Learning chemistry of p-block elements by problem...

The Elias Website for P-Block Elements Page 1

Learning chemistry of p-block elements by problemsolving

Concept based questions and their solutions

Prof. Anil J. Elias

Department of ChemistryIndian Institute of Technology, Delhi

New Delhi, India

Chemistry of the p-block elements is an indispensable and integral component of inorganicchemistry but quite often detested by students as the topic appears too descriptive and eachelement seems to have properties and reactivity different from the rest. The objective of thiswebsite is to help students acquire interest in main group chemistry and to gain confidence bytackling problems related to the p block elements. An enthusiastic student loves to solveproblems. The following collection of more than 120 problems (most of them reasoningquestions) based on the concepts involving chemistry of the p-block elements is especiallymeant for the beginner to this area. Searching the web for solutions to specific questions, ahabit followed by many youngsters these days can often lead to incorrect and obtuse answersas such answers are often not provided by learned experts in the area. Therefore, accurate,literature substantiated solutions to all the given problems are also provided at the end of thisdocument. Solving these problems will help one to understand the physical and chemicalproperties and the unique reaction chemistry of the p block elements.

The fabulous sparkle of p-block compounds

Diamond Ruby Sapphire Emerald Lapis Lazuli

Allotrope of carbon;Has the highesthardness (10 in

Mohs scale) andhighest thermalconductivity for anybulk material

Gemstone varietyof the mineralcorundum(Al2O3) whichcontainchromium asimpurity.

Gemstone varietyof the mineralcorundum(Al2O3) with bothtitanium and ironpresent asimpurities.

Emerald is avariety of themineral beryl

(Be3Al2(SiO3)6),colored green bytrace amountsof chromium and/or vanadium.

A silicate mineralwith the formula(Na,Ca)8(AlSiO4)6

(S,SO4,Cl)1-2. Theintense blue color isdue to the presenceof the S3

- radicalanion in the crystal.

Introduction: Problem solving strategies for p-block elements

Each element of the p-block is unique to a considerable degree. While to some extent anelement will show properties expected of the group to which it belongs (e.g. group valency)additional factors such as whether it belong to the first row or not (d orbital participation,pπ-pπ versus pπ-dπ bonding) or whether they are the heaviest main group metals (inert paireffect) also need to be considered while finding solutions to problems of p-block elements. In


addition, there are properties which are unique to a specific element or a specific isotope of ap- block element (e.g. highest electro negativity of fluorine, liquid nature of gallium at roomtemperature and high nuclear cross section of 10B isotope). While these properties help toaddress issues of application, factors such as ease of availability (natural abundance) andcost relative to other elements/compounds and toxicity do matter when one considerapplication which requires substances in large quantities (reason why Gd and Cd are notpreferred over boron in nuclear reactors). Unique properties of some main group compoundsbased on structure-property relationships (e.g. inertness of SF6, hydrophobicity of siliconesand its flexibility over a wide temperature range) are also of importance while tacklingproblems. Simple concepts such as resonance and hybridization need to be invoked when twovery similar compounds are compared on issues such as stability and acidity. Finally oneshould also be familiar with general trends on going from left to right of the periodic table(e.g. increasing acidity of oxides) or from top to bottom in a period (decreasing ionizationenthalpies) and diagonal relationships (e.g. Li and Mg forming nitrides). Rare gas basedcompounds do hold key for the best examples of compounds obeying the VSEPR rule inpredicting molecular structures (of course with the exception of XeF6).

The basic and advanced levels of problems given are meant for students of pre-university (+2 CBSE/ICSE of India) and B.S/M.S programs respectively. Solutions to the problems are provided at the endof this document and for more details one can contact the author at [email protected]. Please notethat the author reserves copyright. While this material is free for use by recognized educationalinstitutions as part of question papers, using the solutions in any printed form as part of a book (orsupplying to question banks) involving financial gains will invoke legal action for copyright violation.

The Boron Group

Group 13 Signature properties of theelement

Signature properties andapplications of compounds

BElectron deficient, high cross sectionfor thermal neutrons (10B), Formationof hydride clusters, No d orbitals toexpand valency.

Hydroboration-anti Markownokoff,Lewis acid, Boron hydrides as rocketfuel, carboranes in boron nuclear capturetherapy.

AlLight weight, low density, mostabundant metal in the earth’s crust,Coating of oxide provides passivity.

Al2O3 amphoteric, AlCl3, Lewis acid forFriedel-Crafts reaction, LiAlH4, a strongreducing agent. Alumino silicates inclays and zeolites as porous solids andcatalysts, Alumina in gemstones.

GaLiquid metal (m. p 29.7 C), expandson freezing. wets glass surface.

Gallium arsenide has better electronicproperties than silicon due to higherelectron mobility

InGalinstan an alloy of Ga, In and Sn(m.p. -19 C) is a substitute formercury.

InCl3 a Lewis acid stable to air andmoisture and tolerable to acid sensitivefunctional groups in organic reactions.

TlExtreme toxicity due to size and chargesimilarity to potassium; Inert pair effectstabilizes +1 state

CpTl, the only water stablecyclopentadienyl precursor was useful inorganometallic synthesis.

Level: Basic

1. BF63 does not exist while AlF6

3 exists. Why?

2. AlCl3 dimerizes but BCl3 does not dimerize. Why?

3. Solid TlCl3 is unstable and decomposes at 40 °C, losing chlorine to give TlCl.Why?


4. Why is B3N3H6 (borazine, borazole) called inorganic benzene?

5. The Lewis acidity of BF3 is lesser than that of BBr3. Why?

6. Hydrated aluminium trichloride, AlCl3.6H2O does not bring about Friedel-Crafts reactions. Why?

7. Why does AlCl3 and AlMe3 dimerize?

8. What is the difference between structures of B2H6 and Al2Cl6?

9. Gallium and caesium both have melting points 29.7 and 28.4 C respectively.Since human body temp is 37 C, holding these metals in a glove-free hand inprinciple can result in their melting. However, only one of these metals can beheld in a glove-free hand and melted. Which is the metal and why?

10. BCl3 and BF3 hydrolyzes readily but CCl4 does not. Why?

Level: Advanced

11. Melting point of boron is 2453 K while that of gallium is 303 K. Why?

12. Why control rods used in nuclear reactors are made of boron?

13. Thallium (I) compounds are highly poisonous. Why?

14. What is boron neutron capture therapy?

15. Gallium is not stored in glass and metal containers. Why?

16. BF3 does not dimerize. Why?

17. What is Galinstan? What is it used for?

18. Why is gallium arsenide a better semi conducting material than silicon?

19. What is white graphite?

The Carbon Group

Level: Basic

20. Unlike CCl4, SiCl4 hydrolyzes readily. Why?

21. Silicon forms double bonds with difficulty while carbon forms double bondsreadily. Why?

22. (SiH3)3N is a weaker base while (CH3)3N is stronger. Why?


23. Which among the following compounds can form strong Lewis acid baseadducts with BF3 though their N and O atoms. Explain why?

H3Si-NCS, CH3-O-CH3, H3Si-O-SiH3, Me3N

24. The catenation shown by carbon is better than that of silicon. Why?

25. Graphite conducts electricity while hexagonal boron nitride does not. Givereason

26. The covalence of carbon does not expand beyond four. Why?

27. The hybrization of carbon in fullerenes and carbon nanotubes have beendescribed as sp2. Is this statement true or is there a better statement?

28. Sn2+ ions is solution are good reducing agents. Why?

29. PbO is much more stable than PbO2 which decomposes readily to PbO

30. Silicones are hydrophobic while silicates are not. Why?

Group 14 Signature properties of theelement and its compounds


CAllotropes-graphite sp2, diamond sp3

and fullerenes, maximum catenationfor an element. Ratio of 12C to 14Cbasis for radiocarbon dating todetermine age of fossils.

CO the strongest bond (1072 kJ/mol) andligand, N- heterocyclic carbenes (NHC’s)stable carbenes, CO2 green house gas.Carbon in sp, sp2 and sp3 hybridizationform the foundation of organic chemistry.

SiSecond most abundant element onthe earth’s crust. Well knownsemiconductor and solar cellmaterial. Does not form Si=O, dorbital availability makes reactivitydifferent from carbon. Si2+ known

Silicone polymers are the most industriallyimportant p block compounds withproperties such as hydrophobicity, lowglass transition temp, oxygen permeabilityand non-toxicity. Si=Si and SiSi unstablebut have been prepared by using stericallyhindered substituents.

GeExistence was predicted byMendeleev in 1871 and to resemblesilicon in its properties. He suggestedtherefore the name ekasilicon(symbol Es). His predictions for theproperties of germanium areremarkably close to the reality. Ge2+

known.

