NCERT Solutions for Class 12 Chemistry Part 1 Chapter 7
The p-Block Elements Class 12
Chapter 7 The p-Block Elements Exercise Solutions
In text : Solutions of Questions on Page Number : 169
Why are pentahalides more covalent than trihalides?
Why is BiH3the strongest reducing agent amongst all the hydrides of Group 15 elements?
Why is N2less reactive at room temperature?
Mention the conditions required to maximise the yield of ammonia.
Ammonia is prepared using the Haber's process. The yield of ammonia can be maximized under the following conditions:
(i) High pressure (𝖺¼200 atm)
(ii) A temperature of 𝖺¼700 K
(iii)
How does ammonia react with a solution of Cu2+?
What is the covalence of nitrogen in N2O5?
From the structure of N2O5, it is evident that the covalence of nitrogen is 4.
Bond angle in is higher than that in PH3. Why? Answer :
In PH3, P is sp3hybridized. Three orbitals are involved in bonding with three hydrogen atoms and the fourth one contains a lone pair. As lone pair-bond pair repulsion is stronger than bond pair-bond pair repulsion, the tetrahedral
shape associated with sp3bonding is changed to pyramidal. PH3combines with a proton to form in which the lone pair is absent. Due to the absence of lone pair in , there is no lone pair-bond pair repulsion. Hence, the
bond angle in is higher than the bond angle in PH3.
What happens when white phosphorus is heated with concentrated NaOHsolution in an inert atmosphere of CO2?
What happens when PCl5is heated?
Write a balanced equation for the hydrolytic reaction of PCl5in heavy water.
Q11 :
Answer :
Since there are three OH groups present in H3PO4, its basicity is three i.e., it is a tribasic acid.
What happens when H3PO3is heated?
List the important sources of sulphur.
Write the order of thermal stability of the hydrides of Group 16 elements.
The thermal stability of hydrides decreases on moving down the group. This is due to a decrease in the bond dissociation enthalpy (H-E) of hydrides on moving down the group.
Why is H2O a liquid and H2S a gas?
H2O has oxygen as the central atom. Oxygen has smaller size and higher electronegativity as compared to sulphur. Therefore, there is extensive hydrogen bonding in H2O, which is absent in H2S. Molecules of H2S are held together only by weak van der Waal's forces of attraction.
Which of the following does not react with oxygen directly? Zn, Ti, Pt, Fe
Complete the following reactions:
(i) C2H4 + O2 →
Answer :
Why does O3act as a powerful oxidising agent?
Therefore, ozone acts as a powerful oxidising agent.
How is O3estimated quantitatively?
What happens when sulphur dioxide is passed through an aqueous solution of Fe(III) salt?
Comment on the nature of two S-O bonds formed in SO2molecule. Are the two S-O bonds in this molecule equal?
The electronic configuration of S is 1s2 2s2 2p6 3s2 3p4.
Both S-O bonds are equal in length (143 pm) and have a multiple bond character.
How is the presence of SO2detected?
SO2is a colourless and pungent smelling gas.
Mention three areas in which H2SO4plays an important role.
Sulphuric acid is an important industrial chemicaland is used for a lot of purposes. Some important uses of sulphuric acid are given below.
(i) It is used in fertilizer industry. It is used to make various fertilizers such as ammonium sulphate and calcium super phosphate.
(ii) It is used in the manufacture of pigments, paints, and detergents.
(iii)
Write the conditions to maximize the yield of H2SO4by Contact process.
Manufacture of sulphuric acid by Contact process involves three steps.
1. Burning of ores to form SO2
2. Conversion of SO2to SO3by the reaction of the former with O2 (V2O5is used in this process as a catalyst.)
3. Absorption of SO3in H2SO4to give oleum (H2S2O7)
It can be noticed that
Considering the parameters such as bond dissociation enthalpy, electron gain enthalpy and hydration enthalpy, compare the oxidising power of F2and Cl2.
Fluorine is a much stronger oxidizing agent than chlorine. The oxidizing power depends on three factors.
1. Bond dissociation energy
2. Electron gain enthalpy
3. Hydration enthalpy
Give two examples to show the anomalous behaviour of fluorine.
Anomalous behaviour of fluorine
(i) It forms only one oxoacid as compared to other halogens that form a number of oxoacids.
(ii)
Sea is the greatest source of some halogens. Comment.
Give the reason for bleaching action of Cl2.
Coloured substances + [O] → Oxidized colourless substance
Name two poisonous gases which can be prepared from chlorine gas.
Two poisonous gases that can be prepared from chlorine gas are
(i) Phosgene (COCl2)
(ii)
Why is ICl more reactive than I2?
