Chapter 7 The p Block Elements Solutions
Question - 51 : - Justify the placement of O, S,Se, Te and Po in the same group of the periodic table in terms of electronicconfiguration, oxidation state and hydride formation.
Answer - 51 : -
The elements of group 16 arecollectively called chalcogens.
(i) Elementsof group 16 have six valence electrons each. The general electronicconfiguration of these elements is ns2 np4,where n varies from 2 to 6.
(ii) Oxidation state:
As these elements have sixvalence electrons (ns2 np4), theyshould display an oxidation state of −2. However, only oxygen predominantlyshows the oxidation state of −2 owing to its high electronegativity. It alsoexhibits the oxidation state of −1 (H2O2), zero (O2),and +2 (OF2). However, the stability of the −2 oxidation statedecreases on moving down a group due to a decrease in the electronegativity ofthe elements. The heavier elements of the group show an oxidation state of +2,+4, and +6 due to the availability of d-orbitals.
(iii) Formation ofhydrides:
These elements form hydrides offormula H2E, where E = O, S, Se, Te, PO. Oxygen andsulphur also form hydrides of type H2E2. These hydridesare quite volatile in nature.
Question - 52 : - Why is dioxygen a gas but sulphura solid?
Answer - 52 : -
Oxygen is smaller in size ascompared to sulphur. Due to its smaller size, it can effectively form pπ−pπ bondsand form O2 (O==O) molecule. Also, the intermolecular forces inoxygen are weak van der Wall’s, which cause it to exist as gas. On the otherhand, sulphur does not form M2 molecule but exists as apuckered structure held together by strong covalent bonds. Hence, it is asolid.
Question - 53 : - Knowing the electron gainenthalpy values for O → O− and O → O2− as −141 and 702 kJ mol−1 respectively,how can you account for the formation of a large number of oxides having O2− speciesand not O−?
Answer - 53 : -
Stability of an ionic compounddepends on its lattice energy. More the lattice energy of a compound, morestable it will be.
Latticeenergy is directly proportional to the charge carried by an ion. When a metalcombines with oxygen, the lattice energy of the oxide involving O2− ionis much more than the oxide involving O− ion. Hence, the oxidehaving O2− ions are more stable than oxides having O−.Hence, we can say that formation of O2− is energetically morefavourable than formation of O−
Question - 54 : - Which aerosols deplete ozone?
Answer - 54 : -
Freons or chlorofluorocarbons(CFCs) are aerosols that accelerate the depletion of ozone. In the presence ofultraviolet radiations, molecules of CFCs break down to form chlorine-freeradicals that combine with ozone to form oxygen.
Question - 55 : - Describe the manufacture of H2SO4 bycontact process?
Answer - 55 : -
Sulphuric acid is manufactured bythe contact process. It involves the following steps:
Step (i):
Sulphur or sulphide ores areburnt in air to form SO2.
Step (ii):
By a reaction with oxygen, SO2 isconverted into SO3 in the presence of V2O5 asa catalyst.
Step (iii):
SO3 produced isabsorbed on H2SO4 to give H2S2O7 (oleum).
This oleum is then diluted toobtain H2SO4 of the desired concentration.
In practice, the plant isoperated at 2 bar (pressure) and 720 K (temperature). The sulphuric acid thusobtained is 96-98% pure.
Question - 56 : - How is SO2 an airpollutant?
Answer - 56 : -
Sulphur dioxide causes harm tothe environment in many ways:
1. Itcombines 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 invery low concentrations, SO2 causes irritation in therespiratory tract. It causes throat and eye irritation and can also affect thelarynx to cause breathlessness.
3. It isextremely harmful to plants. Plants exposed to sulphur dioxide for a long timelose colour from their leaves. This condition is known as chlorosis. Thishappens because the formation of chlorophyll is affected by the presence ofsulphur dioxide.
Question - 57 : - Why are halogens strong oxidisingagents?
Answer - 57 : -
The general electronicconfiguration of halogens is np5, where n = 2-6. Thus,halogens need only one more electron to complete their octet and to attain thestable noble gas configuration. Also, halogens are highly electronegative withlow dissociation energies and high negative electron gain enthalpies.Therefore, they have a high tendency to gain an electron. Hence, they act asstrong oxidizing agents.
Question - 58 : - Explain why fluorine forms onlyone oxoacid, HOF.
Answer - 58 : -
Fluorine forms only one oxoacidi.e., HOF because of its high electronegativity and small size.
Question - 59 : - Explain why inspite of nearly thesame electronegativity, oxygen forms hydrogen bonding while chlorine does not.
Answer - 59 : -
Both chlorine and oxygen havealmost the same electronegativity values, but chlorine rarely forms hydrogenbonding. This is because in comparison to chlorine, oxygen has a smaller sizeand as a result, a higher electron density per unit volume.
Question - 60 : - Write two uses of ClO2.
Answer - 60 : -
Uses of ClO2:
(i) Itis used for purifying water.
(ii) Itis used as a bleaching agent.