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CHEMISTRY STATISTICS AT A GLANCE Total Number of students who took the examination 37,269 Highest Marks Obtained 100 Lowest Marks Obtained 6 Mean Marks Obtained 67.32 Percentage of Candidates according to marks obtained Details 0-20 94 0.25 94 0.25 Number of Candidates Percentage of Candidates Cumulative Number Cumulative Percentage 21-40 544 1.46 638 1.71 Mark Range 41-60 61-80 14873 12258 39.91 32.89 15511 27769 41.62 74.51 Marks Obtained 39.91 40.00 32.89 Percentage of Candidates 35.00 25.49 30.00 25.00 20.00 15.00 10.00 5.00 0.25 1.46 0.00 0-20 21-40 41-60 Marks Obtained 43 61-80 81-100 81-100 9500 25.49 37269 100.00 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: (Henry s, aldol condensation, absence, do not, ohm, Raoult s, increases, common ion effect, easily, three, solubility product, ohm-1, two, four, ohm-1cm2, cannizzaro, ohm-1cm-1, zero, decreases, presence) (i) Ideal solutions obey_________ law and they _______ form azeotropic mixtures. [5] Benzaldehyde undergoes ________ reaction due to _________ of -hydrogen atom. (iii) The solubility of silver chloride ________ in the presence of sodium chloride because of_____________. (iv) The unit of conductance is _________ and that of specific conductance is _______. (ii) (v) (b) When the concentration of a reactant of first order reaction is doubled, the rate becomes __________ times, but for ___________ order reaction, the rate remains same. Complete the following statements by selecting the correct alternative from the [5] choices given: (i) Electrochemical equivalent is the amount of substance which gets deposited from its solution on passing electrical charge equal to: (1) 96,500 Coulombs (2) 1 Coulomb (3) 60 Coulombs (4) 965 Coulombs (ii) The complex ion [Ni(CN)4]2- is: (1) Square planar and diamagnetic (2) Tetrahedral and paramagnetic (3) Square planar and paramagnetic (4) Tetrahedral and diamagnetic (iii) Wohler s synthesis is used for the preparation of: (1) (2) (3) (4) Glycine Amino acids Urea Proteins 44 (iv) When SO2 gas is passed through acidified K2Cr2O7 solution, the colour of the solution changes to: (1) Red (2) Black (3) Orange (4) Green (v) In the equation CH3COOH + Cl2 (1) CH3CH2Cl A, the compound A is: (2) ClCH2COOH (3) CH3Cl (4) CH3COCl (c) Answer the following questions: [5] (i) What is the order of reaction whose rate constant has the same unit as the rate of reaction? (ii) What is the pH value of a solution whose hydroxyl ion concentration is 1 10-2 M? (iii) Calculate the number of coulombs required to deposit 5 4g of Al when the electrode reaction is: Al3+ + 3e- Al [Atomic Weight of Al = 27 g/mol]. (iv) Write the reaction to prepare acetaldehyde from hydrogen gas and an acid chloride. (v) (d) The edge length of unit cell of a body centered cubic (bcc) crystal is 352 pm. Calculate the radius of the atom. Match the following: (i) (ii) (iii) (iv) (v) Weak electrolyte Colour in crystals Acetone Sorensen Ammonical silver nitrate [5] (a) (b) (c) (d) (e) 45 pH of a solution Iodoform Tollen s reagent Ostwald dilution law F - centre Comments of Examiners (a) (i) Some candidates wrote Henry s instead of Raoult s in the first blank. For the second blank, instead of do not a few candidates wrote incorrect answers. (ii) Instead of Cannizzaro some candidates wrote aldol condensation . In place of absence some wrote presence . (iii) A number of candidates wrote increases instead of decreases in the first blank. Some candidates wrote solubility product instead of common ion effect in the second blank. (iv) Some candidates wrote wrong units for conductance and specific conductance. (v) Many candidates wrote four instead of two for the first blank. In the second blank also, instead of zero , some candidates wrote two . (b) (i) Some candidates wrote 96,500 coulombs instead of 1 coulomb . (ii) Instead of square planer and diamagnetic several candidates wrote wrong alternatives. (iii) A few candidates wrote protein instead of urea . (iv) Instead of green some candidates gave wrong options. (v) A number of candidates attempted this part incorrectly. (c) (i) A number of candidates gave the answer as first and second order reaction which was incorrect. (ii) Most candidates calculated the pH value = 2 instead of the correct value 12. (iii) Some candidates calculated the change in terms of Faraday whereas according to the question, the answer had to be written in Coulombs; incorrect responses were also given. (iv) Several candidates were not able to write the conditions for the reaction. (v) A number of candidates calculate the radius of bcc unit cell incorrectly as the used the wrong formula. (d) This part was attempted correctly by most of the candidates. 