this,  Only p-orbital electron may be involved in  Bonding. In fact in Ga, In and Tl, both +1 and  +3 oxidation states are observed. The relative  Stability of +1 oxidation state progressively  Increases for heavier elements: Al<Ga<In<Tl. In  Thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly  Oxidising in character. The compounds in  +1 oxidation state, as expected from energy  Considerations, are more ionic than those in  +3 oxidation state.

Important trends and anomalous properties of boron – certain important trends can be observed in the chemical behaviour of group 13 elements. The tri-chlorides, bromides and iodides of all these elements being covalent in nature are hydrolysed in water. Species like tetrahedral [M(OH)4] and octahedral [M(H2O)6]3+, except in boron, exist in aqueous medium. The monomeric trihalides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as NH3 to complete octet around boron. It is due to the absence of d orbitals that the maximum covalence of B is 4. Since the d orbitals are available with Al and other elements, the maximum covalence can be expected beyond 4. Most of the other metal halides (e.g., AlCl3) are dimerised through halogen bridging (e.g., Al2Cl6). The metal species completes its octet by accepting electrons from halogen in these halogen bridged molecules.

i) Reactivity towards air  Boron is unreactive in crystalline form.  Aluminium forms a very thin oxide layer on  The surface which protects the metal from  Further attack. Amorphous boron and  Aluminium metal on heating in air form B2O3  And Al2O3 respectively. With dinitrogen at high  Temperature they form nitrides. The nature of these oxides varies down the  Group. Boron trioxide is acidic and reacts with  Basic (metallic) oxides forming metal borates.  Aluminium and gallium oxides are amphoteric  And those of indium and thallium are basic in  Their properties.

ii) Reactivity towards acids and alkalies Boron does not react with acids and alkalies Even at moderate temperature; but aluminium  Dissolves in mineral acids and aqueous alkalies  And thus shows amphoteric character.  Aluminium dissolves in dilute HCl and  Liberates dihydrogen.

2Al(s) + 6HCl (aq) → 2Al3+ (aq) + 6Cl (aq)   + 3H2 (g)

However, concentrated nitric acid renders  Aluminium passive by forming a protective  Oxide layer on the surface.  Aluminium also reacts with aqueous alkali  And liberates dihydrogen.

2Al (s) + 2NaOH(aq) + 6H2O(l)  →  2 Na+ [Al(OH)4]  (aq) + 3H2 (g)

Sodium  Tetrahydroxoaluminate(III).

iii) Reactivity towards halogens  These elements react with halogens to form  Trihalides (except TlI3).  2E(s) + 3 X2 (g) → 2EX3 (s) (X = F, Cl, Br, I)

Borax-  It is the most important compound of boron.  It is a white crystalline solid of formula  Na2B4O7⋅10H2O. In fact it contains the  Tetranuclear units and correct  Formula; therefore, is Na2 [B4O5 (OH) 4].8H2O.  Borax dissolves in water to give an alkaline  Solution.

Na2B4O7 + 7H2O → 2NaOH + 4H3BO3

On heating, borax first loses water  Molecules and swells up. On further heating it  Turns into a transparent liquid, which solidifies  Into glass like material known as borax  Bead.  Na2B4O7.10H2O → Na2B4O7 → 2NaBO2   + B2O3 

Metaborate Boric Anhydride The metaborates of many transition metals  Have characteristic colours and, therefore,  Borax bead test can be used to identify them  In the laboratory. For example, when borax is  Heated in a Bunsen burner flame with CoO on  A loop of platinum wire, a blue coloured  Co(BO2) 2 bead is formed.

Orthoboric acid, H3BO3 is a white crystalline  Solid, with soapy touch. It is sparingly soluble  In water but highly soluble in hot water. It can  Be prepared by acidifying an aqueous solution  Of borax.

Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4B(OH)3

It is also formed by the hydrolysis (reaction  With water or dilute acid) of most boron  Compounds (halides, hydrides, etc.). It has a layer structure in which planar BO3 units are  Joined by hydrogen .

 

[A] MCQ

 

1) Boron is … in crystalline form.

a) unreactive

b) highly reactive

c) less reactive

d) only a) or c)

Ans – a) unreactive

 

2) Orthoboric acid is …

a) Amorphous

b) Crystalline

c) Polyamorphous

d) None of above

Ans- b) crystalline

 

3) Aluminium and gallium oxides are … in their properties.

a) acidic

b) Basic

c) amphoteric

d) None of above

Ans- c) amphoteric

 

4) Indium and thallium are … in  their properties.

a) acidic

b) Alkali

c) amphoteric

d) basic

Ans- d) basic

 

5) Aluminium is a highly … metal.

a) electronegative

b) Neutral

c) electropositive

d) None of above

Ans- c) electropositive

 

[B]Short Answers

 

1) Explain the important trends and anomalous properties of boron.

