Chapter 11 - The p-Block Elements

Assertion (A): Components of a mixture of red and blue inks can be
separated by distributing the components between stationary
and mobile phases in paper chromatography.
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Exemplar Problems, Chemistry 156
Reason (R) : The coloured components of inks migrate at different rates
because paper selectively retains different components
according to the difference in their partition between the two
phases.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.

Assertion (A) : Sulphur present in an organic compound can be estimated
quantitatively by Carius method.
Reason (R) : Sulphur is separated easily from other atoms in the molecule
and gets precipitated as light yellow solid.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.

Assertion (A) : Pent- 1- ene and pent- 2- ene are position isomers.
Reason (R) : Position isomers differ in the position of functional group or
a substituent.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.

Assertion (A) : Energy of resonance hybrid is equal to the average of energies
of all canonical forms.
Reason (R) : Resonance hybrid cannot be presented by a single structure.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.

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 choices given
below each question.

 Assertion (A) : Simple distillation can help in separating a mixture of
propan-1-ol (boiling point 97°C) and propanone (boiling point
56°C).
Reason (R) : Liquids with a difference of more than 20°C in their boiling
points can be separated by simple distillation.
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155 Organic Chemistry – Some Basic Principles and Techniques
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct.
(iv) A is not correct but R is correct.

Match the intermediates given in Column I with their probable structure in
Column II.
Column I Column II
(i) Free radical (a) Trigonal planar
(ii) Carbocation (b) Pyramidal
(iii) Carbanion (c) Linear

Match the terms mentioned in Column I with the terms in Column II.
Column I Column II
(i) Carbocation (a) Cyclohexane and 1- hexene
(ii) Nucleophile (b) Conjugation of electrons of C–H
σ bond with empty p-orbital present
at adjacent positively charged
carbon.
(iii) Hyperconjugation (c) sp2 hybridised carbon with empty
p-orbital
(iv) Isomers (d) Ethyne
(v) sp hybridisation (e) Species that can receive a pair of
electrons
(vi) Electrophile (f) Species that can supply a pair of
electrons

In the following questions more than one correlation is possible between
options of Column I and Column II. Make as many correlations as you can.

 Match the type of mixture of compounds in Column I with the technique of
separation/purification given in Column II.
Column I Column II
(i) Two solids which have different (a) Steam distillation
solubilities in a solvent and which
do not undergo reaction when
dissolved in it.
(ii) Liquid that decomposes at its (b) Fractional distillation
boiling point
(iii) Steam volatile liquid (c) Simple distillation
(iv) Two liquids which have boiling (d) Distillation under reduced
points close to each other pressure
(v) Two liquids with large difference in (e) Crystallisation
boiling points.

By mistake, an alcohol (boiling point 97°C) was mixed with a hydrocarbon
(boiling point 68°C). Suggest a suitable method to separate the two
compounds. Explain the reason for your choice.

Give three points of differences between inductive effect and resonance effect.

Write structural formulae for compounds named as-
(a) 1-Bromoheptane (b) 5-Bromoheptanoic acid

Three students, Manish, Ramesh and Rajni were determining the extra
elements present in an organic compound given by their teacher. They
prepared the Lassaigne’s extract (L.E.) independently by the fusion of the
compound with sodium metal. Then they added solid FeSO4 and dilute
sulphuric acid to a part of Lassaigne’s extract. Manish and Rajni obtained
prussian blue colour but Ramesh got red colour. Ramesh repeated the test
with the same Lassaigne’s extract, but again got red colour only. They were
surprised and went to their teacher and told him about their observation.
Teacher asked them to think over the reason for this. Can you help them by
giving the reason for this observation. Also, write the chemical equations to
explain the formation of compounds of different colours.

Write structures of various carbocations that can be obtained from
2-methylbutane. Arrange these carbocations in order of increasing stability.

In DNA and RNA, nitrogen atom is present in the ring system. Can Kjeldahl
method be used for the estimation of nitrogen present in these? Give reasons.

