NCERT Exemplar Problems Class 12 Chemistry Chapter 7 The p-Block Elements

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NCERT Exemplar Problems Class 12 Chemistry Chapter 7 The p-Block Elements

Multiple Choice Questions

Single Correct Answer Type
Question 1. On addition of cone. H2SO4 to a chloride salt, colourless fumes are evolved but in case of iodide salt, violet fumes come out. This is because
ncert-exemplar-problems-class-12-chemistry-p-block-elements-1
Solution: (c) HI formed during reaction is oxidized to I2 which is violet in colour.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-2

Question 2. In qualitative analysis when H2S is passed through an aqueous solution of
salt acidified with dil. HCl, a black precipitate is obtained. On boiling the precipitate with dil. HNO3, it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives
ncert-exemplar-problems-class-12-chemistry-p-block-elements-3
Solution: (b) Black precipitate of copper sulphide is formed which gives blue colour of copper nitrate on boiling with dilute HNO3. When aqueous solution of ammonia is added to it, deep blue colour of
ncert-exemplar-problems-class-12-chemistry-p-block-elements-4

Question 3. In a cyclotrimeta phosphoric acid molecule, how many single and double bonds are present? .
(a) 3 double bonds; 9 single bonds
(b) 6 double bonds; 6 single bonds
(c) 3 double bonds; 12 single bonds
(d) Zero double bonds; 12 single bonds
Solution: (c) Cyclotrimetaphosphoric acid contains three double bonds and 9 single bonds as shown below
ncert-exemplar-problems-class-12-chemistry-p-block-elements-5
a, b, and c are three pi-bonds and numerics 1 to 12 are sigma bonds.

Question 4. Which of the following element can be involved in pπ-dπ bonding ?
(a) Carbon (b) Nitrogen (c) Phosphorus (d) Boron
Solution: (c) Phosphorus can be involved in pπ-dπ bonding due to presence of vacant d orbitals C, N, and B do not have o’ orbitals.

Question 5. Which of the following pairs of ions are isoelectronic and isostructural?
ncert-exemplar-problems-class-12-chemistry-p-block-elements-6
Solution: (a) CO2- and NO3 are isoelectronic with 32 electrons and sp2 hybridisation
hence trigonal planar structure.

Question 6. Affinity for hydrogen decreases in the group from fluorine to iodine. Which of the halogen acids should have highest bond dissociation enthalpy?
(a) HF (b) HCl (c) HBr (d) HI
Solution: (a)
HF On moving top to bottom HCl Size of halogen atom increases HBr H-X bond length increases HI Bond dissociation enthalpy decreases

Question 7. Bond dissociation enthalpy of E – H (E = element) bonds is given below. Which of the compounds will act as strongest reducing agent?
ncert-exemplar-problems-class-12-chemistry-p-block-elements-7
Solution: (d) SbH3 will act as strongest reducing agent due to minimum bond enthalpy.

Question 8. On heating with concentrated NaOH solution in an inert atmosphere of CO2, white phosphorus gives a gas. Which of the following statement is incorrect about the gas?
(a) It is highly poisonous and has smell like rotten fish.
(b) Its solution in water decomposes in the presence of light.
(c) It is more basic than NH3
(d) It is less basic than NH3
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-8

Question 9. Which of the following acid forms three series of salts?
(a) H2PO2 (b) H3BO3 (C)H3PO4(d)H3PO3
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-111
H3PO4 has 3 -OH groups i.e., has three ionisable H-atoms and hence forms three series of salts. These three possible series of salts of H3PO4 are as follows: .
NaH2PO4, NaHPO4 and Na3PO4

Question 10. Strong reducing behaviour of H3PO2 is due to
(a) low oxidation state of phosphorus
(b) presence of two -OH groups and one P – H bond
(c) presence of one -OH group and two P – H bonds
(d) high electron gain enthalpy of phosphorus
Solution: (c) H3PO2 is a monobasic acid. It acts as a reducing agent as it has two P – H bonds.

Question 11. On heating lead nitrate forms oxides of nitrogen and lead. The oxides formed
are
(a)NO2 , PbO (b) NO2, PbO (c) NO, PbO (d) NO, PbO2
Solution: (b) 2Pb(NO3)2 —> 2PbO + 4NO2 + O2

Question 12. Which of the following element does not show allotropy?
(a) Nitrogen (b) Bismuth (c) Antimony (d) Arsenic
Solution: (a) Nitrogen does not show allotropy due to its weak N – N single bond.
Therefore, ability of nitrogen to form polymeric structure or more than one structure become less. Hence, nitrogen does not show allotropy.