Why is helium used in diving apparatus?
Balance the following equation: XeF6+ H2O → XeO2F2+ HF
Balanced equation
Why has it been difficult to study the chemistry of radon?
Exercise : Solutions of Questions on Page Number : 207
Discuss the general characteristics of Group 15 elements with reference to their electronic configuration, oxidation state, atomic size, ionisation enthalpy and electronegativity.
General trends in group 15 elements
(i) Electronic configuration: All the elements in group 15 have 5 valence electrons. Their general electronic configuration is ns2 np3.
(ii) Oxidation states: All these elements have 5 valence electrons and require three more electrons to complete their octets. However, gaining electrons is very difficult as the nucleus will have to attract three more electrons. This can take place only with nitrogen as it is the smallest in size and the distance between the nucleus and the valence shell is relatively small. The remaining elements of this group show a formal oxidation state of -3 in their covalent compounds. In addition to the -3 state, N and P also show -1 and -2 oxidation states.
All the elements present in this group show +3 and +5 oxidation states. However, the stability of +5 oxidation state decreases down a group, whereas the stability of +3 oxidation state increases. This happens because of the inert pair effect.
First ionization decreases on moving down a group. This is because of increasing atomic sizes. As we move down a group, electronegativity decreases, owing to an increase in size.
(iv)
Why does the reactivity of nitrogen differ from phosphorus?
Discuss the trends in chemical reactivity of group 15 elements.
General trends in chemical properties of group - 15
The elements of group 15 react with hydrogen to form hydrides of type EH3, where E = N, P, As, Sb, or Bi. The stability of hydrides decreases on moving down from NH3to BiH3.
The elements of group 15 form two types of oxides: E2O3and E2O5, where E = N, P, As, Sb, or Bi. The oxide with the element in the higher oxidation state is more acidic than the other. However, the acidic character decreases on moving down a group.
The group 15 elements react with halogens to form two series of salts: EX3and EX5. However, nitrogen does not form NX5as it lacks the d-orbital. All trihalides (except NX3) are stable.
Why does NH3form hydrogen bond but PH3does not?
How is nitrogen prepared in the laboratory? Write the chemical equations of the reactions involved.
NO and HNO3are produced in small amounts. These are impurities that can be removed on passing nitrogen gas through aqueous sulphuric acid, containing potassium dichromate.
How is ammonia manufactured industrially?
Ammonia is prepared on a large-scale by the Haber's process.
Illustrate how copper metal can give different products on reaction with HNO3.
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 sp3hybridisation in NH3and only s-p bonding between hydrogen and other elements of the group].
Hydride NH3 PH3 AsH3 SbH3
H-M-H angle 107° 92° 91° 90°
Why does R3P=O exist but R3N=O does not (R = alkyl group)?
Explain why NH3is basic while BiH3is only feebly basic.
NH3is distinctly basic while BiH3is feebly basic.
Nitrogen has a small size due to which the lone pair of electrons is concentrated in a small region. This means that the charge density per unit volume is high. On moving down a group, the size of the central atom increases and the charge gets distributed over a large area decreasing the electron density. Hence, the electron donating capacity of group 15 element hydrides decreases on moving down the group.
Nitrogen exists as diatomic molecule and phosphorus as P4. Why?
Write main differences between the properties of white phosphorus and red phosphorus.
White phosphorus |
Red Phosphorus |
|
It is a soft and waxy solid. It possesses a garlic smell. |
It is a hard and crystalline solid, without any smell. |
|
It is poisonous. |
It is non-poisonous. |
|
It is insoluble in water but soluble in carbon disulphide. |
It is insoluble in both water and carbon disulphide. |
|
It undergoes spontaneous combustion in air. |
It is relatively less reactive. |
|
In both solid and vapour states, it exists as a P4 molecule.
|
It exists as a chain of tetrahedral P4 units.
|
|
|
|
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Q14 :
Answer :
Give the disproportionation reaction of H3PO3.
Can PCl5act as an oxidising as well as a reducing agent? Justify.
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.
The elements of group 16 are collectively called chalcogens.
(i) Elements of group 16 have six valence electrons each. The general electronic configuration of these elements is ns2np4, where nvaries from 2 to 6.
Asthese elements have six valence electrons (ns2 np4), they should display an oxidation state of -2. However, only oxygen predominantly shows the oxidation state of -2 owing to its high electronegativity. It also exhibits the oxidation state of -1 (H2O2), zero (O2), and +2 (OF2). However, the stability of the -2 oxidation state decreases on moving down a group due to a decrease in the electronegativity of the elements. The heavier elements of the group show an oxidation state of +2, +4, and +6 due to the availability of d-orbitals.
