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CBSE Class 12 Pre Board 2021 : Chemistry - Set 2 (CBSE Gulf Sahodaya Qatar Chapter, Doha)

10 pages, 193 questions, 1 questions with responses, 1 total responses,

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PRE-BOARD EXAMINATION-2020-21 SUBJECT - CHEMISTRY Class: XII (CBSE) Total Marks: 80 Date . 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) Acidity of phenols: The reactions of phenol with metals (e.g., sodium, aluminium) and sodium hydroxide indicate its acidic nature. The hydroxyl group, in phenol is directly attached to the sp2 hybridised carbon of benzene ring which acts as an electron withdrawing group. Due to this, the charge distribution in phenol molecule, as depicted in its resonance structures, causes the oxygen of OH group to be positive. The reaction of phenol with aqueous sodium hydroxide indicates that phenols are stronger acids than alcohols and water. Let us examine how a compound in which hydroxyl group attached to an aromatic ring is more acidic than the one in which hydroxyl group is attached to an alkyl group. The ionisation of an alcohol and a phenol takes place as follows: Due to the higher electronegativity of sp2 hybridised carbon of phenol to which OH is attached, electron density decreases on oxygen. This increases the polarity of O H bond and results in an increase in ionisation of phenols than that of alcohols. Now let us examine the stabilities of alkoxide and phenoxide ions. In alkoxide ion, the negative charge is localised on Page 1 of 10

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