46 Suggestions for teachers Teach ideal and non-ideal solutions, Raoult s Law and azeotropic mixtures with examples. Explain the named organic reactions along with conditions. Ask students to learn common ion effect and its application on the solubility of weak electrolytes. Teach the terms conductance , specific conductance , etc. along with the units. Explain the relationship between the change in concentration and rate of reaction, for different order of reactions. The relationship between electrochemical equivalent and chemical equivalent should be explained to students. The geometry of hybridization and magnetic property of coordination compound must be explained with the help of valence bond theory. Sufficient practice should be given on the calculations of order of reaction. Give practice on numerical based on calculation of pH and pOH value. Stress must be laid on named reactions along with conditions. Ask students to write complete and balanced equations. The relationship between edge length (a) and radius (r) for various types of cubic unit cell must be explained to students. MARKING SCHEME Question 1 (a) (i) Raoult s, do not (b) (c) (ii) Cannizzaro, absence (iii) decreases, common ion effect (iv) ohm-1, ohm-1cm-1 (v) two, zero (i) (2) or 1 Coulomb (ii) (1) or Square planar and diamagnetic (iii) (3) or Urea (iv) (v) (4) or Green (2) or ClCH2COOH (i) Zero order reaction, rate = k[A]o k= (ii) pOH = - log10 [OH-] [OH-] = 1 10-2 M pOH = 2, [ ] = rate = k pH = 14 2 = 12 (iii) Al3+ + 3e Al 1 mole 3 mole 1 mole 27 g 3 Faraday 27 g 27 g if Al is deposited by 3 F 5 4 g Al is deposited by 3 5 4 27 = 0 6 F Coulomb = Faraday 96,500 = 0 6 96,500 = 57,900 coulomb (iv) (v) / 4 CH3COCl+ H2 CH3CHO + HCl For bcc structure radius of sphere = r = a = 352 p m (edge length of unit cell) Radius of atom (r) = 3 4 352 = 152 42 p m 47 3 4 (d) Match the following: (i) Weak electrolyte (d) Ostwald dilution law (ii) (e) F - centre (iii) Acetone (b) Iodoform (iv) Sorensen (a) pH of a solution (v) Ammonical silver nitrate (c) Tollen s reagent Colour in crystals 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) A 10% aqueous solution of cane sugar (mol. wt. 342) is isotonic with 1 754% aqueous solution of urea. Find the molecular mass of urea. [2] (ii) The molecular weight of an organic compound is 58 g mol-1. What will be the boiling point of a solution containing 48 grams of the solute in 1200 grams of water? [2] [Kb for water = 0 513oC kg mole-1; (b) (iii) What will be the value of van t Hoff factor(i) of benzoic acid if it dimerises in aqueous solution? How will the experimental molecular weight vary as compared to the normal molecular weight? [1] (i) Determine the pH value of 0 001 M acetic acid solution if it is 2% ionised at this concentration. How can the degree of dissociation of this acetic acid solution be increased? [2] (ii) The solubility product of PbCl2 at 298K is 1 7 10-5. Calculate the solubility of PbCl2 in g/lit. at 298K. [2] Atomic Weights: (c) Boiling point of water = 100oC.] [Pb = 207 and Cl = 35 5] Graphite is anisotropic with respect to conduction of electric current. Explain. 