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Studymate Solutions to CBSE Board Examination 2012-2013

       Series : SKS/1                                                            Code No. 56/1/1
                                                            Candidates must write the Code on
Roll No.                                                    the title page of the answer-book.



   Code number given on the right hand side of the question paper should be written on the title page of
    the answer-book by the candidate.

   Please check that this question paper contains 30 questions.

   Please write down the Serial Number of the questions before attempting it.

   15 minutes time has been allotted to read this question paper. The question paper will be distributed at
    10.15 a.m. From 10.15 a.m. to 10.30 a.m., the student will read the question paper only and will not
    write any answer on the answer script during this period.




                                         CHEMISTRY

[Time allowed : 3 hours]                                                           [Maximum marks : 70]


General Instructuions:
(i)     All questions are compulsory.
(ii)    Questions numbered 1 to 8 are very short-answer questions and carry 1 mark each.
(iii) Questions numbered 9 to 18 are short-answer questions and carry 2 marks each.
(iv) Questions numbered 19 to 27 are also short-answer questions and carry 3 marks each.
(v)     Questions numbered 28 to 30 are long-answer questions and carry 5 marks each.
(vi) Use Log Tables, if necessary. Use of calculators is not allowed.




                                                    -(1)-
STUDYmate

1.   How many atoms constitute one unit cell of a face-centered cubic crystal?
           1     1
Ans. 8 ×     +6×   =4
           8     2
     In one unit cell, 4 atoms are present.

2.   Name the method used for the refining of Nickel metal.
Ans. Mond’s Process

3.   What is the covalency of nitrogen in N2O5?
Ans. 4 (Four)
                                             O            O
                                                 N–O–N
                                             O            O


4.   Write the IUPAC name of CH3 – CH – CH2 – CH = CH2

                                        Cl
Ans. 4-Chloropent-1-ene

5.   What happens when CH3 – Br is treated with KCN?
Ans. Nucleophilic substitution occurs
                +–
     CH3Br + KC  N  CH3CN + KBr

6.   Write the structure of 3-methyl butanal.
            CH3       O

Ans. H3C – C – CH2 – C – H

          H
     3-Methyl butanal

7.   Arrange the folllwing in increasing order of their basic strength in aqueous solution:
     CH3NH2, (CH3)3N, (CH3)2NH

Ans. (CH3)2 N H > CH3 N H2 > (CH3)3 N

8.   What are the three types of RNA molecles which perform different functions?
Ans. m – RNA
     t – RNA
     r – RNA

9.   18 g of glucose, C6H12O6 (Molar mass = 180 g mol–1) is dissolved in 1 kg of water in a sauce
     pan. At what temperature will this solution boil?
     (Kb for water = 0.52 K kg mol–1, boiling point of pure water = 373.15 K)
Ans. Tb – Tb° = Kb × m
                             18 1 kg
     Tb – 373.15 = 0.52 ×      
                            180 1 kg
     Tb = 0.052 + 373.15
     Tb = 373.202 K

                                                  -(2)-
STUDYmate

10.    The conductivity of 0.20 M solution of KCl at 298 K is 0.025 S cm–1. Calculate its molar
       conductivity.
                 k  1000
Ans. m        =           ; m is molar conductivity.
                     M
                 0.025  1000
               =
                      0.20
               = 125 S cm2 mol–1.


11.    Write the dispersed phase and dispersion medium of the following colloidal systems:
       (a)     smoke                                        (b)      milk
                                                    OR
       What are lyophilic and lyophobic colloids? Which of these sols can be easily coagulated on the
       addition of small amounts of electrolytes?
Ans.
                             Dispersed phase       Dispersion medium
        Smoke                      Solid                    Gas or Air
        Milk                       Liquid                         Liquid

                                                    OR
Ans. Lyophilic colloids are liquid loving. Certain substances mixed with a suitable liquid (dispersion
     medium) directly forms the colloid. Such colloids are lyophilic colloids. Example, gum, gelatin,
     starch, etc.
       Lyophobic colloids are liquid hating. When substances like metals or metal sulphides are
       mixed with the dispersion medium do not form the colloidal sol.
       Such type of sols can be prepared by special methods.
       Example: Metal sulphides.
       On addition of small amount of electrolyte lyophobic collides can be precipitated.


12.    Write the differences between physisorption and chemisorption with respect to the following:
       (a)     specificity                                  (b)      temperature dependence
       (c)     reversibility and                            (d)      enthalpy change
Ans.                    Physisorption                                       Chemisorption
        1. Physisorption is non- specific in nature.        1. Chemisorption is highly specific in
           As physisorption involves Vander                    nature as chemisorptions includes
           Waal’s interaction between adsorbate                chemical bond formation between
           and adsorbent.                                      adsorbate and adsorbent.
        2. On increasing temperature                        2. Chemisorption increases with increase
           physisorption decreases. It is because              in temperature. After reaching a
           desorption occurs rapidl y.                         maximum value, extent of adsorption
           Physisorption occurs usually at low                 decreases with temperature.
           temperature.
        3. Physisorption is reversible in nature.           3. Chemisorption is irreversible in
                                                               nature.
        4. In case of physisorption enthalpy                4. In case of chemisorptions enthalpy
           change is small.                                    change is high.
           For example: 20-40 kJ mol–1 (approx).               For example: 40-400 kJ mol–1 (approx).




                                                    -(3)-
STUDYmate

13.   (a)   Which solution is used for the leaching of silver metal in the presence of air in the
            metallurgy of silver?
      (b)   Out of C and CO, which is better reducing agent at the lower temperature range in the
            blast furnance to extract iron from the oxide ore?
Ans. (a)    Aqueous NaCN
      (b)   CO is a better reducing agent at lower temperature.
            rG° would be more negative for reduction in blast furnace to extract Fe from the oxide
            ore by using CO.


14.   What happens when
      (a)   PCl5 is heated?                                       (b)   H3PO3 is heated?

                         
Ans. (a)               
            PCl5  g   PCl3  g   Cl2  g                 (b)           
                                                                        4H3PO3  3H3PO4 + PH3
                                                                                  


15.   (a)   Which metal in the first transition series (3d series) exhibits +1 oxidation state most
            frequently and why?
      (b)   Which of the following cations are coloured in aqueous solutions and why?
            Sc3+, V3+, Ti4+, Mn2+
            (At. nos. Sc = 21, V = 23, Ti = 22, Mn = 25)
Ans. (a)    Copper shows +1 oxidation state extensively. Cu+ has 3d10 configuration.
      (b)   V3+ = 3d2+ 4s0
            Mn2+ = 3d5 4s0
            V3+ as well as Mn2+ both have unpaired electrons in their d-orbitals which can show d-
            d transition.


