Formatting page ...
D.A.V. PUBLIC /MODEL SCHOOLS, WEST BENGAL ZONE PRE- BOARD EXAMINATION Session: 2020-2021 Class: XII Maximum Marks :70 Subject: Chemistry Time: 3 Hours General Instructions : a) There are 33 questions in this question paper of 6 pages. All questions are compulsory. b) Section A : Q. No. 1 to 16 are objective type questions . Q. No. 1 and 2 are case based questions having four MCQs or Assertion Reason type based on given passage each carrying 1 mark. Q. No. 3 to 16 are MCQs and Assertion Reason type questions carrying 1 mark each. c) Section B : Q. No. 17 to 25 are short answer Type I questions and carry 2 marks each. d) Section C: Q. No. 26 to 30 are short answer Type II questions and carry 3 marks each. e) Section D : Q. No. 31 to 33 are long answer questions and carry 5 marks each. f) There is no overall choice . However internal choices have been provided . g) Use of calculators and log tables is not permitted. _____________________________________________________________________________________________ 1 SECTION A (OBJECTIVE TYPE ) Read the passage given below and answer the following questions: (1x4=4) Alcohols and phenols react with active metals such as Na , K and Al to yield corresponding alkoxides / phenoxides and hydrogen. The acidic character of alcohols is due to the polar nature of O-H bond. Alcohols are however weaker acids than water. Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors. The hydroxyl group in phenol is directly attached to the sp2 hybridized carbon of benzene ring which acts as an electron withdrawing group. Due to this, the charge distribution in phenol molecule causes the oxygen of -OH group to be positive. Presence of substituents affects the acidic strength of alcohols and of phenols. The following questions are multiple choice questions. Choose the most appropriate answer: (i)The correct decreasing order of acidity of the following compounds is p-Nitrophenol(I), o- Nitrophenol(II), Phenol(III), m- Nitrophenol(IV) a) I>II>III>IV c) I>II>IV>III b) II>I>IV>III d) II>I>III>IV (ii) Which one of the following is the strongest conjugate base? a) (CI3)3COb) (CH3)3COc) (CCl3)3CO d) (CBr3)3COOR Which one of the following is the correct decreasing order of acidity? a) Water > Alcohol > Phenol > Acetic acid b) Acetic acid > Phenol > o-nitrophenol > alcohol c) Chloroacetic acid > phenol > water > alcohol > 1-alkyne d) p-nitrophenol > chloroacetic acid > water > 1-alkyne (iii)The most acidic compound is a) p-Nitrophenol b) o-cresol c) Phenol d) Anisole (iv)Which one of the following reacts fastest with Na metal? a) C6H5CH2OH b) F3CCH2OH c) Cl CH2OH CH2OH d) O 2 Read the passage given below and answer the following questions: Colloidal particles always carry an electric charge which may be either positive or negative . For example , when AgNO3 solution is added to KI solution , a negatively charged colloidal sol is obtained . The presence of equal and similar charges on colloidal particles provide stability to the colloidal sol and if , somehow , charge is removed , coagulation of sol occurs . Lyophobic sols are readily coagulated compared to lyophilic sols. In these questions (Q.No. i-iv),a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices : (a ) Assertion and reason both are correct statements and reason is correct explanation for assertion . (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion . (c) Assertion is correct statement but reason is wrong statement . (d) Assertion is wrong statement but reason is correct statement . i) Assertion: Colloidal particles always carry an electric charge . Reason: The nature of charge is the same on all the particles in a given colloidal solution . ii) Assertion: The stability of the lyophobic sols is due to the presence of charge on colloidal particles. Reason: The charge on colloidal particles do not allow the formation of their aggregates . iii) Assertion: Ba2+ is more effective than Na+ in carrying out the coagulation of a negative sol . Reason : The greater the valence of the flocculating ion added, the greater is its power to cause precipitation . iv) Assertion: Prolonged dialysis causes coagulation . Reason : During dialysis , the colloidal particles move towards oppositely charged electrodes, get discharged and precipitated . OR Assertion: Lyophilic colloids are called protective colloids . Reason : Lyophilic colloids have a unique property of protecting lyophobic colloids. (1x4=4) 3 4 5 6` 7 8 9 10 11 Following questions (No.3-11) are multiple choice questions carrying 1 mark each. When a primary amine reacts with chloroform in ethanolic KOH, then the product is : (a) isocyanide (b) aldehyde (c) cyanide (d) alcohol The sequence of amino acids