Explain, how is the electronegativity of carbon atoms related to their state of
hybridisation in an organic compound?

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 choices given
below each question.

Assertion (A): If aluminium atoms replace a few silicon atoms in three
dimensional network of silicon dioxide, the overall structure
acquires a negative charge.
Reason (R) : Aluminium is trivalent while silicon is tetravalent.
(i) Both A and R are correct and R is the correct explanation of A.
(ii) Both A and R are correct but R is not the correct explanation of A.
(iii) Both A and R are not correct
(iv) A is not correct but R is correct.

Match the species given in Column I with properties given in Column II.
Column I Column II
(i) Diborane (a) Used as a flux for soldering metals
(ii) Galluim (b) Crystalline form of silica
(iii) Borax (c) Banana bonds
(iv) Aluminosilicate (d) Low melting, high boiling, useful for
measuring high temperatures
(v) Quartz (e) Used as catalyst in petrochemical
industries

Aluminium dissolves in mineral acids and aqueous alkalies and thus shows
amphoteric character. A piece of aluminium foil is treated with dilute
hydrochloric acid or dilute sodium hydroxide solution in a test tube and on
bringing a burning matchstick near the mouth of the test tube, a pop sound
indicates the evolution of hydrogen gas. The same activity when performed
with concentrated nitric acid, reaction doesn’t proceed. Explain the reason.

If a trivalent atom replaces a few silicon atoms in three dimensional network
of silicon dioxide, what would be the type of charge on overall structure?

Carbon and silicon both belong to the group 14, but inspite of the
stoichiometric similarity, the dioxides, (i.e., carbon dioxide and silicon dioxide),
differ in their structures. Comment.

Draw the structure of boric acid showing hydrogen bonding. Which species
is present in water? What is the hybridisation of boron in this species?

Explain the nature of boric acid as a Lewis acid in water.

What is silica-gel used as?

Why does CO₂ have a linear shape with no dipole moment

Write the resonance structures of carbon dioxide.

How are silicones manufactured?

What are silicates?

What is dry ice?

What are silicones?

Silicon dioxide is treated with hydrogen fluoride. Explain?

Why is CO considered poisonous ?

What is water gas?

Graphite is used as lubricant. Give reason.

Diamond is the hardest substance known. Why?

How are fullerenes obtained?

Why does graphite conduct electricity?

Silicon has no allotropic form analogous to graphite. Why?

What is the common name of recently developed allotrope of carbon i.e. C60
molecule?

Why does carbon show different allotropic forms?

Why does the heavier elements do not form pπ – pπ multiple bond as carbon
do?

Why does the covalence of carbon not expand beyond four?

Why is carbon different from other member of the group?

PbI₄does not exist. Why?

Why is lead unaffected by water?

Why CCl₄ behaves as an electron precise molecule?

Some halides of group 14 elements form complexes of the type [ Mx₆]^2- Give reason.

Why carbon does not form ionic compounds?

Which of the following reacts with water and aqueous solution becomes acidic:

SiCl₄ or CCl₄?

C and S are always tetravalent but Ge, Sn And Pb show divalency. Why?

Name the metalloid found in Group 14 element?

What is the electronic configuration of Group -14 elements?

Why does BF₃ act as Lewis acids?

Why do boron have unusual high melting point?

The atomic radius of Ca is less than that of Al. Why?

How do Boron and Aluminium interact with Na OH?

Why is boron metalloid?

Boron is unable to form BF₆^3- ion why?

Arrange the following halides of boron in the increasing order of acidic

character: BF₃, BCl₃, BBr₃, BI₃.

Give two examples of electron deficient molecules.

How does boron interact with Na OH?

Where does metalloids and non – metals exist?

The elements B, Al, Ca, In and Tl are placed in the same group of the periodic
table. Give reason.

Why the elements of group 13 are called p-block elements?