16.   Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction. Give
      two reasons for the same.
Ans. Chlorobenzene is extremely less reative towards nucleophilic substitution reaction due to
     conjugation.




      (a)   Partial double bond character between carbon of benzene and halogen causes cleavage
            of bond with difficulty.
      (b)   Phenyl cation is quite unstable, So SN1 mechanism is not possible too.


17.   Explain the mechanism of the following reaction:
                                                     
                              H
      2CH3  CH2  OH  CH3CH2  O  CH2  CH3  H2O
                          
                             413K                    
Ans. Step 1: Formation of Protonated alcohol
                                                                    H
            CH3CH2 – OH + H                     CH3 – CH2 – O – H




                                                          -(4)-
STUDYmate

      Step 2: Formation of protonated ether
                                  H                                      H
                                                                             H
            H3C – CH2 – O     +      C – OH2          CH3 – CH2 – O – C            + H2O
                       H      CH3      H                             H       CH3
      Step 3: Formation of ether

            H3C – CH2 – O – CH2 – CH3           CH3 – CH2 – O – CH2 – CH3          +   H
                        H

18.   How will you convert:
      (a)   Propene to Propan-2-ol                        (b)   Phenol to 2, 4, 6–trinitrophenol
Ans. Conversion
      Propene  Propan-2-ol
                         H O/H
      CH3  CH  CH2  CH3  CH CH3
                          2
                                
                                        |
                                       OH
      Phenol  2,4,6 trinitrophenol (Picric acid)
       OH                         OH
                            NO2         NO2
             conc. HNO3


                                   NO2
                              (Picric acid)


19.   (a)   What type of semiconductor is obtained when silicon is doped with boron?
      (b)   What type of magnetism is shown in the following alignment of magnetic moments?
                                                   
      (c)   What type of point defect is produced when AgCl is doped with CdCl2?
Ans. (a)    Silicon is an element of group 14, Boron belongs to group 13.
            So, a p-type semiconductor is resulted.
      (b)   Ferromagnetism (as all magnetic moments lie in same direction)
      (c)   AgCl with CdCl2 causes impurity defect leading to cationic vacancies.


20.   Determine the osmotic pressure of a solution prepared by dissolving 2.5 × 10–2 g of K2SO4 in
      2L of water at 25°C, assuming that it is completely dissociated.
      (R = 0.0821 L atm K–1 mol–1, Molar mass of K2SO4 = 174 g mol–1)
Ans. K2SO4  2K+ + SO42–
      i=2+1=3
      Volume of solution (v) = 2L
      Mass of K2SO4 (WB) = 2.5 × 10–2
            = 0.025 g
      Molar mass of K2SO4 (MB) = 174 g/mol
      T = 25°C = 298.15 K
       = i CRT
         WBRT
      =i M V
           B


                                                  -(5)-
STUDYmate

            3  0.025  0.0821  298.15
      =
                      174  2
      = 5.27 × 10–3 atm.


21.   Calculate the emf of the following cell at 298 K:
      Fe(s) | Fe2+ (0.001 M) || H+ (1 M) | H2(g) (1 bar), Pt(s)
      (Given E°cell = +0.44 V)
Ans. Cell Reaction
               Fe  2H  Fe2  H2
                         (n 2)
                                

      According to Nernst equation
                               0.0591     [H ]2
               Ecell  Eo 
                        cell          log
                                  2       [Fe2 ]
                          0.0591
               = 0.44           (3)
                             2
               = 0.44 + 0.0887
               = 0.528 V


22.   How would you account for the following?
      (a)      Transition metals exhibit variable oxidation states.
      (b)      Zr (Z = 40) and Hf (Z = 72) have almost identical radii.
      (c)      Transition metals and their compounds act as catalyst.
                                                    OR
      Complete the following chemical equations:
      (a)      Cr2O7  + 6Fe2+ + 14H+ 
                   2


      (b)      2CrO2  + 2H+ 
                   4

      (c)      2MnO4 + 5C2O2  + 16H+ 
                   
                           4
Ans. (a)       Transition metals exhibit variable oxidation states due to the participation of ns and (n
               – 1) d electrons in bond formation. Both ns and (n – 1) d electrons have almost equal
               energies.
      (b)      Zr (160 pm) and Hf (159 pm) have almost identical radii due to Lanthanoid contraction.
               Lanthanoid contraction is steady decrease in the size of lanthanoid ions (M3+) with the
               increase in atomic number. The reason for this is a gradual increase in the effective
               nuclear charge experienced by the outer electron.
      (c)      Transition metal and their compounds act as catalysts,
               (i)    because of their variable valencies. Transition metals sometimes form unstable
                      intermediate compounds and provide a new path with lower activation energy for
                      the reaction.
               (ii)   in some cases they provide suitable surface for the reaction to take place. The
                      reactants are adsorbed on the surface of the catalyst where reaction occurs.
                                                    OR
Ans. (a)       Cr2O7 + 14H+ + 6Fe2+  2Cr3+ + 6Fe3+ +
                   2–                                       7H2O
      (b)      2CrO4 + 2H+  Cr2 O7 + H2O
                   2–              2–

      (c)      2MnO4 + 5C2O4 + 16H+  2Mn2+ + 8H2O
                    –      2–                               + 10CO2



                                                    -(6)-
STUDYmate

23.   Write the IUPAC names of the following coordination compounds :
      (i)     [Cr(NH3)3Cl3]                                           (ii)     K3[Fe(CN)6]
      (iii)   [CoBr2(en)2]+, (en = enthylenediamine)
Ans. (i)      Triamminetrichloridochromium(III)
      (ii)    Potassium hexacyanoferrate(III)
      (iii)   Dibromidobis(ethane–1,2–diamine)cobalt(III)


24.   Give the structures of A, B and C in the following reactions:
                                                   H O/H             NH
              C6H5 N2Cl   A  B  C
                              CuCN
      (i)                         2      3 
                                                                       
                              Sn  HCl             NaNO  HCl                H O/H
      (ii)    C6H5 NO2  A  B  C
                                 2       2  
                                                      273K                      
                                                                                           O
               N2 Cl                     CN                     COOH                       C – NH2

Ans. (i)                  CuCN                     H2O/H                      NH 3
                                                                               
                                       (A)                      (B)                        (C)
                   NO2                       NH2                             N2 Cl                 OH
                             Sn/HCl                   NaNO2 + HCl                       H 2O/H
      (ii)                                                 273 K                            
              Nitrobenzene               Aniline                Benzene diazonium                Phenol
                                                                     chloride


25.   Write the names and structures of the monomers of the following polymers:
      (i)     Buna – S                                                (ii)     Neoprene
      (iii)   Nylon – 6, 6
                                                                                   CH = CH2

Ans. (i)      Buna-S                CH2 = CH – CH = CH2                 +
                                             1,3–Butadiene
                                                                              Styrene
      (ii)    Neoprene              CH2 = C – CH = CH2

                                              Cl
                                      2–Chloro–1,3–butadiene
                                                                               COOH
      (iii)   Nylon 6,6                  H2N(CH2)6NH2 + (CH2)4
                                    Hexamethylene diamine
                                                                               COOH
                                                                       Adipic acid



26.   After watching a programme on TV about the adverse effects of junk food and soft drinks on
      the health of school children, Sonali, a student of Class XII, discussed the issue with the
      school principal. Principal immediately instructed the canteen contractor to replace the fast
      food with the fibre and vitamins rich food like sprouts, salad, fruits etc. This decision was
      welcomed by the parents and the students.
      After reading the above passage, answer the following questions:
      (a)     What values are expressed by Sonali and the Principal of the school?
      (b)     Give two examples of water-soluble vitamins.