48 [1] Comments of Examiners (a) (i) A number of candidates calculated the number of moles incorrectly. A few candidates calculated the molecular weight of urea directly. (ii) Calculation of elevation of boiling point ( Tb) was done correctly by many candidates but a few candidates subtracted this value from boiling point of water instead of adding Tb to the boiling point of water to obtain the correct boiling point of solution. (iii) Several candidates wrote that i < 1 instead of i = 0.5 or . Experimental molecular weight = 2 x normal molecular weight was also not mentioned by a few candidates. (b) (i) The pH value was calculated correctly by most of the candidates. However, a number of candidates were not able to answer the second part of the question, i.e. How can the degree of dissociation of this acetic acid solution be increased? . (ii) Most of the candidates calculated the solubility of PbCl2 in terms of moles per litre only but not in terms of g/litre, as asked in the question. (c) Many candidates were not able to explain clearly the term anisotropy with reference to the electrical conductivity in graphite. MARKING SCHEME Question 2 10 (a) (i) No. of moles of cane sugar = 342 = 0 0292 No. of moles of urea = (ii) 1 754 cane sugar = urea(isotonic solution) n1 RT/V = n2 RT/V 0 0292 = 1 754/x x = 60 06 1200 g of water contains 48 g of solute 1000 g contains 48 1000/1200 = 40 g of solute Molality = 40/58 = 0 689 mol / kg Tb = kb molality = 0 513 0 689 = 0 353oC 49 Suggestions for teachers Stress upon writing all the steps involved in solving the numerical problems i.e. the formula, substitution and calculation of answer with correct unit. Explain the difference between the boiling point of pure solvent and the solution. The abnormal molecular weights, Van t Hoff factor, degree of dissociation, degree of association must be explained clearly to students. Stress upon calculation of pH value by using correct formula. The concept of Ostwald dilution law and its application must be explained in detail to students. Use of the formula i.e. ksp = 4s3 for BaCl2, conversion of solubility from moles/lit. to g/lit. and vice versa must be explained clearly to students. The concept of anisotropy and free electrons in graphite must be explained to students. B.P. = 100 + 0 353 = 100 353oC (iii) Vant Hoff factor (i) = observed colligative property / normal colligative property Since benzoic acid dimerised, i = (b) (i) or 0 5 Experimental mol. wt. = twice the normal mol.wt. 2 0 = 100 = 0 02 pH = -log C pH = -log 0 001 0 02 = -log 2 10-5 pH = 4 69 (ii) The degree of dissociation of this acetic acid can be increased by diluting the solution Solubility product (ksp) = 4S3 = 1 7 10-5 Solubility (S) = 0 01619 mol L-1 Mol mass of PbCl2 = 278 Solubility in g/lit. = 0 01619 278 = 4 50 g/lit. (c) Graphite exists in the form of layer structure. The electrical conductivity is more parallel to the layer whereas the electrical conductivity is less perpendicular to the layer. Question 3 (a) (i) (ii) (b) (i) In a body centred and face centred arrangement of atoms of an element, what will be the number of atoms present in respective unit cells? Justify your answer with calculation. A compound AB has a simple cubic structure and has molecular mass 99. Its density is 3 4 g cm-3. What will be the edge length of the unit cell? [2] 2NO(g) N2(g) + O2(g); H = heat Ke = 2 5 102 at 298K what will happen to the concentration of N2 if: (1) Temperature is decreased to 273K. [2] For the reaction: [2] (2) Pressure is reduced. (ii) In a first order reaction, 10% of the reactant is consumed in 25 minutes. Calculate: [2] (1) The half-life period of the reaction. (2) The time required for completing 87 5% of the reaction. (c) Water acts as Bronsted acid as well as a Bronsted base. Give one example each to illustrate this statement. 50 [2] Comments of Examiners (a) (i) Most of the candidates wrote the answer directly without showing the calculation. (ii) While many candidates were able to calculate the value of a3, the value of edge length was not calculated correctly in many cases. Some candidates substituted the value of z = 4 instead of z = 1. (b) (i) A few candidates were confused regarding whether the given reaction is exothermic or endothermic, hence gave wrong answers. The second part was not attempted correctly by many candidates. (ii) While a number of candidates were able to calculate t1/2 correctly, the time required for completing 87.5% of the reaction was not calculated correctly