KENDRIYA VIDYALAYA SANGATHAN, KOLKATA REGION 2nd Pre Board Examination – 2014-15 Class XII Subject: Chemistry; Full marks: 70, Time allowed: 03 hours Marking Scheme: 1 (a) (b) 2 3 4 5 6 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 = ? ½ ½ 1 We know: ½ ½ = 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 = 1½ 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 (π) 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 ½, ½ 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 ΔG° 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 ΔhydHΘ than Cu+(aq) which compensates ΔiH2Θ 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 → CH3 FeCl3 + H2O(excess) → Fe3+ + 3Cl – 2Fe3+ + 6OH–→ 2Fe(OH)3 or Fe2O3.3H2O 3 + OH– KVS Kolkata region, MARKING SCHEME of 2nd Pre Board Examination2014-15, Class XII CHEMISTRY 1 1 1 ½+½ 1 1 1 ½ ½ 1 1 1 1 1 ½,½ 1 1 1 1 1 ½,½ ½ ½ 1 ½ ½ (b) 20 (a) Answer in NCERT Part II page no. 395 Diagram for oct, void with circles Correct derivation: = 0.414 1 ½ 1½ (b) 21 (a) (b) (c) Covalent solid HOOC(CH2)8COOH, NH2(CH2)6NH2 Poly β-hydroxy butyrate-co- β –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 α-D(+) glucose & monomer of cellulose is β-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 ½,½ 1 ½ ½ ½,½ 1 22 (a) (b) (c) 23 (a) (b) (c) (d) 24 (a) (b) or (a) (b) 2NO(g) + O2 → 2NO2 & 2NO2 → (colourless, A) 25 (a) (b) (c) (d) (reddish, B) (B) N2O4 log [Al3+]2 / [Zn2+]3 Ecell = E°cell – log x2 / y3 ½ 1 1 1 1 1 1 1 1 1 1 ½ ½ 1 1,1 (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 Λ°(H2O)=Λ°(HCl) + Λ°(NaOH) - Λ°(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 – ½ KVS Kolkata region, MARKING SCHEME of 2nd Pre Board Examination2014-15, Class XII CHEMISTRY 1 1 1 ½ ½ ½ ½ or (a) (b) S m2 mol-1 H+ + e → ½ H2 so, n = 1, pH= -log[H+] = log = 10 & E°H+/H2 = 0.0V Nernst Equation: EH+/H2 = E°H+/H2 – (c) (d) 26 (a) (b) (c) or (a) (b) (c) 1 ½ ½ 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 ½ ½ ½ ½ ½ ½ 2 ½ each ½ ½ 1 1 1 ½ each