                                                              -(7)-
STUDYmate

Ans. (a)    They are good educated citizens who believe in the saying “healthy mind in a healthy
            body.”
            This shows they value education and are caring towards the school inmates.
      (b)   Vitamin B and Vitamin C


27.   (a)   Which one of the following is a food preservative?
            Equanil, Morphine, Sodium benzoate
      (b)   Why is bithional added to soap?
      (c)   Which class of drugs is used in sleeping pills?
Ans. (a)    Sodium Benzoate
      (b)   To impart antiseptic properties
      (c)   Narcotic Analgesics


28.   (a)   A reaction is second order in A and first order in B.
            (i)     Write the differential rate equation.
            (ii)    How is the rate affected on increasing the concentration of A three times?
            (iii)   How is the rate affected when the concentrations of both A and B are doubled?
      (b)   A first order reaction takes 40 minutes for 30% decoposition. Calculate t1/2 for this
            reaction.
            (Given log 1.428 = 0.1548)
                                                  OR
      (a)   For a first order reaction, show that time required for 99% completion is twice the time
            required for the completion of 90% of reaction.
      (b)   Rate constant ‘k’ of a reaction varies with temperature ‘T’ accordig to the equation:
                                Ea    1
            log k  log A             
                              2.303 R  T 
                                                                                         1
            where Ea is the activation energy. When a graph is plotted for log k Vs.       , a straight
                                                                                         T
            line with a slope of –4250 K is obtained. Calculate ‘Ea’ for the reaction.
            (R = 8.314 JK–1 mol–1)
                    dx
Ans. (a)    (i)         k[A]2 [B]
                    dt
            (ii)    The rate will become 9 times.
            (iii)   The rate will become 8 times.
      (b)           t = 40 min.
            Finding rate constant by the given data.
                         2.303    A
                    k         log 0
                           t      At
                         2.303      1
                    k         log
                           40      0.7
                        2.303
                             log1.428
                          40
                      2.303
                            0.1548
                        40
                    = 0.0089 min–1


                                                  -(8)-
STUDYmate

                           0.693
            Now, t1/2 
                             k
                            0.693
                    t1/2 
                           0.0089
                    = 77.865 min.
                                                   OR
                        2.303         a
Ans. (a)     t 90%           log
                          k       a  0.9a
                      2.303
                           log10
                        k
                      2.303
                    
                        k
                      2.303          a
            t 99.9%        log
                        k       a  0.999a
                        2.303          a
                             log
                          k       a  0.001a
                               2.303
                    t99.9%          3
                                 k
                   t 99.9%  t90%  3
                                          E a
      (b)   The slope of the reaction = 2.303 R

                   Ea
            Now, 2.303 R  4250 K

                            Ea = 4250 × 2.303 × 8.314
                               = 81375.35 Joules
                               = 81.375 kJ
29.   (a)   Give reasons for the following:
            (i)     Bond enthalpy of F2 is lower than that of Cl2.
            (ii)    PH3 has lower boiling point than NH3.
      (b)   Draw the structures of the following molecules:
            (i)     BrF3
            (ii)    (HPO3)3
            (iii)   XeF4
                                                   OR
      (a)   Account for the following :
            (i)     Helium is used in diving apparatus.
            (ii)    Fluorine does not exhibit positive oxidation state.
            (iii)   Oxygen shows catenation behaviour less than sulphur.
      (b)   Draw the structures of the following molecules:
            (i)     XeF2
            (ii)    H2S2O8
Ans. (a)    (i)     In F2, the bond between two F-atoms is formed by the head on overlapping of 2p
                    orbitals, while in Cl2 it is formed by the head on overlapping of 3p orbitals.
                    2p being smaller in size than 3p has more interelectronic repulsions, which makes
                    it weaker, thus its bond enthalpy is lower than that of Cl2.
            (ii)    Due to presence of H-bonding in NH3 its boiling point becomes more than PH3.

                                                   -(9)-
STUDYmate

     (b)    (i)     BrF3
                                                     F




                                                     Br               F



                                                  F
                                             T-shaped
            (ii)    (HPO3)3
                                                 O
                                                         O                O
                                                 P               P
                                        HO                                OH
                                                 O               O
                                                          P
                                                 O              OH
                                                 Cyclic trimer
            (iii)   XeF4


                                             F                        F

                                                         Xe

                                             F                        F

                                                 Square Planar
                                                         OR
Ans. (a)    (i)     Deep sea divers depend upon compressed air for their oxygen supply. Thus, a
                    deep sea diver has both N2 and O2 dissolve considerably in the blood and other
                    body fluids.
                    Oxygen is used up for metabolism but due to high partial pressure and greater
                    solubility of N2, it will remain dissolved and will form bubbles when the diver
                    comes to the atmospheric pressure. These bubbles affect nerve impulses and
                    give rise to a disease called bends or decompression sickness. To avoid bends and
                    also the toxic effects of high concentration of nitrogen in the blood, the cylinders
                    used by the divers are filled with air diluted with helium.
            (ii)    Fluorine does not exhibit positive oxidation state because fluorine is the most
                    electronegative element.
            (iii)   Because of stronger S–S bonds as compared to O–O bonds, sulphur has a stronger
                    tendency for catenation than oxygen.
                           F

                                                                               O    O

     (b)    (i)            Xe                                  (ii)       H–O–S–O–O–S–O–H

                                                                               O    O

                           F

                                                      -(10)-
STUDYmate

30.   (a)   Although phenoxide ion has more number of resonating structures than carboxylate
            ion, carboxylic acid is a stronger acid than phenol. Give two reasons.
      (b)   How will you bring about the following conversions?
            (i)     Propanone to propane
            (ii)    Benzoyl chloride to benzaldehyde
            (iii)   Ethanal to but-2-enal
                                                            OR
      (a)   Complete the following reactions:
                                Conc. KOH                                                Br /P
            (i)     2H  C  H                                (ii)    CH3COOH 
                                                                                       2
                                                                                         
                         |
                         |
                         O
                           CHO
                                    HNO3/H2SO4
            (iii)
                                     273-283 K

      (b)   Give simple chemical tests to distinguish between the following pairs of compounds:
            (i)     Ethanal and Propanal                             (ii)    Benzoic acid and Phenol
Ans. (a)    The two reasons are :
            (i)     In carboxylate anion, the resonating structures are exactly equal in energy,
                    equivalent resonating forms. In phenol the energies of the resonating forms differ.