Ans- Certain important trends can be observed in the chemical behaviour of group 13 elements. The tri-chlorides, bromides and iodides of all these elements being covalent in nature are hydrolysed in water. Species like tetrahedral [M(OH)4] and octahedral [M(H2O)6]3+, except in boron, exist in aqueous medium. The monomeric trihalides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as NH3 to complete octet around boron. It is due to the absence of d orbitals that the maximum covalence of B is 4. Since the d orbitals are available with Al and other elements, the maximum covalence can be expected beyond 4. Most of the other metal halides (e.g., AlCl3) are dimerised through halogen bridging (e.g., Al2Cl6). The metal species completes its octet by accepting electrons from halogen in these halogen bridged molecules.

 

2) Write the short on borax.

Ans- It is the most important compound of boron.  It is a white crystalline solid of formula  Na2B4O7⋅10H2O. In fact it contains the  Tetranuclear units and correct  Formula; therefore, is Na2 [B4O5 (OH) 4].8H2O.  Borax dissolves in water to give an alkaline  Solution.

Na2B4O7 + 7H2O → 2NaOH + 4H3BO3 

On heating, borax first loses water  Molecules and swells up. On further heating it  Turns into a transparent liquid, which solidifies  Into glass like material known as borax  Bead.  Na2B4O7.10H2O → Na2B4O7 → 2NaBO2   + B2O3  

Metaborate Boric Anhydride The metaborates of many transition metals  Have characteristic colours and, therefore,  Borax bead test can be used to identify them  In the laboratory. For example, when borax is  Heated in a Bunsen burner flame with CoO on  A loop of platinum wire, a blue coloured  Co(BO2) 2 bead is formed.

 

3) Write short note on orthoboric acid .

Ans- Orthoboric acid, H3BO3 is a white crystalline  Solid, with soapy touch. It is sparingly soluble  In water but highly soluble in hot water. It can  Be prepared by acidifying an aqueous solution  Of borax.  Na2B4O7 + 2HCl + 5H2O → 2NaCl + 4B(OH)3  It is also formed by the hydrolysis (reaction  With water or dilute acid) of most boron  Compounds (halides, hydrides, etc.). It has a layer structure in which planar BO3 units are  Joined by hydrogen .

 

[C]Long Answers

 

1) Explain oxidation state and trends in chemical reactivity of boron family elements.

Ans- Oxidation state and trends in chemical  Reactivity  Due to small size of boron, the sum of its first  Three ionization enthalpies is very high. This  Prevents it to form +3 ions and forces it to form  Only covalent compounds. But as we move from  B to Al, the sum of the first three ionisation  Enthalpies of Al considerably decreases, and  Is therefore able to form Al3+ ions. In fact,  Aluminium is a highly electropositive metal.  However, down the group, due to poor  Shielding effect of intervening d and f orbitals,  The increased effective nuclear charge holds ns  Electrons tightly (responsible for inert pair  Effect) and thereby, restricting their  Participation in bonding. As a result of this,  Only p-orbital electron may be involved in  Bonding. In fact in Ga, In and Tl, both +1 and  +3 oxidation states are observed. The relative  Stability of +1 oxidation state progressively  Increases for heavier elements: Al<Ga<In<Tl. In  Thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly  Oxidising in character. The compounds in  +1 oxidation state, as expected from energy  Considerations, are more ionic than those in  +3 oxidation state.

2) Explain the trends in chemical reactivity of boron family elements.

Ans- The trends in chemical reactivity of boron family elements are as follows-

i) Reactivity towards air Boron is unreactive in crystalline form. Aluminium forms a very thin oxide layer on  The surface which protects the metal from  Further attack. Amorphous boron and  Aluminium metal on heating in air form B2O3  And Al2O3 With dinitrogen at high  Temperature they form nitrides. The nature of these oxides varies down the  Group. Boron trioxide is acidic and reacts with  Basic (metallic) oxides forming metal borates.  Aluminium and gallium oxides are amphoteric  And those of indium and thallium are basic in  Their properties.

ii) Reactivity towards acids and alkalies Boron does not react with acids and alkalies Even at moderate temperature; but aluminium  Dissolves in mineral acids and aqueous alkalies  And thus shows amphoteric character.  Aluminium dissolves in dilute HCl and  Liberates dihydrogen

 

Case Study – 3

Group 14 elements: the carbon  family- Carbon, silicon, germanium, tin lead and Flerovium are the members of group 14. Carbon  Is the seventeenth most abundant element by  Mass in the earth’s crust. It is widely  Distributed in nature in free as well as in the  Combined state. In elemental state it is available  As coal, graphite and diamond; however, in  Combined state it is present as metal  Carbonates, hydrocarbons and carbon dioxide  Gas (0.03%) in air. One can emphatically say  That carbon is the most versatile element in the  World. Its combination with other elements  Such as dihydrogen, dioxygen, chlorine and  Sulphur provides an astonishing array of  Materials ranging from living tissues to drugs  And plastics.