by many candidates. (c) Concept of Bronsted acid and Bronsted base was not clear to many candidates. Most of the candidates could not give proper examples of water acting as Bronsted acid and as Bronsted base. Suggestions for teachers Explain the calculations to find out the number of atoms in various types of cubic unit cells i.e. simple cubic, body centered cubic and face centered cubic. Give more practice in solving numerical problems based on density, edge length, etc. Factors affecting chemical equilibrium using Le Chatelier s principle should be explained to students. Sufficient practice in numericals based on half-life period (t1/2) should be given. Bronsted Lowry s concept and acidbase conjugate pairs should be explained clearly with examples. MARKING SCHEME Question 3 1 (a) (i) BCC corner atoms = 8 8 = 1 Body centred atom = 1 1= 1 Total number of atoms 1 + 1 = 2 1 FCC corner atoms = 8 8 = 1 1 Face centred atoms = 6 = 3 2 Total number of atoms 1 + 3 = 4 (ii) = 3 Simple cubic structure Z = 1 M = 99, NA = 6 023 1023 , density = 3 4g/cm3 a3 = = 1 99 3 4 6 023 1023 a3 = 4 834 10-23cm a = 3 64 10-8 cm 51 (b) (i) (ii) (1) The reaction is exothermic hence decrease in temperature will favour the forward reaction, i.e. concentration of N2 will increase. (2) Pressure has no effect on equilibrium. (1) k= k= 2 303 2 303 25 log 10 100 log 10 90 k = 0 0042 min-1 t1/2 = (2) 0 693 2 303 0 693 = 0 0042 = 165 min t = 0 0042 log 10 t = 495 14 min (c) 100 12.5 HCl(aq) + H2O(l) H3O+(aq)+ Cl- (aq) acid-1 base-2 acid-2 base-1 H2O(l) + NH3(l) NH4+(aq) + OH-(aq) acid-1 base-2 acid-2 base-1 Question 4 (a) (i) Consider the following cell reaction at 298 K: [3] 2Ag+ + Cd 2Ag + Cd2+ The standard reduction potentials (Eo) for Ag+/Ag and Cd2+/Cd are 0 80V and 0 40V respectively: (1) Write the cell representation. (2) (3) (ii) (b) (c) What will be the emf of the cell if the concentration of Cd2+ is 0 1 M and that of Ag+ is 0 2 M? Will the cell work spontaneously for the condition given in (2) above? What is a buffer solution? How is it prepared? Explain the buffer action of a basic buffer with a suitable example. Explain the following: [2] (i) When NaCl is added to AgNO3 solution, a white precipitate is formed. (ii) An aqueous solution of ammonium chloride is acidic in nature. A 0 05 M NH4OH solution offers the resistance of 50 ohms to a conductivity cell at 298K. If the cell constant is 0 50 cm-1 and molar conductance of NH4OH at infinite dilution is 471 4 ohm-1 cm2 mol-1, calculate: (i) Specific conductance (ii) Molar conductance [2] (iii) Degree of dissociation 52 [3] Comments of Examiners (a) (i) The cell representation was not given correctly by many candidates; the calculation of emf of the cell by using Nernst equation was also not correct, in some cases. The third part of the question was generally answered correctly by most candidates. (ii) Many candidates explained acidic buffer and its action instead of basic buffer (b) (i) Some of the candidates did not mention that the white precipitate is due to the formation of AgCl. (ii) Several candidates mentioned anionic hydrolysis instead of cationic hydrolysis . (c) (i) Specific conductance (k) was calculated correctly in most cases. (ii) While most candidates calculated molar conductance correctly, the unit was not mentioned in several cases. (iii) The degree of dissociation ( ) was not calculated correctly by many candidates. Suggestions for teachers Give more practice in cell representation; Numericals based on Nernst equation should be taught with examples. The relationship between Gibbs free energy (G) and emf of the cell (E) must be explained clearly. Theory of precipitation that I.P. > ksp should be explained to students. Explain cationic and anionic hydrolysis to students by giving suitable examples. Explain clearly the calculations of specific conductance and degree of dissociation. MARKING SCHEME Question 4 (a) (i) (1) Cd(s) / Cd2+(aq) // Ag+(aq) / Ag Eocell= Eocathode Eoanode (2) = 0 80 (-0 40) = 1 2V Ecell = Eocell = 1 2 - 0 0591 = 1 18V log [ 2+ ][ ]2 log10 [0 1] [0 2]2 [ + ]2 [cd] G = -nFEo (3) (ii) 0 0591 Since Eo is