Q29 of 30 Page 1

(a) Give reasons for the following:

(i) Bond enthalpy of F2 is lower than that of Cl2.


(ii)PH3 has lower boiling point than NH3.


(b)Draw the structures of the following molecules:


(i)BrF3


(ii)(HPO3)3


(iii)XeF4


OR


(a) Account for the following:


(i) Helium is used in diving apparatus.


(ii) Fluorine does not exhibit positive oxidation state.


(iii)Oxygen shows catenation behavior less than sulphur.


(b)Draw the structures of the following molecules:


(i)XeF2


(ii)H2S2O8



(a)


(i) Bond enthalpy of F2 is lower than that of Cl2 because F atom is small in size and due to this the electron- electron repulsions between the lone pairs of F-F electrons are very large due to which F-F bond is weakened. Less energy would be required to break this bond. Thus, the bond dissociation energy of F2 is lower than that of Cl2.


(ii) Electronegative elements like N,F,O can form intermolecular hydrogen bonding with water to stabilise the molecule. PH3 has lower boiling point than NH3 because NH3 molecule possess N atom due to which there are intermolecular hydrogen bondings which binds them strongly, whereas, PH3 has weaker van der Waals forces.


(b)


(i)


Structure of BF3


(ii)


Structure of (HPO3)3


(iii)


Structure of XeF4


OR


(a)


(i) Air contains a large amount of nitrogen and the solubility of gases in liquids increases with increase in pressure. When sea divers dive deep into the sea, large amount of nitrogen dissolves in their blood. When they come back to the surface, small air bubbled are formed as solubility of nitrogen decreases and it separates from the blood. This leads to a dangerous medical condition called bends.


So, helium mixed with oxygen under pressure is given to sea- divers for artificial respiration because of its very low solubility in blood.


(ii) 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.


(iii) S-S bond is stronger than O-O bond and sulphur has an extra d orbital present. Oxygen atom is smaller in size as compared to sulphur due to lone pair of electrons in O-O bonds in oxygen experiences more repulsions as compared to the S-S bonds and thus, S-S forms a strong bond.


(b)


(i)



Structure of XeF2


(ii)



Structure of H2S2O8


More from this chapter

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26

After watching a programme on TV about the adverse effects of junk food and soft drinks on the health of school children, Sonali, a student of Class XII, discussed the issue with the school principal. Principal immediately instructed the canteen contractor to replace the fast food with the fibre and vitamins rich food like sprouts, salad, fruits etc. This decision was welcomed by the parents and the students.

After reading the above passage, answer the following questions:


(a) What values are expressed by Sonali and the Principal of the school?


(b) Give two examples of water-soluble vitamins.


 27

(a) Which one of the following is a food preservative?

Equanil, Morphine, Sodium benzoate


(b) Why is bithional added to soap?


(c) Which class of drugs is used in sleeping pills?


 28

(a) A reaction is second order in A and first order in B.

(i) Write the differential rate equation.


(ii) How is the rate affected on increasing the concentration of A three times?


(iii) How is the rate affected when the concentrations of both A and B are doubled?


(b) A first order reaction takes 40 minutes for 30% decomposition. Calculate t1,2 for this reaction.


(Given log 1.428 =0.1548)


OR


(a) For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction.


(b) Rate constant 'k' of a reaction varies with temperature 'T' according to the equation:



Where, Ea is the activation energy. When a graph is plotted for log k Vs. a straight line with a slope of –4250 K is obtained. Calculate 'Ea' for the reaction.


(R = 8.314 JK–1 mol-1).


30

(a) Although phenoxide ion has more number of [5] resonating structures than Carboxylate ion, Carboxylic acid is a stronger acid than phenol. Give two reasons.

(b) How will you bring about the following conversions?


(i) Propanone to propane


(ii) Benzoyl chloride to benzaldehyde


(iii) Ethanal to but-2-enal


OR


(a) Complete the following reactions:


(i)


(ii)


(iii)


(b) Give simple chemical tests to distinguish between the following pairs of compounds: ‘


(i)Ethanal and Propanal


(ii)Benzoic acid and Phenol