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CBSE Class 12 Pre Board 2019 : Chemistry - Prelim 1 (St Xavier's Sr. Sec. School, Delhi)

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Pre Board Examination 2019 Std. 12 21-01-2019 Set 1 Max. Marks : 70 Time : 3 hrs. CHEMISTRY General instructions: i) All questions are compulsory. ii) Section A : Q. No. 1 to 5 are short answer questions and carry 1 marks each. iii) Section B : Q. No. 6 to 12 are short answer questions and carry 2 marks each. iv) Section C : Q. No. 13 to 24 are short answer questions and carry 3 marks each. v) Section D : Q. No. 25 to 27 are long answer questions and carry 5 marks each. vi) There is no overall choice. However an internal choice has been provided in two questions of one mark, two questions of two marks, four questions of 3 marks and all the questions of five marks weightage. You have to attempt only one of the choices in such questions. vii) Use log tables if necessary, use of calculator not allowed. Section A 1. Examine the given defective crystal : X+ YX+ Y- YX+ YX+ X+ YX+ Y- Y- X+ X+ Ye- X+ X+ Y- a) b) Is the above defect stoichiometric or non-stoichiometric? Write the term used for the electron occupied sites. (OR) There is an increase in conductivity when silicon is doped with phosphorous. Give reason. (1) 2. Of physisorption and chemisorption, which type of adsorption has a higher enthalpy of adsorption? 3. On the basis of crystal field theory, write the electronic configuration of d in terms of t2g and eg in an octahedral field when o < P. (OR) Which of the following is a more stable complex and why? 3+ 3+ [Co(NH3)6 ] and [Co(en)3 ] (1) 4. Identify the compound which on hydrogenation produces an optically active compound from the following compounds: (1) 5. Write the structure of the product obtained when glucose is oxidised with nitric acid. (1) (1) 4 Section B 6. 3.9 g of benzoic acid dissolved in 49 g of benzene shows a depression in freezing point of 1.62 K. Calculate the van t Hoff factor and predict the nature of the solute (associated or dissociated) -1 -1 (Molar mass of benzoic acid = 122 g mol , Kf for benzene = 4.9 K Kg Mol ) (OR) Calculate the molality of ethanol solution in which the mole fraction of water is 0.88. (2) 7. Identify the reaction and write the IUPAC name of the product formed. Br2 / Red Phosphorous a) CH3 CH2 COOH b) O Cl H2 Pd BaSO4 (OR) Write the structures and IUPAC names of cross aldol condensation products only of ethanal and propanal. (2) Std. 12 CHEMISTRY (Set 1) Page 2 For the reaction A + B P, the rate is given by Rate = [ A ] [ B ]2 a) How is the rate affected if the concentration of B is doubled? b) What is the overall order of the reaction if A is present in large excess? (2) 9. Give chemical reactions for the following: a) Cannizzaro reaction (2) 10. Write the mechanism of acid catalysed dehydration of ethanol to yield ethene. (2) 11. Name the two most important allotropes of sulphur. Which one of the two is stable at room temperature? (2) 12. Give reasons for the following: a) The enthalpies of atomisation of transition metals are high. b) Transition metals and their many compounds act as catalysts. (2) 8. b) Wurtz reaction Section C 13. Niobium crystallises in body centred cubic structure. If the atomic radius is 141.1 pm, calculate the density of niobium. (Atomic mass = 93u) 14. a) b) (3) Account for the following: i) NaCl solution boil at a higher temperature than water. ii) Mixture of ethanol and acetone shows positive deviation from Raoult s law. Blood cells are isotonic with 0.9 % sodium chloride solution. What happens if we place blood cells in a solution containing i) 1.2 % sodium chloride solution. ii) 0.4 % sodium chloride solution. (3) 15. An organic compound A with molecular formula C8H8O gives positive DNP and iodoform tests. It does not reduce Tollen s or Fehling s reagent and does not decolourise bromine water also. On oxidation with chromic acid, it gives carboxylic acid B with molecular formula C7H6O2. Deduce the structures of A and B (3) 16. Which one of the following compounds will undergo faster hydrolysis reaction by SN1 mechanism? Justify your answer. CH2Cl (or) CH3 CH2 CH2 Cl (OR) A compound is formed by the substitution of two chlorine atoms for two hydrogen atoms in propane. Write the structures of isomers possible. Give the IUPAC name of the isomer which can show enantiomerism. 