                         O                      O
                                                                O                 O          etc.
                    R–C–O              R–C=O
            (ii)    In the resonating structures of phenol, the negative charge is placed on O-atom
                    in one of the resonating form and on C-atom in another resonating form. In
                    carboxylic acid, the negative charge is only on O-atom.
                    We know that the stability of resonating form is more if negative charge is on
                    more electronegative atom. So the resonating forms of carboxylate anion are more
                    stable than phenol.
                          O
                          ||
                                   Zn  Hg/conc. HCl
      (b)   (i)     CH3  C  CH3  CH3  CH2  CH3
                                                     
                    This is known as Clemmenson’s Reduction.
                     O

                     C—Cl
                              Pd-BaSO4, S                   CHO
            (ii)                                                     + HCl
                              boiling xylene

                    This reaction is called Rosenmund reduction
                                         H
                                         |
            (iii)                               dil NaOH/100C
                    CH3  CH  O  CH3  C  O  CH3  CH  CH  CHO
                                                                
                    The above reaction is aldol condensation
                                                            OR
                                     conc.
Ans. (a)    (i)     2H – C – H
                                                            +
                                                 HCOOK + CH3OH                                   [Canizzaro reaction]
                                      KOH
                         O
                          O                                     O
                                             P/Br 2
            (ii)    CH3 – C – O – H                    CH2 – C – OH                                    [HVZ reaction]

                                                       Br
                                                      -Bromoacetic acid

                                                            -(11)-
STUDYmate

                          CHO    HNO3/H 2SO 4
                                                          CHO
            (iii)                                                    + H2O
                                  273-283 K

                                                    NO2
                                                m-Nitrobenzaldehyde
    (b)     (i)     We can use iodoform test to distinguish between ethanal and propanal.
                         H
                    CH3 – C = O + 3I2 + NaOH  CHI3 + HCOONa + 3HI
                                                        yellow ppt
                    This test is not given by propanal.
            (ii)    We can use sodium bicarbonate to distinguish between benzoic acid and phenol.
                    Benzoic acid is a stronger acid than phenol, so it reacts with NaHCO3 (weak base)
                    liberating CO2.
                    C6H5–COOH + NaHCO3  C6H5COONa + H2O + CO2 (brisk effervescence)
                    Phenol shows no reaction with NaHCO3.
                    We can also used neutral FeCl3. It reacts with phenol to give violet coloured
                    solution.


                                           ×·×·×·×·×




                                                     -(12)-
STUDYmate


        Studymate Solutions to CBSE Board Examination 2012-2013

      Series : SKS/1                                                        Code No. 56/1/2
                         UNCOMMON QUESTIONS ONLY
1.     What type of stoichiometric defect is shown by AgCl?
Ans. Frenkel Defect


                                                  CH3
2.     Write the IUPAC name of CH3CH = CH – C – CH3

                                                  Br
Ans. 4-Bromo-4-methylpent-2-ene.


4.     What type of bonding helps in stabilising the -helix structure of proteins?
                                     O

Ans. Hydrogen bonding between – C – and –NH– groups of the peptide bond.


6.     What inspired N. Bartlett for carrying out reaction between Xe and PtF6?
Ans. Neil Bartlett prepared a red compound having the formula O+2PtF6–. He realised that the first
     ionization enthalpy of molecular oxygen (1175 kJ mol–1) is similar to Xe (1170 kJ mol–1). So, he
     was inspired to synthesise a similar compound using Xenon.


7.     What happens when ethyl chloride is treated with aqueous KOH?
Ans. CH3 – CH2 – Cl + aq. KOH  CH3 – CH2 – OH + KCl
       Nucleophillic substitution reaction takes place and ethanol is formed.


8.     Write the structure of 4-chloropentan-2-one.
             H   O   H   H
Ans. H – C – C – C – C – CH3
             H       H   Cl

11.    What is the difference between oil/water (O/W) type and water/oil (W/O) type emulsions?
       Give an example of each type.
Ans. In oil/water type emulsion, water is the dispersion medium. Example: milk and vanishing
     cream.
       In water/oil type emulsion, oil is the dispersion medium. Example: butter and cream.


17.    (a)   Which of the following ores can be concentrated by froth floatation method and why?
             Fe2O3, ZnS, Al2O3
       (b)   What is the role of silica in the metallurgy of copper?
Ans. (a)     ZnS because sulphide ores have wettabilty with pine oil which adsorb ore particles on
             them.
       (b)   It acts as flux so that another impurity slag may be formed.



                                                 -(13)-
STUDYmate

18.   (a)   Why does p-dichlorobenzene have a higher m.p. than its o– and m–isomers?
      (b)   Why is () – Butan-2-ol is optically inactive?
Ans. (a)    p-dichlorobenzene have a higher m.p. because its packing is more efficient due to
            symmetry.
      (b)   Racemic mixture.


23.   Write the names and structures of the monomers of the following polymers:
      (a)   Polystyrene
      (b)   Dacron
      (c)   Teflon
                        CH = CH2

Ans. (a)    Styrene

      (b)   Ethylene glycol + Terephthalic acid/Benene-1, 4-dicarboxylic acid

            HOH2C – CH2OH + HOOC                 COOH

      (c)   Tetra fluoroethene
            CF2 = CF2

                                         ×·×·×·×·×




                                                 -(14)-
STUDYmate


        Studymate Solutions to CBSE Board Examination 2012-2013

      Series : SKS/1                                                      Code No. 56/1/3
                         UNCOMMON QUESTIONS ONLY
1.     What type of substances would make better permanent magnets, ferromagnetic or
       ferrimagnetic?
Ans. Ferromagnetic substances


3.     What is the composition of ‘Copper matte’?
Ans. Copper matte chiefly consists of Cu2S and some uncharged FeS.


5.     What is a glycosidic linkage?
Ans. Glycosidic linkage formed when two mono-saccharides are held together with the loss of
     small molecule’s like H2O. It is an oxide linkage which joins two monosaccharides.


7.     Which compound in the following pair undergoes faster SN1 reaction?
                                            Cl             Cl
                                                 and

         Cl
Ans.



