CBSE Class 12 Chemistry Previous Year Question Paper 2020

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SECTION A


  1. Why halogens have maximum negative electron gain enthalpy?                   [1]

  2. Why fluorine shows anomalous behaviour as compared to other halogen?    [1]

  3.  Arrange the hydrogen halides (HF to HI) in the decreasing order of their reducing character      [1]

  4. Why fluorine is a stronger oxidizing agent than chlorine?                             [1]

  5. What are the sizes of X and X’ in the interhalogen compounds?                   [1]

  6.  Out of   , which will undergo SN1 reaction faster with OH- [1]

  7. Write the IUPAC name of ?                                                   [1]

  8. What type of linkage is present in polysaccharide?                                       [1]

  9. Name an artificial sweetener whose use is limited to cold drinks.                 [1]

  10. Name the polymer which is used for making non-stick utensils.                 [1]

  11. Kohlrausch given the following relation for strong electrolytes:                  [1]


    Which of the following equality holds?


  12. In an electrochemical process, a salt bridge is used:                                 [1]

    (a)   as a reducing agent.

    (b)   as an oxidizing agent.

    (c)   to complete the circuit to that current can flow.

    (d)  None of these

  13. In a chemical reaction X → Y. It is found that the rate of reaction doubles when the concentration of X is increased four times. The order of the reaction with respect to X is

    1. 1
    2. 0
    3. 2
    4. 1/2                                                                                                   [1]
  14. Which of the following will give a white precipitate upon reacting with AgNO3?         [1]

    (a)   K2[Pt(en)2Cl2]

    (b)   [CO(NH3)3Cl3]

    (c)   [Cr(H2O)6]Cl3

    (d)  [Fe(H2O)3Cl3]

  15. Copper matte contains                                                                              [1]

    (a)   Cu2S, Cu2O and silica

    (b)   Cu2S, CuO and silica

    (c)   Cu2S, FeO and silica

    (d)  Cu2S, FeS and silica

  16. Assertion (A): 0.1 M solution of KCl has greater osmotic pressure than 0.1 M solution of glucose at same temperature.
    Reason(R): In solution, KCl dissociates to produce more number of particles.

    A)   Both Assertion (A) and Reason (R) are correct statement, and Reason (R) is the correct explanation of the Assertion (A).

    B)   Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not correct explanation of the Assertion (A).

    C)  Assertion (A) is correct, but Reason (R) is wrong statement.

    D)  Assertion (A) is wrong, but Reason (R) is correct statement.                 [1]

  17. Assertion (A): Conductivity of an electrolyte increases with decrease in concentration.
    Reason (R): Number of ions per unit volume decreases on dilution.         [1]

    A)   Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion (A).

    B)   Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion (A).

    C)  Assertion (A) is correct, but Reason (R) is wrong statement.

    D)  Assertion (A) is wrong, but Reason (R) is correct statement.  

  18. Assertion (A): Ortho and para-nitrophenols can be separated by steam distillation.
    Reason (R): Ortho isomer associates through intermolecular hydrogen bonding while Para isomer associates through intramolecular hydrogen bonding.              [1]
    A) Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion
    B) Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion C) Assertion (A) is correct, but Reason (R) is wrong statement.
    D) Assertion (A) is wrong, but Reason (R) is correct statement.

  19. Assertion (A): Oxidation of ketones is easier than aldehydes.
    Reason (R): C-C bond of ketones is stronger than C-H bond of aldehydes. [1]

    (a) Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion (A).

    (b) Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion (A).

    (c) Assertion (A) is correct, but Reason (R) is wrong statement.

    (d)  Assertion (A) is wrong, but Reason (R) is correct statement.

  20. Assertion (A): Low spin tetrahedral complexes are rarely observed.
    Reason (R): Crystal field splitting energy is less than pairing energy for tetrahedral complexes.                                                                                                [1]

    (a) Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion (A).

    (b)  Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion (A).

    (c) Assertion (A) is correct, but Reason (R) is wrong statement.

    (d) Assertion (A) is wrong, but Reason (R) is correct statement.

  21. Write the role of                                                                                        [2]

    (a)   Dilute NACN in the extraction of Gold.

    (b)   CO in the extraction of Iron.

    OR


    How is leaching carried out in the case of low grade copper ores? Name the method used for refining of copper metal.