in each polypeptide chain in a protein is called a) primary structure b) secondary structure c) tertiary structure d) quarternary structure OR Which amino acid has no asymmetric carbon ? a) Threonine b) Histidine c) -alanine d) Glycine If the elevation of boiling point of a solution of non volatile, non electrolyte and non associating solute in a solvent (Kb = x K kg mol-1) is y K, then the depression of freezing point of the solution of the same concentration would be ( Kf of the solvent= z K kg mol-1) : a) 2 b) c) d) 2 Which of the following pairs has the same size ? a) Zr4+ , Hf4+ b )Zn2+ , Hf4+ 2+ 2+ c) Fe , Ni d) Zr4+ , Ti4+ OR Which of the following ions cannot show transition ? a) Ti4+ b) Cr3+ c) Mn3+ d) Cu2+ Which of the following is Hinsberg s reagent ? (a) C6H5N2Cl (b) C6H5CH2Cl (c) C6H5SO2Cl (d) None of the above. OR On heating aniline with fuming H2SO4 at 453-473 K , the compound formed will be a) Aniline disulphate b) Sulphanilic acid c) Aniline 2,4,6- trisulphonic acid d) None of the above The coordination number, oxidation number, number of d-electrons and unpaired electrons in K3[CoCl6] are respectively a) 6, 3, 6, 0 b) 6, 3, 6, 4 c) 5, 4, 5, 5 d) 6, 4, 5, 1 OR 3+ Which is true for [Co(NH3)6] ? a) oct< P b) oct> P c) oct = P d) all of the above. The magnetic nature of elements depends on the presence of unpaired electrons. Identify the configuration of transition element ,which shows highest magnetic moment. (a) 3d7 (b) 3d5 (c) 3d8 (d) 3d2 The isomer of bromobutane with lowest b.p. is a) n-butyl bromide b) Isobutyl bromide c) tert-butyl bromide d) sec-butyl bromide Which kind of defect is shown by the given crystal ? K+ Cl- K+ Cl- Cl- K+ K+ Cl- K+ Cl- K+ Cl- Cl- K+ Cl- Cl- K+ Cl- K+ K+ 1 1 1 1 1 1 1 1 1 (a) Schottky defect (c) Schottky and Frenkel defects 12 13 14 15 16 17 18 19 20 (b) Frenkel defect (d) Substitution disorder In the following questions (Q. No. 12-16 ) a statement of assertion followed by a statement of reason is given . Choose the correct answer out of the following choices . (a ) Assertion and reason both are correct statements and reason is correct explanation for assertion . (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion . (c) Assertion is correct statement but reason is wrong statement . (d) Assertion is wrong statement but reason is correct statement . Assertion: Proteins are made up of -amino acids. Reason : During denaturation, secondary and tertiary structures of proteins are destroyed. Assertion: Ozone is powerful oxidising agent in comparison to O2. Reason : Ozone is diamagnetic but O2 is paramagnetic. Assertion : Molarity of a solution in liquid state changes with temperature. Reason : The volume of a solution changes with the change in temperature. OR Assertion : When non-volatile solute is added to solvent, the vapour pressure of the solution increases. Reason : As number of solvent molecules escaping from the surface is reduced , the vapour pressure of the solution is also reduced. Assertion : The -hydrogen in aldehydes and ketones is acidic in nature . Reason : The -hydrogen in carbonyl compounds is due to the strong electron donating nature of the carbonyl group. Assertion : In Lucas test, 30 alcohols react immediately. Reason : An equimolar mixture of anhydrous ZnCl2 and conc. HCl is called Lucas reagent . SECTION B The following questions , Q. No. 17-25 are short answer Type I and carry 2 marks each. With the help of resonating structures, explain that haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction. OR Carry out the following conversions in not more than 2 steps : (i) But-1-ene to n-Butyl iodide. (ii) Chlorobenzene to p-Nitrophenol An aqueous solution of 2% non-volatile solute exerts a pressure of 1.004 bar at the normal boiling point of the solvent . What is the molar mass of the solute? (Given : p0A = 1.013 bar) When a coordination compound CrCl3.6H2O is mixed with AgNO3 , 2 moles of AgCl are precipitated per mole of the compound . Write i) the structural formula of the complex ii) IUPAC name of the complex OR Write the state of hybridisation , shape and IUPAC name of the complex [CoF6]3( Atomic No. of Co = 27 ) The rate constant for a first order reaction is 60 S-1.How much time will it take to reduce the initial concentration of the reactant to 1/10 th of its initial value ? OR 1 1 1 1 1 2 2 2 2 For the reaction, 2A + B C + D, the following kinetic data were obtained in three separate experiment, all at 298K temperature. Experiment No. 21 22 23 24 25 Initial conc. Of A Initial conc. Of B Initial rate of formation of C(mol /L/min) 0.1 M 1.2 x 10-3 0.2 M 1.2 x 10-3 0.1 M 2.4 x 10-3 1 0.1 M 2 0.1 M 3 0.2 M Write the rate law of the reaction. (i) The rate constant for a