8.     Write the structure of p-Methylbenzaldehyde molecule.
         CHO


Ans.
         CH3


9.     What is the difference between multi-molecular and macromolecular colloids? Give one
       example of each.
Ans. When large number of atoms/molecules of a substance on dissolution aggregate together to
     form species having size in the colloidal range, then species thus formed is called
     multimolecular colloids. E.g. sulphur sol consisting of S8 particles.
       When macromolecules in a suitable solvents form solutions in which the size of the
       macromolecules is in colloidal range, then such systems are macromolecular colloids e.g.
       starch, cellulose.


17.    Account for the following:
       (i)     The C–Cl bond length in chlorobenzene is shorter than that in CH3 – Cl.
       (ii)    Chloroform is stored in closed dark brown bottles.


                                                  -(15)-
STUDYmate

Ans. As resonance imparts partial double bond character

      (i)     to C–Cl bond in chlorobenzene.


                        Cl                       Cl

      (ii)    As it get oxidised in presence of sunlight

              O2  2CHCl3  2COCl2  2HCl
                           sunlight
                                    
                                             Poisonous gas



23.   Give the structures of products A, B and C in the followng reactions:
                        KCN                  LiAlH             HNO
      (i)     CH3CH2Br  A  B  C
                                4
                                      0C
                                          2
                                            
                               NH         NaOH Br                  CHCl  Alc. KOH
      (ii)    CH3COOH  A  B  C
                        
                          3      2
                                        3
                                             
                        KCN                              LiAlH                               HNO , 0C
Ans. (i)      CH3CH2Br  CH3CH2CN  CH3CH2CH2NH2  CH3CH2CH2OH
                                       4
                                                         2
                                                            
                               (A)             (B)               Pr op 1ol

                               NH                        NaOH/Br                      CHCl
      (ii)    CH3 COOH  CH3CONH2  CH3NH2  CH3 NC
                           3             2                3
                                                             
                                                   Alc. KOH
                               (A)            (B)              (C)


27.   Write the names and structures of the monomers of the following polymers:
      (i)     Bakelite                                       (ii)      Nylon-6
      (iii)   Polythene
                OH

Ans. (i)                 &         HCHO
                              Formaldehyde
              Phenol
                  H
                              O
                    N
                        C
              CH2            CH2
      (ii)
              CH2            CH2
                    CH2
              Caprolactam
      (iii)   CH2 = CH2             Ethene


                                                     ×·×·×·×·×




                                                              -(16)-

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English cbse 2011-12_solutions
 

