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ISC Board Exam 2014 : Chemistry Paper 1 (Theory) with Model Answers

22 pages, 69 questions, 29 questions with responses, 61 total responses,

ISC 12th
Indian School Certificate Examination (ISC), New Delhi

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CHEMISTRY A. STATISTICS AT A GLANCE Total number of students taking the examination Highest marks obtained Lowest marks obtained Mean marks obtained 35,095 100 14 65.06 Percentage of candidates according to marks obtained Number of candidates Percentage of candidates Cumulative Number Cumulative Percentage 0-20 72 0.21 72 0.21 21-40 549 1.56 621 1.77 34 Mark Range 41-60 15676 44.67 16297 46.44 61-80 12065 34.38 28362 80.81 81-100 6733 19.19 35095 100 B. ANALYSIS OF PERFORMANCE CHEMISTRY PAPER 1 (THEORY) PART I (20 Marks) Answer all questions. Question 1 (a) Fill in the blanks by choosing the appropriate word/words from those given in the brackets: [5] (increases, formic acid, decreases, less, zero, small, paired, atoms, unpaired, ions, pentagonal bipyramidal, electrical, more, ethylamine, molecules, propanoic acid, methylamine, chemical) (i) An electrochemical cell converts ________ energy to _________ energy. (ii) The crystal of graphite is made up of ______________ while that of sodium chloride is made up of __________________. (iii) Ethyl isocyanide, on hydrolysis with dilute sulphuric acid, gives ______________ and _________________ . (iv) The molar conductance of a solution _______________ with dilution, while its specific conductance _______________ with dilution. (v) (b) The Van t Hoff factor of acetic acid solution is ____________than one and the value of normal colligative property is ____________ than the observed colligative property of this solution. Complete the following statements by selecting the correct alternative from the [5] choices given:(i) Of the following terms used for denoting concentration of a solution, the one which does not get affected by temperature is: (1) (2) (3) (4) (ii) Molarity Molality Normality Formality The solubility of calcium hydroxide is s mol litre-1. The solubility product under the same condition will be: (1) 4s3 (2) 2s3 (3) 2s2 (4) s3 35 [5] (iii) A current liberates 0 50g of hydrogen in 2 hours. The weight of copper (at.wt.= 63 5) deposited at the same time by the same current through copper sulphate solution is: (1) 63 5 g (2) 31 8 g (3) 15 9 g (4) 15 5 g (iv) Natural rubber is a: (1) Polyester (2) Polyamide (3) Polyisoprene (4) Polysaccharide (v) Among the following halogens, the one which does not form an oxyacid is: (1) Fluorine (2) Chlorine (3) Bromine (4) Iodine (c) Answer the following questions: [5] (i) What is the [OH-] concentration of an acid whose pH is 5 at 25oC? (ii) What happens when a nickel rod is dipped into a copper sulphate solution? Justify your answer. 0 0 ENi 2 0 25v and ECu 0 34v 2 Ni Cu (iii) Write the equation for the preparation of acidanilide from aniline. (iv) Define Raoult s law for the elevation of boiling point of a solution. (v) An ionic compound is made up of A cations and B anions. If A cations are present at the alternate corners and B anion is present on the body of the diagonal, what is the formula of the ionic compound? 36 (d) Match the following: [5] (i) Molal depression constant (a) Infinite dilution (ii) Acetaldehyde (b) mol l -1 sec-1 (iii) Rate of reaction (c) Iodoform (iv) Optical activity (d) K Kg mol-1 (v) (e) Lactic acid Kohlrausch s law Comments of Examiners (a) (i) Some candidates wrote the answers in the reverse order. The correct answer was chemical and electrical respectively, but some candidates wrote electrical and chemical instead. (ii) The crystal of graphite is made up of atoms. Some candidates wrote that the crystal of graphite is made up of molecules, which is not correct. Many candidates wrote that NaCl is made up of molecules instead of ions. (iii)Some candidates wrote the formula instead of writing in words. Several candidates wrote propanoic acid in place of formic acid and methyl amine in place of ethyl amine . (iv)Several candidates wrote the answer as decreases and increases while the correct answer was vice versa. (v) Candidates were not clear whether Vant Hoff