CBSE Class 12 Chemistry 2020 Question Paper

This question paper comprises four Sections - A, B, C and D. There are 37 questions in the question paper. All questions are compulsory. Section A from Questions no. 1 to 20 are very short answer type questions, carrying 1 mark each. Answer these questions in one word or one sentence.

Section B from Questions no. 21 to 27 are short answer type questions, carrying 2 marks each. Section C from Questions no. 28 to 34 are long answer type-I questions, carrying 3 marks each. Section D from Questions no. 35 to 37 are long answer type-II questions, carrying 5 marks each.

Section A

Read the given passage and answer the questions number 1 to 5 that follow :

The substitution reaction of alkyl halide mainly occurs by SN1 or SN2 mechanism. Whatever mechanism alkyl halides follow for the substitution reaction to occur, the polarity of the carbon halogen bond is responsible for these substitution reactions. The rate of SN1 reactions are governed by the stability of carbocation whereas for SN2 reactions steric factor is the deciding factor. If the starting material is a chiral compound, we may end up with an inverted product or racemic mixture depending upon the type of mechanism followed by alkyl halide. Cleavage of ethers with HI is also governed by steric factor and stability of carbocation, which indicates that in organic chemistry, these two major factors help us in deciding the kind of product formed.

1. Predict the stereochemistry of the product formed if an optically active alkyl halide undergoes substitution reaction by SN1 mechanism.

2. Name the instrument used for measuring the angle by which the plane polarised light is rotated.

3. Predict the major product formed when 2-Bromopentane reacts with alcoholic KOH.

4. Give one use of CHI3.

5. Write the structures of the products formed when anisole is treated with HI.

Questions number 6 to 10 are one word answers :

6. Identify which liquid will have a higher vapour pressure at 90°C if the boiling points of two liquids A and B are 140°C and 180°C, respectively.

7. Out of zinc and tin, whose coating is better to protect iron objects ?

8. Will the rate constant of the reaction depend upon T if the Eact (activation energy) of the reaction is zero ?

9. Give the structure of the monomer of PVC.

10. Which structural unit present in a detergent makes it non-biodegradable ?

Questions number 11 to 15 are multiple choice questions :

11. Out of the following, the strongest base in aqueous solution is

(A) Methylamine
(B) Dimethylamine
(C) Trimethylamine
(D) Aniline

12. Iodoform test is not given by

(A) Ethanol
(B) Ethanal
(C) Pentan-2-one
(D) Pentan-3-one

13. Out of the following transition elements, the maximum number of oxidation states are shown by

(A) Sc (Z = 21)
(B) Cr (Z = 24)
(C) Mn (Z = 25)
(D) Fe (Z = 26)

14. Hardening of leather in tanning industry is based on

(A) Electrophoresis
(B) Electro-osmosis
(C) Mutual coagulation
(D) Tyndall effect

15. What is the correct IUPAC name of the given compound ?

(A) 2,2-Dimethylbutanoic acid
(B) 2-Carboxyl-2-methylbutane
(C) 2-Ethyl-2-methylpropanoic acid
(D) 3-Methylbutane carboxylic acid

For questions number 16 to 20, two statements are given - one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (i), (ii), (iii) and (iv) as given below :

  1. Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion (A).
  2. Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion (A).
  3. Assertion (A) is correct, but Reason (R) is incorrect statement.
  4. Assertion (A) is incorrect, but Reason (R) is correct statement.

16. Assertion (A) : Au and Ag are extracted by leaching their ores with a dil. solution of NaCN.
Reason (R) : Impurities associated with these ores dissolve in NaCN.

17. Assertion (A) : F – F bond in F2 molecule is weak.
Reason (R) : F atom is small in size.

18. Assertion (A) : Linkage isomerism arises in coordination compounds because of ambidentate ligand.
Reason (R) : Ambidentate ligand like NO2 has two different donor atoms i.e., N and O.

19. Assertion (A) : Sucrose is a non-reducing sugar.
Reason (R) : Sucrose has glycosidic linkage.

20. Assertion (A) : The molecularity of the reaction H2 + Br2 → 2HBr appears to be 2.
Reason (R) : Two molecules of the reactants are involved in the given elementary reaction.

Section B

21. Define the following terms :

  1. Tranquilizers
  2. Antiseptic

OR

Explain the cleansing action of soaps.