High index of refraction and low opticaldispersion of GeO2: useful for wide-anglecamera lenses and optical fibers. GeO2

exhibits the unusual property of having ahigh refractive index for visible light, buttransparency to infrared light. Finds use ininfrared night vision equipments. UnlikeSi=O, Ge=O compounds are known.

Snβ-tin (white tin), which is stableabove room temperature, ismalleable. In contrast, α-tin (nonmetallic form, or gray tin) is stablebelow 13.2 °C and is brittle. In coldconditions, β-tin tends to transformspontaneously into the brittle α-tin, aphenomenon known as "tin pest”.

Sn2+ stable oxidation state and a goodreducing agent.Bronze, an alloy of tin and copper was thefirst alloy, used in large scale since 3000BC. Tin/lead soft solders, typicallycontaining 60% or more of tin.

PbInert pair effect, toxicity, damages

the nervous system and causes braindisorders as lead is a neurotoxin thataccumulates both in soft tissues andthe bones.

Protective shield in nuclear reactors and X-ray sources. Tetraethyllead was used as anantiknock agent. PbO2 unstable due to inertpair effect.


31. The tin buttons on the coats of Napoleon’s troops were believed to haveliterally crumbled to a powder after being exposed to the bone-chilling cold ofa Russian winter. Why?

Level: Advanced

32. Stanley Kipping, the father of silicone chemistry tried to make keto analoguesof silicon but failed. Why?

33. While carbon readily form double bond with oxygen the heavier group 14elements do not. The first Ge=O containing molecule was discovered only in2012 by K. Tamao which required kinetic stabilization of the molecule usingbulky substituents on germanium. Why is there this difference in behaviour?

34. Commercial production of silicones was made inexpensive by making theindustrial preparation of Me2SiCl2 possible from inexpensive startingmaterials. What is this process?

35. How is semiconductor grade silicon prepared from SiO2? What is the key stepin this purification of silicon which reduced impurities manyfold?

36. Why was silicone based shoes and outfits used in the lunar expeditions ofUSA?

37. Silane is a reducing agent under normal conditions while methane is not.Why?

38. The Si-Si and Si-H bonds are almost of the same strength as the C-C and C-Hbonds. However silanes explode or catch fire on exposure to air and reactvigorously with alkaline water while alkanes does not. Why.

39. What property of lead makes it useful as a protective shield in nuclear reactorsand similar nuclear facilities?

40. Sodalite (molecular sieves 4A) and ZSM-5 both are zeolites. But only ZSM-5brings about shape selective catalysis such as conversion of toluene to p-xylene selectively. Why?

41. Sodalite is preferred as a water softening agent than ZSM-5 why?

42. Why is CO is a very strong ligand? Although oxygen is more electronegativethan carbon still carbon of CO binds to metals in forming organometalliccomplexes Why?

43. What is the basic difference between graphite and graphene?


The Nitrogen Group



NLewis base, N≡N has the secondhighest bond enthalpy for any bond(946 kJ/mol). Azides (N3

) andpentazenium (N5

+) explosivelyconvert to N2. NH3 is one of the mostwidely produced inorganic nitrogencompounds and fertilizer precursor.

A range of oxides are known, N2O(laughing gas), NO (vasodilator), NO2

(brown), N2O3 (blue), N2O4 and N2O5.HNO3 oxidizing acid, NO2 and NO3 groupspart of many explosives (TNT, RDX etc).NCl3 explosive. Hydrazine, N2H4 reducingagent.

PThree allotropes while, red and black.Common oxidation states-3, +3 and+5. Most stable black with hexagonalrings. White P used in incandescentbombs while red P is used inmatches.

White phosphorus reacts with O2 of airgiving P4O10 Driving force for manyreactions P=O bond formation in +5 state.Wittig reaction and Arbuzov rearrangementare examples. Phosphates, a component ofbiological molecules like DNA and ATP.

AsMost common allotropes are metallicgray, yellow and black. Yellowarsenic, similar to yellow phosphorushas a tetrahedral structure. Thisunstable allotrope, being molecular,is the most volatile, least dense andmost toxic.

Cadet's fuming liquid (cacodyl), claimed asthe first synthetic organometalliccompound, was synthesized in 1760.Arsenic contamination of groundwater is aproblem that affects millions of peoplearound the world.

SbAntimony is mainly used asits trioxide in making flame-proofingcompounds. The formation ofhalogenated antimony compounds isthe reason for the flame retardingeffect of antimony trioxide,

SbF5 (a strong Lewis acid) reaction withK2MnF6 –only reaction which yields F2 gas.Fluoroantimonic acid (H2FSbF6), one of thestrongest superacids is 1016 times strongerthan 100% sulfuric acid and can protonateall organic compounds.

BiInert pair effect, Bismuth has

unusually low toxicity for a heavymetal. Longest ever radioactive half-life for an element is for bismuth-209: over twenty billion billion yearswhich is more than a billion times theestimated age of the universe - decaysby emitting alpha-particles

Bismuth subsalicylate is used asan antidiarrheal. It is the active ingredientin drugs such as "Pink Bismuth". Pepto-Bismol- a combination of bismuth subsalicylate and bismuth subcitrate is used totreat peptic ulcers.

Level: Basic

44. NH3 is a better electron donor than PH3. Why?

45. Between NH3 and NF3 which will have a higher dipole moment. Which willhave a higher bond angle? Justify.

46. NF5 does not exist while PF5 exists. Why?

47. Me3P and Et3P catches fire on exposure to air while Me3N and Et3N does not.Why?

48. Boiling point of NH3 is 238.5 K while that of PH3 is only 185.5K. Explain.


49. N-N single bonds is weaker than P-P single bond. Why?

50. H3PO2 is a good reducing agent while H3PO4 is not although both havepentavalent phosphorus atoms. Why?

51. NO2 dimerizes while ClO2 does not. Why?

52. The bond dissociation energy of carbon monoxide is +1084 kJ mol-1 while thatof nitrogen, N2, is +945 kJ mol-1. CO has a stronger bond but it is morereactive than N2. Why?

53. CN is known but CP is not known. Why?

54. Both PCl3 and PCl5 can be used for converting ethanol to ethyl chloride. Howdoes these reaction differ?

55. PCl5 is ionic in solid state and covalent in gaseous state. Why?

56. Yellow phosphorus fumes in air while red phosphorus does not. Why?

57. Between Bi+5 and Sb+5 which is a better oxidizing agent and why?

58. Between PCl4+ and PCl4

which is not likely to exist and why?

59. Zinc phosphide Zn3P2 is a well known rat bait and poison. Why do rats getattracted to it and how does it kill them?

Level: Advanced

60. Are N2O3 and N2O5 acid anhydrides? If so of which acids?

61. Reaction of Cu with dilute HNO3 gave a colorless gas A. This gas was foundto react with freshly prepared FeSO4 to give a brown solution. Gas A reactedwith O2 to give a brown colored gas B which on cooling gave a colorless gasC. Reaction of A and B gave a blue colored solid D. Identify the compoundsA-D.

62. In contrast to the corresponding Na and K salts, the thermal decomposition ofthe ammonium salts NH4Cr2O7, NH4MnO4, NH4ClO4 are exothermic and evenexplosive. Why and what is the common product in all of thesedecompositions.

63. NCl3 is explosive while NF3 is stable. Why?

64. NaN3 is used in protection air bags of automobiles and airplane escape chutesWhy?


65. Between yellow, red and black phosphorus, which is the most stable allotropeof phosphorus and why?

66. Almost all well known explosives such as TNT, nitrocellulose and RDX hasnitro group/s present. Explain the chemical basis.

67. The formal oxidation state of H3PO2 is 1 while that of H3PO4 is 5 how?

68. What is the difference in the nature of products obtained when LiNO3, NaNO3

and NH4NO3 are heated.

69. Select the most suitable match for (a) laughing gas, (b) a brown toxic gas and(c) a vasodilator gas from NaNO3, NH4NO3, LiNO3 and Cu + dil HNO3.

70. BiF5 is a very strong fluorinating agent which can even convert hydrocarbonsto fluorocarbons while PF5 cannot. Why?

71. Thermal decomposition of potassium dichromate is less exothermic while thatof ammonium dichromate is more exothermic. Why?

72. The triple bond between N and N in N2 is very strong while the quadrupolebond between Re and Re in Re2Cl82- is not as strong. Why?

73. A waxy white solid A (normally stored under water) on exposure to air selfignites and forms a while solid B. B on reaction with water gives a polybasicacid C which is present in living systems. A on reaction with conc NaOHsolution gives a poisonous gas D having smell of rotten fish which catchesfire in the impure form. Identify compounds A-D.