The valence shell electronic configuration of  These elements is ns2np2 .The inner core of the  Electronic configuration of elements in this  Group also differs.

Covalent Radius  There is a considerable increase in covalent  Radius from C to Si, thereafter from Si to Pb a  Small increase in radius is observed. This is  Due to the presence of completely filled d and f  Orbitals in heavier members.

Ionization Enthalpy  The first ionization enthalpy of group 14  Members is higher than the corresponding  Members of group 13. The influence of inner  Core electrons is visible here also. In general the  Ionisation enthalpy decreases down the group.  Small decrease in ∆i  H from Si to Ge to Sn and  Slight increase in ∆i  H from Sn to Pb is the  Consequence of poor shielding effect of  Intervening d and f orbitals and increase in size  Of the atom.

Electronegativity  Due to small size, the elements of this group  Are slightly more electronegative than group  13 elements. The electronegativity values for  Elements from Si to Pb are almost the same.

Physical Properties  All members of group14 are solids. Carbon and  Silicon are non-metals, germanium is a metalloid,  Whereas tin and lead are soft metals with low  Melting points. Melting points and boiling points  Of group 14 elements are much higher than those  Of corresponding elements of group 13.

(i) Reactivity towards oxygen  All members when heated in oxygen form  Oxides. There are mainly two types of oxides,  i.e., monoxide and dioxide of formula MO and  MO2 respectively. SiO only exists at high  Temperature. Oxides in higher oxidation states  Of elements are generally more acidic than  Those in lower oxidation states. The dioxides  CO2, SiO2 and GeO2 are acidic, whereas  SnO2 and PbO2 are amphoteric in nature.  Among monoxides, CO is neutral, GeO is  Distinctly acidic whereas SnO and PbO are  Amphoteric.

(ii) Reactivity towards water  Carbon, silicon and germanium are not  Affected by water. Tin decomposes steam to  Form dioxide and dihydrogen gas.

Sn + 2H2O → SnO2 + 2H2   

Lead is unaffected by water, probably  Because of a protective oxide film formation.

(iii) Reactivity towards halogen  These elements can form halides of formula  MX2 and MX4 (where X = F, Cl, Br, I). Except  Carbon, all other members react directly with  Halogen under suitable condition to make  Halides. Most of the MX4 are covalent in nature.  The central metal atom in these halides  Undergoes sp3 hybridisation and the molecule  Is tetrahedral in shape. Exceptions are SnF4  And PbF4, which are ionic in nature.

Carbon atoms have the tendency to link  With one another through covalent bonds to  Form chains and rings. This property is called  Catenation. This is because C—C bonds are  Very strong. Down the group the size increases and electronegativity decreases, and, thereby,  Tendency to show catenation decreases. This  Can be clearly seen from bond enthalpies  Values. The order of catenation is C > Si > Ge ≈ Sn. Lead does not show catenation.

 

[A] MCQ

 

1)  Which of the following is not the member of group 14 ?

a) boron

b) silicon

c) germanium

d) tin

Ans- a) boron

 

2) … does not show catenation.

a) Carbon

b) Lead

c) Silicon

d) Germanium

Ans- b) Lead

 

3) Which of following elements are affected by water ?

a) carbon

b) silicon

c) germanium

d) All the above

Ans- d) All the above

 

4) The valence shell electronic configuration of Group 14 elements is …

a) ns2np4

b) ns2np5

c) ns2np2

d) ns2np3

Ans- c) ns2np2

 

5) Half-life of 14C is  … years.

a) 6570

b) 4570

c) 5770

d) 1970

Ans- c) 5770

 

[B]Short Answers

 

1) Explain the ionisation enthalpy of carbon family elements (group 14) .

Ans- The first ionization enthalpy of group 14  Members is higher than the corresponding  Members of group 13. The influence of inner  Core electrons is visible here also. In general the  Ionisation enthalpy decreases down the group.  Small decrease in ∆i  H from Si to Ge to Sn and  Slight increase in ∆i  H from Sn to Pb is the  Consequence of poor shielding effect of  Intervening d and f orbitals and increase in size  Of the atom.

 

2) What is catenation ?

Ans- Carbon atoms have the tendency to link  With one another through covalent bonds to  Form chains and rings. This property is called  Catenation. This is because C—C bonds are  Very strong. Down the group the size increases and electronegativity decreases, and, thereby,  Tendency to show catenation decreases. This  Can be clearly seen from bond enthalpies  Values. The order of catenation is C > Si > Ge ≈ Sn. Lead does not show catenation.