17. a) b) (3) What is the role of t-butyl peroxide in the polymerisation of ethene? Identify the monomers in the following polymer [ NH - (CH2)6 NH CO (CH2)4 CO - ]n 18. c) Arrange the following polymers in the increasing order of their intermolecular forces: Polystyrene, Terylene, Buna S (3) a) b) c) What is the problem arises in using alitame as an artificial sweetner? Differentiate between disinfectants and antiseptics. Which of the following drug is an antibiotic? Morphine, Equanil, Chloramphenicol, Aspirin (3) Std. 12 19. 21. A colloidal sol is prepared by the given method in figure. What is the charge of AgI colloidal particles in the test tube? How is the sol formed, represented? b) Which of the following electrolytes is the most effective for the coagulation of Fe(OH)3 sol which is a positively charged sol? NaCl, Na2SO4 , Na3PO4 Define peptization. What are essential and non-essential aminoacids? Give one example for each. How many peptide linkages are present in a tripeptide? (OR) Give three reactions of glucose which cannot be explained by its open chain structure. a) b) c) (3) Write the principle of vapour phase refining. Write the role of dilute NaCN in the extraction of silver. What is the role of collectors in the froth floatation process? (OR) Write the principle of electrolytic refining. Why does copper obtained in the extraction from copper pyrites have a blistered appearance? What is the role of depressant in the froth floatation process? (3) Account for the following: a) Zr and Hf have almost similar atomic radii. 3+ 2+ 3+ 3+ b) Out of Sc , Co , and Cr ions, only Sc is colourless in aqueous solution. c) Complete the following equation: + MnO4 + 8H + 5e (3) c) 23. (3) a) b) a) b) 22. Page 3 a) c) 20. CHEMISTRY (Set 1) For a chemical reaction R P the variation in the concentration ln[R] vs time plot is given. y a) b) c) Predict the order of the reaction. In[R] What is the slope of the curve? What is the unit for rate constant for this reaction? x t(s) (OR) The rate constant of a first order decomposition of H2O2 is given by the following equation: 1.0 x 10 4 K T Calculate Ea for this reaction and rate constant k if its half life period be 200 minutes. -1 -1 (R = 8.314 J K mol ). log k 14.2 24. 3- Write the state of hybridisation, shape and IUPAC name of the complex [CoF6] (Atomic no. of Co = 27 ) Section D 25. (3) a) Complete the following reactions: i) ii) O + NH2OH COOH COOH + NH3 strong heating (3) Std. 12 b) c) a) b) c) 26. a) b) a) b) c) 27. a) b) c) a) b) c) CHEMISTRY (Set 1) Page 4 Give chemical tests to distinguish between the following pairs of compounds: i) Acetophenone and benzophenone ii) Aniline and benzylamine Carry out the following conversions: i) Ethanoic acid to propanoic acid ii) Nitrobenzene to benzoic acid (OR) Account for the following: i) Direct nitration of aniline yields significant amount of meta derivative. ii) Aniline does not undergo Friedel Crafts reaction. Arrange the following in the increasing order of basic strength in aqueous solution: NH3, CH3NH2, (CH3)2NH, (CH3)3N Write structures of A and B. NaOH + Br2 OH i) CH3 CH2 CN A B partial hydrolysis Sn + HCl (i) NaNO2 + HCl ii) C6H5NO2 A B (5) + (ii) H2O / H How would you account for the following: i) N2 is less reactive at room temperature. ii) Reducing character increases from HF to HI. iii) Chlorine water is a powerful bleaching agent. Draw the structures of the following moecules: i) Dibasic oxoacid of phosphorous. ii) Chloric acid (OR) Complete the following equations: i) Cu + H2SO4 ii) SO3 + H2O Arrange the following in the order of the property indicated against each set i) F2, I2, Cl2, Br2 (increasing order of bond dissociation enthalpy) ii) NH3, AsH3, SbH3, BiH3, PH3 (decreasing base strength) On adding NaOH to ammonium sulphate a colourless gas with pungent order is 2+ evolved which forms a blue coloured complex with Cu ion. Identify the gas. (5) When a current of 0.75 A is passed through copper sulphate solution for 25 min, 0.369 g of copper is deposited at the cathode. Calculate the atomic mass of copper. Write the products of electrolysis of aqueous solution of NaCl with platinum electrodes. A voltaic cell is set up at 25o C with the following half cells: Al / Al3+ (0.001 M) and Ni / Ni2+ ( 0.50 M ) Write an equation for the reaction that occurs when the cell generates an electric current and determine the cell potential 1 1 (Eo Ni2+/ Ni = - 0.25 V, Eo Al3+ / Al = - 1.66 V, R = 8.314 J K mol , 1F = 96500C) (OR) How do metallic and ionic substances differ in conducting electricity? What type of battery is mercury cell? Why is it more advantageous than dry cell? Calculate e.m.f and G for the following cell at 298K. 2+ + Mg|Mg (0.01M) || Ag (0.0001M) | Ag 2+ + 1 1 (Eo Mg | Mg = 2.37V, Eo Ag |Ag = 0.80V, R = 8.314 J K mol , 1F = 96500C) (5) -x-x-x-x-x-x-

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