74. Which among the following inorganic salts (a) on heating with NaOH releasea gas which will show a brown ppt with HgI4

2- and (b) on heating as such givewater and release a neutral gas which will react only with Mg and Li? Explain.[NaNO3; NH4NO3; NH4NO2; (NH4)2CO3]

The Oxygen Group

Level: Basic

75. Sulphur is diamagnetic in the solid state and paramagnetic in the vapourphase. Why?

76. SF6 is used as a dielectric in electrical transformers. Why?

77. H2O is liquid whereas H2S is gas at room temperature. Why?

78. H2S is more acidic than H2O. Why?

79. TeF6 hydrolyzes in water to give Te(OH)6 but SF6 does not react even withsteam at high pressure. Why?


80. SF6 is known while OF6 is unknown. Why?

81. Although less electronegative than fluorine the ability to stabilize higheroxidation states of transition metals is more for oxygen than fluorine. Why?

82. The bleaching action of SO2 on colors is reversible while that with O3 orbleaching powder s irreversible. Why?

83. Rubber tyres kept near electric spark generating equipments (e.g. motors) forlong periods of time develop cracks on the surface. Give reason

84. Between ozone and Cl2 which is better for purification of drinking water andwhy?

85. Reaction of SO3 with SCl2 was found to give a reactive liquid A and a gas B.Gas B in presence of moisture was found to bleach colours in a reversiblemanner. Liquid A on reaction with yellow phosphorus gave another liquidcompound C as the major product. C on hydrolysis gave an acid D which onheating released a colorless gas E with smell of rotten fish. Identifycompounds A-E.

86. For the industrial synthesis of H2O2 why 2-ethylhydroquinine or similar 2-alkyl hydroquinone is used although in principle hydroquinone can bringabout the reaction?



OLewis base, second highest electronegativity, stabilizes elements in highoxidation states. O2 paramagnetic.

O3 and H2O2 good oxidizers. O3 isdiamagnetic.

SOldest known element; mentioned inthe Bible (brimstone). crown shaped S8

is the most common type present in (orthorhombic) and (monoclinic)forms. S2-S20 known. S2 paramagnetic.In 1839 Charles Goodyear discoveredvulcanization of natural rubber using S.

Multiple oxidation state known from -2to +6. H2SO4 oxidizing and dehydrating,SOCl2 and SO2Cl2 chlorinating agents,Burning S produces SO2. SO3 used inH2SO4 synthesis. SF6 an unreactivehexafluoride used as dielectric medium.SF4 is a well known fluorinating agent.

SeRed-colored α, β and γ forms areproduced from solutions of polymericblack selenium by varying evaporationrates of CS2 solvent. They all haveSe8 rings as in sulfur. Gray Se, a semiconductor shows photoconductivity

Selenium used as photoconductor inXerox machines. Selenocysteine, the 21st

essential amino acid. Selenium is acomponent of the antioxidantenzymes glutathione peroxidase andthioredoxin reductase.

TeCdTe based thin film solar cells,accounts for about 6% of all solar cellsinstalled in 2010. Cadmiumtelluride photovoltaics is the only thinfilm photovoltaic technology tosurpass crystalline silicon photovoltaics in cheapness .

The conjugate base of teflic acidTeF5OH, the teflate anion, F5TeO isknown to resist oxidation. This propertyhas allowed the preparation severalhighly unusual species such as thehexateflates M(OTeF5)6

- (M = As, Sb,Bi).Xenon forms the cation Xe(OTeF5)+

PoRadioactive, first discovered by Marie

and Pierrie Curie from pitchblende.


87. Ozone generators are used to remove bad smell from rooms affected by moldand mildew and fire accident. Why?

88. Sulphur has a greater tendency to form catenation compared to oxygen. Why ?

89. Why is H2O2 always stored in dark plastic bottles?

90. An yellow solid A forms a volatile foul smelling hydride B which gives ablack precipitate with lead nitrate solution. On heating in air a forms acolorless pungent gas C. The gas C in presence of moisture was found totemporarily bleach flowers. C in the presence of a vanadium catalyst wasfound to get oxidized to a low boiling liquid D. This on reaction with waterwas found to result in dibasic acid E. Predict the compounds A to E.

91. What property of selenium was utilized by Xerox corporation in making thefirst photocopier machines?

The Halogen group



FSmall size, pale yellow, highest electronegativity, shows only -1 oxidation state.Henry Moissan isolated F2 first time byelectrolysis of KHF2 and dry HF in 1886.Gaseous uranium hexafluoride is used toseparate uranium-235, from uranium-238.First IE is higher than that of any elementexcept neon and helium.

Aqueous HF a weak acid, strongest Hbonding. HF etches glass forming SiF4.The fluoride ion, reduces tooth decay andfor this reason is used in toothpaste. Thelargest commercial use is in refrigerant(CFC and HCFC) gases; Teflon, animportant polymer is used in electricalinsulation and cookware.

ClYellow-green gas. a strong oxidizingagent. Used as bleach and in waterpurification. Commercially produced frombrine by electrolysis. Used in themanufacture of a wide range of consumerproducts, about two-thirds of them organicchemicals such as polyvinyl chloride,

Exists in odd oxidation states from −1 to+7. Solutions of chlorine in water containchlorine, HCl and HOCl: Perchloric acid,a powerful oxidizer is a stronger acidthan sulfuric and nitric acids. CFC’sunder UV generate chlorine free radicals,the major cause of ozone depletion.

BrCorrosive dark red liquid with b.p. 58.8C. At high temperatures, organobrominecompounds convert to free bromineatoms, which has the effect of stoppingfree radical chain reactions. Tyrianpurple, an organobromo dye extractedfrom a shell fish was first used by theancient Phoenicians as early as 1600 BC.

Shows oxidation states from -1 to + 7. N-Bromosuccinimide is commonly used as asubstitute for elemental bromine, beingeasier to handle, and reacting more mildlyand thus more selectively. CF3Br (Halon1301) was used as a fire suppressant inaircrafts. Bromide salts are used as anti-epileptic drugs.

ISublimable brown solid; purple vapours,Lewis acid, X-ray contrast material.Present in thyroid hormones, thyroxineand triiodo-thyronine. Poor watersolubility. Iodide and iodate are used assalt additives to counter iodine deficiencydisordes, a leading cause of intellectualdisabilities among children.

Shows oxidation states from -1 to + 7.maximum positive oxidation state in IF7.Iodometry an analytical redox titrationmethod. Iodide is the best halide leavinggroup, MeI, is a good methylating agent.I2 used to activate magnesium whenpreparing Grignard reagents. I2 reactsviolently with aluminium.

AsRadioactive most stable isotope is

astatine-210, with a half-life of 8.1 hours.


Level: Basic

92. Fluorine does not show multiple oxidation states while other halogens showwhy. Fluorine does not show positive oxidation states. Why?

93. In reactions involving KMnO4 under acidic conditions one does not use HClas the acid Why? Another haloacid can in principle be used for the same butnot used for two reasons. Explain

94. The negative election gain enthalpy of fluorine is less than that of chlorine.Why?

95. An organohalogen compound contains the same number of Cl, Br and I. TheLassagine’s extract (sodium fusion extract) of this compound was mixed withCCl4 and the mixture was reacted with small amount of Cl2 gas. The colorwhich the CCl4 layer will show initially will be [yellowish green/pinkishviolet/brownish red]

96. Chlorofluorocarbons are highly volatile. Why?

97. ICl is more reactive than I2. Why?

98. Chlorine water on standing loses its yellow colour. Why?

99. Only one hydrogen halide is liquid at near room temperature. Which one andwhy?

100. Hydrofluoric acid is always stored in plastic bottles and reacted inplastic/Teflon vessels. Why?

101. Teflon is rigid even at room temperature while silicone is highly flexible.Why?

102. Teflon coating provides a non stick surface. Why?

103. Good alternatives to ozone depleting chlorofluorocarbons (CFC’s) arehydrofluorocarbons (HFC’s). Why?

104. What is the shape of IO2F2 ? Between SO2F2 and XeO2F2 which will have

the same shape as IO2F2 ?

105. The covalent radius of S and Cl are comparable (102 and 99 pm) So are thesizes of the HSO4

anion and ClO4 anion (221 and 225 pm). However

perchloric acid (Pka -10) is a stronger acid than sulphuric acid (pka -1.9).Why?

106. Common reagent grade conc. HF (49%) is a weak acid but 100% HF(anhydrous HF) is a very strong acid. Why?


107. The pKa values indicating acidity of chlorus acid, chloric acid and perchloricacid are +2, -1 and -8 respectively. Explain why the acidity increases in thisorder?

108. In titrations involving release of I2 excess of I- is used. Why?

109. While Cl2 and Br2 solutions in water can be used for bleaching purpose, F2

cannot be used. Why?