3) Explain the following properties of group 14 elements .

i) Covalent radius

ii) Electronegativity

Ans- Covalent Radius – There is a considerable increase in covalent  Radius from C to Si, thereafter from Si to Pb a  Small increase in radius is observed. This is  Due to the presence of completely filled d and f  Orbitals in heavier members

Electronegativity- Due to small size, the elements of this group  Are slightly more electronegative than group  13 elements. The electronegativity values for  Elements from Si to Pb are almost the same.

 

[C]Long Answers

 

1) Explain trends in chemical reactivity of group 14 .

Ans- Trends in chemical reactivity of group 14 is as follow.

(i) Reactivity towards oxygen  All members when heated in oxygen form  Oxides. There are mainly two types of oxides,  i.e., monoxide and dioxide of formula MO and  MO2 respectively. SiO only exists at high  Temperature. Oxides in higher oxidation states  Of elements are generally more acidic than  Those in lower oxidation states. The dioxides  CO2, SiO2 and GeO2 are acidic, whereas  SnO2 and PbO2 are amphoteric in nature.  Among monoxides, CO is neutral, GeO is  Distinctly acidic whereas SnO and PbO are  Amphoteric.

(ii) Reactivity towards water  Carbon, silicon and germanium are not  Affected by water. Tin decomposes steam to  Form dioxide and dihydrogen gas.

Sn + 2H2O → SnO2 + 2H2  

Lead is unaffected by water, probably  Because of a protective oxide film formation.

(iii) Reactivity towards halogen  These elements can form halides of formula  MX2 and MX4 (where X = F, Cl, Br, I). Except  Carbon, all other members react directly with  Halogen under suitable condition to make  Halides. Most of the MX4 are covalent in nature.  The central metal atom in these halides  Undergoes sp3 hybridisation and the molecule  Is tetrahedral in shape. Exceptions are SnF4  And PbF4, which are ionic in nature.

2) Write short note on carbon family.

Ans- Carbon, silicon, germanium, tin lead and Flerovium are the members of group 14. Carbon  Is the seventeenth most abundant element by  Mass in the earth’s crust. It is widely  Distributed in nature in free as well as in the  Combined state. In elemental state it is available  As coal, graphite and diamond; however, in  Combined state it is present as metal  Carbonates, hydrocarbons and carbon dioxide  Gas (0.03%) in air. One can emphatically say  That carbon is the most versatile element in the  World. Its combination with other elements  Such as dihydrogen, dioxygen, chlorine and  Sulphur provides an astonishing array of  Materials ranging from living tissues to drugs  And plastics.

The valence shell electronic configuration of  These elements is ns2np2 .The inner core of the  Electronic configuration of elements in this  Group also differs.

Case Study – 4

Carbon exhibits many allotropic forms; both crystalline as well as amorphous. Diamond and graphite are two well-known crystalline forms of carbon. In 1985, third form of carbon known as fullerenes was discovered by H.W.Kroto, E.Smalley and R.F.Curl. For this discovery they were awarded the Nobel Prize in 1996.

Diamond It has a crystalline lattice. In diamond each carbon atom undergoes sp3 hybridisation and linked to four other carbon atoms by using hybridised orbitals in tetrahedral fashion. The C–C bond length is 154 pm. The structure extends in space and produces a rigid three- dimensional network of carbon atoms. It is very difficult to break extended covalent bonding and, therefore, diamond is a hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dyes and in the manufacture of tungsten filaments for electric light bulbs.

Graphite Graphite has layered structure. Layers are held by van der Waals forces and distance between two layers is 340 pm. Each layer is composed of planar hexagonal rings of carbon atoms. C—C bond length within the layer is 141.5 pm. Each carbon atom in hexagonal ring undergoes sp2 hybridisation and makes three sigma bonds with three neighbouring carbon atoms. Fourth electron forms a π bond. The electrons are delocalised over the whole sheet. Electrons are mobile and, therefore, graphite conducts electricity along the sheet. Graphite cleaves easily between the layers and, therefore, it is very soft and slippery. For this reason graphite is used as a dry lubricant in machines running at high temperature, where oil cannot be used as a lubricant.

Fullerenes Fullerenes are made by the heating of graphite in an electric arc in the presence of inert gases such as helium or argon. The sooty material formed by condensation of vapourised Cn  small molecules consists of mainly C60 with smaller quantity of C70 and traces of fullerenes consisting of even number of carbon atoms up to 350 or above. Fullerenes are the only pure form of carbon because they have smooth structure without having ‘dangling’ bonds. Fullerenes are cage like molecules. C60 molecule has a shape like soccer ball and called Buckminsterfullerene. It contains twenty six- membered rings and twelve five-membered rings. A six membered ring is fused with six or five membered rings but a five membered ring can only fuse with six membered rings. All the carbon atoms are equal and they undergo sp 2 hybridisation. Each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalised in molecular orbitals, which in turn give aromatic character to molecule. This ball shaped molecule has 60 vertices and each one is occupied by one carbon atom and it also contains both single and double bonds with C–C distances of 143.5 pm and 138.3 pm respectively. Spherical fullerenes are also called bucky balls in short.