3 chem cbse_2012-13_12th_11-03-13

  • 1. Studymate Solutions to CBSE Board Examination 2012-2013 Series : SKS/1 Code No. 56/1/1 Candidates must write the Code on Roll No. the title page of the answer-book.  Code number given on the right hand side of the question paper should be written on the title page of the answer-book by the candidate.  Please check that this question paper contains 30 questions.  Please write down the Serial Number of the questions before attempting it.  15 minutes time has been allotted to read this question paper. The question paper will be distributed at 10.15 a.m. From 10.15 a.m. to 10.30 a.m., the student will read the question paper only and will not write any answer on the answer script during this period. CHEMISTRY [Time allowed : 3 hours] [Maximum marks : 70] General Instructuions: (i) All questions are compulsory. (ii) Questions numbered 1 to 8 are very short-answer questions and carry 1 mark each. (iii) Questions numbered 9 to 18 are short-answer questions and carry 2 marks each. (iv) Questions numbered 19 to 27 are also short-answer questions and carry 3 marks each. (v) Questions numbered 28 to 30 are long-answer questions and carry 5 marks each. (vi) Use Log Tables, if necessary. Use of calculators is not allowed. -(1)-
  • 2. STUDYmate 1. How many atoms constitute one unit cell of a face-centered cubic crystal? 1 1 Ans. 8 × +6× =4 8 2 In one unit cell, 4 atoms are present. 2. Name the method used for the refining of Nickel metal. Ans. Mond’s Process 3. What is the covalency of nitrogen in N2O5? Ans. 4 (Four) O O N–O–N O O 4. Write the IUPAC name of CH3 – CH – CH2 – CH = CH2 Cl Ans. 4-Chloropent-1-ene 5. What happens when CH3 – Br is treated with KCN? Ans. Nucleophilic substitution occurs +– CH3Br + KC  N  CH3CN + KBr 6. Write the structure of 3-methyl butanal. CH3 O Ans. H3C – C – CH2 – C – H H 3-Methyl butanal 7. Arrange the folllwing in increasing order of their basic strength in aqueous solution: CH3NH2, (CH3)3N, (CH3)2NH Ans. (CH3)2 N H > CH3 N H2 > (CH3)3 N 8. What are the three types of RNA molecles which perform different functions? Ans. m – RNA t – RNA r – RNA 9. 18 g of glucose, C6H12O6 (Molar mass = 180 g mol–1) is dissolved in 1 kg of water in a sauce pan. At what temperature will this solution boil? (Kb for water = 0.52 K kg mol–1, boiling point of pure water = 373.15 K) Ans. Tb – Tb° = Kb × m 18 1 kg Tb – 373.15 = 0.52 ×  180 1 kg Tb = 0.052 + 373.15 Tb = 373.202 K -(2)-
  • 3. STUDYmate 10. The conductivity of 0.20 M solution of KCl at 298 K is 0.025 S cm–1. Calculate its molar conductivity. k  1000 Ans. m = ; m is molar conductivity. M 0.025  1000 = 0.20 = 125 S cm2 mol–1. 11. Write the dispersed phase and dispersion medium of the following colloidal systems: (a) smoke (b) milk OR What are lyophilic and lyophobic colloids? Which of these sols can be easily coagulated on the addition of small amounts of electrolytes? Ans. Dispersed phase Dispersion medium Smoke Solid Gas or Air Milk Liquid Liquid OR Ans. Lyophilic colloids are liquid loving. Certain substances mixed with a suitable liquid (dispersion medium) directly forms the colloid. Such colloids are lyophilic colloids. Example, gum, gelatin, starch, etc. Lyophobic colloids are liquid hating. When substances like metals or metal sulphides are mixed with the dispersion medium do not form the colloidal sol. Such type of sols can be prepared by special methods. Example: Metal sulphides. On addition of small amount of electrolyte lyophobic collides can be precipitated. 12. Write the differences between physisorption and chemisorption with respect to the following: (a) specificity (b) temperature dependence (c) reversibility and (d) enthalpy change Ans. Physisorption Chemisorption 1. Physisorption is non- specific in nature. 1. Chemisorption is highly specific in As physisorption involves Vander nature as chemisorptions includes Waal’s interaction between adsorbate chemical bond formation between and adsorbent. adsorbate and adsorbent. 2. On increasing temperature 2. Chemisorption increases with increase physisorption decreases. It is because in temperature. After reaching a desorption occurs rapidl y. maximum value, extent of adsorption Physisorption occurs usually at low decreases with temperature. temperature. 3. Physisorption is reversible in nature. 3. Chemisorption is irreversible in nature. 4. In case of physisorption enthalpy 4. In case of chemisorptions enthalpy change is small. change is high. For example: 20-40 kJ mol–1 (approx). For example: 40-400 kJ mol–1 (approx). -(3)-
  • 4. STUDYmate 13. (a) Which solution is used for the leaching of silver metal in the presence of air in the metallurgy of silver? (b) Out of C and CO, which is better reducing agent at the lower temperature range in the blast furnance to extract iron from the oxide ore? Ans. (a) Aqueous NaCN (b) CO is a better reducing agent at lower temperature. rG° would be more negative for reduction in blast furnace to extract Fe from the oxide ore by using CO. 14. What happens when (a) PCl5 is heated? (b) H3PO3 is heated?  Ans. (a)  PCl5  g   PCl3  g   Cl2  g  (b)  4H3PO3  3H3PO4 + PH3  15. (a) Which metal in the first transition series (3d series) exhibits +1 oxidation state most frequently and why? (b) Which of the following cations are coloured in aqueous solutions and why? Sc3+, V3+, Ti4+, Mn2+ (At. nos. Sc = 21, V = 23, Ti = 22, Mn = 25) Ans. (a) Copper shows +1 oxidation state extensively. Cu+ has 3d10 configuration. (b) V3+ = 3d2+ 4s0 Mn2+ = 3d5 4s0 V3+ as well as Mn2+ both have unpaired electrons in their d-orbitals which can show d- d transition. 16. Chlorobenzene is extremely less reactive towards a nucleophilic substitution reaction. Give two reasons for the same. Ans. Chlorobenzene is extremely less reative towards nucleophilic substitution reaction due to conjugation. (a) Partial double bond character between carbon of benzene and halogen causes cleavage of bond with difficulty. (b) Phenyl cation is quite unstable, So SN1 mechanism is not possible too. 17. Explain the mechanism of the following reaction:  H 2CH3  CH2  OH  CH3CH2  O  CH2  CH3  H2O  413K  Ans. Step 1: Formation of Protonated alcohol H CH3CH2 – OH + H CH3 – CH2 – O – H -(4)-
  • 5. STUDYmate Step 2: Formation of protonated ether H H H H3C – CH2 – O + C – OH2 CH3 – CH2 – O – C + H2O H CH3 H H CH3 Step 3: Formation of ether H3C – CH2 – O – CH2 – CH3 CH3 – CH2 – O – CH2 – CH3 + H H 18. How will you convert: (a) Propene to Propan-2-ol (b) Phenol to 2, 4, 6–trinitrophenol Ans. Conversion Propene  Propan-2-ol H O/H CH3  CH  CH2  CH3  CH CH3 2  | OH Phenol  2,4,6 trinitrophenol (Picric acid) OH OH NO2 NO2 conc. HNO3 NO2 (Picric acid) 19. (a) What type of semiconductor is obtained when silicon is doped with boron? (b) What type of magnetism is shown in the following alignment of magnetic moments?       (c) What type of point defect is produced when AgCl is doped with CdCl2? Ans. (a) Silicon is an element of group 14, Boron belongs to group 13. So, a p-type semiconductor is resulted. (b) Ferromagnetism (as all magnetic moments lie in same direction) (c) AgCl with CdCl2 causes impurity defect leading to cationic vacancies. 20. Determine the osmotic pressure of a solution prepared by dissolving 2.5 × 10–2 g of K2SO4 in 2L of water at 25°C, assuming that it is completely dissociated. (R = 0.0821 L atm K–1 mol–1, Molar mass of K2SO4 = 174 g mol–1) Ans. K2SO4  2K+ + SO42– i=2+1=3 Volume of solution (v) = 2L Mass of K2SO4 (WB) = 2.5 × 10–2 = 0.025 g Molar mass of K2SO4 (MB) = 174 g/mol T = 25°C = 298.15 K  = i CRT WBRT =i M V B -(5)-