110. Which oxide of halogen is used for the estimation of CO in automobileexhaust gases and why?

111. How does aqua regia dissolve gold and platinum?

Rare gas groupLevel: Basic

112. Xenon is the only element among rare gases which form compounds readily.Why?



HeLowest mp (-272.2C) and bp (-268.9C) among elements. Extractedby fractional distillation from naturalgas, which can contain up to 7%helium. liquid helium is used to coolcertain metals an compounds toextremely low temperatures requiredfor super conductivity,

Second most abundant element in theuniverse (after hydrogen), Is the leastwater soluble monatomic gas, and haspoor lipid solubility. No knowncompounds. The Large Hadron Colliderat CERN uses 96 metric tons of liquid

helium to maintain the temperature at1.9 kelvin

NeIndustrially produced bycryogenic fractional distillation ofliquefied air.

No known compounds. Neon plasma hasthe most intense light discharge atnormal voltages and currents of all thenoble gases.

ArThe third most common gas inthe Earth's atmosphere, at 0.93%.Argon is produced industrially by the

fractional distillation of liquid air

Argon fluorohydride (HArF) is the onlyknown neutral compound of argon(synthesized by mixing argon and HFon a CsI surface at −265 °C, under UVradiation.) HArF is stable only if keptbelow 17 K (−256 °C).

KrDiscovered in 1898 by Sir WilliamRamsay, and Morris Travers (foundingdirector of India’s first science instituteIISc, in Bangalore), from residue leftafter evaporating nearly all componentsof liquid air.

KrF2 (stable at -78 C) oxidizes gold toits highest (+5) oxidation state (AuF5).Also known are a few highly unstablekrypton compounds such as Kr(OTeF5)2.

Xe

The rare gas which forms maximumnumber of compounds. Lowest firstionization potential among stable raregases. Obtained commercially as a byproduct of the separation of air intooxygen and nitrogen.

Di, tetra and hexa fluorides, tri and tetraoxides, oxyfluorides, pentafluoroaryls,-N(SO2F)2, -BF2, -OH, -CCH derivativesknown. Forms square planar complexwith gold. AuXe4

2+. XeF2, a convenientfluorinating agent.

RnMost stable radioactive isotope 222Rn,

has a half-life of only 3.8 days.Epidemiological studies have shown linkbetween breathing high concentrations ofradon and incidence of lung cancer.


113. Noble gases have very low boiling points. Why?

114. XeF2 is a very convenient fluorinating agent Why?

115. Which is the rare xenon compound which does not obey the VSEPR rule.What is the expected shape of this molecule according to VSEPR theory

116. XeF5, prepared and structurally characterized in 1991 was found to obey the

VSEPR rule. Predict its shape. What is its hybridization and whichinterhalogen compound has the same hybridization as this compound?

117. XeOF5, structurally characterized in 1995 by K Seppelt obeys VSEPR rule.

Predict it structure

118. Structures of xenon fluorides cannot be explained by conventional valencebond theory. Why?

119. Oxygen is less electronegative than fluorine. But unlike fluorine it is able toform compounds with xenon in its highest formal oxidation state of +8 (XeF6

versus XeO4). Why?

120. XeOF2 on reaction with CsF gives XeOF3. Predict the shapes of the reactant

and the product.

Level: Advanced

121. Other than having fluorine what is common when one considers the type ofchemical bonding involved in XeF2, ClF3, SF4 and PF5.

122. Xenon oxides are acidic. Why?

123. Iodide ion and xenon are isoelectronic and of comparable size. Can xenon,like iodide act as a ligand in coordination chemistry?

124. Why is a mixture of helium and oxygen used for deep sea diving than air(N2+O2) or pure O2?


Learning chemistry of p-block elements by problem solving

Solutions to problems

Prof. Anil J. Elias

Department of chemistryIndian Institute of Technology, Delhi

Hauz Khas, New Delhi, India

1. Boron being a first row element cannot expand its valency beyond 4 due tonon availability of suitable d orbitals of comparable energy. Aluminium beingin the second row can expand its valency beyond 4 due to availability of dorbitals.

Related question with the same answer: Reaction of BF3 with excess F- forms BF4-

while AlF3 gives AlF63-. Why?

(It should be mentioned that while this is the classic explanation for such differences there isan upcoming school of thought which ignores d orbital participation due to the highpromotional energies required to promote a p electron to a d orbital. In such a case a TBPstructure of PF5 will be considered as a combination of sp2 and a 3 centre 4 electron bondinvolving the axial p orbital. This theory is supported by the discovery of linear F3

ionsimilar to I3

as d orbitals of fluorine cannot be invoked for the central sp3d hybridization insuch a situation. (See Sun. Chem. Educator, 2002, 7, 11)

2. This is due to the large size difference between Al and B. The relative size ofchlorine being large, boron in unable to accommodate more than 3 Cl units.(Although there are claims of BCl4

there are no examples which arestructurally characterized by X-ray diffraction studies). The reason why BF3

does not dimerize is entirely different as covalently bound fluorine shows verylittle intermolecular interaction due to high electro negativity and also there isconsiderable pπ-pπ interaction between B and F evident from the shorter B-Fbond distance of 1.30 Å. BF4

also exists.

3. This is due to inert pair effect. Inert pair effect occurs when electrons arepulled closer to the nucleus, making them more stable and more difficult toionize. The s-orbital electrons are affected in this way since they have agreater penetrating power. In thallium the outer-shell s electrons are not aswell shielded as those in the inner core. They are therefore pulled into theinner core of electrons and thus become inert rendering an oxidation state twounits less than the group oxidation state.

Related question: TlCl is more stable than TlCl3 Why?

4. B3N3H6 is planar having all B-N bonds equal with bond distances between B-N and B=N values. It is also a liquid. Borazine is iso-structural with benzene.The six B-N bonds have length of 1.436 Å [between that of the B -N (1.51Å )and the B=N (1.31 Å)]. The carbon-carbon bond in benzene is similar havingbond length of 1.42 Å. However chemical reactivity of borazine are notsimilar to benzene.


Properties Borazine Benzene

Mol wt 80.5 78.1

b.p 55°C 80°C

density 0.81 0.81

Critical temp 252°C 288°C

planarity planar planar

B-N/C-CDistance Ä

1.46 1.42

5. The F atom is small compared to the larger Cl and Br atoms, and the lone pairelectrons in the pz orbital of F readily overlaps with the empty pz orbital ofboron. As a result, the π donation of F to B is greater than that of Cl or Brresulting in lower Lewis acidity of BF3 compared to BBr3.

6. Friedel-Crafts reaction requires formation of AlCl4 thereby converting an

alkyl or acyl halide to an electrophile such as R+ or RC(O)+.. This is possibleonly if AlCl3 retains its Lewis acidity. By forming AlCl3.6H2O which is hexacoordinate Al(H2O)6, ( as seen from crystal structure) its Lewis acidity is lost.

7. AlCl3 and AlMe3 are electron deficient. By dimerization AlCl3 can get onemore pair of electrons (lone pair on Cl) to complete its octet. Dimerisationobserved of AlMe3 is akin that of B2H6 (electron deficient).

8. Both B2H6 and Al2Cl6 form dimers having bridging H and Cl respectivelyperpendicular to the plane having the terminal H2B and Cl2Al units. Whilethe bridging hydrogens are involved in an electron deficient 3 centre 2electron bond in B2H6, the bridging chlorines are involved in a 3 centre 4electron bond in Al2Cl6.

9. Gallium is stable to air and moisture while Caesium, like the rest of the alkalimetals react vigorously with moisture and even more explosively than theother alkali metals with water.

10. BCl3 and BF3 are electron deficient having an empty p orbital and are alsotriangular planar is shape. They can easily accommodate the lone pair of awater molecule becoming tetrahedral in the process. CCl4 does not have emptyp orbital and unlike silicon cannot use d orbitals to expand its valency.

Level: Advanced

11. The crystal lattice of gallium shows that Ga2 dimers are the fundamentalbuilding blocks. So there is less interatomic attractions which are normallyfound in a metal. In contrast boron consists of B12 icosahedra which are boundto each other strongly.


12. Boron has two naturally occurring and stable isotopes, 11B (80.1%) and 10B(19.9%).The 10B isotope has one of the highest nuclear cross sections forthermal neutrons (propensity to absorb thermal neutrons). In a nuclear chainreaction, the rate is determined by the number of thermal neutrons produced insuccessive nuclear reactions. In fact Gd has even better propensity for thermalneutrons but its natural abundance is less than boron.

Related question: During the Chernobyl nuclear disaster helicopters dropped tons ofboric acid on the burning reactor. Why?

13. Thallium and potassium has similar ionic radius and charge. (Thallium (I) hasionic radius 1.47 Å while that of potassium is 1.33 Å). Tl+ like K+ gets into thecell of living organisms and once inside, the slight differences become moresignificant and the functioning of the cell is impaired. The most importantsites where there is significant damage is central nervous system and hairfollicles resulting is hair loss all over the body.

