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NCERT Solutions for Class 12 Chemistry Chapter 7 The p Block Elements
Why are pentahalides more covalent than trihalides?
Higher the positive oxidation state of central atom, more will be its polarising power which, in turn, increases the covalent character of bond formed between the central atom and the other atom.
Why is BiH3 the strongest reducing agent amongst all the hydrides of group 15 element?
Because BiH3 is the least stable among the hydrides of group 15.
Why is N2 less reactive at room temperature?
Because of strong pπ – pπ overlap resulting into the triple bond. N = N due to which the bond dissociation energy of N2 is very high rendering it less reactive.
Mention the conditions required to maximise the yield of ammonia.
N2 + 3H2 2NH3; ∆fH = -46.1 kj mol-1
In accordance with Le Chatelier’s principle high pressure and low temperature would favour formation of ammonia. The optimum conditions for the production of ammonia are a pressure of about 200 atm, a temperature of about 700K and use of a catalyst such as iron oxide with small amount of K2O and Al2,O3 as promoters.
How does ammonia react with a solution of Cu2+?
NH3 in the form of solution reacts with Cu2+ to form a complex with deep blue colour.
What is the covalence of nitrogen in N2O5?
From the structure of N2O5, it is evident that covalence of nitrogen is four.
Bond angle in PH4+ is higher than in PH3. Why?
Both are sp3 hybridised. In PH4+ all the four orbitals are bonded whereas in PH3 there is a lone pair of electrons on P, which is responsible for lone pair-bond pair repulsion in PH3 reducing the bond angle to less than 109° 28′.
What happens when white phosphorus is heated with concentrated NaOH solution in an inert atmosphere of CO2?
When white phosphorus is heated with concentrated NaOH solution in an inert atmosphere of CO2, phosphine gas is liberated.
What happens when PCl5 is heated?
On heating PCl5 sublimes and is converted to PCl3 on stronger heating.
Write a balanced equation for the hydrolytic reaction of PCl5 in heavy water.
PCl5 + D2O → POCl3, + 2DCl
What is the basicity of H3PO4?
Three P – OH groups are present in the molecule of H3PO4. Therefore, its basicity is three.
What happens when H3PO3 is heated?
On heating phosphorous acid dispropotionates to give orthophosphoric acid and phosphine.
4H3PO3 → 3H3PO4 + PH3
List the important sources of sulphur.
Combined sulphur exists as sulphates, such as gypsum, epsom, baryte and sulphides such as galena, zinc blende, copper pyrites, etc. Traces of sulphur occur as hydrogen sulphide in volcanoes. Few organic materials like eggs, proteins, garlic, onion, mustard, hair and wool contain sulphur. 0.03 – 0.1% sulphur is present in the earth’s crust.
Write the order of thermal stability of the hydrides of group 16 element.
H2O > H2S > H2Se > H2Te
Why is H2O a liquid and H2S a gas?
Because of small size and high electronegativity of oxygen, molecules of water are highly associated through hydrogen bonding resulting in its liquid state.
Which of the following does not react with oxygen directly? Zn,Ti, Pt, Fe.
Platinum does not react with oxygen directly
Complete the following reactions :
- C2H4 + O2 →
- 4Al + 3 O2 →
- C2H4 + 3O2 → 2CO2 + 2H2O
- 4Al + 3O2 → 2Al2O3
Why does O3 act as a powerful oxidising agent?
Due to the ease with which ozone liberates nascent oxygen atoms, it acts as a powerful oxidising agent.
O3 → O2 + O
How is O3 estimated quantitatively?
When ozone reacts with an excess of KI solution buffered with a borate buffer (pH = 9.2), iodine is liberated which can be titrated against standard solution of sodium thiosulphate. This is used as a method of estimation of ozone quantitatively.
What happens when sulphur dioxide is passed through an aqueous solution of Fe(lll) salt?
When sulphur dioxide is passed through an aqueous solution of ferric ions, ferric ions are reduced to ferrous ions.
Comment on the nature of two S — O bonds formed in SO2 molecule. Are the two S — O bonds in this molecule equal?
Both the S—O bonds are covalent and have equal strength due to resonating structures.
How is the presence of SO2 detected?
Presence of SO2 is detected by bringing a paper dipped in acidified potassium dichromate near the gas. If the paper turns green, it shows the presence of SO2 gas.
Mention three areas in which H2SO4 plays an important role.
- In manufacture of fertilisers.
- In manufacture of pigments, paints and dyestuff intermediates.