  22. State Raoult’s law for a solution containing volatile component. What is the similarity between Raoult’s law and Henry’s law?                                                      [2]

  23. Draw the structures of the following:                                                         [2]
      i. H2S2O7

      ii.BrF5

  24.  Define adsorption with an example. What is the role of adsorption in heterogeneous catalysis?    

    OR

    Define Brownian movement. What is the cause of Brownian movement in colloidal? How is it responsible for the stability of Colloidal Sol?              [2]

  25. Identify the monomers in the following polymers.                                      [2]

  26. Discuss the nature of bonding in metal carbonyls.                                     [2]

  27. How do antiseptic differ from disinfectants? Name a substance which can be used as a disinfectant as well as antiseptic.                                                               [2]

  28. Identify A, B, C, D, E and F in the following:                                            [3]

  29. A 0.01 m aqueous solution of AlCl3 freezes at – 0.068°C. Calculate the percentage of  dissociation.[Given Kf for Water=1.86 K Kg mol-1]                                    [3]

  30. Define the following terms with a suitable example in each:                       [3]

    1. Polysaccharides
    2. Denatured protein
    3. Fibrous protein
  31. When a steady current of 2A was passed through two electrolytic cells A and B containing electrolytes ZnSO4 and CuSO4 connected in series, 2g of Cu were deposited at the cathode of cell B. How long did the current flow?
    What mass of Zn was deposited at cathode of cell A?
    [Atomic mass: Cu = 63.5 g mol-1, Zn = 65 g mol-1, 1F = 96500 C mol-1]   [3]

  32. Write three differences between lyophobic sol and lyophilic sol.                 [3]

    OR
    Define the following terms:
    i.   Protective colloid

    ii. Zeta potential

    iii.  Emulsifying agent

  33. Give the structure of final products expected from the following reactions: [3]
    i.   Hydroboration of propane followed by oxidation with H2O2 in alkaline medium.
    ii.   Dehydration of (CH3)3C- OH by heating it with 20% H3PO4 at 358 K.

     OR

    How can you convert the following?

      i.        Phenol to o-hydroxy benzaldehyde.

     ii.        Methanol to ethanol

    iii.        Phenol to phenyl ethanoate.

  34.  Give reason:                                                                                             [3]
      i.  Aniline does not undergo Friedal-Crafts reaction.

      ii.  Aromatic primary amines cannot be prepared by Gabriel’s phthalimide synthesis.

      iii.  Aliphatic amines are stronger bases than ammonia.


  35. a) A first order reaction is 25% complete in 40 minutes. Calculate the value of rate constant. In what time will the reaction be 80% completed?
    b) Define order of reaction. Write the condition under which a bimolecular reaction follows first order kinetics.                                                                    [5]

    OR

    a) A first reaction is 50% complete in 10 minutes at 300 K and in 10 minutes at 320K. Calculate activation energy (Ea) for the reaction. (R = 8.314 J K-1 mol-1)
    b) Write the two conditions for collisions to be effective collisions.
    C) How order of reaction and molecularity differ towards a complex reaction?
    [Given: log 2 =0.3010, log 3 = 0.4771, log 4 =0.6021, log 5 = 0.6991]

  36.  
    a) Give reason.                                                                                          [5]

    i. Transition metals and their compounds show catalytic activities.

    ii. Separation of a mixture of Lanthanoid elements is difficult.
    iii. Zn, Cd and Hg are soft and have low melting point.

    b) Write the preparation of the following:

     i. Na2Cr2O7 from Na2CrO4

    ii. K2MnO4 from MnO2
    OR


    a) Account for the following:

       Ti3+ is coloured whereas Sc3+ is colourless solution.

        Cr2+ is a strong reducing agent.
    b) Write two similarities between chemistry of lanthanoids and actinoids.
    c)Complete the following ionic equation:  
    MNO subscript 4 superscript 2 minus end superscript plus 4 straight H to the power of plus rightwards arrow with blank on top


  37. a) Write the products formed when benzaldehyde reacts with the following reagents:                                                                                                               [5]

                   i.        CH3CHO in presence of dilute NaOH

                  ii.       

                 iii.        Conc. NaOH

b)Distinguish between following:

                 i.       CH subscript 3 minus CH equals CH minus CO minus CH subscript 3 space and space CH subscript 3 space minus CH subscript 2 minus CO minus CH equals CH subscript 2

                ii.        Benzaldehyde and Benzoic acid

OR

a) Write the final products in the following:

b)Arrange the following in the increasing order of their reactivity towards addition reaction:

C)Draw the structure of 2, 4 DNP derivate of acetaldehyde.