14. BNCT is a highly dedicated type of radiation therapy that can selectivelytarget a tumour at the cellular level without causing radiation damage to theadjacent normal cells and tissues. BNCT is based on the fact that boron-10 hasa high propensity to capture thermal neutrons. The boron-11 formed onnuclear capture undergoes decay leading to high energy He and Li particles asshown below.

10B + nth → [11B] → α + 7Li + 2.31 MeV.

However the range of this radiation is very small almost the diameter of aliving cell (5-9 microns). So if a cancer cell is made to accumulate boron 10the malignant cell gets selectively destroyed by irradiating with thermalneutrons.

15. Upon solidification, gallium metal expands by 3.1% of its volume. Therefore,storage in either glass or metal containers has to be avoided, due to thepossibility of container rupturing with freezing.

16. BF3 does not dimerize as there is considerable pπ- pπ interaction between Band F evident from the shorter B-F bond length of 1.30 Å. In additioncovalently bound fluorine atoms show very little intermolecular interactiondue to high electro negativity and very low electron cloud polarisability.

17. Galinstan is a eutectic alloy mainly consisting of gallium, indium and tin.These alloys are liquids at room temperature, typically melting at −19 °C. Dueto the low toxicity and low reactivity of its component metals, galinstan findsuse as a replacement for many applications that previously employedtoxic mercury. Galinstan tends to wet and adhere to many materials, includingglass, which limits its use compared to mercury. It is commercially used as amercury replacement in thermometers due to its nontoxic properties, but theinner tube must be coated with gallium oxide to prevent the alloy from wettingthe glass surface.

18. Compared to silicon, gallium arsenide has a higher saturated electronvelocity and higher electron mobility, allowing gallium arsenide transistors to


function at frequencies in excess of 250 GHz. Unlike silicon junctions, GaAsdevices are relatively insensitive to heat owing to their wider bandgap. Also,GaAs devices tend to have less noise than silicon devices, especially at highfrequencies. These properties recommend GaAs circuitry in mobilephones, satellite communications etc. Another advantage of GaAs is that it hasa direct band gap, which means that it can be used to emit light efficiently(infrared LED). Silicon has an indirect bandgap and so is very poor at emittinglight. As a wide direct band gap material with resulting resistance to radiationdamage, GaAs is an excellent material for space electronics and opticalwindows in high power applications.

19. White graphite is hexagonal boron nitride which has the same molecularstructure and lubricating properties of graphite.

The Carbon group

20. This is a widely debated question. While the standard answer to the question isthat silicon can accommodate the lone pair of the attacking OH-/ H2O group inits available low lying d orbitals, carbon is unable to do it due to absence of dorbitals. However, a related issue crops up which is that although CCl4 doesnot get hydrolyzed, CH3Cl does get hydrolyzed under basic conditions (oftenthe example used by organic chemists to depict SN

2 transition statemechanism). So the explanation should takes care of the latter issue as well.

The facts related to these issues are

The silicon atom has accessible ‘empty’ d-orbitals, whereas carbon does not .Theoretical studies have shown that among tetrahalides of group 14, carbondoes not use d orbitals since they are of high energy compared to silicon,germanium and tin.

The C-Cl bond dissociation enthalpy is 327 kJ mol-1 in CCl4 while Si-Cl bonddissociation enthalpy in SiCl4 is 406 kJ mol-1

The C-Cl electronegativity difference is 0.5, while for Si-Cl is 1.2. Thiscorresponds to an approximate ionic character of 6% for the C-Cl bond and30% for the Si-Cl bond.

Covalent radius of carbon is 77 pm while that of silicon is 118 pm

Now the SN2 transition state mechanism doesn’t actually form five bonds at

any given time (a difference between a transition state and an intermediate)The C-Cl bond breaks at the same time as the C-O bond is forming. In SN

2, thenucleophile attacks from the opposite side of the breaking bond. This is not aproblem when the bonds are C-H (CH3Cl situation) as hydrogen is small butwith CCl4, the chlorine atoms are quite large and can hinder the approach ofthe nucleophile from the backside. Also if one were to consider an SN

1 typemechanism, the CCl3

+ carbocation is not stable enough.


So the final answer will be (a) the d-orbitals of silicon are available to acceptthe lone pair of electrons from the attacking hydroxide ion/ water moleculeand the Si-Cl bond does not have to break first; in any case this is easier forSi-Cl bond to break compared with C-Cl given the much higher polarity of theformer. (b) The silicon atom being larger in size than carbon means that thereis less steric hindrance in SiCl4 for the formation of the necessary 5-coordinatetransition state. It may be also be possible that it is a true 5-coordinateintermediate as well.

21. The 3p orbitals of silicon which should involve in pπ- pπ bonding are largerand diffuse in nature compared to the 2p orbitals of oxygen. So pπ-pπ bondingis less strong in Si-Si π bonds. Similarly pπ-dπ or dπ-dπ bonds which canform between silicon units are less compact and less strong compared to pπ-pπbond of O2.

22. The lone pair on the nitrogen involves in a pπ-dπ bonding interaction with theempty d orbitals of silicon. As a result the basicity of (SiH3)3N is less.

23. CH3-O-CH3 and Me3N. Lewis basicity of silicon bonded O and N atoms arereduced due to pπ-dπ bonding. In other words the electron density on the Nand O is shared with the empty available d orbitals of silicon where ever it sbonded directly to these elements.

24. The C-C bond enthalpy is 348 kJ/mol while the Si-Si bond enthalpy is 297kJ/mol . Therefore catenation of carbons is better than that of silicon.

25. The carbon atoms of graphite are sp2 hybridized leading to unsaturated bondshaving π electron density which is delocalized making it a conductor. Inhexagonal boron nitride the structure is almost same but due to the electronegativity difference between B and N there is no delocalization of electrondensity in BN rather the electron density is localized on the nitrogen atomsalone. Hence boron nitride is not a good conductor.

26. Carbon cannot use its d orbitals to expand its valency and form hypervalentcompounds like the second row elements of the p block.

27. The six membered rings of fullerenes have alternating single and double bondswhich are not delocalized. Hence fullerenes such as C60 have two differentbond lengths and two different bond angles (1.45 and 1.39 A and 108 and 120deg.). The bond angles in graphite and diamond are 120 deg and 109 deg.respectively. So a better statement will be to say that it consist of both sp2 andsp3 hybridizations.

28. Sn2+ stability can be attributed to some amount of inert pair effect. Tin caneasily get oxidized to +4 since the inert pair effect is not so strong on itcompared to lead and +4 is the group valency.

29. This is due to inert pair effect. See solution 3 for details


30. Methyl groups which are the substituents on the Si-O polymer back bonebrings in the hydrophobicity. Silicates do not have organic non polar groupsand the terminus are ionic sites.

31. This phenomenon of tin going from metallic form (white tin) to brittle form (grey tin) is well known as ‘tin pest’. White tin starts the transition to thegrey form at 13.2 C. The transformation happens faster at lower temperaturesand when the tin is in pure form.

Level: Advanced

32. Silicon does not form double bonds with oxygen to give silanones unlikecarbon. The basic reason is the very high energy of the Si-O single bond,which gives silanones a strong tendency to oligomerize to siloxanes.

33. The E=O bond [where E is a heavy group 14 atom] is inherently polarised,owing to the larger electronegativity difference between E and O atoms,compared to the C=O bond in ordinary ketones, This means that the doublebond in heavier ketone analogues is highly reactive and prone to formingpolymeric chains, such as the -Si-O-Si-O-Si- chains of polysiloxanes, Forstable bonds to form between two atoms, the participating valence orbitalsof the two atoms should have similar sizes and similar energies, such asbetween carbon and oxygen; two elements in the same period. This is not casefor silicon and oxygen, as a results, even if bonds form, the compounds tendto be very unstable.

34. This process, well known as the Rochow-Müller process discovered byEugene Rochow prepares methylchlorosilanes especially Me2SiCl2 using analloy of silicon and copper with methyl chloride at high temperatures. Theprocess uses very inexpensive starting materials and hence reduces cost.

35. Impure SiO2 is first reduced to ferrosilicon which is normally having impurityof iron. This is chlorinated using Cl2 to give SiCl4, a liquid and FeCl3, a solid.The liquid SiCl4 is distilled repeatedly to get very pure SiCl4.This is thenreduced to get ultrapure silicon.

36. Dimethyl silicones have a glass transition temperature which is quite low (-125 °C). Therefore materials made of this polymer do not become hard andbrittle till it reaches the lower limit of its glass transition temperature. Thethermal decomposition of silicones also sets in above 200 C. The surface ofthe moon during a lunar day can vary from -153 to + 107 C.