Question 13. Maximum covalency of nitrogen is
(a) 3 (b) 5 (c) 4 (d) 6
Solution: (c) Maximum covalency of nitrogen is four as it cannot extend its valency beyond four [ NH4, R4N] due to absence of vacant d orbitals.

Question 14. Which of the following statement is wrong?
(a) Single N – N bond is stronger than the single P – P bond.
(b) PH3 can act as a ligand in the formation of coordination compound with transition elements.
(c) NO2 is paramagnetic in nature.
(d) Covalency of nitrogen in N2O2 is four.
Solution: (a) Nitrogen forms pπ-pπ multiple bond and the bond strength is very high. Single N – N bond is weaker than single P – P bond.

Question 15. A brown ring is formed in the ring test for NO3 ion. It is due to the formation of
ncert-exemplar-problems-class-12-chemistry-p-block-elements-10
Solution: (a) When freshly prepared solution of FeSO4 is added in a solution containing NO3 ion, it leads to formation of a brown coloured complex. This is known as brown ring test of nitrate.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-11
Hence, 2 moles of ammonia will produce 2 moles of NO.

Question 16. Elements of group 15 form compounds in +5 oxidation state. However, bismuth forms only one well characterized compound in +5 oxidation state. The compound is
(a) Bi2O5 (b) BiF5 (c) BiCl5 (d)Bi2S5
Solution: (b) Bismuth commonly shows +3 oxidation state instead of +5 oxidation state due to inert pair effect. But Bi forms BiF5 compound due to high electronegativity and small size of fluorine atom.

Question 17.On heating ammonium dichromate and barium azide separately we get
(a) N2 in both cases
(b) N2 with ammonium dichromate and NO with barium azide
(c) N2O with ammonium dichromate and N2 with barium azide
(d) N2O with ammonium dichromate and N2O with barium azide
Solution: (a)
ncert-exemplar-problems-class-12-chemistry-p-block-elements-12

Question 18. In the preparation of HNO3, we get NO gas by catalytic oxidation of ammonia. The moles of NO produced by the oxidation of two moles of NH3 will be (a) 2 (b) 3 (c) 4 (d) 6
Solution: (a) Two moles of NH3 will produce 2 moles of NO on catalytic oxidation of ammonia in preparation of nitric acid
ncert-exemplar-problems-class-12-chemistry-p-block-elements-13

Question 19. The oxidation state of central atom in the anion of compound NaH2PO2 will be
(a) +3 (b) +5 (c) +1 (d) -3
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-14

Question 20. Which of the following is not tetrahedral in shape?
(a) NH4 (b) SiCl4 (c) SF4 (d) S02
Solution: (c) SF4 trigonal bipyramidal structure.

Question 21. Which of the following pairs are’peroxoacids of sulphur?
(a) H2S04 and H2S2O8 (b) H2SO5 and H2S2O7
(c) H2S2O7 and H2S2O8 (d) H2S2O6 and H2S2O7
Solution: (a) Peroxoacids of sulphur must contain one -O – O – bond as shown below
ncert-exemplar-problems-class-12-chemistry-p-block-elements-15

Question 22. Hot cone. H2SO4 acts as moderately strong oxidizing agent. It oxidises both metals and non-metals. Which of the following element is oxidised by cone. H2SO4 into two gaseous products?
(a) Cu (b) S (c) C (d) Zn
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-16

Question 23. A black compound of manganese reacts with a halogen acid to give greenish
yellow gas. When excess of this gas reacts with NH3 an unstable trihalide is formed. In this process, the oxidation state of nitrogen changes from
(a) -3 to +3 (b) -3 to 0 (c) -3 to +5 (d) 0 to -3
Solution: (a) Mn02 + 4HCl -» MnCl2 + 2H2O + Cl
(greenish yellow gas)
When excess of Cl2 reacts with NH3 the products are NCl3 and HCl.
NH3 + 3Cl2 —> NCl3 +3HCl
O.S. (-3) O.S.(+3)

Question 24. In the preparation of compounds of Xe, Bartlett had taken O2+Pt F6 as a base compound. This is because
(a) both O2 and Xe have same size
(b) both O2 and Xe have same electron gain enthalpy
(c) both O2 and Xe have almost same ionization enthalpy
(d) both Xe and O2 are gases.
Solution: (c) Bartlett had taken 02 Pt as a base compound because 02 and Xe both have almost same ionization enthalpy. The ionization enthalpies of noble gases are the highest in their respective periods due to their stable electronic configurations.