- In detergent industry
Write the conditions to maximise the yield of H2SO4 by Contact process.
The key step in the manufacture of sulphuric acid is oxidation of SO2 to SO3 in presence of V2O5 catalyst.
The reaction is exothermic and reversible. Hence, low temperature and high pressure are the favourable conditions for maximum yield of SO3. In practice a pressure of 2 bar and temperature of 720 K is maintained.
Why is Ka2 « Ka1 for H2SO4 in water?
H2SO4 is a very strong acid in water largely because of its first ionisation to H3O+ and HSO4– The ionisation of HSO4– to H3O+ and SO42- is very very small. That is why, Ka2« Ka1.
Considering the parameters such as bond dissociation enthalpy, electron gain enthalpy and hydration enthalpy, compare the oxidising power of F2 and Cl2.
Fluorine is a better oxidising agent than chlorine because E°F2/F- is higher than E°Cl2/Cl- It is mainly due to low bond dissociation energy, high hydration energy and lower electron gain enthalpy, non-availability of d-orbitals in valence shell, that results in higher reduction potential of F2 than chlorine.
Give two examples to show the anomalous behaviour of fluorine.
- Ionisation enthalpy, electro-negativity and electrode potential are higher for fluorine than the expected trends of other halogen.
- Fluorine does not show any positive oxidation state except in HOF.
Sea is the greatest source of some halogens. Comment.
Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium but sodium chloride being the maximum makes sea water saline. Various sea weeds contain upto 0.5% iodine.
Give the reason for bleaching action of Cl2.
Chlorine bleaches by oxidation Cl2 + H2O → HCl + HOCl → HCl + [O]
The nascent oxygen reacts with dye to make it colourless.
Name two poisonous gases which can be prepared from chlorine gas.
COCl2 (phosgene), CCl3NO2 (tear gas)
Why is ICI more reactive than l2?
In general, interhalogen compounds are more reactive than halogens due to weaker X-X’ bonding than X-X bond. Thus, ICI is more reactive than I2.
Why is helium used in diving apparatus?
A mixture of helium and oxygen does not cause pain due to very low solubility of helium in blood as compared to nitrogen.
Balance the following equation :
XeF6 + H2O → XeO2F2 + 4HF
Why has it been difficult to study the chemistry of radon?
Radon is radioactive with very short half-life which makes the study of chemistry of radon difficult.
Discuss the general characteristics of Group 15 elements with reference to their electronic configuration, oxidation state, atomic size, ionisation enthalpy and electronegativity.
The valence shel 1 electronic configuration of group 15 elements is ns2np3. Due to half- filled p-orbitals, these elements have extra stability associated with them.
The common oxidation states of these elements are – 3, +3 and +5. The tendency to exhibit -3 oxidation state decreases down the group. The stability of +5 state decreases and that of +3 state increases down the group due to inert pair effect.
The size of group 15 elements increases down the group. There is a considerable increase in covalent radius from N to P. However, from As to Bi only a small increase in covalent radius is observed. This is due to presence of completely filled d and or f orbitals in heavier members.
Down the group, ionisation enthalpy decreases due to increase in atomic size. Due to stable half-filled configuration, they have much greater value than that of group 14 elements.
The electronegativity value, in general, decreases down the group with increasing atomic size. However, amongst the heavier elements, the difference is not that much pronounced.
Why does the reactivity of nitrogen differ from phosphorus?
Nitrogen has a unique ability to form pπ – pπ multiple bonds with itself and with other elements having small size and high electronegativity. Consequently, its bond enthalpy is very high and reactivity is less. Another factor which affects the chemistry of N is the absence of d-orbitals in its valence shell. As a result, not only the covalency of N is restricted to four, it can also not form dn – pn bonds. P on the other hand, does not form pn – pn bonds and hence it only forms a P – P single bond, which can easily be broken. Also, phosphorus has vacant d-orbitals and can form dn – dn bond with transition metals to form compounds which can act as ligands.
Discuss the trends in chemical reactivity of group 15 elements.
Nitrogen has very low reactivity due to unavailability of vacant d-orbital and high bond dissociation energy of N ≡ N bond.
(a) Hydrides : General formula for hydrides is MH3, e.g., NH3, PH3, ASH3, SbH3, BiH3. All these hydrides are covalent in nature and have pyramidal structure (sp3 hybridized).
(b) Halides – Elements of group 15 form two types of halides viz. trihalides and pentahalides. The halides are predominantly basic (Lewis bases) in nature and have lone pair of electrons (central atom is sp3 hybridized). The pentahalides are thermally less stable than the trihalides.