reaction of zero order in A is 0.0030 mol L-1 S-1. How long will it take for the initial concentration to fall from 0.10 M to 0.075 M ? (ii) What is the nature of (rate vs concentration of the reactant ) graph of a zero order reaction? Write the mechanism for the acidic dehydration of excess of ethanol with conc. sulphuric acid at 413 K. Account for the following : i) Bleaching of flowers by chlorine is permanent while that by sulphur dioxide is temporary . ii) On addition of ozone gas to KI solution , violet vapours are obtained . The treatment of alkyl chloride with aqueous KOH leads to the formation of alcohols but in the presence of alcoholic KOH, alkene is the major product. Explain with suitable reason. In a solid, oxide ions (O2-) are arranged in ccp, cation (A3+) occupy one -sixth of the tetrahedral voids and cation (B3+) occupy one-third of the octahedral voids. What is the formula of the compound? 2 2 2 2 2 SECTION C The following questions 26-30 are Short Type II carrying 3 marks each . 26 27 28 29 3 i) Why do transition elements show variable oxidation states ? ii) Explain why CuSO4.5H2O is blue while ZnSO4.7H2O is colourless ? iii) Which out of Fe2+ and Fe3+ ions is more paramagnetic and why ? OR i) Chromium has high negative reduction potential value, but it does not liberate hydrogen from dilute acids. Why ? ii) The E0 value of Zn is more negative than expected. Why ? iii) The transition elements have great tendency for complex formation. Why ? 3 Arrange the following as specified : i) C2H5NH2 ,(C2H5)2NH , (C2H5)3N and NH3 (in decreasing order of basic strength in aqueous phase) ii) CH3CH2CH3 , CH3CH2NH2 , CH3CH2OH (in increasing order of dipole moment) iii) C2H5NH2 , C6H5NHCH3 , (C2H5)2NH and C6H5NH2 (in decreasing order of the pKb values) OR i) Give a chemical test to distinguish between (C2H5)2NH and (C2H5)3N. ii) Write the reaction of benzyl chloride with NH3 followed by reaction of product so formed with chloromethane. iii) Ethylamine is soluble in water whereas aniline is not. Why ? 3 An element crystallizes in a fcc lattice with cell edge of 400 pm . The density of the -3 element is 7 g cm . How many atoms are present in 280 g of the element ? (i) Give a chemical equation that confirms the presence of the following in structure 2+1 of glucose: (a) A carbonyl group (b) A primary alcoholic group. (ii) Write down any two basic structural differences between DNA and RNA. 30 31 32 33 i) Account for the following: (a) Acidic character increases from H2O to H2Te. (b) F2 is more reactive than ClF3,whereas ClF3 is more reactive than Cl2 . ii) Complete the following chemical reaction equation: Cl2 (g) + NaOH (aq) (hot and conc.) SECTION D Q.No 31 to 33 are long answer type carrying 5 marks each : i) Draw structure and name the shape of the following : a) XeF2 b) ICl3 ii) What happens when (support your answer with equation) a) Sulphur dioxide gas is passed through an aqueous solution of a Fe(III) salt ? b) Xenon hexafluoride is subjected to complete hydrolysis ? c) Concentrated sulphuric acid is poured over cane sugar ? OR i) X2 is a greenish yellow gas with pungent smell and used in purification of water. On dissolving water , it gives a solution which turns blue litmus red. When it is passed through NaBr solution Br2 is obtained. (a) Identify the gas. (b) What are the products obtained when X2 reacts with excess of ammonia and ammonia reacts with excess of X2. Give chemical equations. ii) Complete: a) FeSO4 + O3 + H2SO4 b) FeS2 + O2 A compound X (C2H4O) on oxidation gives Y ( C2H4O2) . X undergoes haloform reaction. On treatment with HCN, X forms a product Z which on hydrolysis gives 2Hydroxypropanoic acid . i) Write down structures of X and Y . ii) Name the product when X reacts with dil.NaOH. iii) Write down the equations for the reactions involved. OR i) Give IUPAC name of Salicyclic acid. ii) Benzaldehyde reduces Tollen s reagent but not the Fehling s or Benedict s solution . Why? iii) Write the products formed when (CH3)3C-CHO reacts with the following: a) Zinc amalgam and dilute hydrochloric acid b) Concentrated sodium hydroxide solution c) Semicarbazide and a weak acid. i) Represent the cell in which the following reaction takes place: 2Al(s) + 3Ni2+(0.1M) 2Al3+(0.01M) + 3Ni(s) Calculate the emf if E0cell = 1.41 V. ii) How does molar conductivity vary with increase in concentration for strong electrolyte and weak electrolyte ? Explain. (iii) How can you obtain limiting molar conductivity ( 0m) for weak electrolyte? Give an example. OR Calculate e.m.f and G for the following cell at 298 K: 2+ Mg(s) /Mg (0.01 M) // Ag+ (0.0001 M) / Ag(s) Given: [E0Mg2+/Mg = - 2.37 V , E0Ag+/Ag = + 0.80 V] xxx-xxx 2+1 2+3 3+2 1+1+3 1+1+3 2+2+1 3+2
|