positive, G will be negative so the cell will work spontaneously. Buffer solutions are those solutions which resist the change in their pH value when small quantity of acid or alkali is added to it. Preparation of buffer By taking aqueous solution of a weak acid and its salt with a strong base. or By taking aqueous solution of a weak base and its salt with a strong acid. 53 Buffer action of basic buffer NH4+(aq) + OH-(aq) NH4OH(aq) NH4+(aq) + Cl-(aq) NH4Cl(aq) On adding NaOH NH4++ OHFrom buffer NH4OH(aq) from NaOH On adding HCl OH-(aq)+ H3O+ From buffer 2H2O(l) from HCl Hence, there is no change in pH of buffer solution. Buffer action of any basic buffer solution may be given. (b) (i) NaCl AgNO3 Na+ + ClAg+ + NO3- NaCl + AgNO3 AgCl + NaNO3 white ppt (ii) (c) (i) Ammonium chloride is a salt of strong acid and weak base, hence due to cationic hydrolysis, the aq solution of ammonium chloride is acidic in nature. Specific conductance = = 1 1 cell constant 0 50 50 = 0 01 ohm-1 cm-1 (ii) Molar conductance (^ ) = = 200 ohm-1 cm2 mol-1 (iii) Degree of dissociation ( ) = 1000 ^ ^ = = 1000 0 01 200 471.4 0 05 = 0.4242 SECTION B Answer any two questions Question 5 (a) (b) Write the IUPAC names of the following: (i) [Co(NH3)4SO4]NO3 (ii) K[Pt(NH3)Cl3] What type of isomerism is exhibited by the following pairs of compounds: (i) [PtCl2(NH3)4]Br2 and [PtBr2(NH3)4]Cl2 (ii) [Cr(SCN)(H2O)5]2+ and [Cr(NCS)(H2O)5]2+ 54 [2] [1] (c) How does K2[Pt Cl4] get ionised when dissolved in water? Will it form precipitate when AgNO3 solution is added to it? Give a reason for your answer. [2] Comments of Examiners (a) (i) Some candidates wrote amine instead of Suggestions for teachers ammine . A few candidates wrote the wrong Given sufficient practice in writing oxidation state. IUPAC names. (ii) Several candidates wrote platinumate or Calculation of oxidation state of platinum instead of platinate . central metal atom should be given (b) (i) While most candidates wrote the correct type of more emphasis. isomerism some wrote ionic and structural Isomerism of coordination isomerism instead of ionization isomerism . compounds should be explained to (ii) Some candidates wrote ligand isomerism instead students with examples. of linkage isomerism . (c)The ionization of K2[PtCl4], was not correctly mentioned by a number of candidates. A few candidates wrote that precipitate will be formed when AgNO3 solution is added. MARKING SCHEME Question 5 (a) (i) Tetraamminesulphato cobalt(III) nitrate (b) (c) (ii) Potassium ammine trichloridoplatinate(II) (i) Ionisation isomerism (ii) Linkage isomerism K2[Pt Cl4] 2K+ + [Pt Cl4]2It will not form white precipitate with AgNO3 solution because Cl- ion is not free to form white precipitate of AgCl. Question 6 (a) Give balanced equations for the following reactions: (i) Silver nitrate is added to dilute solution of sodium thiosulphate. (ii) Potassium dichromate is treated with acidified ferrous sulphate solution. [3] (iii) Phosphorous reacts with conc. sulphuric acid. (b) How will you obtain pure potassium permanganate (KMnO4) crystals from its ore, pyrolusite? Give the steps involved and the reactions. 55 [2] Comments of Examiners (a) Most of the candidates wrote either incorrect or incomplete equations. In many cases, the equations were unbalanced. (b) The conversion of pyrolusite (MnO2) to pure potassium permanganate was not answered correctly by the candidates. Candidates were not able to write the steps correctly. Suggestions for teachers More practice should be given in writing complete and balanced equations. Emphasis should be given on writing complete and correct equations of preparation of KMnO4 from MnO2 (pyrolusite ore) MARKING SCHEME Question 6 (a) (i) 2AgNO3 + Na2S2O3 Ag2S2O3 + 2NaNO3 white ppt Ag2S2O3+ H2O Ag2S + H2SO4 black (ii) (iii) (b) K2Cr2O7 + 7H2SO4 + 6FeSO4 K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O P4 + 10 H2SO4 4H3PO4 + 10SO2+ 4H2O Conversion of pyrolusite (MnO2) to potassium manganate 2MnO2 + 4KOH + O2 2K2MnO4 + 2H2O Or 2MnO2 + 2K2CO3 + O2 2K2 MnO4+ 2CO2 Oxidation of potassium manganate to potassium permanganate. Or 3K2MnO4 + 2CO2 