(c) Oxides – All the elements of this group form two types of oxides ie., M2O3 and M2O5 and are called trioxides and pentoxides
Why does NH3 form hydrogen bond but PH3 does not?
Hydrogen bond is formed between electronegative atom and hydrogen atom. Nitrogen is an electronegative atom and electronegativity decreases down the group so PH3 cannot form H – bond.
How is nitrogen prepared in the laboratory? Write the chemical equations of the reactions involved.
In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite.
Small amounts of NO and HNO3 are also formed in this reaction. These impurities can be removed by passing the gas through aqueous sulphuric acid containing potassium dichromate. It can also be obtained by the thermal decomposition of ammonium dichromate.
Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide.
How is ammonia manufactured industrially?
Industrially ammonia is manufactured by Haber’s process.
In accordance with Le-Chatelier’s principle, high pressure and low temperature would favour the formation of ammonia. The optimum conditions for production of ammonia are a pressure of 200 × 105 Pa (about 200 atm), a temperature of -700 K and the use of a catalyst such as iron-oxide with small amounts of K2O and Al2O3 to increase the rate of attainment of equilibrium.
Illustrate how copper metal can give different products on reaction with HNO3.
acid is a strong oxidising agent and attacks most metals. The products of oxidation depend upon the concentration of the acid, temperature and the nature of the material undergoing oxidation. Copper with . cold dil. HNO3 forms nitric oxide (NO).
Give the resonating structures of NO2 and N2O5.
The HNH angle value is higher than HPH, HAsH and HSbH angles. Why?
[Hint : Can be explained on the basis of sp3 hybridisation in NH3 and only s-p bonding between hydrogen and other elements of the group].
The actual bond angles are
The decreased bond angle in other hydrides can be explained by the fact that the sp3 hybridisation becomes less and less distinct with increasing size of the central atom i.e., pure p-orbitals are utilised in M-H bonding.
Why does R3P = O exist but R3 N = O does not (R = alkyl group)?
In R3 N = O, covalency required is five. The maximum covalency of nitrogen is four as it does not possess d-orbitals in the valence shell i.e., it cannot extend its valency beyond four. On the other hand, other members have d-orbitals and can utilise these orbitals to show covalency of five or six e.g., R3P = O, PCl5, [SbF6]– etc. These can form dπ – pπ bonds.
Explain why NH3 is basic while BiH3 is only feebly basic.
The basic character decreases from NH3 to BiH3. The basic nature is due to the presence of lone pair of electrons on the central atom. NH3 is the strongest electron pair donor due to its small size as the electron density of the electron pair is concentrated over a small region. As the size increases the electron density gets diffused over a large region and hence the ability to donate the electron pair (basic nature) decreases.
Nitrogen exists as diatomic molecule and phosphorus as P4. Why?
Nitrogen is diatomic gaseous molecule at ordinary temperature due to its ability to form pπ – pπ multiple bonds. The molecule has one σ and two π – bonds. Phosphorus exists as discrete tetratomic tetrahedral molecules as these are not capable of forming multiple bonds due to repulsion between non-bonded electrons of the inner core.
Write main differences between the properties of white phosphorus and red phosphorus.
Structure of white and red phosphorus are given below :
Why does nitrogen show catenation properties less than phosphorus?
The single N-N bond is weaker than the single P-P bond because of high interelectronic repulsion of the non-bonding electrons, owing to the small bond length. As a result the catenation tendency is weaker in nitrogen.
Give the disproportionation reaction of H3PO3.
The acids in +3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states, e.g., phosphorous acid on heating disproportionates to give orthophosphoric acid and phosphine.
4H3PO3 → 3H3PO4 + PH3
Can PCl5 act as an oxidising as well as a reducing agent? Justify.
The oxidation state of P in PCl5 is +5. As P has five electrons in its valence shell, it cannot increase its oxidation state beyond +5 by donating electrons, therefore, PCl5 cannot act as a reducing agent. However, it can decrease its oxidation number from +5 to +3 or some lower value, so, PCl5 acts as an
oxidising agent. For example, it oxidises Ag to AgCl, Sn to SnCl4.
Justify the placement of O, S, Se, Te and Po in the same group of the periodic table in terms of electronic configuration, oxidation state and hydride formation.
All these elements have same ns2np4 (n = 2 to 6) valence shell electronic configuration and hence are justified to be placed in group 16 of the periodic table.