factor is less or more than one. Many candidates were confused whether acetic acid will associate or disassociate in solution. (b) (i) Some candidates wrote molarity instead of molality . (ii) The candidates were not very clear regarding the relationship between solubility (S) and solubility product (Ksp) for different kinds of weak electrolytes and hence wrote wrong answers. (iii)Many candidates wrote 15.5 or 31.8 as answer whereas the correct answer was 15.9. (iv)Some candidates gave the answers as polyamide or polyester , whereas the correct answer was polyisoprene . (v) Most of the candidates were not able to write the correct option i.e. Fluorine. 37 Suggestions for teachers Explain the working of galvanic cell properly. The difference between electrolytic and electrochemical cell should be taught properly with the help of a diagram. In solid state, the type of crystals and their constituent particles must be clearly taught to students. Characteristics of different types of crystalline solids should be explained in tabular form. Tell students that the blanks must be filled with the options as given in brackets. The chemical properties of alkyl isocyanides should be taught in detail with specific examples. The relationship between specific conductance and molar conductance must be explained properly. The effect of dilution on specific and molar conductance should be explained to students. Students must be explained that only molality has weight term which is not affected by temperature whereas other options have volume term which depend on temperature. (c) (i) Some candidates calculated pOH value instead of [OH-] concentration. Many candidates calculated the [OH-] concentration as 10-5 moles per litre instead of 10-9 moles/litre. (ii) Some candidates wrote that Cu will displace Ni which was not correct. Some candidates did not mention that lower reduction potential of Nickel is responsible for displacing Copper from CuSO4 solution when Ni rod is dipped into CuSO4 solution. (iii)A number of candidates were confused as they did not understand that acid anilide is also known as acetanilide. Some candidates wrote the reaction between aniline and acetic acid instead of aniline with acid chloride. (iv)Most of the candidates defined Raoult s law directly as, lowering of vapour pressure is directly proportional to molality instead of The elevation of boiling point is directly proportional to molality. (v) Candidates were able to score marks in this part. (d)Most of the candidates attempted this part correctly. Some candidates wrongly matched molal depression constant with mol lit-1 sec-1 and rate of reaction with K kg mole-1 . MARKING SCHEME Question 1. (a) (i) chemical, electrical (ii) atoms, ions (iii) formic acid, ethylamine ( or vice versa) (iv) increases, decreases (v) less, more ( or vice versa) 38 The relationship between solubility (S) and solubility product (Ksp) must be explained to candidates by taking different kinds of weak electrolytes. The relationship between Faraday and equivalent weight should be told properly to candidates. The calculation of [H+] concentration and [OH-] concentration and their relationship with pH value must be explained to students. Nomenclature of organic compounds should be taught in detail. Insist that students write complete and balanced equation. Definition of Raoult s law should be taught in different terms i.e. relative lowering of vapour pressure and elevation in boiling point. Calculation of number of atoms, cations and anions in different types of crystals must be taught properly. Explain the method for deriving units for the rate of reaction and molal depression constant. (b) (i) (2) Molality (ii) (1) 4s3 (iii) (3) 15 9 (iv) (3) Polyisoprene (v) (1) Fluorine (c) (i) [OH-] = 10-9 moles/lit. (ii) Ni displaces Cu from the solution due to its lower reduction potential. (iii) C6H5NH2+CH3COCl C6H5NHCOCH3+HCl [Any Acid Chloride or any acid anhydride in place of CH3COCl] (iv) The elevation of b.p. is directly proportional to molality. (v) AB2 (d) (i) Molal depression constant (d) K kg mol-1 (ii) Acetaldehyde (c) Iodoform (iii) Rate of reaction (b) mol l-1sec-1 (iv) Optical activity (e) lactic acid (v) Kohlraush s law (a) Infinite dilution PART II (50 