37. Because of the greater electronegativity of hydrogen in comparison to silicon,in silane the hydrogen atoms have a partial negative charge and the silicona partial positive charge. This Si-H bond polarity is the opposite of thatobserved in the C-H bonds of methane. Silicon (1.90) is less electronegativethan hydrogen (2.20); as a result, the silicon-hydrogen bond possesses somehydridic character. In the presence of a strong electrophile, organosilanescontaining an Si-H bond (hydrosilanes) serve as hydride donors or reducingagents.


38. Answer same as above

39. Lead can effectively moderate certain kinds of radiation because of itshigh density and high atomic number. Principally, it is effective atstopping gamma rays and x-rays. Lead’s high density is caused by thecombination of its high atomic mass and the relatively small size of its bondlengths and atomic radius. The high atomic mass means thatmore electrons are needed to maintain a neutral charge and the small bondlength and a small atomic radius means that many atoms can be packed into aparticular lead structure. Because of lead’s density and large number ofelectrons, it is well suited to scattering x-rays and gamma-rays. These raysform photons, a type of boson, which impart energy onto electrons when theycome into contact with them. Without a lead shield, the electrons within aperson’s body would be affected, which could damage their DNA andcause cancer.

40. Shape selective catalysis shown by zeolites depends upon the pore size of thechannels found on a zeolite, its cavity size at the junctions of these channelsas well as to the ratio of Si to Al of a zeolite. The pore size of zeolite 4A isonly 4 Å which is too small for small organic molecules such as toluene toenter the channel and undergo catalysis.

41. The structural composition of sodalite (Si: Al ratio of 1:1) has alternatingsilicate and aluminate units resulting in a number of anionic sites(correspondiong to every AlO4

¯ unit present) which are neutralised bydisplaceable Na+ ions. The Si: Al ratio of ZSM-5 is 3:1. The significantly largeamount of Na+ ions per molecule of the sodalite makes it a better choice as awater softening agent than ZSM-5.

42. CO is a very strong ligand because it not only donates a pair of electrons to ametal but also receives electron density from a metal in its low lyingantibonding orbitals (known as back bonding). The highest occupiedmolecular orbital of CO is slightly antibonding in nature and is a carbon basedorbital. Hence the donation of the electron is from the carbon and not theoxygen.

43. Graphene can be described as a one-atom thick layer of graphite having asingle planar sheet of sp2-bonded carbon atoms that are densely packed in ahoneycomb crystal lattice. Many layers of graphene stacked togethereffectively form crystalline flake graphite. It is very light, with a 1-square-meter sheet weighing only 0.77 milligrams. Graphene is the basic structuralelement of carbon allotropes including graphite, carbon nanotubesand fullerenes.

The Nitrogen groupLevel: Basic

44. In NH3 the lone pair of electrons occupies an sp3 hybrid orbital which hasformed from the more compact 2s and 2p orbital while in PH3 it is an sp3


hybrid orbital formed from the relatively less compact 3s and 3p orbitals. Theformer will have more directionality and hence more stronger H bonds.

45. Higher dipole moment will be for NH3 since the bond dipoles reinforce theeffect of the lone pair ( =1.47D) making the molecule more basic. For NF3

the bond dipoles which are directed towards F reduce the effect of the lonepair and hence a lower dipole moment ( =0.23D).Since the fluorine end of the N-F bond is more electron rich, the lone pair onN can approach more closely to N in NF3. This lone pair exerts greaterrepulsion on the bondinf N-F pairs resulting in lower F-N-F bond angles (102vs 107 in NH3). In addition, the longer N-F bonds result in lesser bond pair-bond pair repulsion reducing the bond angles further.

46. N and P both have 5 valence electrons out of which two are paired in the 2sand 3s orbitals. NF5 to exist its nitrogen has to promote one of its 2s electronsto a d orbital. But there are no d orbitals available for first row elements. InPF5 one of the 3s electrons is promoted to the empty 3d orbital and an sp3dhybrid results with orbitals half filled. These can pair with the electrons fromthe five fluorine atoms.

47. Alkyl phosphines are electron rich, small and volatile and unlike alkyl aminesthey can easily expand the phosphorus valence using 3d orbitals forming avery strong and a stable dπ-pπ P=O bond. The heat generated in this oxidationreaction is sufficient to make the molecule go beyond its auto ignitiontemperature and making it pyrophoric.

48. The high electronegativity of N in NH3 molecule facilitates intermolecularhydrogen bonding resulting in a higher boiling point. In PH3 the lowerelectronegativity and larger size results in a weaker, less directional hydrogenbond which is less significant and hence resulting in a lower boiling point.

49. The smaller size and shorter bond length of the single N-N bond results ingreater interelectronic repulsion between the non-bonded electrons of bothnitrogen atoms thereby weakening the bond in comparison to an analogous P-P bond.

50. The two P-H bonds present on H3PO2 imparts reducing character to H3PO2

while no P-H bonds are present in H3PO4.

51. The unpaired electron on NO2 facilitates its dimerisation at low temperature.On bringing to higher temperature it reverts to monomeric brown NO2.Although ClO2 is having an unpaired electron, the presence of an extra lonepair of electrons on the Cl atoms and its repulsion prevents ClO2 fromdimerization.

52. At the basic level one can see that unlike N2 which is not polar, CO is polardue to a positive dipole moment. This makes the CO more reactive than N2. Atthe advanced level one has to compare the MO diagrams of N2 and CO whichare isoelectronic. While the former is simple and symmetrical, the latter iscomplicated by the fact that it involves two atoms with electronegativity


difference and hence a complex MO picture. The basic difference in the MOpicture of CO (See Gupta and Elias, Basic Organometallic Chemistry, 2013) whencompared to N2 is the HOMO which is a 2p orbital is partly antibonding withrespect to O while it is bonding in the case of N2. This orbital, which in MOterminology is a carbon based orbital can therefore donate its electron pairespecially to metals thus acting as a good donor ligand and gaining energysince it is getting rid of antibonding electrons. The partly antibonding natureof the HOMO also indicates the actual bond order should be less than 3 whileit is 3 in case of N2 where the HOMO is clearly bonding in nature.

53. This statement is factually incorrect. A better statement would have been CN

is stable as a ligand but CP is relatively unstable. The cyaphide ion, P≡C− ,the phosphorus analogue of cyanide is not known as a salt but has beenobserved in the gas phase. Theoretical studies reveal that the -1 charge in thision is located mainly on the carbon. The molecule exist as a terminal ligand ina few metal complexes such as trans-[(dppe)2Ru(H)(C≡P)] (prepared by Prof.R. J. Anjelici in 1992) where dppe is a chelating biphosphine ligand. Thereason for its instability compared to CN is due to the fact that p-p bondbetween elements of different periods are not very strong due to poor overlapof interacting orbitals of different sizes.

54. In the case of PCl3 the side product is H3PO3 while the side product in the caseof PCl5 is Cl3P=O.

55. PCl5 exists as PCl4+PCl6

‾ in the solid state and hence it is ionic.

56. Yellow phosphorus is having 4 P units bonded in a tetrahedral arrangementwhile red phosphorus is a polymer of this tetrahedron where one of the P-Pbonds of the tetrahedron is broken and acts as a bridge connecting the 4 atomP units. Therefore, red phosphorus has less angle strain in its P4 unit. Yellow Pfumes in air getting converted to P4O10, a process which releases the anglestrain. Red P also gets oxidized but requires more activation (like pressure,heat etc)

57. Bi+5 will be a better oxidizing agent as the side product of such an oxidationwill be Bi+3 which is highly stable due to inert pair effect than Sb+3.

58. PCl4+ obeys octet rule and is obtained by removing an electronegative chlorine

from PCl5. PCl4 will be a 10 electron species and its synthesis will requireremoval of a Cl+ from PCl5 which is unlikely. PCl3 is also has octet so additionof a Cl to it to realize PCl4

is going to destabilize PCl3. Hence PCl4+ is more

likely to exist (in fact PCl5 exists as PCl4+PCl6

in the solid state).

59. Tablets of zinc phosphide on slow hydrolysis liberates PH3 gas. PH3 has agarlic/rotten fish smell which attracts rats to the bait. Once consumed, theacidic digestive juices make the hydrolysis faster and the poisonous PH3

released kills the rat from inside.


Level: Advanced

60. N2O3 is the anhydride of nitrous acid (HNO2) and N2O5 is the anhydride ofHNO3.

61. A = NO, B=NO2, C=N2O4, D= N2O3

62. The anion is a strong oxidizer which can oxidize the ammonium cation. Thecommon product of all these reactions is N2 gas which is thermodynamicallyvery stable.