  • 6. STUDYmate 3  0.025  0.0821  298.15 = 174  2 = 5.27 × 10–3 atm. 21. Calculate the emf of the following cell at 298 K: Fe(s) | Fe2+ (0.001 M) || H+ (1 M) | H2(g) (1 bar), Pt(s) (Given E°cell = +0.44 V) Ans. Cell Reaction Fe  2H  Fe2  H2 (n 2)  According to Nernst equation 0.0591 [H ]2 Ecell  Eo  cell log 2 [Fe2 ] 0.0591 = 0.44  (3) 2 = 0.44 + 0.0887 = 0.528 V 22. How would you account for the following? (a) Transition metals exhibit variable oxidation states. (b) Zr (Z = 40) and Hf (Z = 72) have almost identical radii. (c) Transition metals and their compounds act as catalyst. OR Complete the following chemical equations: (a) Cr2O7  + 6Fe2+ + 14H+  2 (b) 2CrO2  + 2H+  4 (c) 2MnO4 + 5C2O2  + 16H+   4 Ans. (a) Transition metals exhibit variable oxidation states due to the participation of ns and (n – 1) d electrons in bond formation. Both ns and (n – 1) d electrons have almost equal energies. (b) Zr (160 pm) and Hf (159 pm) have almost identical radii due to Lanthanoid contraction. Lanthanoid contraction is steady decrease in the size of lanthanoid ions (M3+) with the increase in atomic number. The reason for this is a gradual increase in the effective nuclear charge experienced by the outer electron. (c) Transition metal and their compounds act as catalysts, (i) because of their variable valencies. Transition metals sometimes form unstable intermediate compounds and provide a new path with lower activation energy for the reaction. (ii) in some cases they provide suitable surface for the reaction to take place. The reactants are adsorbed on the surface of the catalyst where reaction occurs. OR Ans. (a) Cr2O7 + 14H+ + 6Fe2+  2Cr3+ + 6Fe3+ + 2– 7H2O (b) 2CrO4 + 2H+  Cr2 O7 + H2O 2– 2– (c) 2MnO4 + 5C2O4 + 16H+  2Mn2+ + 8H2O – 2– + 10CO2 -(6)-
  • 7. STUDYmate 23. Write the IUPAC names of the following coordination compounds : (i) [Cr(NH3)3Cl3] (ii) K3[Fe(CN)6] (iii) [CoBr2(en)2]+, (en = enthylenediamine) Ans. (i) Triamminetrichloridochromium(III) (ii) Potassium hexacyanoferrate(III) (iii) Dibromidobis(ethane–1,2–diamine)cobalt(III) 24. Give the structures of A, B and C in the following reactions: H O/H NH C6H5 N2Cl   A  B  C  CuCN (i)  2  3   Sn  HCl NaNO  HCl H O/H (ii) C6H5 NO2  A  B  C  2  2  273K  O N2 Cl CN COOH C – NH2 Ans. (i) CuCN H2O/H NH 3  (A) (B) (C) NO2 NH2 N2 Cl OH Sn/HCl NaNO2 + HCl H 2O/H (ii) 273 K  Nitrobenzene Aniline Benzene diazonium Phenol chloride 25. Write the names and structures of the monomers of the following polymers: (i) Buna – S (ii) Neoprene (iii) Nylon – 6, 6 CH = CH2 Ans. (i) Buna-S CH2 = CH – CH = CH2 + 1,3–Butadiene Styrene (ii) Neoprene CH2 = C – CH = CH2 Cl 2–Chloro–1,3–butadiene COOH (iii) Nylon 6,6 H2N(CH2)6NH2 + (CH2)4 Hexamethylene diamine COOH Adipic acid 26. After watching a programme on TV about the adverse effects of junk food and soft drinks on the health of school children, Sonali, a student of Class XII, discussed the issue with the school principal. Principal immediately instructed the canteen contractor to replace the fast food with the fibre and vitamins rich food like sprouts, salad, fruits etc. This decision was welcomed by the parents and the students. After reading the above passage, answer the following questions: (a) What values are expressed by Sonali and the Principal of the school? (b) Give two examples of water-soluble vitamins. -(7)-
  • 8. STUDYmate Ans. (a) They are good educated citizens who believe in the saying “healthy mind in a healthy body.” This shows they value education and are caring towards the school inmates. (b) Vitamin B and Vitamin C 27. (a) Which one of the following is a food preservative? Equanil, Morphine, Sodium benzoate (b) Why is bithional added to soap? (c) Which class of drugs is used in sleeping pills? Ans. (a) Sodium Benzoate (b) To impart antiseptic properties (c) Narcotic Analgesics 28. (a) A reaction is second order in A and first order in B. (i) Write the differential rate equation. (ii) How is the rate affected on increasing the concentration of A three times? (iii) How is the rate affected when the concentrations of both A and B are doubled? (b) A first order reaction takes 40 minutes for 30% decoposition. Calculate t1/2 for this reaction. (Given log 1.428 = 0.1548) OR (a) For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction. (b) Rate constant ‘k’ of a reaction varies with temperature ‘T’ accordig to the equation: Ea 1 log k  log A    2.303 R  T  1 where Ea is the activation energy. When a graph is plotted for log k Vs. , a straight T line with a slope of –4250 K is obtained. Calculate ‘Ea’ for the reaction. (R = 8.314 JK–1 mol–1) dx Ans. (a) (i)  k[A]2 [B] dt (ii) The rate will become 9 times. (iii) The rate will become 8 times. (b) t = 40 min. Finding rate constant by the given data. 2.303 A k log 0 t At 2.303 1 k log 40 0.7 2.303  log1.428 40 2.303   0.1548 40 = 0.0089 min–1 -(8)-
  • 9. STUDYmate 0.693 Now, t1/2  k 0.693 t1/2  0.0089 = 77.865 min. OR 2.303 a Ans. (a) t 90%  log k a  0.9a 2.303  log10 k 2.303  k 2.303 a t 99.9%  log k a  0.999a 2.303 a  log k a  0.001a 2.303 t99.9%  3 k  t 99.9%  t90%  3 E a (b) The slope of the reaction = 2.303 R Ea Now, 2.303 R  4250 K Ea = 4250 × 2.303 × 8.314 = 81375.35 Joules = 81.375 kJ 29. (a) Give reasons for the following: (i) Bond enthalpy of F2 is lower than that of Cl2. (ii) PH3 has lower boiling point than NH3. (b) Draw the structures of the following molecules: (i) BrF3 (ii) (HPO3)3 (iii) XeF4 OR (a) Account for the following : (i) Helium is used in diving apparatus. (ii) Fluorine does not exhibit positive oxidation state. (iii) Oxygen shows catenation behaviour less than sulphur. (b) Draw the structures of the following molecules: (i) XeF2 (ii) H2S2O8 Ans. (a) (i) In F2, the bond between two F-atoms is formed by the head on overlapping of 2p orbitals, while in Cl2 it is formed by the head on overlapping of 3p orbitals. 2p being smaller in size than 3p has more interelectronic repulsions, which makes it weaker, thus its bond enthalpy is lower than that of Cl2. (ii) Due to presence of H-bonding in NH3 its boiling point becomes more than PH3. -(9)-
  • 10. STUDYmate (b) (i) BrF3 F Br F F T-shaped (ii) (HPO3)3 O O O P P HO OH O O P O OH Cyclic trimer (iii) XeF4 F F Xe F F Square Planar OR Ans. (a) (i) Deep sea divers depend upon compressed air for their oxygen supply. Thus, a deep sea diver has both N2 and O2 dissolve considerably in the blood and other body fluids. Oxygen is used up for metabolism but due to high partial pressure and greater solubility of N2, it will remain dissolved and will form bubbles when the diver comes to the atmospheric pressure. These bubbles affect nerve impulses and give rise to a disease called bends or decompression sickness. To avoid bends and also the toxic effects of high concentration of nitrogen in the blood, the cylinders used by the divers are filled with air diluted with helium. (ii) Fluorine does not exhibit positive oxidation state because fluorine is the most electronegative element. (iii) Because of stronger S–S bonds as compared to O–O bonds, sulphur has a stronger tendency for catenation than oxygen. F O O (b) (i) Xe (ii) H–O–S–O–O–S–O–H O O F -(10)-