Marks) Answer six questions choosing two from Section A, two from Section B and two from Section C. SECTION A Answer any two questions. Question 2 (a) (i) (ii) (b) (i) What will be the vapour pressure of a solution containing 5 moles of sucrose (C12H22O11) in 1 kg of water, if the vapour pressure of pure water is 4 57 mm of Hg? [C = 12, H = 1, O = 16] A 2 molal solution of sodium chloride in water causes an elevation in the boiling point of water by 1 88 K. What is the value of Van t Hoff factor? What does it signify? [Kb = 0 52 K kg mol-1] [3] Write the mathematical expression relating the variation of rate constant of a reaction with temperature. [4] 39 [2] (ii) How can you graphically find the activation energy of the reaction from the above expression? (iii) The slope of the line in the graph of log k (k = rate constant) versus (c) Calculate the activation energy of the reaction. Define Frenkel defect in solid crystal. Comments of Examiners (a) (i) Instead of substituting the value of number of moles as 5 some candidates took the weight of sucrose as 5 grams . Several candidates did wrong substitution and took approximate values, hence did not get the correct answer. (ii) Most of the candidates calculated the correct value of Van t Hoff factor. However, a few could not write the importance or significance of this value. (b) (i) Some candidates wrote incorrect mathematical expressions. Instead of writing K = Ae-Ea/RT candidates were confused with the base factor i.e. base e or base 10 . (ii) Many candidates did not plot the graph. Some candidates did not mention the axis of the graph. In some cases, the slope was not written with a negative sign. (iii) A few candidates used a wrong formula Ea = - slope x R instead of using the correct formula Ea = - 2.303 x slope x R. Some candidates used a different numerical value of R (gas constant). Many candidate wrote the answer with a wrong unit. (c) Some candidates wrote that anions leave their original site and move to an interstitial position which is incorrect. MARKING SCHEME Question 2. (a) (i) o Po water P solution =X sucrose Po water X sucrose n2 5 5 0 083 n1 n2 5 1000 60 5 18 40 1 is 5841. T [1] Suggestions for teachers Enough practice should be given in numericals. Students must be told to solve the numerical step wise i.e. first the formula then substitution and finally the calculation. Answers should be given with the correct unit. The relationship between the value of Van t Hoff factor (i) and disassociation, or association of solute must be clearly explained to students. Mention the significance of (i) as = 1, i < 1, i > 1. Teach students to write all the correct mathematical expressions (i.e. all the forms of Arrhenius equation). Explain the calculation of activation energy (Ea) by graphical representation. The various imperfections found in crystal lattice should be properly explained to students. Po P = 0 083 Po Po P = 0 083 Po = 0 083 4 57 = 0 38 P = Po - 0 38 = 4 57 - 0 38 = 4 19 mm of Hg (ii) Tb (exp ected ) = 0 52 2 = 1 04 K Tb ( observed ) = 1 88 i= (b) (i) Tb ( observed ) Tb (exp ected ) 1 88 1 80 2 1 04 NaCl dissociates in two particles. Ea k = Ae RT k = rate constant A = collision factor Ea = activation energy (ii) Slope = log k (iii) 1 T Ea = 2 303 slope R Ea 2 303R = 2 303 5841 8 314 = 111838 4 J mol-1 = 111 8 KJ mol-1 (c) In a solid crystal lattice when a cation leaves its original site and moves to an interstitial position, the defect is known as Frenkel defect. Question 3 (a) Explain giving reasons why: [4] (i) Ionic solids conduct electricity in molten state, but not in solid state. (ii) Solution of sodium chloride has no effect on litmus, but a solution of zinc chloride turns blue litmus red. 41 (b) In a crystal of diamond: [2] (i) How many carbon atoms are present per unit cell? (ii) What type of lattice does diamond crystallize in? (iii) How many carbon atoms surround each carbon atom? (iv) How are they arranged? (c) (i) What is standard hydrogen electrode? [1] (ii) 0 05 M NaOH solution offered a resistance of 31 6 ohm in a conductivity cell at 298 K. If the cell constant of the cell is 0 367 cm-1, calculate the molar conductivity of the NaOH solution. [3] Comments of Examiners (a) (i) Most of the candidates failed