Question 25. In solid state PCl5 is a
(a) covalent solid
(b) octahedral structure
(c) ionic solid with [PCl6]+ octahedral and [PCl4] tetrahedral
(d) ionic solid with [PCl4]+ tetrahedral and [PCl6] octahedral
Solution: (d) Structure of PCl5 in solid state
ncert-exemplar-problems-class-12-chemistry-p-block-elements-17

Question 26. Reduction potentials of some ions are given below. Arrange them in decreasing order of oxidizing power.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-18
Solution: (c) The higher the reduction potential, the higher is its tendency to get reduced. Hence, the order of oxidizing power is
BrO4 > IO4 > CIO4

Question 27. Which of the following is isoelectronic pair?
(a) ICl2, ClO2 (b) BrO2,BrF2+ (c) ClO2, BrF (d) CN, O3
Solution: (b) Isoelectronic pair have same number of electrons
ncert-exemplar-problems-class-12-chemistry-p-block-elements-19

More than One Correct Answer Type

Question 28. If chlorine gas is passed through hot NaOH solution, two changes are observed in the oxidation number of chlorine during the reaction. These are
—— and ——-
Solution: (a) 0 to +5

Question 29. Which of the following options are not in accordance with the properly mentioned against them?
ncert-exemplar-problems-class-12-chemistry-p-block-elements-20
Solution: (b, c) Bond dissociation enthalpy order is Cl2 > Br2 > F2 > I2
Ionic character of metal halides increases as electronegativity of halogen increases
MI < MBr < MCI < MF

Question 30. Which of the following is correct for P4 molecule of white phosphorus?
(a) It has 6 lone pairs of electrons (b) It has six P – P single bonds
(c) It has three P – P single bonds (d) It has four lone pairs of electrons,
Solution: (b, d) Structure of P4molecule can be represented as
ncert-exemplar-problems-class-12-chemistry-p-block-elements-21
It has total four lone pairs of electrons situated at each P-atom.
It has six P – P single bond.

Question 31. Which of the following statements are correct?
(a) Among halogens, radius ratio between iodine and fluorine is maximum.
(b) Leaving F – F bond, all halogens have weaker X – X bond than X – X’ bond in interhalogens.
(c) Among interhalogen compounds maximum number of atoms ate present in iodine fluoride.
(d) Interhalogen compounds are more reactive than halogen compounds.
Solution: (a, c, d) Among halogens radius ratio between iodine and fluorine is maximum
because iodine has maximum radius while fluorine has minimum radius. Also, due to highest radius ratio maximum number of atoms are present in iodine fluoride.
Interhalogen compounds are more reactive than halogen compounds because A – B bond of dissimilar halogen is weaker than A – A or B – B bond of halogens.

Question 32. Which of the following statements are correct for SO2 gas?
(a) It acts as a bleaching agent in moist conditions.
(b) Its molecule has a linear geometry.
(c) Its dilute solution is used as disinfectant.
(d) It can be prepared by the reaction of dilute H2SO4 with metal sulphide.
Solution: (a, c) SO2 acts as a bleaching agent in presence of moisture. This is due to reducing nature of SO2
SO2 + 2H2O ->• H2SO4 + 2H
Coloured matter + H –> colourless matter. Its dilute solution is used as disinfectant.

Question 33. Which of the following statements are correct?
(a) All three N – O bond lengths in HNO3 are equal.
(b) All P – Cl bond lengths in PCl5 molecule in gaseous state are equal.
(c) P4 molecule in white phosphorus have angular strain therefore white phosphorus is very reactive.
(d) PCl5 is ionic in solid state in which cation is tetrahedral and anion is octahedral.
Solution: (c, d)
(a) All the three N – O bond lengths in HNO3 are not equal.
(b) All P – Cl bond lengths in PCl5 molecule in gaseous state are not equal. Axial bond is longer than equatorial bond.
(c) P4 molecule in white phosphorus have angular strain therefore white phosphorus is very reactive.
(d) PCl5 is ionic in solid state in which cation is tetrahedral and anion is octahedral.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-22

Question 34. Which of the following orders are correct as per the properties mentioned against each?
ncert-exemplar-problems-class-12-chemistry-p-block-elements-23
Solution: (a, d) Acidic strength of oxides in a group decreases down the group and increases along a period from left to right.
Thermal stability of hydrides of group 16 decreases down the group.

