Answer :

(a) Dimerisation is a process of association (addition reaction) of two similar molecules in order to gain stability which was absent in the monomer form.


NO2 being unstable dimerises to N2O4 to exist as a stable molecule.


NO2 beforehand, was an odd electron molecule (7+16=23) due to which it tries to unite with another of its kind to become stable.


That is why it dimerises.


(b) The bleaching action of chlorine is oxidative in nature that’s why it is so powerful and the effect is permanent. It needs moisture for its bleaching action. Chlorine reacts with water to form hydrochloric (HCl)andhypochlorous (HOCl) acids.



HOCl acid being unstable, easily dissociates into nascent oxygen as shown:



This nascent oxygen apowerful oxidising agent and is responsible for the bleaching nature of chlorine.


(c) Electron gain enthalpy is defined as the amount of energy released when an electron is added to an isolated neutral gaseous atom.


Both O and S lies in the group 16 of the periodic table and O lies above S. The negative electron gain enthalpy is more in sulphur than in oxygen.


As a result:


Oxygen, the first element has the smaller size and sulphur with larger size,as a result, due to smaller size, the inter-electronic repulsion in smaller 2p-orbitals of oxygen is much more than the repulsion in 3p-orbitals of sulphur.


So, the incoming new electron feels less attraction in oxygen and more attraction in sulphur due to this attraction, the addition of electron becomes easy and thus electron gain enthalpy becomes more negative in sulphur.


(d) Fluorine being the most electronegative atom does not exhibit positive oxidation state.Also it does not have d-orbitals for octet expansion and therefore, it shows only a negative oxidation state of -1. So, the only oxoacid it can produce is HOF, where F has -1 state.


(e) Due to weak dispersion forces, noble gases as they are monoatomic have no interatomic forces and therefore, they are liquefied at very low temperatures. Hence, they have low boiling points.


But intermolecular forces increases with atomic size, as a result helium has a lower boiling than neon, followed by argon etc, down the group.


OR


(a) (i)


Due to the presence of five electron pairs around sulphur adopt trigonal bipyramidal geometry with see-saw shape.Also, one position is occupied by a lone pair. This lone pair finds a position which tries to minimize the number of 90° repulsions it has with the bonding electron pairs as lesser the repulsions, more stable is the molecule. It occupies an equatorial position with two 90° repulsions. The bonded electrons occupy the axial and equatorial positions. As the axial S-F bonds are slightly bent away from the lone pair,we can conclude that all the bonds in SF4 are not equivalent.


(ii)


Bent T-shaped and due to the presence of 3 bond pairs and 2 lone pairs, on central atom, the shape is trigonal bipyramidal.


(b)(i) When chlorine gas is passed through hot and concentrated sodium hydroxide solution, chlorine(0) gets sodium chlorate(+5) along with sodium chloride and water.



(ii) When xenon hexafluoride is subjected to complete hydrolysis, xenon trioxide (XeO3) and hydrofluoric acid (HF) is formed.



(iii)Dehydration reaction occurs when concentrated sulphuric acid is poured over cane sugar. A black residue of carbon is left behind along with evaporation of water molecules.



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