CBSE Class 12 Board Exam Chemistry Paper 2018 2019 2020 with Solutions

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Class 12 Subject Chemistry Paper Set 1 with Solutions

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 S_N1 or S_N2 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 S_N1 reactions are governed by the stability of carbocation whereas for S_N2 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.

Question 1: (Marks 1)

Predict the stereochemistry of the product formed if an optically active alkyl halide undergoes substitution reaction by S_N1 mechanism.

Answer :

Racemic Mixture

Question 2: (Marks 1)

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

Answer :

Polarimeter

Question 3: (Marks 1)

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

Answer :

Pent-2-ene / CH₃CH=CHCH₂CH₃

Question 4: (Marks 1)

Give one use of CHI₃ .

Answer :

Antiseptic

Question 5: (Marks 1)

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

Answer :

CH₃I + C₆H₅OH

Questions number 6 to 10 are one word answers :

Question 6: (Marks 1)

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.

Answer :

Question 7: (Marks 1)

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

Answer :

Question 8: (Marks 1)

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

Answer :

Question 9: (Marks 1)

Give the structure of the monomer of PVC.

Answer :

CH₂=CH-Cl

Question 10: (Marks 1)

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

Answer :

Branched hydrocarbon part

Questions number 11 to 15 are multiple choice questions :

Question 11: (Marks 1)

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

(A) Methylamine

(B) Dimethylamine

(D) Aniline

Answer :

(B) Dimethylamine

Question 12: (Marks 1)

Iodoform test is not given by

(A) Ethanol

(B) Ethanal

(D) Pentan-3-one

Answer :

(D) Pentan-3-one

Question 13: (Marks 1)

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

(A) Sc (Z = 21)

(B) Cr (Z = 24)

(D) Fe (Z = 26)

Answer :

Question 14: (Marks 1)

Hardening of leather in tanning industry is based on

(A) Electrophoresis

(B) Electro-osmosis

(D) Tyndall effect

Answer :

Question 14: (Marks 1)

What is the correct IUPAC name of the given compound

Answer :

(A) 2,2-Dimethylbutanoic 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

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

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

(iii) Assertion (A) is correct, but Reason (R) is incorrect statement.

(iv) Assertion (A) is incorrect, but Reason (R) is correct statement.

Question 16: (Marks 1)

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.

Answer :

(iii) Assertion (A) is correct, but Reason (R) is incorrect statement.

Question 17: (Marks 1)

Assertion (A) : F – F bond in F2 molecule is weak.

Reason (R) : F atom is small in size.

Answer :

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

Question 18: (Marks 1)

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.

Answer :

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

Question 19: (Marks 1)

Assertion (A) : Sucrose is a non-reducing sugar.

Reason (R) : Sucrose has glycosidic linkage.

Answer :

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

Question 20: (Marks 1)

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.

Answer :

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

SECTION B

Question 21: (Marks 2)

Define the following terms :

(a) Tranquilizers

(b) Antiseptic

Explain the cleansing action of soaps.

Answer :

(a) The drugs which are used to control stress / anxiety / tension / mild or severe mental diseases

(b) The drugs which are used to kill or to prevent the growth of micro-organism, applied externally on living tissues.

Soap molecules form micelle around the oil droplet or dirt in such a way that hydrophobic part interacts with the oil droplet and hydrophilic part projects out. Micelles can be washed away on rinsing with water. Thus soap helps in emulsification and washing away of oil and fats.

Question 22: (Marks 2)

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]

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.

Answer :

For a 5% solution of urea (Molar mass = 60 g/mol), calculate the osmotic

Question 23: (Marks 2)

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

Answer :

(a) 1^st order

(b) No, due to exponential relation / the curve never touches the x-axis.

Question 24: (Marks 2)

Draw the shape of the following molecules :

(a) XeOF₄

(b) BrF₃

Answer :

Draw the shape of the following molecules

Question 25: (Marks 2)

Give the formulae of the following compounds :

(a) Potassium tetrahydroxidozincate (II)

(b) Hexaammineplatinum (IV) chloride

Answer :

(a) K2[Zn(OH)₄]

(b) [Pt(NH₃)₆]Cl₄

Question 26: (Marks 2)

What happens when

(a) Propanone is treated with methylmagnesium iodide and then hydrolysed, and

(b) Benzene is treated with CH₃COCl in presence of anhydrous AlCl₃?

Answer :

a) (CH₃)₃C-OH / tertiary butyl alcohol is formed

b) C₆H₅COCH₃ / acetophenone is formed (or correct chemical equation)

Question 27: (Marks 2)

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

(a) Bakelite

(b) Neoprene

Answer :

a) C₆H₅OH + HCHO , Phenol + formaldehyde

b) CH₂= C(Cl) – CH=CH₂, Chloroprene

SECTION C

Question 28: (Marks 3)

Give the structures of A and B in the following sequence of reactions

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

(i) Aniline and Ethanamine

(ii) Aniline and N-methylaniline

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

Butanol, Butanamine, Butane

Answer :

(a) (A) → CH₃CONH₂ (B) → CH₃NH₂

(b) (A) → C₆H₅NH₂ (B) → C₆H₅N₂Cl

a) (i) Add Ice cold (NaNO₂ + HCl) followed by phenol or β-Naphthol to both the compounds.

Aniline forms orange red dye while ethylamine doesn’t.

ii) Add CHCl₃ and KOH (alc.) to both the compounds.

Aniline gives foul smelling isocyanide while N-Methylaniline doesn’t.

(or any other suitable chemical test)

b) Butanol > Butanmine > Butane

Question 29: (Marks 3)

Give the plausible explanation for the following :

(a) Glucose doesn’t give 2,4-DNP test.

(b) The two strands in DNA are not identical but are complementary.

Answer :

(a) Because the – CHO group in glucose is involved in hemiacetal formation and thus is not free / due to cyclic structure of glucose -CHO group is not free.

(b) Because the hydrogen bonds are formed between specific pairs of bases.

Question 30: (Marks 3)

Account for the following :

(a) Sulphurous acid is a reducing agent.

(b) Fluorine forms only one oxoacid.

Complete the following chemical reactions :

(a) MnO₂ + 4 HCl →

(b) XeF₆ + KF →

Answer :

(a) Because sulphur readily gets oxidized itself to more stable +6 state.

(b) Because of absence of d-orbital in Fluorine.

(a) MnO₂+ 4HCl → MnCl₂ + Cl₂ + 2H₂O

(b) XeF₆ + KF → K + [XeF₇]^-

Question 31: (Marks 3)

Explain the role of the following :

(a) NaCN in the separation of ZnS and PbS.

(b) SiO₂ in the metallurgy of Cu containing Fe as impurity.

Answer :

(a) NaCN act as a depressant.

(b) SiO₂ act as a flux. / used to remove FeO as slag

Question 32: (Marks 3)

Give three points of difference between physisorption and chemisorption.

Answer :

Give three points of difference between physisorption and chemisorption.

Question 33: (Marks 3)

How will the rate of the reaction be affected when

(a) Surface area of the reactant is reduced,

(b) Catalyst is added in a reversible reaction, and

Answer :

(a) Decreases.

(b) Increases

Question 34: (Marks 3)

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)

Answer :

Calculate the mass of ascorbic acid (Molar mass