Question 35. Which of the following statements are correct?
(a) S – S bond is present in H2S2O6.
(b) In peroxosulphuric acid (H2SO5) sulphur is in +6 oxidation state.
(c) Iron powder along with Al2O3 and K2O is used as a catalyst in the preparation of NH3 by Haber’s process
(d) Change in enthalpy is positive for the preparation of SO3 by catalytic
oxidation of SO2.
Solution: (a, b)
ncert-exemplar-problems-class-12-chemistry-p-block-elements-24

Question 36. In which of the following reactions cone. H2S04 is used as an oxidizing reagent?
ncert-exemplar-problems-class-12-chemistry-p-block-elements-25
Solution: (b, c) In the above given four reactions, (b) and (c) represent oxidizing behaviour of H2S04. As we know that oxidizing agent reduces itself as oxidation state of central atom decreases.
Here,
ncert-exemplar-problems-class-12-chemistry-p-block-elements-26

Question 37. Which of the following statements are true?
(a) Only type of interactions between particles of noble gases are due to weak dispersion forces.
(b) Ionisation enthalpy of.molecular oxygen is very close to that of xenon.
(c) Hydrolysis of XeF6 is a redox reaction.
(d) Xenon fluorides are not reactive.
Solution: (a, b) Weak dispersion forces are present between particles of noble gases. Ionization enthalpy of molecular oxygen is very close to that of xenon.

Short Answer Type Questions

Question 38. In the preparation of H2S06 by contact process, why is S03 not absorbed directly in water to form H2S04?
Solution: The S03 is absorbed directly in H2S04, acid fog is formed which is difficult to condense.

Question 39. Write a balanced chemical equation, for the reaction showing catalytic oxidation of NH3 by atmospheric oxygen.
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-27

Question 40. Write the structure of pyrophosphoric acid.
Solution: H4P2O7 is pyrophosphoric acid. Its structure is
ncert-exemplar-problems-class-12-chemistry-p-block-elements-28

Question 41. PH3 forms bubbles when passed slowly in water but NH3 dissolves. Explain.
Solution: NH3 forms hydrogen bonds with water and therefore soluble in water but
PH3 cannot form hydrogen bonds with water and, therefore, is not soluble in water. It escapes as gas.

Question 42. In PCl5, phosphorus is in sp3d hybridised state but all its five bonds are not equivalent. Justify your answer with reason.
Solution: In PCl5, phosphorus undergoes sp3d hybridisation and a trigonal bipyramidal configuration comes into existence.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-29
The equatorial P–Cl bonds are equivalent while two axial bonds are different and larger than equatorial bonds.

Question 43. Why is nitric oxide paramagnetic in gaseous state but the solid obtained on cooling is diamagnetic?
Solution: Nitric oxide (NO) in the gaseous state is monomer having one unpaired electron as it is paramagnetic in nature.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-30

Question 44. Give reason to explain why ClF3 exists but FCl3 does not exist.
Solution: ClF3 is known because chlorine can exhibit +3 oxidation state due to promotion of
ncert-exemplar-problems-class-12-chemistry-p-block-elements-31
Each of the three singly occupied orbitals overlap with p-orbital of each of three fluorine atoms to form ClF3 (trigonal bipyramidal geometry, T-shaped molecule).
No (i-orbitals are present in the valency shell of fluorine and thus excitation is not possible. Fluorine, therefore, does not exhibit positive oxidation state and so the formation of FCl3 is not possible.

Question 45. Out of H2O and H2S, which one has higher bond angle and why?
Solution: Oxygen is more electronegative than sulphur therefore bond pair of electrons of O – H bond will be closer to oxygen. As a result, there will be more bond pair-bond pair repulsion between bond pairs of two O – H bonds.

Question 46. SF6 is known but SCl6 is not. Why?
Solution: Due to small size, six fluorine atoms can be accommodated around sulphur atom while chlorine atoms being larger in size are difficult to accommodate.
The other reason is that the fluorine being highly electronegative and oxidising in nature is capable of unpairing the paired orbitals of the values shell of sulphur atom and thereby showing the highest, oxidation state of +6 while chlorine is not able to do this.