Why is dioxygen a gas but sulphur a solid?
Knowing the electron gain enthalpy values for O → O-and O → O2-as -141 and 702 kJ mol-1respectively, how can you account for the formation of a large number of oxides having O2-species and not O-?
Answer :
Stability of an ionic compound depends on its lattice energy. More the lattice energy of a compound, more stable it will be.
Which aerosols deplete ozone?
Describe the manufacture of H2SO4by contact process?
Sulphuric acid is manufactured by the contact process. It involves the following steps:
Sulphur or sulphide ores are burnt in air to form SO2.
By a reaction with oxygen, SO2is converted into SO3in the presence of V2O5as a catalyst.
SO3produced is absorbed on H2SO4to give H2S2O7(oleum).
This oleum is then diluted to obtain H2SO4of the desired concentration.
How is SO2an air pollutant?
Sulphur dioxide causes harm to the environment in many ways:
1. It combines with water vapour present in the atmosphere to form sulphuric acid. This causes acid rain. Acid rain damages soil, plants, and buildings, especially those made of marble.
2. Even in very low concentrations, SO2causes irritation in the respiratory tract. It causes throat and eye irritation and can also affect the larynx to cause breathlessness.
3.
Why are halogens strong oxidising agents?
Explain why fluorine forms only one oxoacid, HOF.
Explain why inspite of nearly the same electronegativity, oxygen forms hydrogen bonding while chlorine does not.
Write two uses of ClO2.
Uses of ClO2:
(i) It is used for purifying water.
(ii)
Why are halogens coloured?
Almost all halogens are coloured. This is because halogens absorb radiations in the visible region. This results in the excitation of valence electrons to a higher energy region. Since the amount of energy required for excitation differs for each halogen, each halogen displays a different colour.
Write the reactions of F2and Cl2with water.
(i)
Q29 :
Answer :
(i)
(ii) HCl can be prepared from Cl2on treating it with water.
What inspired N. Bartlett for carrying out reaction between Xe and PtF6?
Neil Bartlett initially carried out a reaction between oxygen and PtF6. This resulted in the formation of a red compound, .
Later, he realized that the first ionization energy of oxygen (1175 kJ/mol) and Xe (1170 kJ/mol) is almost the same.
What are the oxidation states of phosphorus in the following:
(i) H3PO3 (ii) PCl3 (iii) Ca3P2
Answer :
.
(i)
(ii) PCl3
(iii)
(iv) Na3PO4
(v)
Write balanced equations for the following:
(ii) Chlorine gas is passed into a solution of NaI in water.
How are xenon fluorides XeF2, XeF4and XeF6obtained?
With what neutral molecule is ClO-isoelectronic? Is that molecule a Lewis base?
ClO- is isoelectronic to ClF. Also, both species contain 26 electrons in all as shown. Total electrons ClO-= 17 + 8 + 1 = 26
In ClF = 17 + 9 = 26
ClF acts like a Lewis base as it accepts electrons from F to form ClF3.
How are XeO3and XeOF4prepared?
(i) XeO3can be prepared in two ways as shown.
(ii) XeOF4can be prepared using XeF6.
Arrange the following in the order of property indicated for each set:
(ii) HF, HCl, HBr, HI - increasing acid strength.
Answer :
(i) Bond dissociation energy usually decreases on moving down a group as the atomic size increases. However, the bond dissociation energy of F2 is lower than that of Cl2 and Br2. This is due to the small atomic size of fluorine. Thus, the increasing order for bond dissociation energy among halogens is as follows:
I2< F2< Br2< Cl2
(ii) HF < HCl < HBr < HI
The bond dissociation energy of H-X molecules where X = F, Cl, Br, I, decreases with an increase in the atomic size. Since H-I bond is the weakest, HI is the strongest acid.
(iii) BiH3 ┰¤SbH3 < AsH3 < PH3 < NH3
Which one of the following does not exist?
(iii) XeF2 (iv) XeF6
Give the formula and describe the structure of a noble gas species which is isostructural with:
(ii)
(i)
Why do noble gases have comparatively large atomic sizes?
List the uses of Neon and argon gases.
Uses of neon gas:
(i) It is mixed with helium to protect electrical equipments from high voltage.
(ii) It is filled in discharge tubes with characteristic colours.
(iii) It is used in beacon lights.
(i) Argon along with nitrogen is used in gas-filled electric lamps. This is because Ar is more inert than N.
(ii) It is usually used to provide an inert temperature in a high metallurgical process.
(iii) It is also used in laboratories to handle air-sensitive substances.