to write that in solid state ions are held by strong electrostatic forces of attraction hence not free to conduct electricity whereas in molten state, ions are free to conduct electricity. Some candidates were confused between electrons and ions. (ii) Many candidates failed to write that NaCl is a salt of strong acid and strong base and does not hydrolyse hence the solution is neutral; zinc chloride is a salt of strong acid and weak base and undergoes cationic hydrolysis, hence the solution is acidic. (b) (i) Most of the candidates answered this part correctly. (ii) Some candidates gave the answer as, hexagonal closed packing or octahedral. (iii) In this part, some candidates wrote eight or twelve instead of four. (iv) Instead of writing that they are arranged in a tetrahedral manner, some candidates wrote sp3. (c) (i) For standard hydrogen electrode, the conditions i.e. [H+] should be 1M, temp = 298 K, pressure 1 atm were missing from many of the answers given by candidates. (ii) Most of the candidates wrote wrong unit of molar conductance. 42 Suggestions for teachers More emphasis should be laid on reasoning type of questions in chemical bonding. Teachers should explain the meaning of cation hydrolysis and anion hydrolysis and that a salt of strong acid and strong base does not undergo hydrolysis. The behaviour or limits to various type of solutions should be explained. Structure of diamond should be taught with the ball and stick model or with a 3-D diagram. The following points must be emphasized: Type of lattice No. of atoms per unit cell Coordination number of each carbon atom Hybridization Arrangement of atoms Properties Standard hydrogen electrode should be taught with diagram along with all relevant conditions. Derivation of unit with the help of formula should be clearly explained to students. MARKING SCHEME Question 3. (a) Ionic solids are bad conductors in the solid state because their constituent ions are held together in fixed positions by strong attractive forces. However, they conduct electricity in the molten state because of the presence of free ions. (ii) (b) (i) ZnCl2 is a salt of weak base and strong acid and undergoes cation hydrolysis. NaCl is a salt of strong acid and strong base and does not hydrolyse. Diamond crystallizes in (i) (ii) Face centred cubic (fcc) structure. (iii) Four carbon atoms surround each carbon (iv) (c) Eight carbon atoms are present in a unit cell of diamond. They are arranged in a tetrahedral manner. (i) Standard Hydrogen Electrode (SHE) Standard hydrogen electrode is set up by passing pure hydrogen gas at 1 atm pressure in a solution of H+ (H3O+)ions of concentration of 1 mol L-1 in contact with a platinised platinum foil at 25oC. A standard hydrogen electrode can be represented as Pt, H2 (g) (1 atm) / H+(1 mol) at 25oC (ii) Conductance = 1 / R = 1 / 31.6 ohm-1 Specific conductance , = ( 1/ 31.6) ohm-1 ( 0.367 cm-1) Molar conductance = 1000 0.0116 1000 = 232 ohm-1 cm2 mol-1 = C 0.05 Question 4 (b) (i) O 2(g) SO3(g) is 61 7 at 60oC. What is its unit? Kc for the reaction SO2(g) + 1 2 Calculate Kp for the reaction and write its unit. [3] (ii) (a) What happens to the equilibrium in a reversible reaction if a catalyst is added to it? [1] State the effect of the following on the reaction 2SO2(g)+O2(g) 2SO3(g)+189 4 kJ at equilibrium: (i) Temperature is increased. (ii) Concentration of SO2 is increased. (iii) Pressure is decreased. (iv) Helium is added at constant pressure. 43 [2] (c) (i) 0 3605 g of a metal is deposited on the electrode by passing 1 2 amperes of current for 15 minutes through its salt solution. The atomic weight of the metal is 96. What is the valency of the metal? [3] (ii) Explain why phenolphthalein is used as an indicator in acid-base titration. [1] Comments of Examiners (a) (i) Some candidates wrote the unit of Kc and Kp incorrectly. The calculation of Kp was also not done correctly by a few candidates. (ii) Many candidates wrote that equilibrium attains quickly but did not mention that equilibrium remains unaffected. They did not mention that the catalyst increases the rate of both forward and backward reaction. (b) (i) Some candidates misunderstood that the reaction is endothermic, hence gave wrong answers. (ii) The effect