63. Despite the similarities of the Pauling electronegativities of nitrogen andchlorine, the NCl3 molecule is very polar with negative charge residing onnitrogen. Theoretical studies indicate that the nitrogen in NCl3 is in the 3oxidation state and the chlorine atoms in the +1 oxidation state. In contrast,due to the higher electronegativity of fluorine, the N atom of NF3 has a +3oxidation state and fluorines -1 oxidation state. Halogen in the +1 oxidationstate is a good oxidizing agent and it can oxidize the nitrogen to the highlystable N2 and get itself reduced to Cl2, both stable gases with a release ofenergy. This does not happen in NF3 as the fluorine in -1 state is not anoxidizer and F2 is not as stable as F. Size of Cl also matters as three Cl on Nis an unstable situation.

Related question: NCl3 on hydrolysis gives NH3 and HClO while PCl3 on hydrolysis givesH3PO3 and HCl. Why?

64. Automobile airbag formulations contains mixtures of oxidizers and sodiumazide and other agents including ignitors and accelerants. An electroniccontroller detonates this mixture during an automobile crash with suddenrelease of N2 gas filling the airbag:

2 NaN3 → 2Na + 3 N2

The same reaction occurs upon heating NaN3 to approximately 300 °C. Thesodium that is formed is converted by reaction with other ingredients, suchas potassium nitrate and silica to harmless compounds like sodiumsilicates. Sodium azide is also used in airplane escape chutes for quick fillingof N2 gas.

65. Black phosphorus is the thermodynamically most stable form of phosphorus.It is obtained by heating white phosphorus under high pressures (12,000atmospheres). In appearance and properties it is like graphite. The structurehas puckered sheets of interlinked six-membered rings having no angle strain.The tetrahedral and linked tetrahedral arrangement of phosphorus atoms ofwhite and red phosphorus have angle strain.

66. The nitro group is one of the most common explosophores (functional groupthat makes a compound explosive). This property of both nitro and nitrate isbecause their thermal decomposition yields molecular nitrogen, N2 plusconsiderable energy, due to the exceptionally high bond dissociation enthalpyof molecular nitrogen.

67. Although the Pauling electronegativity scale gives very close electronegativityvalues to both P and H, other detailed calculations such as Mulliken-Jaffe


electronegativity scale confers slightly more electronegativity to sp3

hybridized P making it compared to H. H3PO2 has two direct P-H bondswhile H3PO4 has none. Hence the H3PO2 has a formal +1 oxidation state.

68. LiNO3 give Li2O and NO2, NaNO3 gives NaNO2 and O2 while NH4NO3 givesN2O and H2O.

69. (a) laughing gas = N2O obtained from NH4NO3

(b) brown toxic gas = NO2 obtained from heating LiNO3

(c) vasodilator gas = NO obtained from Cu + dil HNO3

70. Due to inert pair effect, bismuth favours +3 oxidation state unlike P which hasno inert pair effect. Hence BiF5 is a strong fluorinating agent.

71. Dichromate is a good oxidizer and can oxidize ammonium ion to N2 withrelease of considerbale amount of energy, a consequence of the high stabilityof N2. This does not happen with the potassium salt.

72. In principle a quadrupole bond should be stronger than a triple bond. Butunlike N2 which has a triple bond resulting from p-p interaction, thequadrupole bond of Re2Cl8

2- involves bonding overlap (1, 2, 1) betweend orbitals. d Orbitals are larger in size (especially 5d) and more diffuse innature compared to the p orbitals (especially 2p of N2). Hence in this case thebonding overlap is not strong for Re2Cl8

2‾.

73. A= White phosphorus; B= P4O10; C= H3PO4; D=PH3

74. (a) The gas is ammonia and it is released when NH4NO3 is heated withNaOH. Note that direct heating of NH4NO3 releases N2O. Ammoniumcarbonate on heating as such also gives NH3 and NH4HCO3.

(b) Heating of NH4NO2 as such is explosive and gives N2 (which reacts withLi and Mg forming nitrides) and H2O.

Oxygen groupLevel: Basic

75. In the vapour phase sulphur exists as S2 whose highest occupied molecularorbital is similar to that of O2 having two unpaired electrons. Hence in thevapour phase it is paramagnetic like O2.

76. SF6 is one of the most dense gases (density 6.12g/L compared to air 1.225g/L).It has very high arc quenching properties which makes it useful in circuitbreakers and transformers. It is non inflammable and very inert. It is non-toxic, moderately inexpensive and a good insulator with a breakdown strengthof about 3 times that of air.

77. The electro negativity of oxygen (3.5) is much higher than that of sulfur (2.6).Hence strong hydrogen bonds are formed in water than in H2S where only veryweak hydrogen bonding is present making it a gas at room temperature (b.p.-60 C).

78. The bond dissociation energy of O-H on H2O is 463 kj/mol while that S-H ofH2S is 347 kj/mol. Hence H2S is more acidic than H2O.


79. The relatively larger size of Te compared to S makes it more accessible toattacking reagents such as water getting hydrolyzed to Te(OH)6. In SF6 thesulphur is sterically hindered by the six fluorine atoms making it quiteunreactive.

80. Oxygen cannot expand its valency being a first row main group element as itdoes not have d orbitals of lower energy.

81. Oxygen, second only to fluorine in electronegativity, unlike fluorine can formdouble bonds with metals thereby reducing the crowding and at the same timestabilizing higher oxidation states.

82. SO2 in presence of moisture forms H2SO3 which bleaches by a reductionreaction, while ozone and bleaching power bleaches by an oxidation reaction.The reversibility of the former reaction is due to the fact that oxygen in theatmosphere can reoxidize the reduced colors back to its normal color.

83. Spark generating equipments generate also ozone and ozone has been found toreact with double bonds of organic compounds (in this case the double bondspresent in natural rubber), breaking the polymer chain which in turn becomesvisible as cracks.

84. Ozone is better as it does not leave residues of chlorine containing compounds(HCl, HClO) or dissolved chlorine in the drinking water.

85. A=SOCl2 ; B= SO2; C= PCl3; D= H3PO3; E = PH3

86. The process involves use of a solvent mixture having a nonpolar (benzene)and polar solvent (higher secondary alcohols). The 2-ethyl anthraquinone hasbetter solubility in this solvent mixture than simple anthraquinone.

87. Smell is often due to unsaturated volatile organic compounds and ozone canoxidize these compounds thereby removing the smell.

88. The ability of an element to catenate is primarily based on the bond energy ofthe element to itself, which decreases with more diffuse orbitals overlapping toform the bond. However in the case of oxygen, due to small size there will beinterelectronic repulsion between lone pair of electrons present on eachoxygen atom which will make the single O-O bond weaker than the S-S bond.Hence sulphur has a greatere tendency to form catenation.

89. Hydrogen peroxide degradation has been found to be catalyzed by UV light.As the peroxide breaks down, it forms oxygen gas. This gas can build up andexpand, therefore H2O2 also has to be stored in a dark preferably plastic bottle.

90. A=S; B=H2S; C=SO2; D=SO3; E= H2SO4

91. Selenium is a photoconductor. Which means a fine uniform coating of it on asurface (preferably a drum) can retain the image which has been exposed to itwith a specific ionic charge. A toner (carbon powder) made of the oppositecharge will cling to this image when it is rolled over it. This clinging tonerpowder in the image of the shape is then transferred to a paper and heated tofuse on the surface of the paper.


The Halogen groupLevel: Basic

92. Fluorine does not have available d orbitals of lower energy in its valence shelland therefore cannot expand its octet unlike the other halogens. Being themost electronegative element it exhibits only -1 oxidation state.

93. KMnO4 being a very strong oxidizing agent can oxidize Cl‾ to Cl2. This is truefor other halide ions as well except fluoride. But fluoride cannot be used as theHF acid is not a strong acid in aqueous solution and also it can react withglass.

94. The negative electron gain enthalpy of fluorine should have been more thanthat of chlorine. This is due to the small size of fluorine atom. As aconsequence of the small size, there are strong interelectronic repulsions in therelatively small sized 2p orbital. As a result the incoming electron does notexperience much attraction.

95. The colour of the CCl4 layer will initially be pinkish violet which is thecharacteristic colour of I2 in CCl4. This is because the ease of oxidation of I‾by Cl2 is more compared to the ease of oxidation of Br‾.

96. The high volatility of CFC’s is due to the fact that there is very littleintermolecular attractive forces due to absence of H atoms. In addition theirmolecular weights are low and electron density on the F atoms of the C-Fbond are not polarisable due to the high electonegativity of fluorine.