Question 47. On reaction with Cl2, phosphorus forms two types of halides ‘A’ and ‘B’. Halide A is yellowish-white powder but halide ‘B’ is colourless oily liquid. Identify A and B and write the formulas of their hydrolysis products.
Solution: When dry chlorine is passed over white phosphorus heated gently in a retort, first phosphorus trichloride is formed. PCl3 further reacts with Cl2 and forms phosphorus pentachloride PCl5.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-32

Question 48. In the ring test of NO3 ion.Fe2+ion reduces nitrate ion to nitric oxide, which
combines with Fe2+ (aq.) ion to form brown complex. Write the reactions involved in the formation of brown ring.
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-33

Question 49. Explain why does the stability of oxoacids of chlorine increase in the order given below:
HClO < HClO2 < HClO3 < HClO4
Solution: Oxygen is more electronegative than chlorine, therefore dispersal of negative charge present on chlorine increases from CIO to ClO4 ion because number of oxygen atoms attached to chlorine increases. Therefore, stability of ions will increase in the order given below:
ClO<ClO2<ClO3<ClO5
Thus due to increase in stability of conjugate base, acidic strength of corresponding acid increases in the following order HClO < HClO2 < HClO5 < HClO4

Question 50. Explain why ozone is thermodynamically less stable than oxygen.
Solution: Ozone is thermodynamically unstable with respect to oxygen because it
ncert-exemplar-problems-class-12-chemistry-p-block-elements-34

Question 51. P4O6 reacts with water according to equation P4O6 + 6H2O
Calculate the volume of 0.1 M NaOH solution required to neutralize the acid formed by dissolving 1.1 g of P4O6 in H2O.
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-35

Question 52. White phosphorus reacts with chlorine and the product hydrolysis in the presence of water. Calculate the mass of HCl obtained by the hydrolysis of the product formed by the reaction of 62 g of white phosphorus with chlorine in the presence of water.
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-36

Question 53. Name three oxoacids of nitrogen. Write the disproportionation reaction of that oxoacid of nitrogen in which nitrogen is in +3 oxidation state.
Solution: The three oxyacids of nitrogen are:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-37

Question 54. Nitric acid forms an oxide of nitrogen on reaction with P4O10. Write the reaction involved. Also write the resonating structures of the oxide of nitrogen formed.
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-38
Reactivity: White phosphorus is very reactive in comparison to red phosphorus. This is due to angular strain in white phosphorus on account of bond angles (60°).

Question 55. Phosphorus has three allotropic fonns —(i) white phosphorus (ii) red phosphorus and (iii) black phosphorus. Write the difference between white and red phosphorus on the basis of their structure and reactivity.
Solution: Yellow or white phosphorus: It is a discrete tetrahedral molecule. The molecular formula is P4. The four phosphorus atoms lie at the comers of a regular tetrahedron. Each P atom is linked to each of the three other atoms by single covalent bonds i.e., six P – P bonds are present and each P atom has a lone pair.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-39
ncert-exemplar-problems-class-12-chemistry-p-block-elements-40

Question 56. Give an example to show the effect of concentration of nitric acid on the formation of oxidation product.
Solution: Dilute and concentrated nitric acid give different oxidation products on reaction with metals. For example, zinc with very dilute HNO3 (6%) forms NH4NO3, with dilute HNO3(20%) forms N2O and with cone. HNO3 (70%) forms NO2.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-41

Question 57. PCl5 reacts with finely divided silver on heating and a white silver salt is obtained, which dissolves on adding excess aqueous NH3 solution. Write the reactions involved to explain what happens.
Solution: PCl5 reacts with silver to form white silver salt (AgCl). This dissolves in aqueous ammonia to form soluble complex.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-42

Question 58. Phosphorus forms a number of oxoacids. Out of these oxoacids phosphinic acid has strong reducing property. Write its structure and also write a reaction showing its reducing behaviour.
Solution: Phosphinic acid is hypophosphorus acid (H3PO2) which acts as a reducing agent due to possession of P – H bonds.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-43

Matching Column Type Questions

Question 59. Match the compounds given in Column I with the hybridization and shape given in Column II and mark the correct option.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-44
Solution: (a) (A -> 1), (B -> 3), (C -> 4), (D -» 2)
ncert-exemplar-problems-class-12-chemistry-p-block-elements-45
ncert-exemplar-problems-class-12-chemistry-p-block-elements-46

Question 60. Match the formulas of oxides given in Column I with the type of oxide given in Column II and mark the correct option.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-47
Solution: (b) (A —> 4), (B->1), (C-> 2), (D ->3)
ncert-exemplar-problems-class-12-chemistry-p-block-elements-48

Question 61. Match the items of Column I and Column II and mark the correct option.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-49
ncert-exemplar-problems-class-12-chemistry-p-block-elements-50
Solution: (a) (A -> 4), (B -» 3), (C -> 1), (D -» 2)
(a) H2SO4 is used in storage batteries
(b) CCl3NO2 is known as tear gas.
(c) Cl2 has highest electron gain enthalpy.
(d) Sulphur is a member of chalcogen i.e., one producing elements.