of increase in concentration of SO2 was given correctly by most of the candidates. (iii)A few candidates wrote that the equilibrium will shift towards right instead of writing that it will shift to the left or backward. (iv)Some of the candidates gave the answer as no effect instead of writing that the equilibrium will shift to the left or backward. (c) (i) Some of the candidates wrote wrong formulas for the calculation of eq. wt. As the eq wt. was wrong the valency was also calculated incorrectly. (ii) Most of the candidates were unable to give the pH range of 8.3 to 10. Several candidates were confused with the change in colour given by phenolphthalein in acidic and basic mediums. 44 Suggestions for teachers Derivation of unit with the help of formula should be taught and adequate practice should be given in solving numericals. The properties of catalysts should be explained with proper examples. The characteristics of chemical equilibrium must be taught. Le Chatelier s principle should be explained with proper examples. The following points need to be emphasized: Change of concentration at equilibrium Change of temperature Change in pressure Effect of catalyst Addition of inert gas Explain the use of Faraday s laws of electrolysis, in calculation of eq. wt. and valency. Explain the pH range of phenolphthalein (8.3 to 10) and also HPh H+ + Phcolourless pink Due to CIE it remains colourless in acidic medium and gives pink colour in basic medium. MARKING SCHEME Question 4. (a) (i) Unit = [mol/lit]-1/2 1 1 n 1 1 2 2 Kp = Kc(RT) n Kp = 61 7 [0 0821 (60+273)] = 11.797 atm 1 2 1 2 (ii) Temperature is increased the equilibrium will shift to the backward direction as the increase in temperature will be compensated by absorbing heat. Concentration of SO2 is increased the equilibrium will shift to the right so that additional SO2 is used up to form SO3. (iii) Pressure is decreased the equilibrium will shift to the left to produce more number of moles. (iv) (c) (i) (ii) (b) When a catalyst is added, the state of equilibrium is not distributed but equilibrium is attained quickly. This is because the catalyst increases the rate of forward and backward reaction to the same extent. Helium is added at constant pressure it will result in increase in volume. As a result, the number of molecules per unit volume will decrease. This stress is relieved by the formation of more molecules and the equilibrium will shift to the left. (i) W= Eg wt / i t 96500 Valency (ii) Egwt 96500 0 3605 = 32.21 1 2 15 60 96 2 98 3 32 18 Phenolphthalein is a weak organic acid which changes its colour between pH value of 8.3 to 10. It gives pink colour in alkaline medium but is colourless in acidic medium due to common ion effect. 45 SECTION B Answer any two questions Question 5 (a) Write the formula of the following compounds: (i) (b) Triamminetriaquachromium(III)chloride (ii) [2] Potassiumhexacyanoferrate(III) Name the types of isomerism shown by the following pairs of compounds: (i) and [CoCl2(NH3)4]Cl.H2O (ii) (c) [CoCl(H2O)(NH3)4]Cl2 [Pt(NH3)4][PtCl6] [2] and [Pt(NH3)4Cl2][PtCl4] For the complex ion of [Co(NH3)6]3+ : [1] (i) State the hybridisation of the complex. (ii) State the magnetic nature of the complex. Comments of Examiners (a) Many candidates gave incorrect formula. (i) In place of [Cr(NH3)3(H2O)3]Cl3 some candidates wrote [Cr(NH2)3(H2O)3]Cl3. (ii) In place of K3[Fe(CN)6] some candidates wrote K4[Fe(CN)6]. (b) Many candidates gave incorrect answers. (i) Instead of hydrate isomerism some wrote hydration isomerism . (ii) Instead of coordination isomerism they wrote coordinate isomerism . (c) Many candidates wrote sp3 d2 hybridization in place of d2 sp3 hybridization. For the magnetic nature of the complex, the word paramagnetic was written in place of diamagnetic . 