(ii) Oxidation states : They need two more electrons to form dinegative ions by acquiring the nearest noble gas configuration. So, the minimum oxidation state of these elements should be -2. Oxygen predominantly and sulphur to some extent being electronegative show an oxidation state of -2. Since these elements have six electrons in the valence shell, therefore, the maximum oxidation state they can show is, + 6. Other positive oxidation states shown by these elements are +2 and +4. Although, oxygen due to the absence of d-orbitals does not show oxidation states of +4 and +6. Thus, on the basis of minimum and maximum oxidation states, these elements are justified to be placed in the same group i.c., group 16 of the periodic table.
(iii) Formation of hydrides : All the elements complete their respective octets by sharing two of their valence electrons with ls-orbilal of hydrogen to form hydrides of the general formula EH2 i.c., H2O, H2S, H2Se, H2Te and H2Po. Therefore, on the basis of formation of hydrides of the general formula, EH2, these elements are justified to be placed in group 16 of the periodic table.
Why is dioxygen a gas but sulphur a solid?
Oxygen atom has the tendency to form multiple bonds (pπ – pπ interaction) with other oxygen atom on account of small size while this tendency is missing in sulphur
atom. The bond energy of oxygen-oxygen double bond (0 = 0) is quite large (about three times that of oxygen-oxygen single bond, O – O = 34.9 kcal mol-1) while sulphur-sulphur double bond (S = S) is not so large (less than double of sulphur-sulphur single bond, S – S = 63.8 kcal mob1). As a resul —O—O—O— chains are less stable as compared to O = O molecule while —S—S—S— chains are more stable than S = S molecule. Therefore, at room temperature, while oxygen exists as a diatomic gas molecule, sulphur exists as S8 solid.
Knowing the electron gain enthalpy values for O → O– and O → O2- as -141 and 702 kJ mol-1 respectively, how can you account for the formation of large number of oxides having O2- species and not O– ?
This can be explained with the help of electronic configuration.
As O2- has most stable configuration amongst these. So, formation of O2- is much more easier. In solid state, large amount of energy (lattice enthalpy) is released to form divalent O2- ions. It is greater lattice enthalpy of O2- which compensates for the high energy required to remove the second electron.
Which aerosols deplete ozone?
CFC – Chlorofluorocarbon (freons) : These compounds commonly known as freons are introduced into the atmosphere from aerosol sprays and refrigerating equipments. They undergo photochemical decomposition and destroy ozone as shown by the following sequence of reactions.
Describe the manufacture of H2SO4 by Contact process?
Sulphuric acid is manufactured by the Contact process which involves three steps :
- burning of sulphur or sulphide ores in air to generate SO2.
- conversion of SO2 to SO3 by the reaction with oxygen in the presr nee of a catalyst (V2O5), and
- absorption of SO3 in H2SO4 to give oleum (H2S2O7).
A flow diagram for the manufacture of sulphuric acid is shown in the figure. The SO2 produced is purified by removing dust and other impurities such as arsenic compounds. The key step in the manufacture of H2SO4
is the catalytic oxidation of SO2 with O2 to give SO3 in the presence of V2O5 (catalyst).
The reaction is exothermic, reversible and the forward reaction leads to a decrease in volume. Therefore, low temperature and high pressure are the favourable conditions for maximum yield. But the temperature should not be very low otherwise rate of reaction will become slow.
In practice, the plant is operated at a pressure of 2 bar and a temperature of 720 K. The SO3 gas from the catalytic converter is absorbed in concentrated H2SO4 to produce oleum. Dilution of oleum with water give H2SO4 of the desired concentration. In the industry, two steps are carried out simultaneously to make the
process a continuous one and also to reduce the cost.
How is SO2 an air pollutant?
SO2 present in atmosphere combines with water to give sulphuric acid.
2SO2 + O2 + 2H2O → 2H2SO4
This is called acid rain. Acid rain causes extensive damage to building and statues made of marble, limestone, etc. due to its reaction to give CaSO4.
CaCO3 + H2SO4 → CaSO4 + CO2 + H2O
This way SO2 acts as pollutant.
Why are halogens strong oxidising agents?
General electronic configuration of halogen is
It will easily accept one electron to fulfill its shell. This easily acceptance of electron makes halogens strong oxidising agents.
Explain why fluorine forms only one oxoacid, HOF.
Due to high electronegativity and absence of d orbitals, F does not form oxoacids such as HOFO, HOFO2 and HOFO3 in which the oxidation state of F is +3, +5 and +7. It just forms one oxoacid, i.e., HOF in which the oxidation state of F is + 1.