Uses of Carbon Graphite fibres embedded in plastic material form high strength, lightweight composites. The composites are used in products such as tennis rackets, fishing rods, aircrafts and canoes. Being good conductor, graphite is used for electrodes in batteries and industrial electrolysis. Crucibles made from graphite are inert to dilute acids and alkalies. Being highly porous, activated charcoal is used in adsorbing poisonous gases; also used in wateof filters to remove organic contaminators and in airconditioning system to control odour. Carbon black is used as black pigment in black ink and as filler in automobile tyres. Coke is used as a fuel and largely as a reducing agent in metallurgy. Diamond is a precious stone and used in jewellery. It is measured in carats (1 carat = 200 mg).

Carbon Monoxide Direct oxidation of C in limited supply of oxygen or air yields carbon monoxide. 2C(s) O (g) → 2CO(g)

On small scale pure CO is prepared by dehydration of formic acid with concentrated H2SO4 at 373 K

On commercial scale it is prepared by the passage of steam over hot coke. The mixture of CO and H2 thus produced is known as water gas or synthesis gas.

When air is used instead of steam, a mixture of CO and N2 is produced, which is called producer gas.

Water gas and producer gas are very important industrial fuels. Carbon monoxide in water gas or producer gas can undergo further combustion forming carbon dioxide with the liberation of heat. Carbon monoxide is a colourless, odourless and almost water insoluble gas. It is a powerful reducing agent and reduces almost all metal oxides other than those of the alkali and alkaline earth metals, aluminium and a few transition metals. This property of CO is used in the extraction of many metals from their oxides ores.

In CO molecule, there are one sigma and two π bonds between carbon and oxygen, :C ≡ O: . Because of the presence of a lone pair on carbon, CO molecule acts as a donor and reacts with certain metals when heated to form metal carbonyls. The highly poisonous nature of CO arises because of its ability to form a complex with haemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. This prevents haemoglobin in the red blood corpuscles from carrying oxygen round the body and ultimately resulting in death.

 

[A] MCQ

 

1) In diamond each carbon atom undergoes … hybridisation.

a) sp

b) sp2

c) sp3

d) sp3d

Ans- c) sp3

 

2) Carbon atom in hexagonal ring undergoes … hybridisation.

a) sp

b) sp2

c) sp3

d) sp3d

Ans- b) sp2

 

3) C—C bond length within the layer in graphite is … pm.

a) 5

b) 5

c) 180

d) 90

Ans- b)141.5

 

4) Fullerenes was discovered by …

a) W.Kroto

b) Smalley

c) F.Curl

d) All the above

Ans- d) All the above

 

5) The C–C bond length in diamond is … pm.

a) 5

b) 5

c) 180

d) 154

Ans- d) 154

 

[B]Short Answers

 

1) Write short note on diamond.

Ans- Diamond has a crystalline lattice. In diamond each carbon atom undergoes sp3 hybridisation and linked to four other carbon atoms by using hybridised orbitals in tetrahedral fashion. The C–C bond length is 154 pm. The structure extends in space and produces a rigid three- dimensional network of carbon atoms. It is very difficult to break extended covalent bonding and, therefore, diamond is a hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dyes and in the manufacture of tungsten filaments for electric light bulbs.

2) Write short note on graphite.

Ans- Graphite has layered structure. Layers are held by van der Waals forces and distance between two layers is 340 pm. Each layer is composed of planar hexagonal rings of carbon atoms. C—C bond length within the layer is 141.5 pm. Each carbon atom in hexagonal ring undergoes sp 2  hybridisation and makes three sigma bonds with three neighbouring carbon atoms. Fourth electron forms a π bond. The electrons are delocalised over the whole sheet. Electrons are mobile and, therefore, graphite conducts electricity along the sheet. Graphite cleaves easily between the layers and, therefore, it is very soft and slippery. For this reason graphite is used as a dry lubricant in machines running at high temperature, where oil cannot be used as a lubricant.

3) What are the uses of carbon?

Ans- Uses of Carbon are as follows-

Graphite fibres embedded in plastic material form high strength, lightweight composites. The composites are used in products such as tennis rackets, fishing rods, aircrafts and canoes. Being good conductor, graphite is used for electrodes in batteries and industrial electrolysis. Crucibles made from graphite are inert to dilute acids and alkalies. Being highly porous, activated charcoal is used in adsorbing poisonous gases; also used in water filters to remove organic contaminators and in airconditioning system to control odour. Carbon black is used as black pigment in black ink and as filler in automobile tyres. Coke is used as a fuel and largely as a reducing agent in metallurgy. Diamond is a precious stone and used in jewellery. It is measured in carats (1 carat = 200 mg).