22. For a 5% solution of urea (Molar mass = 60 g/mol), calculate the osmotic pressure at 300 K. [R = 0·0821 L atm K–1 mol–1]

OR

Visha took two aqueous solutions - one containing 7·5 g of urea (Molar mass = 60 g/mol) and the other containing 42·75 g of substance Z in 100 g of water, respectively. It was observed that both the solutions froze at the same temperature. Calculate the molar mass of Z.

23. Analyse the given graph, drawn between concentration of reactant vs. time.

  1. Predict the order of reaction.
  2. Theoretically, can the concentration of the reactant reduce to zero after infinite time ? Explain.

24. Draw the shape of the following molecules :

  1. XeOF4
  2. BrF3

25. Give the formulae of the following compounds :

  1. Potassium tetrahydroxidozincate (II)
  2. Hexaammineplatinum (IV) chloride

26. What happens when

  1. Propanone is treated with methylmagnesium iodide and then hydrolysed, and
  2. Benzene is treated with CH3COCl in presence of anhydrous AlCl3 ?

27. Write the names and structures of monomers in the following polymers :

  1. Bakelite
  2. Neoprene

Section C

28. Give the structures of A and B in the following sequence of reactions :

OR

(a) How will you distinguish between the following pairs of compounds :

  1. Aniline and Ethanamine
  2. Aniline and N-methylaniline

(b) Arrange the following compounds in decreasing order of their boiling points :

Butanol, Butanamine, Butane

29. Give the plausible explanation for the following :

  1. Glucose doesn’t give 2,4-DNP test.
  2. The two strands in DNA are not identical but are complementary.
  3. Starch and cellulose both contain glucose unit as monomer, yet they are structurally different.

30. Account for the following :

  1. Sulphurous acid is a reducing agent.
  2. Fluorine forms only one oxoacid.
  3. Boiling point of noble gases increases from He to Rn.

OR

Complete the following chemical reactions :

  1. MnO2 + 4 HCl →
  2. XeF6 + KF →
  3. I (aq) + H+ (aq) + O2 (g) →

31. Explain the role of the following :

  1. NaCN in the separation of ZnS and PbS.
  2. SiO2 in the metallurgy of Cu containing Fe as impurity.
  3. Iodine in the refining of Ti.

32. Give three points of difference between physisorption and chemisorption.

33. How will the rate of the reaction be affected when

  1. Surface area of the reactant is reduced,
  2. Catalyst is added in a reversible reaction, and
  3. Temperature of the reaction is increased ?

34. Calculate the mass of ascorbic acid (Molar mass = 176 g mol–1) to be dissolved in 75 g of acetic acid, to lower its freezing point by 1·5°C. (Kf = 3·9 K kg mol–1)

Section D

35. (a) Calculate ΔG° for the reaction

Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s).

Given : E° for Zn2+/Zn = – 0·76 V and E° for Cu2+/Cu = + 0·34 V

R = 8·314 JK–1 mol–1

F = 96500 C mol–1.

(b) Give two advantages of fuel cells.

OR

(a) Out of the following pairs, predict with reason which pair will allow greater conduction of electricity :

  1. Silver wire at 30°C or silver wire at 60°C.
  2. 0·1 M CH3COOH solution or 1 M CH3COOH solution.
  3. KCl solution at 20°C or KCl solution at 50°C.

(b) Give two points of differences between electrochemical and electrolytic cells.

36. (a) Account for the following :

  1. Copper (I) compounds are white whereas Copper (II) compounds are coloured.
  2. Chromates change their colour when kept in an acidic solution.
  3. Zn, Cd, Hg are considered as d-block elements but not as transition elements.

(b) Calculate the spin-only moment of Co2+ (Z = 27) by writing the electronic configuration of Co and Co2+.

OR

(a) Give three points of difference between lanthanoids and actinoids.

(b) Give reason and select one atom/ion which will exhibit asked property :

  1. Sc3+ or Cr3+ (Exhibit diamagnetic behaviour)
  2. Cr or Cu (High melting and boiling point)

37. (a) Out of t-butyl alcohol and n-butanol, which one will undergo acid catalyzed dehydration faster and why ?

(b) Carry out the following conversions :

  1. Phenol to Salicylaldehyde
  2. t-butylchloride to t-butyl ethyl ether
  3. Propene to Propanol

OR

(a) Give the mechanism for the formation of ethanol from ethene.

(b) Predict the reagent for carrying out the following conversions :

  1. Phenol to benzoquinone
  2. Anisole to p-bromoanisole
  3. Phenol to 2,4,6-tribromophenol