46 Suggestions for teachers Explain the nomenclature of coordination compounds with examples. Give different examples of isomerism given in the scope of syllabus with correct name and spelling. Explain VBT for bonding of coordination compound. Inner orbital and outer orbital complexes should be taught properly. Also explain that presence of paired electrons represents diamagnetic character and that of unpaired electrons represent paramagnetic character. MARKING SCHEME Question 5. (a) K3[Fe(CN)6] (i) Hydrate isomerism (ii) Coordination isomerism (i) d2sp3 (ii) (c) [Cr(NH3)3(H2O)3] Cl3 (ii) (b) (i) diamagnetic [or solvate isomerism] Question 6 (a) Write balanced chemical equations for the following reactions: (i) Ozone and lead sulphide. (ii) Chlorine is passed through hot concentrated NaOH solution. (iii) Sulphuric acid is treated with phosphorous. [3] (b) Give reasons for the following: [2] (i) Zn+2 salts are white but Cu2+ salts are blue in colour. (ii) Fluorine gives only one oxide but chlorine gives a series of oxides. Comments of Examiners (a) Many candidates gave unbalanced equations. Some candidates failed to write the correct products. (b) (i) Many candidates failed to write about absorption of visible light for excitation of partly filled d orbital electrons. (ii) Most of the candidates wrote that fluorine gives only one oxide due to high electronegativity but did not mention that fluorine does not have d orbital whereas chlorine forms series of oxides due to vacant d orbital. MARKING SCHEME Question 6. (a) (i) PbS + 4 O3 PbSO4 + 4 O2 (ii) 3 Cl2 + 6NaOH 5NaCl + NaClO3 + 3H2O (iii) 5H2SO4+2P 2H3PO4+5SO2+ 2H2O Or P4+10H2SO4 2H3PO4+10SO2+ 4H2O 47 Suggestions for teachers Teach the chemical reactions of inorganic compounds in detail. Tell students that when equations are written, balancing is a must. Explain the variable covalency of chlorine due to the empty d subshell in chlorine whereas fluorine has no d orbital. (i) Cu2+ salts are coloured because they absorb visible light for excitation of electrons to partly filled d-orbitals. Zn2+ salts are colourless because of the absence of partly filled d-orbitals to which the electrons can be oxidized. (ii) (b) Fluorine does not have d orbital. Chlorine has vacant 3d orbital. Electrons jumps from 3s and 3p orbitals to 3d orbitals. Question 7 (a) How is potassium dichromate prepared from a sample of chromite ore? Give balanced equations for the chemical reactions involved. [3] (b) For the molecule IF7: (i) Draw the structure of the molecule. (ii) State the hybridisation of the central atom. (iii) State the geometry of the molecule. [2] Comments of Examiners (a) Most of the candidates were not able to write all the three balanced equations correctly. Equations were either unbalanced or incomplete. Many candidates just described the preparation of potassium dichromate instead of giving balanced equations. (b) Some of the candidates showed just seven bonds between iodine and fluorine but geometry (pentagonal bipyramid) was not shown correctly. A few candidates wrote sp3d2 hybridization instead of sp3d3 hybridization. Suggestions for teachers Preparation of compounds should be taught in terms of reactants, products, conditions, balanced equations, etc. Explain the interhalogen compounds, their geometry, hybridization and structure in detail. MARKING SCHEME Question 7. (a) Finely powdered Chromite ore is mixed with soda ash and quick lime. The mixture is then roasted in a reverberatory furnace in presence of excess of air, a yellow mass containing sodium chromate is obtained. 4 FeCr2O4 + 8 Na2CO3 + 7 O2 2Fe2O3 + 8 Na2CrO4 + 8CO2 (Sodium chromate) Conversion of sodium chromate to sodium dichromate Sodium chromate is treated with concentrated sulphuric acid to form sodium chromate. 2 Na2CrO4 + H2SO4 Na2Cr2O7 + Na2SO4 + H2O Conversion of sodium dichromate to potassium dichromate Sodium dichromate solution is heated to concentrate. Now add a calculated quantity of potassium chloride is added. On cooling, the least soluble potassium dichromate separate out as crystals. 48 (b) (i) (ii) Na2Cr2O7 + 2 KCl F F F I F F F F K2Cr2O7 + 2 NaCl sp3d3 (iii) Pentagonal bipyramid. SECTION C Answer any two questions. Question 8 (a) How can the following conversions be brought about: (i) Acetic acid to methyl cyanide. [2] (ii) Acetaldehyde to formaldehyde. [3] (iii) Nitrobenzene to 2, 4, 6 tribromoaniline. (b) [2] Identify the reagents A, B, C, D, E and F required for the following conversion: A B C D H2SO4 C6H5NO2 C6H5NH2 C6H5N2+ClC6H5Cl C6H5OH E+F [3] Comments of Examiners (a) (i) Some candidates left the reaction in the middle and Suggestions for teachers could not reach the end product. In some cases, for Insist