 

[C]Long Answers

 

1) Write note on fullerene .

Ans- Fullerenes are made by the heating of graphite in an electric arc in the presence of inert gases such as helium or argon. The sooty material formed by condensation of vapourised Cn  small molecules consists of mainly C60 with smaller quantity of C70 and traces of fullerenes consisting of even number of carbon atoms up to 350 or above. Fullerenes are the only pure form of carbon because they have smooth structure without having ‘dangling’ bonds. Fullerenes are cage like molecules. C60 molecule has a shape like soccer ball and called Buckminsterfullerene. It contains twenty six- membered rings and twelve five-membered rings. A six membered ring is fused with six or five membered rings but a five membered ring can only fuse with six membered rings. All the carbon atoms are equal and they undergo sp2 hybridisation. Each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalised in molecular orbitals, which in turn give aromatic character to molecule. This ball shaped molecule has 60 vertices and each one is occupied by one carbon atom and it also contains both single and double bonds with C–C distances of 143.5 pm and 138.3 pm respectively. Spherical fullerenes are also called bucky balls in short.

2) Give the preparation of Carbon monoxide and give detail explanation on it .

Ans- Direct oxidation of C in limited supply of oxygen or air yields carbon monoxide. 2C(s) O (g) → 2CO(g)

On small scale pure CO is prepared by dehydration of formic acid with concentrated H2SO4 at 373 K

On commercial scale it is prepared by the passage of steam over hot coke. The mixture of CO and H2 thus produced is known as water gas or synthesis gas.

When air is used instead of steam, a mixture of CO and N2 is produced, which is called producer gas.

Water gas and producer gas are very important industrial fuels. Carbon monoxide in water gas or producer gas can undergo further combustion forming carbon dioxide with the liberation of heat. Carbon monoxide is a colourless, odourless and almost water insoluble gas. It is a powerful reducing agent and reduces almost all metal oxides other than those of the alkali and alkaline earth metals, aluminium and a few transition metals. This property of CO is used in the extraction of many metals from their oxides ores.

In CO molecule, there are one sigma and two π bonds between carbon and oxygen, :C ≡ O: . Because of the presence of a lone pair on carbon, CO molecule acts as a donor and reacts with certain metals when heated to form metal carbonyls. The highly poisonous nature of CO arises because of its ability to form a complex with haemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. This prevents haemoglobin in the red blood corpuscles from carrying oxygen round the body and ultimately resulting in death.

Case Study – 5

It is prepared by complete combustion of  Carbon and carbon containing fuels in excess  Of air.

In the laboratory it is conveniently  Prepared by the action of dilute HCl on calcium  Carbonate.

CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) +  H2O(l)

Buffer system helps to  Maintain pH of blood between 7.26 to 7.42.  Being acidic in nature, it combines with alkalies  To form metal carbonates.  Carbon dioxide, which is normally present  To the extent of ~ 0.03 % by volume in the  Atmosphere, is removed from it by the process  Known as photosynthesis. It is the process  By which green plants convert atmospheric  CO2 into carbohydrates such as glucose. The  Overall chemical change can be expressed as:

By this process plants make food for  Themselves as well as for animals and human  Beings. Unlike CO, it is not poisonous. But the  Increase in combustion of fossil fuels and  Decomposition of limestone for cement  Manufacture in recent years seem to increase  The CO2 content of the atmosphere. This may  Lead to increase in green house effect and  Thus, raise the temperature of the atmosphere  Which might have serious consequences.  Carbon dioxide can be obtained as a solid  In the form of dry ice by allowing the liquified  CO2 to expand rapidly. Dry ice is used as a  Refrigerant for ice-cream and frozen food.  Gaseous CO2 is extensively used to carbonate  Soft drinks. Being heavy and non-supporter  Of combustion it is used as fire extinguisher. A  Substantial amount of CO2 is used to  Manufacture urea.  In CO2 molecule carbon atom undergoes  Sp hybridisation. Two sp hybridised orbitals  Of carbon atom overlap with two p orbitals of  Oxygen atoms to make two sigma bonds while  Other two electrons of carbon atom are involved

In pπ– pπ bonding with oxyglargeen atom. This  Results in its linear shape [with both C–O bonds  Of equal length (115 pm)] with no dipole  Moment. The resonance structures are shown  Below:  Resonance structures of carbon dioxide

Silicon Dioxide, SiO2  95% of the earth’s crust is made up of silica  And silicates. Silicon dioxide, commonly known  As silica, occurs in several crystallographic  Forms. Quartz, cristobalite and tridymite are  Some of the crystalline forms of silica, and they  Are interconvertable at suitable temperature.  Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is  Covalently bonded in a tetrahedral manner to  Four oxygen atoms. Each oxygen atom in turn  Covalently bonded to another silicon atoms as  Shown in diagram . Each corner is  Shared with another tetrahedron. The entire  Crystal may be considered as giant molecule  In which eight membered rings are formed with  Alternate silicon and oxygen atoms. Silica in its normal form is almost non-  Reactive because of very high Si—O bond  Enthalpy. It resists the attack by halogens,  Dihydrogen and most of the acids and metals  Even at elevated temperatures. However, it is  Attacked by HF and NaOH.