2KMnO4+ MnO2 + 2K2CO3 2K2MnO4 + Cl2 2KMnO4 + 2KCl (or any other correct method) Question 7 (a) (i) Sulphur dioxide acts as an oxidizing agent as well as a reducing agent. Give one reaction each to show its oxidizing nature and its reducing nature. Explain why an aqueous solution of potassium hexacyanoferrate (II) does not give the test for ferrous ion. [3] What is meant by Lanthanide contraction? Write the general electronic configuration of inner transition elements. [2] (ii) (b) 56 Comments of Examiners (a) (i) Most of the candidates were not able to write this answer correctly. The examples given were also not correct. (ii) The ionisation of complex compounds was shown correctly by many candidates but some gave invalid reasons. (b) Incorrect meaning of lanthanide contraction was given by several candidates. The general electronic configuration of inner-transition element was not written correctly in many cases. Suggestions for teachers Explain the properties of oxidising and reducing agents with correct examples. Explain ionisation of co-ordination compounds clearly. Basic idea of complex compound should be given. Explain the general electronic configuration of block elements. MARKING SCHEME Question 7 (a) The oxidation state of S in SO2 is + 4 (i) Which is an intermediate state and may increase or decrease. Hence, SO2 can act both as an oxidizing and reducing agent. Example of oxidizing agent 3Fe+ SO2 2FeO + FeS Example of reducing agent SO2 + I2 + 2H2O H2SO4 + 2HI (ii) Aqueous solution of K4[Fe(CN)6] ionizes as K4[Fe(CN)6] 4K+ + [Fe(CN)6]4- Fe2+ ion is not in free state, hence it does not give the test of Fe2+ ion. (b) On moving from La3+ (At. No. 57) to Lu3+ (At. No. 71) the size of the atoms and ions decreases regularly due to increase in nuclear change. This decrease in size is called Lanthanide contraction. The general electronic configuration of inner transitional elements is ns2 (n-1) d0-1 (n- 2)f 1-14 57 SECTION C Answer any two questions. Question 8 (a) (b) (c) How can the following conversions be brought about: (i) Acetaldehyde to acetaldehyde phenyl hydrazone. [1] (ii) Benzoic acid to aniline. [1] (iii) Methyl chloride to acetone. [2] (iv) Benzene to benzene diazonium chloride. [1] (i) Glycerol (propane 1, 2, 3 triol) is more viscous than ethylene glycol (ethane 1, 2, diol). Explain. [1] (ii) How can urea be detected by Biuret test? [1] [3] Identify the compounds A, B and C: (i) (ii) C2 H5 OH PCl5 C6 H5 COOH A SOCl2 KCN A B NH3 H3 O + B C2 H5 COOH Br2 KOH C 3 C Comments of Examiners (a) Most of the candidates answered this part correctly. Suggestions for teachers Some common errors made by them were: Stress upon writing complete and (i) The product formed was correct but the equation balanced equations along with given was not balanced. proper conditions while giving the (ii) The conversion of benzoic acid to aniline was chemical equations. answered correctly. However, some candidates Properties of alcohols should be failed to write the conditions for the reaction. taught in much more detail. (iii)Many candidates could not complete the reaction Named organic reactions such as after reaching up to acetic acid. biuret test must be explained to the (iv) Incomplete or incorrect equation was given by the students. candidates, temperature 0o 5o was not mentioned. Give more practice to students in (b) (i) The explanation given by a few candidates were not identifying organic compounds. correct. Some failed to mention that the extent of More practice should be given to hydrogen bonding is more in glycerol, hence it is solve such problems. more viscous than ethylene glycol. (ii) Some candidates did not mention heating of urea at above 132oC to form biuret. Instead of violet colour, some candidates wrote pink or blue colour. (c)(i) Most of the candidates identified compounds A, B and C correctly, but some identified compound C as CH3COONH4 instead of CH3CONH2 (ii) Compound A was not identified correctly by some of the candidates. 