that students practice the conversion of CH3COOH to CH3CONH2 heating organic conversions stepwise was not shown. Similarly, for the conversion of with correct reagents and CH3CONH2 to CH3CN heating was not shown. conditions. (ii) The correct reagents were not shown in many cases. Stress upon studying organic (iii)For the conversion of nitrobenzene to aniline, acidic reactions by writing the medium was not used for reduction by several equations. Catalysts and candidates. Instead of Br2/H2O, Br2/CS2 was used by conditions should be explained. several candidates. (b) Many candidates were not able to identify all the reagents correctly. Mainly, the candidates were confused in the identification of E & F. In case of reagent D, only NaOH was written by some candidates in place of NaOH/HCl. 49 MARKING SCHEME Question 8. (a) (i) CH 3COOH NH 3 CH 3CONH 2 H 2O CH 3CONH 2 (ii) K 2Cr2O7 NH 3 Br / KOH CH 3COOH CH 3CONH 2 2 H CH 3CHO CH 3 NH 2 P2O5 CH 3CN H 2O HONO Cu CH 3OH HCHO NaNO2 / HCl [O ] (iii) Sn / HCL C6 H 5 NO2 C6 H 5 NH 2 H Br2 NH2 H 2O Br Br O Br (b) C6H5NO2 A C6H5NH2 B C6H5N2+Cl- C C6H5Cl D C6H5OH H2SO4 E+F OH A = Sn / HCl E= O SO3H B = NaNO2 + HCl & 0oC 5oC OH C = CuCl / HCl F= D = NaOH / HCl O SO3H Question 9 (a) The deficiency of which vitamin will cause the following diseases: (i) (b) Scurvy (ii) Haemorrhages Give one chemical test to distinguish between the following pairs of compounds: (i) [3] Ethanol and 2 propanol. (ii) (c) [2] Aniline and ethylamine. Write the structures of all enantiomers possible for lactic acid. 50 [1] (d) Give balanced equations for the following reactions: [4] (i) Acetaldehyde is heated with hydroiodic acid in the presence of red phosphorous. (ii) Calcium acetate is subjected to dry distillation. (iii) Sodium ethoxide is treated with ethyl bromide. (iv) Benzaldehyde is treated with sodium bisulphite. Comments of Examiners (a) Most of the candidates identified Vitamin C correctly but some were confused regarding the vitamin, the deficiency of which causes haemorrhages. (b) Some candidates gave those chemical tests which are given by both the compounds hence the compounds could be distinguished. In several cases, only the name of the test was given, the reagents used and the observation were not written. (c) Many candidates could not draw the correct optical isomers. A few candidates wrote wrong formula of lactic acid. (d) A number of candidates were unable to write balanced equations; in some cases, by products were not mentioned. Suggestions for teachers Explain the deficiency related diseases of different vitamins in detail to students. Students must be told to give correct chemical tests. If the test is positive with one reagent only then the negative test with other compound should be given. Explain optical isomerism in detail and also about enantiomers and diastereoisomers. Insist that students practice organic reactions with correct conditions and reagents. MARKING SCHEME Question 9. (a) Vitamin C (ii) (b) (i) Vitamin K (i) Ethanol and propan-2-ol : Lucas Test With concentrated HCl + ZnCl2 ethanol no turbidity is formed. Turbidity appears within five minutes in propan-2-ol. (or any other suitable test) (ii) Aniline and ethylamine: Azo dye Test Dissolve the given amine in dil. HCl and cool it in ice-cold water. Now add ice-cold solution of NaNO2 and HCl followed by an ice-cold -naphthol solution. The appearance of brilliant orange-red indicates that the given amine is aniline. Ethylamine does not form any dye and therefore does not show any colour when treated similarly. ( or any other suitable test) 51 (c) CH3 CH3 H C OH HO C H COOH COOH d(+) (d) (i) (ii) CH 3CHO 4 H CH3COO l( ) Re d P C2 H 6 H 2 O 2 I 2 Ca CH3COO (iii) CH3 CO + CaCO3 CH3 C2H5ONa+ C2H5Br C2H5OC2H5+NaBr SO3Na (iv) C6H5 C =0 +NaHSO3 C6H5 C OH H H Question 10 (a) An organic compound A with molecular formula C7H8 on oxidation by chromylchloride in the presence of CCl4 gives a compound B which gives positive tollen s test. The compound B on treatment with NaOH followed by acid hydrolysis gives two products C and D. C on oxidation gives B which on further oxidation gives D. The compound D on distillation with sodalime gives a hydrocarbon E. Below 60oC, concentrated nitric acid reacts with E in the presence of concentrated sulphuric acid forming a compound F. Identify the compounds A, B, C, D, E and F. [3] (b) Give balanced equations for the following name reactions: (i) Clemmensen s reduction. (ii) Kolbe s electrolytic reaction. (iii) Balz-Schiemann s reaction. [3] (c) (i) (ii) [2] (d) Answer the following: (i) What is biuret test? (ii) What do you observe when glucose is treated with bromine water? What is isoelectric point? [2] Write balanced equation for the formation of biuret. 