SiO2 + 2NaOH → Na2SiO3 + H2O  SiO2 + 4HF → SiF4 + 2H2O

Quartz is extensively used as a piezoelectric  Material; it has made possible to develop extremely  Accurate clocks, modern radio and television  Broadcasting and mobile radio communications.  Silica gel is used as a drying agent and as a support  For chromatographic materials and catalysts.  Kieselghur, an amorphous form of silica is used  In filtration plants.

Silicones are a group of organosilicon polymers,  Which have (R2SiO) as a repeating unit. The  Starting materials for the manufacture of  Silicones are alkyl or aryl substituted silicon  Chlorides, RnSiCl(4–n), where R is alkyl or aryl  Group. When methyl chloride reacts with  Silicon in the presence of copper as a catalyst  At a temperature 573K various types of methyl substituted chlorosilane of formula MeSiCl3,  Me2SiCl2, Me3SiCl with small amount of Me4Si  Are formed. Hydrolysis of dimethyl-  Dichlorosilane, (CH3) 2SiCl2 followed by  Condensation polymerisation yields straight  Chain polymers.

A large number of silicates minerals exist in  Nature. Some of the examples are feldspar,  Zeolites, mica and asbestos. The basic structural unit of silicates is SiO44–  In which silicon atom is bonded to four  Oxygen atoms in tetrahedron fashion. In  Silicates either the discrete unit is present or  A number of such units are joined together  Via corners by sharing 1,2,3 or 4 oxygen  Atoms per silicate units. When silicate units  Are linked together, they form chain, ring,  Sheet or three-dimensional structures.  Negative charge on silicate structure is  Neutralised by positively charged metal ions.  If all the four corners are shared with other  Tetrahedral units, three-dimensional network  Is formed.  Two important man-made silicates are  Glass and cement. Zeolites  If aluminium atoms replace few silicon atoms  In three-dimensional network of silicon dioxide,  Overall structure known as aluminosilicate,  Acquires a negative charge. Cations such as  Na+ , K+  Or Ca2+ balance the negative charge.  Examples are feldspar and zeolites.

Zeolites are  Widely used as a catalyst in petrochemical  Industries for cracking of hydrocarbons and  Isomerisation, e.g., ZSM-5 (A type of zeolite)  Used to convert alcohols directly into gasoline.  Hydrated zeolites are used as ion exchangers  In softening of “hard” water.

 

[A] MCQ

 

1) … is used as a  Refrigerant for ice-cream and frozen food.

a) Dry ice

b) Wet ice

c) Crescent Ice

d) Nugget Ice

Ans- a) Dry ice

 

2) H2CO3 is a …

a) strong dibasic acid

b) weak dibasic acid

c) weak diacidic base

d) Strong diacidic base

Ans- b) weak dibasic acid

 

3) … is extensively used as a piezoelectric  Material.

a) Glass

b) Ferrite

c) Quartz

d) Saphire

Ans- c) Quartz

 

4) … an amorphous form of silica is used  In filtration plants.

a) Ferrite

b) Quartz

c) Saphire

d) Kieselghur

Ans- d) Kieselghur

 

5) Which of the following is not an example of silicate mineral ?

a) feldspar

b) mica

c) asbestos

d) hematite

Ans- d) hematite

 

[B]Short Answers

 

1) What are silicates ?

Ans- A large number of silicates minerals exist in  Nature. Some of the examples are feldspar,  Zeolites, mica and asbestos. The basic structural unit of silicates is SiO44–  In which silicon atom is bonded to four  Oxygen atoms in tetrahedron fashion. In  Silicates either the discrete unit is present or  A number of such units are joined together  Via corners by sharing 1,2,3 or 4 oxygen  Atoms per silicate units. When silicate units  Are linked together, they form chain, ring,  Sheet or three-dimensional structures.  Negative charge on silicate structure is  Neutralised by positively charged metal ions.  If all the four corners are shared with other  Tetrahedral units, three-dimensional network  Is formed.  Two important man-made silicates are  Glass and cement. Zeolites  If aluminium atoms replace few silicon atoms  In three-dimensional network of silicon dioxide,  Overall structure known as aluminosilicate,  Acquires a negative charge. Cations such as  Na+ , K+  Or Ca2+ balance the negative charge.  Examples are feldspar and zeolites.