Why do third – period elements expand their covalence above four?

How does metallic and non-metallic character vary in a group?

What is ‘inert pair effect’?

How many groups are there in p-block?

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

Elements of group 14

(a) exhibit oxidation state of +4 only

(b) exhibit oxidation state of +2 and +4

(c) form M^2-and M⁴⁺ion

(d) form M²⁺and M⁴⁺ions

Thermodynamically the most stable form of carbon is

(a) diamond (b) graphite

(c) fullerenes (d) coal

The type of hybridisation of boron in diborane is

(a) sp (b) sp²(c) sp³ (d) dsp²

Boric acid is polymeric due to

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

(c) its monobasic nature (d) its geometry

An aqueous solution of borax is

(a) neutral (b) amphoteric

(c) basic (d) acidic

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

Write balanced equations for:

(i) BF₃ + LiH Ã¢â€ ’

(ii) B₂H₆ + H₂O Ã¢â€ ’

(iii) NaH + B₂H₆ Ã¢â€ ’

(iv) H₃BO₃ 

(v) Al + NaOH Ã¢â€ ’

(vi) B₂H₆ + NH₃ Ã¢â€ ’

A certain salt X, gives the following results.

(i) Its aqueous solution is alkaline to litmus.

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

(iii) When conc. H₂SO₄is added to a hot solution of X, white crystal of an acid Zseparates out.

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

What do you understand by (a) inert pair effect (b) allotropy and(c) catenation?

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

In some of the reactions thallium resembles aluminium, whereas in others it resembles with group I metals. Support this statement by giving some evidences.

(a) Classify following oxides as neutral, acidic, basic or amphoteric:

CO, B₂O₃, SiO₂, CO₂, Al₂O₃, PbO₂, Tl₂O₃

(b) Write suitable chemical equations to show their nature.

What are allotropes? Sketch the structure of two allotropes of carbon namely diamond and graphite. What is the impact of structure on physical properties of two allotropes?

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

Explain why is there a phenomenal decrease in ionisation enthalpy from carbon to silicon?

Give reasons:

(i) Conc. HNO₃can be transported in aluminium container.

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

(iii) Graphite is used as lubricant.

(iv) Diamond is used as an abrasive.

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

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

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

Explain the following reactions

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

(b) Silicon dioxide is treated with hydrogen fluoride;

(c) CO is heated with ZnO;

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

What happens when

(a) Borax is heated strongly,

(b) Boric acid is added to water,

(c) Aluminium is treated with dilute NaOH,

(d) BF₃is reacted with ammonia?

Explain structures of diborane and boric acid.

How is excessive content of CO₂ responsible for global warming?

Suggest a reason as to why CO is poisonous.

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous BF₃is bubbled through. Give reasons.

If B-Cl bond has a dipole moment, explain why BCl₃molecule has zero dipole moment.

Suggest reasons why the B–F bond lengths in BF₃(130 pm) and (143 pm) differ.

Rationalise the given statements and give chemical reactions:

”¢ Lead(II) chloride reacts with Cl₂to give PbCl₄.

”¢ Lead(IV) chloride is highly unstable towards heat.

”¢ Lead is known not to form an iodide, PbI₄.

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

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

Describe the shapes of BF₃and BH₄^-. Assign the hybridisation of boron in these species.

What are electron deficient compounds? Are BCl₃and SiCl₄electron deficient species? Explain.

Write reactions to justify amphoteric nature of aluminium.

Describe the shapes of BF₃and BH₄^-. Assign the hybridisation of boron in these species.

Explain what happens when boric acid is heated.

Is boric acid a protic acid? Explain.

Consider the compounds, BCl₃ and CCl₄. How will they behave with water? Justify.

Why does boron trifluoride behave as a Lewis acid?

How can you explain higher stability of BCl₃ as compared to TlCl₃?

Discuss the pattern of variation in the oxidation states of

(i) B to Tl and (ii) C to Pb.