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CBSE XII Pre Boards 2015 : CHEMISTRY with Answers (KV Kolkata)

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KENDRIYA VIDYALAYA SANGATHAN, KOLKATA REGION 2 Pre Board Examination 2014-15 nd Class XII Subject: Chemistry Time allowed : 3 hours Maximum marks: 70 General instructions: (i) All questions are compulsory. (ii) Question numbers 1 to 5 are very short answer questions & carry 1 mark each. (iii) Question numbers 6 to 10 are short answer questions & carry 2 marks each. (iv) Question numbers 11 to 22 are also short answer questions & carry 3 marks each. (v) Question number 23 is a value based question & carry 4 marks. (vi) Question numbers 24 to 26 are long answer questions & carry 5 marks each. (vii) Use log tables, f necessary. Use of calculators is not allowed. ----------------------------------------------------------------------------------------------------------------------------------------------.1) Write an example of an anti ferro magnetic substance. [1] . 2) What is a colloidion? [1] 3) Write the IUPAC name of ionization isomer of [Co(H2O)5Cl ]SO4. H2O. [1] 4) Which isomer of C4H9Br undergoes SN1 reaction easily & why? [1] .5) Explain: Aniline does not undergo Friedel-Crafts reaction. [1] 6) The vapour pressure of pure benzene at a certain temperature in 0.85 bar. A non volatile, non electrolyte solid weighing 0.5 g when added to 39.0 g of benzene (molar mass 78 g/mol). the Vapour pressure of the solution is then 0.845 bar. What is the molar mass of the solid substance? [2] .7) (a) What is the order & molecularity of the following elementary reaction: 2A + B D; the rate law expression is r=k [A] [B] ? (b) How does a catalyst affect the rate of a chemical reaction ? [2] 8) (a) Which is more stable & why- white phosphorus or red phosphorus? (b) Write the reaction of thermal decomposition of sodium azide. [2] Or (a) Although electron gain enthalpy of fluorine is less negative than that of chlorine, fluorine is a stronger oxidizing agent than chlorine. Why? (b) Complete the equation: Cl2 + NaOH (hot & conc.) nd ? KVS RO, Kolkata 2 Pre Board examination 2014-15 Class XII (Chemistry Question paper) [2] .9) (a) What is misch metal & write one of its uses? (b) A black ore of manganese on fusion with an alkali (KOH) and oxidizing agent (O2) forms a dark green mass. Write the chemical equation involved. [2] .10) (a) Write the IUPAC name of CH3COCH=C(OH)CHO. (b) Convert: acetaldehyde into propan-2-ol. [2] 11) (a) Find the value of i for K4[Fe(CN)6] solution with 100% dissociation. (b) What type of a binary solution is formed when two miscible liquids A & B forms a solution & the solution turns hot? What type of azeotrope does it form? (c) Under what condition reverse osmosis takes place? [3] 12) (a) Give an example of a reaction whose unit of rate of reaction & specific rate constant is same. (b)The rate constant for the first order decomposition of H2O2 is given by the following reaction: log k = 14.34 1.25 x 104 K/T. Calculate Ea for this reaction at 298 K [1+2] 13) (a) What happens when a few drops of AgNO3 solution is added to a test tube containing KI solution & shaken? (b) Why is a delta formed at the meeting point of sea & river? (c) Give an example of a colloidal sol used as (i) an eye lotion & (ii) to cure kalaazar? [3] .14) (a) What is the role of silica in the metallurgy of copper? (b) Is it true that under certain conditions, Mg can reduce Al2O3 and Al can reduce MgO? What are those conditions? (c) Write the principle of zone refining in an element purification process. [3] .15) Explain (a) Cr2+ is a stronger reducing agent than Fe2+. (b) Many copper (I) compounds are unstable in aqueous solution. (c) Lanthanoid elements occur together in nature & difficult to separate. 