2)What are zeolites ?

Ans- Zeolites are  Widely used as a catalyst in petrochemical  Industries for cracking of hydrocarbons and  Isomerisation, e.g., ZSM-5 (A type of zeolite)  Used to convert alcohols directly into gasoline.  Hydrated zeolites are used as ion exchangers  In softening of “hard” water.

 

3) What are silicones ?

Ans- They are a group of organosilicon polymers,  Which have (R2SiO) as a repeating unit. The  Starting materials for the manufacture of  Silicones are alkyl or aryl substituted silicon  Chlorides, RnSiCl(4–n), where R is alkyl or aryl  Group. When methyl chloride reacts with  Silicon in the presence of copper as a catalyst  At a temperature 573K various types of methyl substituted chlorosilane of formula MeSiCl3,  Me2SiCl2, Me3SiCl with small amount of Me4Si  Are formed. Hydrolysis of dimethyl-  Dichlorosilane, (CH3) 2SiCl2 followed by  Condensation polymerisation yields straight  Chain polymers.

 

[C]Long Answers

 

1) Write short note on carbon dioxide .

Ans- It is prepared by complete combustion of  Carbon and carbon containing fuels in excess  Of air.

In the laboratory it is conveniently  Prepared by the action of dilute HCl on calcium  Carbonate.

CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) +  H2O(l)

On commercial scale it is obtained by  Heating limestone.  It is a colourless and odourless gas. Its low  Solubility in water makes it of immense bio-  Chemical and geo-chemical importance. With  Water, it forms carbonic acid, H2CO3 which is  A weak dibasic acid and dissociates in two  Steps:

Buffer system helps to  Maintain pH of blood between 7.26 to 7.42.  Being acidic in nature, it combines with alkalies  To form metal carbonates.  Carbon dioxide, which is normally present  To the extent of ~ 0.03 % by volume in the  Atmosphere, is removed from it by the process  Known as photosynthesis. It is the process  By which green plants convert atmospheric  CO2 into carbohydrates such as glucose. The  Overall chemical change can be expressed as:

By this process plants make food for  Themselves as well as for animals and human  Beings. Unlike CO, it is not poisonous. But the  Increase in combustion of fossil fuels and  Decomposition of limestone for cement  Manufacture in recent years seem to increase  The CO2 content of the atmosphere. This may  Lead to increase in green house effect and  Thus, raise the temperature of the atmosphere  Which might have serious consequences.  Carbon dioxide can be obtained as a solid  In the form of dry ice by allowing the liquified  CO2 to expand rapidly. Dry ice is used as a  Refrigerant for ice-cream and frozen food.  Gaseous CO2 is extensively used to carbonate  Soft drinks. Being heavy and non-supporter  Of combustion it is used as fire extinguisher. A  Substantial amount of CO2 is used to  Manufacture urea.  In CO2 molecule carbon atom undergoes  Sp hybridisation. Two sp hybridised orbitals  Of carbon atom overlap with two p orbitals of  Oxygen atoms to make two sigma bonds while  Other two electrons of carbon atom are involved

In pπ– pπ bonding with oxyglargeen atom. This  Results in its linear shape [with both C–O bonds  Of equal length (115 pm)] with no dipole  Moment. The resonance structures are shown  Below:  Resonance structures of carbon dioxide

 

2) Write short note on Silicon dioxide.

Ans-  Silicon Dioxide, SiO2  95% of the earth’s crust is made up of silica  And silicates. Silicon dioxide, commonly known as silica, occurs in several crystallographic  Forms. Quartz, cristobalite and tridymite are  Some of the crystalline forms of silica, and they  Are interconvertable at suitable temperature.  Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is  Covalently bonded in a tetrahedral manner to  Four oxygen atoms. Each oxygen atom in turn  Covalently bonded to another silicon atoms as  Shown in diagram . Each corner is  Shared with another tetrahedron. The entire  Crystal may be considered as giant molecule  In which eight membered rings are formed with  Alternate silicon and oxygen atoms. Silica in its normal form is almost non-  Reactive because of very high Si—O bond  Enthalpy. It resists the attack by halogens,  Dihydrogen and most of the acids and metals  Even at elevated temperatures. However, it is  Attacked by HF and NaOH.

SiO2 + 2NaOH → Na2SiO3 + H2O  SiO2 + 4HF → SiF4 + 2H2O

Quartz is extensively used as a piezoelectric  Material; it has made possible to develop extremely  Accurate clocks, modern radio and television  Broadcasting and mobile radio communications.  Silica gel is used as a drying agent and as a support  For chromatographic materials and catalysts.  Kieselghur, an amorphous form of silica is used  In filtration plants.

Updated: May 9, 2022 — 5:27 pm

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