Somak Chatterjee|Feb 20th, 2013 06:41pm
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During the Examination:–
Ques: Complete the following chemical reaction equations: (i) P4 + SO2Cl2 → (ii) XeF4 + H2O (CBSE 2010 Set 3, Q 30)
Ques: NF3 is an exothermic compound but NCl3 is endothermic compound. (CBSE 2010 Set 3, Q 30)
Model Ans: The instability of NCl3 is due to the weak NCl bond. This is due to the large difference in the size of nitrogen and chlorine atoms. On the other hand, atoms of both nitrogen (75 pm) and fluorine (72 pm) are small sized. Thus, bonding in NF3 is quite strong and it is an exothermic compound.
Ques: Account for the following: (CBSE 2009 Set 1, Q 23)
(i) NH3 is a stronger base than PH3.
(ii) Sulphur has a greater tendency for catenation than oxygen.
(iii) Bond dissociation energy of F2 is less than that of Cl2.
Model Ans: (i) On moving from nitrogen to phosphorus, i.e., down the group, the atomic size increases. As the size of the central atom increases, the lone pair of electrons occupy a larger volume. Consequently, the electron density, and hence, the basic strength decrease. Thus, NH3 is a stronger base than PH3.
(ii) The tendency for catenation depends upon the bond energy. The bond energy of a sulphur molecule is more than that of an oxygen molecule. Thus, the sulphur − sulphur bond strength is higher, and as a result, the tendency of catenation is also higher. Sulphur shows catenation up to eight atoms.
(iii) Bond dissociation energy of fluorine is less than that of chlorine. It is due to the low value of electron affinity of small-sized fluorine. Also, the value of enthalpy of hydration of fluorine is much higher than that of chlorine.
Ques: A solution prepared by dissolving 1.25 g of oil of winter green (methyl salicylate) in 99.0 g of benzene has a boiling point of 80.31 °C. Determine the molar mass of this compound. (B.P. of pure benzene = 80.10 °C and Kb for benzene = 2.53 °C kg mol−1) (CBSE 2010 Set 3, Q 21)
Ques: Define the term, “homopolymerisation” giving an example.
(2012 CBSE Set 1, Q8 Polymer)
Ques: What is meant by coagulation of a colloidal solution? Describe briefly any three methods by which coagulation of lyophobic sols can be carried out.
(2012 CBSE Set 1, Q11 Surface Chemistry)
Ques: What is meant by (i) Peptide linkage (ii) biocatalysts?
(2012 CBSE Set 2, Q14 Biomolecules)
Ques: Explain the following terms giving one example of each type:
(2012 CBSE Set 3, Q27 Chemistry in Everyday Life)
A little hard work with a little smartness will make you come out with flying colours in the upcoming chemistry examination. So focus on time management, topic-wise revision, self-assessment and then Chemistry will be a pleasant and successful experience for you!!!
All the Best!