  • 11. STUDYmate 30. (a) Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Give two reasons. (b) How will you bring about the following conversions? (i) Propanone to propane (ii) Benzoyl chloride to benzaldehyde (iii) Ethanal to but-2-enal OR (a) Complete the following reactions: Conc. KOH Br /P (i) 2H  C  H   (ii) CH3COOH  2  | | O CHO HNO3/H2SO4 (iii) 273-283 K (b) Give simple chemical tests to distinguish between the following pairs of compounds: (i) Ethanal and Propanal (ii) Benzoic acid and Phenol Ans. (a) The two reasons are : (i) In carboxylate anion, the resonating structures are exactly equal in energy, equivalent resonating forms. In phenol the energies of the resonating forms differ. O O O O etc. R–C–O R–C=O (ii) In the resonating structures of phenol, the negative charge is placed on O-atom in one of the resonating form and on C-atom in another resonating form. In carboxylic acid, the negative charge is only on O-atom. We know that the stability of resonating form is more if negative charge is on more electronegative atom. So the resonating forms of carboxylate anion are more stable than phenol. O || Zn  Hg/conc. HCl (b) (i) CH3  C  CH3  CH3  CH2  CH3  This is known as Clemmenson’s Reduction. O C—Cl Pd-BaSO4, S CHO (ii) + HCl boiling xylene This reaction is called Rosenmund reduction H | (iii) dil NaOH/100C CH3  CH  O  CH3  C  O  CH3  CH  CH  CHO  The above reaction is aldol condensation OR conc. Ans. (a) (i) 2H – C – H + HCOOK + CH3OH [Canizzaro reaction] KOH O O O P/Br 2 (ii) CH3 – C – O – H CH2 – C – OH [HVZ reaction] Br -Bromoacetic acid -(11)-
  • 12. STUDYmate CHO HNO3/H 2SO 4 CHO (iii) + H2O 273-283 K NO2 m-Nitrobenzaldehyde (b) (i) We can use iodoform test to distinguish between ethanal and propanal. H CH3 – C = O + 3I2 + NaOH  CHI3 + HCOONa + 3HI yellow ppt This test is not given by propanal. (ii) We can use sodium bicarbonate to distinguish between benzoic acid and phenol. Benzoic acid is a stronger acid than phenol, so it reacts with NaHCO3 (weak base) liberating CO2. C6H5–COOH + NaHCO3  C6H5COONa + H2O + CO2 (brisk effervescence) Phenol shows no reaction with NaHCO3. We can also used neutral FeCl3. It reacts with phenol to give violet coloured solution. ×·×·×·×·× -(12)-
  • 13. STUDYmate Studymate Solutions to CBSE Board Examination 2012-2013 Series : SKS/1 Code No. 56/1/2 UNCOMMON QUESTIONS ONLY 1. What type of stoichiometric defect is shown by AgCl? Ans. Frenkel Defect CH3 2. Write the IUPAC name of CH3CH = CH – C – CH3 Br Ans. 4-Bromo-4-methylpent-2-ene. 4. What type of bonding helps in stabilising the -helix structure of proteins? O Ans. Hydrogen bonding between – C – and –NH– groups of the peptide bond. 6. What inspired N. Bartlett for carrying out reaction between Xe and PtF6? Ans. Neil Bartlett prepared a red compound having the formula O+2PtF6–. He realised that the first ionization enthalpy of molecular oxygen (1175 kJ mol–1) is similar to Xe (1170 kJ mol–1). So, he was inspired to synthesise a similar compound using Xenon. 7. What happens when ethyl chloride is treated with aqueous KOH? Ans. CH3 – CH2 – Cl + aq. KOH  CH3 – CH2 – OH + KCl Nucleophillic substitution reaction takes place and ethanol is formed. 8. Write the structure of 4-chloropentan-2-one. H O H H Ans. H – C – C – C – C – CH3 H H Cl 11. What is the difference between oil/water (O/W) type and water/oil (W/O) type emulsions? Give an example of each type. Ans. In oil/water type emulsion, water is the dispersion medium. Example: milk and vanishing cream. In water/oil type emulsion, oil is the dispersion medium. Example: butter and cream. 17. (a) Which of the following ores can be concentrated by froth floatation method and why? Fe2O3, ZnS, Al2O3 (b) What is the role of silica in the metallurgy of copper? Ans. (a) ZnS because sulphide ores have wettabilty with pine oil which adsorb ore particles on them. (b) It acts as flux so that another impurity slag may be formed. -(13)-
  • 14. STUDYmate 18. (a) Why does p-dichlorobenzene have a higher m.p. than its o– and m–isomers? (b) Why is () – Butan-2-ol is optically inactive? Ans. (a) p-dichlorobenzene have a higher m.p. because its packing is more efficient due to symmetry. (b) Racemic mixture. 23. Write the names and structures of the monomers of the following polymers: (a) Polystyrene (b) Dacron (c) Teflon CH = CH2 Ans. (a) Styrene (b) Ethylene glycol + Terephthalic acid/Benene-1, 4-dicarboxylic acid HOH2C – CH2OH + HOOC COOH (c) Tetra fluoroethene CF2 = CF2 ×·×·×·×·× -(14)-
  • 15. STUDYmate Studymate Solutions to CBSE Board Examination 2012-2013 Series : SKS/1 Code No. 56/1/3 UNCOMMON QUESTIONS ONLY 1. What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic? Ans. Ferromagnetic substances 3. What is the composition of ‘Copper matte’? Ans. Copper matte chiefly consists of Cu2S and some uncharged FeS. 5. What is a glycosidic linkage? Ans. Glycosidic linkage formed when two mono-saccharides are held together with the loss of small molecule’s like H2O. It is an oxide linkage which joins two monosaccharides. 7. Which compound in the following pair undergoes faster SN1 reaction? Cl Cl and Cl Ans. 8. Write the structure of p-Methylbenzaldehyde molecule. CHO Ans. CH3 9. What is the difference between multi-molecular and macromolecular colloids? Give one example of each. Ans. When large number of atoms/molecules of a substance on dissolution aggregate together to form species having size in the colloidal range, then species thus formed is called multimolecular colloids. E.g. sulphur sol consisting of S8 particles. When macromolecules in a suitable solvents form solutions in which the size of the macromolecules is in colloidal range, then such systems are macromolecular colloids e.g. starch, cellulose. 17. Account for the following: (i) The C–Cl bond length in chlorobenzene is shorter than that in CH3 – Cl. (ii) Chloroform is stored in closed dark brown bottles. -(15)-
  • 16. STUDYmate Ans. As resonance imparts partial double bond character (i) to C–Cl bond in chlorobenzene. Cl Cl (ii) As it get oxidised in presence of sunlight O2  2CHCl3  2COCl2  2HCl sunlight  Poisonous gas 23. Give the structures of products A, B and C in the followng reactions: KCN LiAlH HNO (i) CH3CH2Br  A  B  C  4  0C 2  NH NaOH Br CHCl  Alc. KOH (ii) CH3COOH  A  B  C  3 2  3  KCN LiAlH HNO , 0C Ans. (i) CH3CH2Br  CH3CH2CN  CH3CH2CH2NH2  CH3CH2CH2OH  4  2  (A) (B) Pr op 1ol NH NaOH/Br CHCl (ii) CH3 COOH  CH3CONH2  CH3NH2  CH3 NC 3 2 3   Alc. KOH (A) (B) (C) 27. Write the names and structures of the monomers of the following polymers: (i) Bakelite (ii) Nylon-6 (iii) Polythene OH Ans. (i) & HCHO Formaldehyde Phenol H O N C CH2 CH2 (ii) CH2 CH2 CH2 Caprolactam (iii) CH2 = CH2 Ethene ×·×·×·×·× -(16)-