Question 62. Match the species given in Column I with the shape given in Column II and mark the correct option.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-51
Solution: (b) (A ->3), (B -> 4), (C —> 2), (D —> 1)
ncert-exemplar-problems-class-12-chemistry-p-block-elements-52

Question 63. Match the items of Column I and Column II and mark the correct option.
ncert-exemplar-problems-class-12-chemistry-p-block-elements-53
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-54

Assertion and Reason Type Questions

In the following questions, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
(a) Both Assertion and Reason are correct statements and Reason is the correct explanation of Assertion.
(b) Both Assertion and Reason are correct statements but Reason is not the correct explanation of Assertion.
(c) Assertion is correct but Beason is wrong.
(d) Assertion is wrong but Reason is correct.
(e) Both Assertion and Reason are wrong.

Question 64. Assertion (A): N2 is less reactive than P4.
Reason (R): Nitrogen has more electron gain enthalpy than phosphorus.
Solution: (c) N2 is less reactive due to high bond dissociation energy. Its electron gain enthalpy is less than phosphorus.

Question 65. Assertion (A): HNO3 makes from passive.
Reason (R): HNO3 forms a protective layer of ferric nitrate on the surface of iron.
Solution: (c) Passivity is attained by formation of a thin film of oxide on iron.

Question 66. Assertion (A): HI cannot be prepared by the reaction of KI with concentrated
H2SO4.
Reason (R): HI has lowest H – X bond strength among halogen acids.
Solution: (b) Both statements are correct but are independent of each other.

Question 67. Assertion (A): Both rhombic and monoclinic sulphur exist as S8 but oxygen exists as Oπ.
Reason (R): Oxygen forms pπ-pπ multiple bond due to small size and small bond length but pπ-pπ bonding is not possible in sulphur.
Solution: (a) S exists as S8 but oxygen exists as O2 because oxygen forms pπ-pπ multiple bonds which is not possible in S.

Question 68. Assertion (A): NaCl reacts with concentrated H2SO4 to give colourless fumes with pungent smell. But on adding MnO2 the fumes become greenish yellow.
Reason (R): MnO2 oxidises HCl to chlorine gas which is greenish yellow.
Solution: (a) Colourless fumes of HCl become greenish yellow because MnO2 oxidises HCl to chlorine gas.

Question 69. Assertion (A): SF6 cannot be hydrolysed but SF4 can be.
Reason (R): Six F atoms in SF6 prevent the attack of H2O on sulphur atom of SF6.
Solution: (a) SF6 is sterically protected due to presence of six F atoms around S which prevents the attack of H2O on SF6.

Long Answer Type Questions

Question 70. An amorphous solid “A” bums in air to form a gas “B” which turns lime water milky. The gas is also produced as a by-product during roasting of sulphide ore. This gas decolourises acidified aqueous KMnO4 solution and reduces Fe3+ to Fe+2. Identify the solid “A” and the gas “B” and write the reactions involved.
Solution: A is sulphur (Sg) while B is sulphur dioxide (SO2).
ncert-exemplar-problems-class-12-chemistry-p-block-elements-55

Question 71. On heating lead (II) nitrate gives a brown gas “A”. The gas “A” on cooling changes to colourless solid “B”. Solid “B” on heating with NO changes to a blue solid C. Identify A, B and C and also write reactions involved and draw the structures of B and C.
Solution: The gas A is NO2 . The reactions are explained as under:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-56

Question 72. On heating compound (A) gives a gas (B) which is a constituent of air. This gas when treated with 3 mol of hydrogen (H2 ) in the presence of a catalyst gives another gas (C) which is basic in nature. Gas C on further oxidation in moist condition gives a compound (D) which is a part of acid rain. Identify compounds (A) to (D) and also give necessary equations of all the steps involved. –
Solution:
ncert-exemplar-problems-class-12-chemistry-p-block-elements-57

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