Explain why inspite of nearly the same electronegativity, oxygen forms hydrogen bonding while chlorine does not.
Although O and Cl have about the same electronegativity, yet their atomic size (covalent radii) are much different: O = 66 pm and Cl = 99 pm. Thus, electron density per unit volume of oxygen atom is much higher than that of chlorine atom. Hence, oxygen forms hydrogen bonds while chlorine does not though both have approx, the same electronegativity.
Write two uses of ClO2.
ClO2 is a powerful oxidising agent. ClO2 is used as a bleaching agent for paper pulp and textiles and in water treatment.
Why are halogens coloured?
All halogens are coloured because of absorption of radiations in visible region which results in the excitation of outer electrons to higher level. By absorbing different quanta of radiation, they display different colours as
F2 → yellow
Cl2 → greenish yellow
Br2 → red
I2 → violet
Write the reactions of F2 and Cl2 with water.
Fluorine reacts vigorously with water and oxidises water to oxygen.
Chlorine dissolves in water to form chlorine water. It slowly reacts with the water to form a mixture of hydrochloric acid and hypochlorous acid.
Hypochlorous acid is very unstable. In presence of sunlight, it decomposes to give HCl and nascent oxygen.
This oxygen is responsible for oxidising and bleaching properties of chlorine.
How can you prepare Cl2 from HCI and HCI from Cl2? Write reactions only.
Preparation of chlorine by Deacon’s process : By oxidation of hydrogen chloride gas by atmospheric oxygen
in the presence of CuCl2 at 723 K
What inspired N. Bartlett for carrying out reaction between Xe and PtF6?
What are the oxidation states of phosphorus in the following :
Write balanced equations for the following :
- NaCI is heated with sulphuric acid in the presence of MnO2.
- Chlorine gas is passed into a solution of Nal in water.
How are xenon fluorides XeF2, XeF4 and XeF6 obtained?
Xenon forms three binary fluorides, XeF2, XeF4 and XeF6 by the direct reaction of elements under appropriate experimental conditions.
With what neutral molecule is ClO– isoelectronic? Is that molecule a Lewis base?
Replace O– (9 electrons) in ClO– by F (9 electrons). The resulting neutral molecule is ClF. Since ClF can combine further with F to form ClF3, so, ClF is a Lewis base.
How are XeO3 and XeOF4 prepared?
Arrange the following in the order of property indicated for each set :
- F2, Cl2, Br2, l2 – increasing bond dissociation enthalpy.
- HF, HCI, HBr, HI – increasing acid strength.
- NH3, PH3, ASH3, SbH3, BiH3 – increasing base strength.
- l2 < Br2 < F2 < Cl2
- HF < HCI < HBr < HI
- NH3 > PH3 > AsH3 > SbH, > BiH3
Which one of the following does not exist?
(ii) Amongst all noble gases, only Xe (except KrF2) forms compounds.
Give the formula and describe the structure of a noble gas species which is isostructural with :
(iii) Structure of BrO–3 : The central atom Br has seven electrons. Four of these electrons form two double bonds or coordinate bonds with two oxygen atoms while the fifth electron forms a single bond with O– ion. The remaining two electrons form one lone pair. Hence, in all there are three bond pairs and one lone pair around Br atom in BrO–3 Therefore, according to VSEPR theory, BrO–3 s should be pyramidal.
Here, BrO–3 has 26(7 + 3 × 6 + 1 = 26) valence electrons. A noble gas species having 26 valence electrons is XeO3 (8 + 3 × 6 = 26). Thus, like BrO–3, XeO3 is also pyramidal.
Why do noble gases have comparatively large atomic sizes?
The atomic radii of noble gases are by far the largest in their respective periods. This is due to the reason that noble gases have only van der Waals radii while others have cova lent radii, van der Waals radii, by definition are larger than covalent radii.
List the uses of neon and argon gases.
Uses of neon :
- Neon is used in discharge tubes and fluorescent bulbs for advertisement display purposes.
- Glow of different colours ‘neon signs’ can be produced by mixing neon with other gases.
- Neon bulbs and used in botanical gardens and in green houses.
Uses of argon:
- Argon is used mainly to provide an inert atmosphere in high temperature metallurgical processes such as arc welding of metals and alloys. In the laboratory, it is used for handling substance which are air sensitive.
- It is used in filling incandescent and fluorescent lamps where its presence retards the sublimation of the filament and thus increases the life of the lamp.
- It is also used in “neon signs” for obtaining lights of different colours.
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