58 MARKING SCHEME Question 8 (a) (i) CH3 C = O + H2 N H N.C6H5 CH3 C = NHN C6H5 + H2O H H (ii) C6H5COOH (iii) (iv) (b) (i) (ii) CH3Cl Ca(OH)2 NaOH/CaO KCN (i) HOH/H+ (CH3COO)2Ca distil. conc HNO3 /conc H2 SO4 C6H6 conc Sn/HCl C6H6 CH3CN C6H5NH2 CH3COOH CH3COCH3 Sn/HCl C6H5NO2 / C6H5NO2 6[H] C6H5NH2 0 52 C6H5N2Cl Glycerol is more viscous than ethane 1, 2 diol because, in glycerol, there are three OH groups as compared to two OH groups in ethane 1, 2 diol. Therefore, the extent of hydrogen bonding is more, hence glycerol is more viscous. Biuret test: 2 moles of urea when heated above 132oC H2NCO NH2 + H (c) conc HNO3 /H2 SO4 132 NH.CONH2 H2NCO NHCONH2 + H2O Biuret When alkaline solution of biuret is treated with copper sulphate solution, a violet colour is produced. A = C2H5Cl B = C2H5CN (ii) C = C2H5CONH2 A = C6H5COCl B = C6H5 CONH2 C = C6H5NH2 Question 9 (a) Give balanced equations for the following name reactions: (i) Benzoin condensation (ii) Wurtz-Fittig reaction (iii) Carbylamine reaction [3] (b) Give chemical test to distinguish: (i) Formaldehyde and acetaldehyde (ii) Dimethyl ether and ethyl alcohol. [3] 59 (c) (i) Write the structure of three ethers with molecular formula C4H10O. (ii) Starting with Grignard s reagent, how will you prepare propanoic acid? [4] Comments of Examiners (a) (i) Many candidates wrote the structure of Benzoin Suggestions for teachers incorrectly. Some failed to mention alcoholic To distinguish between compounds KCN. by chemical test, the experiment, (ii) Some candidates missed the condition dry ether , condition and observation must be while some gave the example of Fittig reaction. given. Students must be asked to (iii) A number of candidates did not mention alcoholic write the positive test for one KOH. Some gave incomplete equations - they did compound and negative test for the not mention by-products. other. (b) (i) Correct observations were not written in some Insist that the students must learn cases although the tests given were correct. the structural formula of organic (ii) Some candidates wrote the positive test for ethyl compounds. alcohol but did not write anything about dimethyl Teach preparation of different ether. organic compounds by starting with (c) (i) Most of the candidates were able to write the Grignard s reagent. structure of at least two ethers. However, a number of candidates were not able to write the structure of branched chain ether. (ii) Instead of propanoic acid many candidates wrote the preparation of ethanoic acid . MARKING SCHEME Question 9 (a) (i) Benzoin condensation: O O OH O KCN(alc) C6H5C + H C C6H5 C6H5 C C C6H5 | H H Benzoin (ii) Wurtz-Fittig reaction: C6H5Cl + 2Na + ClCH3 (iii) Carbylamine reaction: RNH2 + CHCl3 + 3KOH(alc) (b) (i) C6H5. CH3 + 2NaCl R N= C + 3KCl + 3H2O Formaldehyde and acetaldehyde: Acetaldehyde on reaction with iodine and alkali gives yellow precipitate of iodoform which has a characteristic odour. Formaldehyde does not give this test. (or any other suitable test.) Dimethyl ether and ethyl alcohol: Ethyl alcohol when reacts with iodine and alkali gives yellow precipitate of iodoform 60 (c) which has characteristic odour. Diethyl ether does not give this test. (or any other suitable test) CH3 O CH2 CH2 CH3 (i) CH3 CH2 O CH2 CH3 CH3 CH3 O CH CH3 (ii) C2H5 +HOH C2H5MgBr + O = C = O O = C O MgBr C2H5 Br / O = C OH + Mg \ OH Question 10 (a) (b) (c) An organic compound A has the molecular formula C7H6O. When A is treated with NaOH followed by acid hydrolysis, it gives two products B and C. When B is oxidized, it gives A, when A and C are each treated separately with PCl5, they give two different products D and E. (i) Identify A, B, C, D and E. (ii) Give the chemical reaction when A is treated with NaOH and name the reaction. Answer the following: [4] (i) What do you observe when glucose solution is heated with Tollen s reagent? (ii) Name the monomers and the type of polymerisation in each of the following polymers: (1) Terylene (2) Polyvinyl chloride [3] Give balanced equations for the following reactions: (i) Ethylamine with nitrous acid. (ii) Diethyl ether with phosphorous pentachloride. (iii) Aniline with acetyl chloride. 