97. The bond dissociation enthalpy of ICl (211.3 kJ/mol) is less compared to Cl2

(242.6 kJ/mol) and more compared to I2 (151.1 kJ/mol). So strictly speaking,ICl should be less reactive than I2. But due to the electronegativity differencebetween I and Cl and less effective overlap of orbitals with different sizes, IClis highly polar and it has dipole moment. It is a source for reactive I+ speciesand hence its higher reactivity.

98. Dissolved Cl2 in water decomposes to HClO and HCl both of which are colorless. HClO also slowly converts to HCl and gives off nascent O which isresponsible for the bleaching action.

99. HF. Due to strong hydrogen bonding between FHF and H+ units.

100. Hydrofluoric acid reacts with glass forming SiF4. This is also called etching ofglass.

101. Adjacent carbon atoms of the teflon backbone has two fluorine’s attached tothem. Fluorine being larger in size than H packs this rigidly and hence teflonis rigid. In contrast, silicone has a Si-O-Si-O back bone and only the siliconatoms have substituents. Hence there is more space between the substitutentsand more flexibility.

102. The fluorine atoms on the surface of teflon which are covalently bound to thecarbon chain due to their high electronegativity does not take part in secondaryinteractions. As a result PTFE is hydrophobic, neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate no hydrogenbonds but only moderate London dispersion forces due to the


high electronegativity of fluorine. PTFE has one of the lowest coefficients offriction against any solid.

103. HFC’s do not have C-Cl or C-Br bonds. They do not harm the ozone layer asthey do not contain chlorine or bromine as while in the atmosphere it is the C-Cl and C-Br bonds which undergo homolyitc cleavage resulting in Cl· and Br·free radicals. In addition, presence of H reduces volatility to some extent as C-H bonds can also have weak intermolecular interactions.

104. TBP. XeO2F2

105. The number of resonance structures possible for ClO4 ion is more that that

possible for HSO4. Greater resonance delocalization makes the perchlorate

anion more stable. The charge is dispersed over five atoms (the Cl and four Oatoms) in the perchlorate anion, but only over four atoms in the hydrogensulfate anion. Hence formation of the former is favoured over the latterresulting in easier cleavage of the H+ from perchloric acid compared tosulphuric acid.

106. Unlike other hydrohalic acids, such as hydrochloric acid, hydrogen fluoride isonly a weak acid in dilute aqueous solution, with acid dissociationconstant (pKa) of 3.19. This is in partly a result of the strength of thehydrogen-fluorine bond which is higher than other hydrohalic acids. Otherfactors such as the tendency of HF, H2O, and F−anions to form clusters also add. In contrast at 100% concentration, HFmolecules undergo homoassociation to form polyatomic ions (suchas bifluoride, FHF) and protons, thus greatly increasing the acidity.

107. If you consider the anions of these acids one can see that the number of Cl=Obonds increase from 1-3 on going from chlorus acid, chloric acid to perchloricacid. The stability of the anions also increase in this order as the negativechange can be shared by more resonance structures when there are more Cl=Obonds. Hence the increase in the acidity.

108. I will react with I2 formed to form I3which is more soluble in water than I2.

In this way possibility of volatile I2 being lost is reduced and solubility of I2 isenhanced.

109. F2 oxidizes water to O2. Hence it cannot be used.

2 F2 + 2 H2O 4H+ + 4F + O2

110. I2O5. Iodine pentoxide easily oxidises carbon monoxide to carbon dioxide atroom temperature:

5CO + I2O5 → I2 + 5CO2

This reaction can be used to analyse the concentration of CO in a gaseoussample by estimation of I2 by iodometric titration.

111. Aqua regia (HNO3:HCl 1:3) dissolves gold. Although individually both HNO3

and HCl cannot dissolve gold, in combination each acid performs a differenttask. Nitric acid, a powerful oxidizer, will actually dissolve a virtuallyundetectable amount of gold, forming gold ions (Au3+). The hydrochloric acidprovides a ready supply of chloride ions (Cl−), which react with these goldions to produce chloroaurate anions, also in solution. The reaction with


hydrochloric acid is an equilibrium reaction which favors formation ofchloroaurate anions (AuCl4

−). Thus the forward reaction is favoured as moreand more gold is made to go into solution and allows further oxidation of goldto take place. Gold dissolves to become chloroauric acid.Appropriate equations are:Au (s) + 3 NO3

(aq) + 6 H+(aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l) andAu3+ (aq) + 4 Cl− (aq) → AuCl4

(aq).The oxidation reaction can also be written with nitric oxide as the productrather than nitrogen dioxide:Au(s) + NO3

(aq) + 4 H+(aq) → Au3+ (aq) + NO (g) + 2 H2O (l).If elemental gold is desired, the chloroaurate ions may be selectively reducedwith sulfur dioxide, hydrazine, oxalic acid, etc.

Rare gas groupLevel: Basic

112. The first ionization potential of xenon is the lowest among all non radioactiverare gases other than radon. Radon, which has a lower first ionization potentialhowever, has a very short half life and hence cannot be handled easily. Thelongest-lived isotope, radon-222, has a half-life of only 3.82 days.

113. Noble gases are monoatomic and except weak dispersion forces no interatomicforces exist between the atoms for the smaller elements such as He and Ne.The electron clouds of the larger rare gas atoms become polarized whichleads to weak Van der Waals forces among the atoms. Overall therefore theyhave low boiling points.

114. It is a crystalline solid which is easily prepared ( b mixing Xe And F2 in theproper molar ratio and keeping the flask in sunlight). The side product of thefluorination is xenon gas which is unreactive and easily separated fromproducts. In addition this evolved xenon can be mixed again with F2 and XeF2

regenerated.

115. XeF6. Since it will be an AX6E unit one should expect a pentagonal bipyramidshape as per VSEPR theory. However, the actual shape is a distortedoctahedron as a result of a face capped lone pair on the octahedron.

116. XeF5 will be AX5E2. The shape of the molecule will be pentagonal planar.

The hybridization will be sp3d3. The interhalogen compound which will havethe same hybridization will be IF7 which has a pentagonal bipyramid structure.(J. Am. Chem. Soc, 1991,113, 3351)

117. XeOF5 will be AX6E. The structure will be pentagonal pyramid with the O

and lone pair occupying the axial positions of the pentagonal bipyramid. (Inorg.

Chem. 1995, 34, 1868)

118. The conventional valence bond theory is based on elements acquiring octet.The rare gases already have octet and the compounds they form arehypervalent compounds in which the valence electron count is an even numberabove 8.


119. Oxygen is only second to fluorine in the electronegativity scale. But it canform double bonds which reduce crowding around an element. Thereforeoxygen is able to form compounds with xenon with formal oxidation state of8.

120. XeOF2 will be of the type AX2YE2 and will have a T shape with O and thetwo lone pairs occupying the equatorial positions of the trigonal bipyramid.XeOF3

will be of the type AX3YE2. In this octahedral arrangement the twolone pairs will be trans to each other for minimum lp-lp repulsion. Hence thepredicted shape for he molecule will be square planar with the three fluorinesand oxygen occupying the plane. (J. Am. Chem. Soc, 2010,132, 10935)

121. They all have 10 valence electrons (hypervalent compounds)

122. Xenon oxides on hydrolysis give hydroxenous and hydroxenic acids. Hencethey are acidic.

123. Yes. In fact a square planar complex of gold with xenon as ligand, AuXe42+

has been reported and structurally characterized.

124. Beyond a limit, oxygen in high pressure will become toxic to biochemicalsystems and so it has to be diluted with a non reactive gas. Therefore, air ismore preferred than pure, O2 for shallow diving. But due to nitrogen narcosis(temporary decline or loss of senses and movement, numbness) it is notpreferred for deep sea diving below 150 feet of sea water. Due to the verylow lipid solubility of helium, it has very little narcotic affect and is thereforeuseful as a diluent for oxygen between 150- 250 feet below sea water andsince its density is less it makes breathing easier as well.

About the author’s association with p-block elements

Anil J. Elias is professor of inorganic chemistry at the Department ofChemistry, Indian Institute of Technology, New Delhi. He has done hisdoctorate in the chemistry of sulfur-nitrogen heterocycles from IIT Madras.Afterwards he has gained post doctoral research experience in the area ofamorphous silicon (IISc, Bangalore), Si-O metal compounds (University ofGoettingen, Germany) and fluorine chemistry (University of Idaho, USA). Afterreturning to India and joining IIT Kanpur and later IIT Delhi, he has beencarrying out research in the chemistry of fluorinated and chlorinatedphosphorus-nitrogen and phosphorus- nitrogen- sulphur heterocycles. Hisother area of research interest is in the organometallic chemistry of sandwichcompounds and development of new metal sandwich based catalysts. Prof.Elias with more than 20 years of teaching experience is the author of two textbooks and the recipient of the 2012 INSA teacher award instituted by theIndian National Science Academy for excellence in teaching sciences.

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