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CBSE Class 11 Pre Board 2021 : Chemistry - Prelim (CBSE Gulf Sahodaya Qatar Chapter, Doha)

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PREBOARD EXAMINATION 2020-21 Class: XI (CBSE) Date . Total Marks: 80 SUBJECT - CHEMISTRY Time: 3 hrs. Read the Instructions Carefully: 1. There are 33 questions in this question paper. All questions are compulsory. 2. Section A: Q. No. 1 to 16 are objective type questions. Q. No. 1 and 2 are passage-based questions carrying 4 marks each while Q. No. 3 to 16 carry 1 mark each. 3. Section B: Q. No. 17 to 25 are short answer questions and carry 2 marks each. 4. Section C: Q. No. 26 to 30 are short answer questions and carry 3 marks each. 5. Section D: Q. No. 31 to 33 are long answer questions carrying 5 marks each. 6. There is no overall choice. However, internal choices have been provided. 7. Use of calculators and log tables is not permitted. SECTION - A 1. Read the passage given below and answer the following questions: (4) The orbital wave function or for an electron in an atom has no physical meaning. It is simply a mathematical function of the coordinates of the electron. However, for different orbitals the plots of corresponding wave functions as a function of r (the distance from the nucleus) are different. Fig. 2.12(a), gives such plots for 1s (n = 1, l = 0) and 2s (n = 2, l = 0) orbitals. According to the German physicist, Max Born, the square of the wave function (i.e., 2 ) at a point gives the probability density of the electron at that point. The variation of 2 as a function of r for 1s and 2s orbitals is given in Fig. 2.12(b). Here again, you may note that the curves for 1s and 2s orbitals are different. It may be noted that for 1s orbital the probability density is maximum at the nucleus and it decreases sharply as we move away from it. On the other hand, for 2s orbital the probability density first decreases sharply to zero and again starts increasing. After reaching a small maxima it decreases again and approaches zero as the value of r increases further. The region where this probability density function reduces to zero is called nodal surfaces or simply nodes. In general, it has been found that ns-orbital has (n 1) nodes, that is, number of nodes increases with increase of principal quantum number n. In other words, number of nodes for 2s orbital is one, two for 3s and so on. These probability density variation can be visualised in terms of charge cloud diagrams [Fig. 2.13(a)]. In these diagrams, the density of the dots in a region represents electron probability density in that region. Page 1 of 9

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