61 [3] Comments of Examiners (a) (i) A number of candidates were able to identify Suggestions for teachers compounds A, B, C, D, & E correctly. Some Give more practice for questions in candidates identified compound C as which identification of compounds C6H5COONa instead of C6H5COOH . A few is based on different chemical candidates were not able to identify compound D reactions. correctly. Give more emphasis on named (ii) The Cannizzaro s reaction was given correctly by reactions. most of the candidates. Insist that the students read the (b) (i) Most of the candidates wrote that silver mirror is observations for different organic formed. Some wrote that a white precipitate is reactions. They should mention formed. colour or ppt. properly. (ii) Many candidates were unable to write the correct Teach the polymers and their monomers of Terylene. Some wrote incorrect monomers in detail. The types of polymerization. polymerisation should also be (c) (i) Incomplete equations was given by some explained in detail. candidates. A few candidates could not write Ask students to write balanced C2H3OH as product. equations with correct reactants and (ii) A number of candidates wrote wrong products products. such as C2H5COCl or C2Cl5-O-C2Cl5 although correct answer was C2H5Cl and POCl3. (iii) In some cases, wrong formula of the product was written - instead of C6H5NHCOCH3, several candidates wrote C6H5NHCH3CO. MARKING SCHEME Question 10 (a) (i) (A) = C6H5CHO (B) = C6H5CH2OH (C) = C6H5COOH (D) = C6H5CHCl2 (E) = C6H5COCl (ii) 2 C6H5CHO + NaOH C6H5COONa + C6H5CH2OH OR Cannizzaro reaction 62 (b) (i) CHO (CHOH)4 + 2[Ag(NH3)2]+ OH Tollen s reagent CH2OH glucose COONH4 (CHOH)4 + 2Ag + 3NH3 + H2O CH2OH Silver mirror OR CHO COOH (CHOH)4 + Ag2O (CHOH)4 + 2Ag Tollen s reagent CH2OH CH2OH glucose Silver mirror (ii) (1) Terylene:Ethylene glycol + terephthalic acid Condensation polymerisation (2) Polyvinyl chloride: Vinyl chloride Addition polymerisation (c) (i) C2H5NH2 + HONO C2H5OH + N2 + H2O (ii) C2H5 O C2H5 + PCl5 2C2H5Cl + POCl3 (iii) CH3CO Cl+H NHC6H5 CH3CONHC6H5 + HCl 63 GENERAL COMMENTS: (a) Topics found difficult by candidates: Numerical problems of relative molecular mass and mole, Van t Hoff factor and its relation with molecular weight, calculation of degree of dissociation Anisotropic nature of graphite, calculation of edge length (a) and radius (r) of unit cell Chemical equilibrium, Le Chatelier s principle Electrolytic conductance, calculation of emf of the cell and cell representation, Nernst equation Ionic equilibria, calculation of pH value, solubility and solubility product Bronsted-Lowry s concept for acid and base. Buffer action of basic buffer Preparation of inorganic compounds General electronic configuration of inner transition elements Conversion of organic compounds, balancing of equations and named organic reactions Chemical tests to distinguish between organic compounds (b) Concepts between which candidates got confused: Van t Hoff factor and molecular weight Anisotropic and isotropic Common ion effect and solubility product Total number of particles in bcc and fcc unit cell and their relationship Paramagnetic and diamagnetic Edge length (a) and radius (r) of various types of cubic unit cell and their relationship Buffer action of acidic and basic buffer Concept of oxidation and reduction of SO2 Gibb s free energy and emf of cell in terms of spontaneity Types of polymerization and polymer (c) Suggestions for candidates: Read questions carefully and understand what is required before attempting the question. Practice numerical problems regularly, solve the numerical stepwise with correct formula and write the answer with correct unit. Learn complete and balanced equations along with the conditions, in inorganic and organic compounds Avoid selective study. Practice writing the IUPAC names for coordination compounds as well as organic compounds. Learn both positive and negative chemical tests in organic reactions. Learn the shapes and hybridization of molecules with diagram. While solving numerical problems, proper steps should be followed, i.e. formula, substitution and correct answer with units. 64

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