16) (a) Write the electronic configuration of central metal ion, Fe2+ in an octahedral complex for a strong ligand field according to crystal field theory. (b) Why is geometrical isomerism not possible in tetrahedral complexes having two nd KVS RO, Kolkata 2 Pre Board examination 2014-15 Class XII (Chemistry Question paper) [3] different types of unidentate ligands coordinated with the central metal ion? (c) Explain: [Ni(CN)4]2- is a tetrahedral diamagnetic complex ion. [3] Or (a) Draw the stereoisomers of [Co(NH3)3(NO3)3] (b) Draw figure to show the splitting of d orbitals in an octahedral crystal field. (c) Explain: [Cu(en)2]2+ is more stable than [Cu(H2O)4]2+. [3] 17) (a) Write the structure of the major products formed when C2H5ONa reacts with (CH3)3CCl. (b) Explain why (i) Grignard reagents should be prepared under anhydrous condition? (ii) CH2=CHCH2-Cl undergoes SN1 reaction & not SN2 reaction as expected. [3] 18) (a) Illustrate Cannizzaro reaction (b) Give a chemical test to distinguih between : (i) HCOOH & CH3COOH (ii) Acetophenone & benzophenone. [3] 19) (a) Account for the following:(i) pKb of aniline is more than that of methylamine. (ii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide. (b) Convert: aniline into 4-nitroaniline as major product. [3] 20) (a) If the radius of the octahedral void is r & radius of the atoms in closed packing is R , derive relation between r & R. (b) Solid A is a very hard electrical insulator in solid as well as in molten state & melts at very high temperature. What type of solid is it? [3] .21) (a) Identify the monomers in the following polymer:-[CO-(CH2)8-CO-NH-(CH2)6-NH]n(b) Write the full form of PHVB. (c) Write the monomers of Bakelite. 22) (a) Give an example of (i) a cationic detergent (ii) an antioxidant. (b) What is the advantage of using sucrolose over aspartame? nd KVS RO, Kolkata 2 Pre Board examination 2014-15 Class XII (Chemistry Question paper) [3] (c) Why are cimetidine or ranitidine better antacids than NaHCO3 or Mg(OH)2 or Al(OH)3? [3] 23)* Neeta observed that her mother got tired after doing every little work. She took her mother to a doctor. The doctor immediately put her mother on dextrose solution, diagnosed pernicious anemia & gave instructions & medicines. Neeta helped her in household work till she recovered & took her care. (a) Name the vitamin whose deficiency causes pernicious anemia. (b) What is the common name of dextrose ? Draw its open structure. (c) Write one important difference between starch & cellulose. (d) Mention the values shown by Neeta. 24) (a) Complete the equation: (i) FeS2 + O2 [4] ? (ii) P4 + NaOH + H2O (iii) H3PO3 ? ? (c) Write the reactions involved in the brown ring test. [5] Or (a) Give reason: (i) PCl5 exists as an ionic crystalline solid. (ii) Chlorine gas is a powerful bleaching agent. (iii) Fluorine exhibits only -1 oxidation state whereas chlorine exhibits +1, +3, +5 and +7 oxidation states also. (b) A colourless gas, A which is an oxide of nitrogen, turns into a reddish brown gas B when brought in contact with air. Gas B again becomes colourless on cooling,forming gas C . Write the reactions involved in the observations. 25) (a) Explain: Rusting is prevented in alkaline medium. (b) Suggest a way to determine the m value of water. (c) Write the cathode reaction of H2-O2 fuel cell. nd KVS RO, Kolkata 2 Pre Board examination 2014-15 Class XII (Chemistry Question paper) [5] (d) Write the Nernst equation for Al(s)/Al3+(xM)//Zn2+(yM)/Zn(s) at 298K [5] Or (a) Write the SI unit of molar conductivity. (b) Calculate the potential of hydrogen electrode in contact with a solution whose pH is 10. (c) Predict the products of electrolysis of aqueous solution of NaCl using Pt electrodes. (d) A mercury cell has a constant cell potential throughout its life. Why? [5] 26) (a) Write the mechanism of dehydration of ethanol by conc.H2SO4 at 443K. (b) An organic compound A having molecularformula C6H6Ogivec a characteristic colour with aqueous FeCl3 solution. A on treatment with CO2& NaOH at 400K under pressure gives B which on acidification gives C . The compound C reacts with acetic anhydride to give D which is a popular pain killer. Identify A, B, C & D. (c) How can we separate the product formed by the reaction of phenol with dilute nitric acid at low temperature(298K)? [5] or (a) How is phenol manufactured from cumene? (b) Explain: (i) Denatured alcohol is unfit to drink. (ii) Phenol undergoes electroplilic substitution at ortho and para positions. (c) An organic compound A having molecular formula C8H16O2, was hydrolysed with dilute H2SO4 to give a carboxylic acid B , and an alcohol C.Oxidation of C with chromic acid produced B . C on dehydration gives but-2-ene as the major product. Identify A, B and C and write the reactions involved. [1+2+2] ----------------xxx-------------- nd KVS RO, Kolkata 2 Pre Board examination 2014-15 Class XII (Chemistry Question paper) KENDRIYA VIDYALAYA SANGATHAN, KOLKATA REGION 2 Pre Board Examination 2014Class I Subject: Chemistry; Full marks: 70, Time allowed: 03 hours Marking Scheme: 1 (a) (b) 3 4 5 6 1 We know: 2 ReO3 MnO 4% solution of nitrocellulose in a mixture of alcohol & ether, used to make ultra filters pentaaquasulphatocobalt(III) chloride monohydrate (CH3)3C-Br, forms stable carbocation (CH3)3C+ Aniline is a base & Friedel Craft reaction involves Lewis base like AlCl3, BF3 etc which reacts with aniline to form undesired product. C6H5NH2 + BF3 C6H5NH2+BF3 Given: p1 =0.845 bar, M1 = 78 g/mol, w2 = 0.5 g, w1 = 39 g, M2 = ? = = Substituting the values: 7 (a) (b) 8 (a) or (b) (a) 9 (b) (a) (b) 10 (a) (b) 11 (a) (b) (c) 12 (a) (b) On correct calculation: M2 = 170g/mol Molecularity = 3, Order = 1 A calatyst increases the rate of a chemical reaction by decreasing its activation energy. Red phosphorus has long polymeric structure but white phosphorus a molecular structure of P4 with high angular strain of 60 + structures 2NaN3 2Na + 3N2 Reasons: (1) low bond dissocian enthalpy of F-F bond than Cl-Cl bond (2) high hydration enthalpy of F- than Cl-. 3Cl2 + 6 NaOH 5NaCl + NaClO3 + 3H2O Mischmetall is an alloy of lanthanoid metals ( 95% )& iron ( 5%) with traces of S, C, Ca & Al, Mg etc + used to produce bullets, shell , lighter flint (any one) 2MnO2 + 4KOH + O2 2K2MnO4 + 2H2O (black mass) (dark green) 2-Hydroxy-4-oxopent-2-enal CH3CHO CH3CHOH CH3 + 4K4[Fe(CN)6] 4K + [Fe(CN)6] so, i =5 Non ideal solution with negative deviation, maximum boilng azeotrope External pressure applied > osmotic pressure ( ) Zero order reaction 2NH3(g) N2(g) + 3H2(g) or any correct example Given: log k = 14.34 1.25 x 104 /T & comparing with Arrhenius equation: log k = log A - 1 1 1 we have = 1.25 x 104 KVS Kolkata region, MARKING SCHEME of 2nd Pre Board Examination2014-15, Class XII CHEMISTRY 1 1 1 1 1 1 1 1 1 1 1 1 , 1 1 1 (a) 13 (b) (c) 14 (a) (b) (c) 15 (a) (b) (c) 16 (a) (b) (c) or (a) (b) (c) 17 (a) (b) (a) (b) 18 (a) 19 On calculation Ea = 23.93 X 104 J/mol = 239.3 kJ/mol Forms negatively charged colloid: AgNO3(aq)+ KI(aq) AgI(s) + KNO3(aq) AgI(s) + KI AgI/I + K+ River water is a colloid of clay , sea water contains electrolytes, so when river water meets sea water, colloidal solution of clay precipitates according to Hardy Schulze law, a delta is formed. (i) Argyrol or silver sol (ii) colloidal antimony SiO2 acts as flux to remove gangue FeO to form slag FeSiO3 : (FeO + SiO2 FeSiO3 ) Yes, below 1350 C Mg can reduce Al2O3 & above 1350 C Al can MgO according to Ellingham diagram of G vs T plots. The principle of Zone refining is - the impurities are more soluble in the melt than in the solid state of the metal. Cr2+ is stronger reducing agent than Fe2+. Because Cr2+(d4) Cr3+(d3, stable due to half filled t2g3) Fe2+(d6) Fe3+(d5, comparatively less stable, more energy state of t2g3 eg2 ) Cu+ undergoes disproportionation reaction as 2Cu+ Cu2+ + Cu Cu2+(aq) has more negative hydH than Cu+(aq) which compensates iH2 of Cu Due to lanthanoid contraction Fe2+: (3d6) so, 3d6 t2g6 eg0 Because the relative positions of the unidentate ligands in the central metal atom or ion are the same w.r.t. each other (labile) Answer in NCERT Part 1 page no. 249 Answer in NCERT Part 1 page no. 245 Answer in NCERT Part 1 page no. 251 [Cu(en)2]2+ is a chelate, more stable than a complex of weak ligand like H2O. 2-methyl prop-1-ene or CH3C(CH3)=CH2 , C2H5OH (i) Grignard reagents reacts with water to form alkanes as RMgX + H2O RH + MgX(OH) (ii) forms stable carbocation (CH2=CH-CH2+) stable through resonance & favours SN1 reaction Statement + one example (i) HCOOH undergoes Tollen s test, but CH3COOH does not. + relevant reaction or any other correct answer (ii) C6H5COCH3 undergoes iodoform test but C6H5COC6H5 does not, + relevant reaction or any other correct answer (i)-NH2 group in aniline undergoes +R effect to delocalize electron pair from N atom & decreases electron density & becomes less basic with more pKb value. -CH3 group shows +I effect to increase electron density on N atom of NH2 group in CH3-NH2 so it is more basic with less pKb value, (ii)The reaction involved are: CH3NH2 + H-OH FeCl3 + H2O(excess) Fe3+ + 3Cl 2Fe3+ + 6OH 2Fe(OH)3 or Fe2O3.3H2O CH3 3 + OH KVS Kolkata region, MARKING SCHEME of 2nd Pre Board Examination2014-15, Class XII CHEMISTRY 1 1 1 + 1 1 1 1 1 1 1 1 , 1 1 1 1 1 , 1 (b) 20 (a) Answer in NCERT Part II page no. 395 Diagram for oct, void with circles Correct derivation: = 0.414 1 1 (b) 21 (a) (b) (c) Covalent solid HOOC(CH2)8COOH, NH2(CH2)6NH2 Poly -hydroxy butyrate-co- hydroxy valerate, Formaldehyde or HCHO, Phenol or C6H5-OH (i) [CH3(CH2)11N+(CH3)3]Br (ii) BHT or BHA or stucture Sucrolose is stable at cooking temperature whereas aspartame is only stable at low temperature. NaHCO3 or Mg(OH)2 or Al(OH)3 are alkaline and trigger more hydrochloric acid secretion in the stomach, but cimetidine or ranitidine bindswith the receptors present in the stomach wall resulting in the release of less amount of energy. Vitamin B12 Glucose Monomer of starch is -D(+) glucose & monomer of cellulose is -D(+) glucose Or any other valid difference Values: caring, concern, helping, dutiful etc (any two) (i) FeS2 +11O2 2Fe2O3 + 8SO2 (ii) P4 + 3NaOH + 3H2O PH3 + 3NaH2PO2 (iii) 4H3PO3 3H3PO4 + PH3 NO3- + 3Fe2+ + 4H+ NO +3Fe3+ + 2H2O [Fe(H2O)6]2+ + NO [Fe(H2O)5NO]3+ + H2O (i) PCl5 exists as [PCl4]+[PCl6](ii) Cl2 reacts with water forming nascent oxygen which is a powerful bleaching agent. Cl2 + H2O 2HCl + O Coloured substance + O Colourless substance (iii) Flourine is the most electronegative element & cannot exhibit any positive oxidation state, but chlorine has vacant 3d orbitals & can expand their their octets & show +1, +3, +5 & +7 oxidation states. 1 , 1 , 1 22 (a) (b) (c) 23 (a) (b) (c) (d) 24 (a) (b) or (a) (b) 2NO(g) + O2 (colourless, A) 25 (a) (b) (c) (d) 2NO2 & 2NO2 (reddish, B) (B) N2O4 1 1 1 1 1 1 1 1 1 1 1 1,1 log x2 / y3 1 1 1 log [Al3+]2 / [Zn2+]3 Ecell = E cell (colourless, C) Hydroxyl group (OH-) of alkaline solution reacts with hydronium ion (H+)to form water which stops the electrochemical reaction according to electrochemical theory of rusting: OH- + H+ H2O (H2O)= (HCl) + (NaOH) - (NaCl) or other example O2(g) + 2H2O(g) + 4e 4OH-(aq) Cell reaction: 2Al(s) + 3Zn2+(aq) 2Al3+(aq) + 3Zn(s) n =6 Ecell = E cell KVS Kolkata region, MARKING SCHEME of 2nd Pre Board Examination2014-15, Class XII CHEMISTRY or (a) (b) S m2 mol-1 H+ + e H2 so, n = 1, = 10 pH= -log[H+] = log & E H+/H2 = 0.0V Nernst Equation: EH+/H2 = E H+/H2 (c) (d) 26 (a) (b) (c) or (a) (b) (c) 1 log = 0 0.059 x10 = - 0.59 V At cathode: H2, NaoH At anode Cl2 Overall cell reaction of a mercury cell involves all reactants & products in pure solid & liquid states with no change in concentration. Zn(Hg) + HgO(s) ZnO (s) + Hg(l) Answer given in NCERT text book Part II page 331 A=Phenol or C6H5-OH, B= Sodium salicylate or structure C= Salicylic acid or structure& D = aspirin or structure Forms o- & p- isomers of nitrophenols, o- nitrophenol is separated by steam distillation Answer given in NCERT text book part II page 324 (i) due to the presence of 5% methanol which is poisonous (ii) OH group in phenol shows +R effect (activating group), increases electron density at o- & p- positions + resonance structure A= C6H5-OH / (Phenol), B = C6 H5 CHO / (benzaldehyde) C = C6 H5 COOH / (Benzoic acid) D= C6 H6 / (benzene) By: Arunesh Gupta: PGT (Chemistry) KV BKP (AFS) KVS Kolkata region, MARKING SCHEME of 2nd Pre Board Examination2014-15, Class XII CHEMISTRY 2 each 1 1 1 each

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