52 Comments of Examiners (a) Most of the candidates identified the compounds Suggestions for teachers correctly. In some cases, in place of C6H5CH2OH Practice must be given in (benzyl alcohol) C6H5OH (phenol) was reported. questions in which identification (b) Many candidates failed to answer this question correctly of compounds is based on especially parts (ii) and (iii). For Clemmensen s different chemical reactions. reduction, the correct condition for the reaction was not Name reactions should be taught given by many candidates. For Kolbe s electrolytic with correct examples. reaction, many candidates wrote Kolbe Schmidt Candidates should be asked to reaction. In part (iii), Balz-Schiemann s reaction, many practice balanced equations with candidates were confused and gave wrong answers. proper reagents and conditions. (c) (i) Instead of giving the observations, some candidates Explain the difference between wrote only balanced chemical reactions. observation and chemical (ii) Some candidates wrote that isoelectric point is a reaction. point whereas it is the pH of solution at which migration of amino acid does not takes place to any electrode. (d) (i) Several candidates forgot to mention heating of urea. (ii) Balanced equation for the formation of biuret was not given by some candidates. MARKING SCHEME Question 10. (a) Identify the compounds from (A) to (F): A = Toluene (or) C6H5CH3 B = Benzaldehyde (or) C6H5CHO C = Benzyl Alcohol (or) C6H5CH2OH D = Benzoic Acid (or) C6H5COOH E = Benzene (or) C6H6 F = Nitrobenzene (or) C6H5NO2 (b) (i) Clemmensen s reduction Zn / Hg CH3CHO + 4(H) Conc.HCl (ACETALDEHYDE) CH3-CH3 + H2O (ETHANE) (or) Zn / Hg CH3COCH3 + 4(H) CH3CH2CH3 Conc. HCl (ii) 2CH3 COOK 2CH3COO +2K+ at anode 2CH3COO 2e + H2O (PROPANE) 2H2O 2H++ 2OH+ C2H6+CO2 at cathode 2H++2e 53 H2 (iii) N2Cl O (c) (i) (ii) (d) (i) (ii) HBF4 N2BF 4 FF O 4 F O +N2+BF3 Decolourises bromine water.(Red colour of bromine water disappear) OR Br2 / H2O CH2OH(CHOH)4CHO CH2OH(CHOH)4COOH (glucose) ( gluconic acid) Isoelectric point At a particular pH of a solution the amino acid molecule should not migrate to either electrode and should exist as a neutral dipolar ion. This pH is known as isoelectric point. Urea on heating strongly (above 132oC) gives biuret, alkaline solution of biuret gives a violet colouration with dilute copper sulphate solution. NH2CONH2+NH2CONH2 NH2CONHCONH2 + NH3 biuret GENERAL COMMENTS: (a) Topics found difficult by candidates in the Question Paper: Relative molecular mass and mole Chemical kinetics Chemical equilibrium Electrolyte conductance Coordination compounds Preparation of inorganic compounds Ionic equilibria Organic conversions and balanced equations Named organic reactions (Balz-Schiemann s reaction) (b) Concepts in which candidates got confused: Van Hoff factor and colligative properties Raoult s Law and elevation of boiling point Calculation of activation by using slope of the graph Unit in numerical Le Chatelier s principle 54 Isomerism and hybridization in coordination compounds Organic conversions Named reactions Biomolecules (c) Suggestions for candidates: Numericals should be practiced regularly Practice organic conversions with correct conditions. Write the correct balanced chemical equations. Avoid selective study Read the question carefully and understand what is required before attempting the answers. Study chemical tests to distinguish between organic compounds. Write the formula then substitute the values and calculate the answers with correct units. Learn to write the keywords in the answer. 55

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Additional Info : ISC 2014 : Board Model Answers (Chemistry Paper 1)
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