CBSE Previous Year Question Papers Class 12 Chemistry with Solutions

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Class 12 Subject Chemistry Paper Set 3 with Solutions

SECTION A

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

The d-block of the periodic table contains the elements of the groups 3 – 12 and are known as transition elements. In general, the electronic configuration of these elements is (n – 1) d^1–10 ns^1–2. The d-orbitals of the penultimate energy level in their atoms receive electrons giving rise to the three rows of the transition metals i.e., 3d, 4d and 5d series. However, Zn, Cd and Hg are not regarded as transition elements. Transition elements exhibit certain characteristic properties like variable oxidation states, complex formation, formation of coloured ions and alloys, catalytic activity, etc. Transition metals are hard (except Zn, Cd and Hg) and have a high melting point.

Question 1: (Marks 1)

Why are Zn, Cd and Hg non-transition elements ?

Answer :

Zn , Cd and Hg have completely filled d¹⁰ configuration in their ground state as well as in their oxidized state.

Question 2: (Marks 1)

Which transition metal of 3d series does not show variable oxidation states ?

Answer :

Scandium / Sc

Question 3: (Marks 1)

Why do transition metals and their compounds show catalytic activity ?

Answer :

Because of multiple oxidation states / ability to form complexes / having large surface area

Question 4: (Marks 1)

Why are melting points of transition metals high ?

Answer :

Involvement of (n-1)d and ns electrons in inter atomic metallic bonding / strong metal-metal bonding.

Question 5: (Marks 1)

Why is Cu²⁺ ion coloured while Zn²⁺ ion is colourless in aqueous solution ?

Answer :

Presence of unpaired eshowing d-d transition in Cu²⁺ , while in Zn²⁺ there is no unpaired electron.

Questions number 6 to 10 are one word answers :

Question 6: (Marks 1)

Name the cell which was used in the Apollo Space Programme.

Answer :

H₂ - O₂ Fuel cell

Question 7: (Marks 1)

How many coulombs are required for the oxidation of 1 mol of H₂O to O₂?

Answer :

2 x 96500 C / 193000 C

Question 8: (Marks 1)

Write the slope value obtained in the plot of ln[R] vs. time for a first order reaction.

Answer :

Slope = -k

Question 9: (Marks 1)

Name the disaccharide which on hydrolysis gives two molecules of glucose.

Answer :

Maltose

Question 10: (Marks 1)

Name the class of the synthetic detergent which is used in toothpaste.

Answer :

Anionic detergent

Questions number 11 to 15 are multiple choice questions :

Question 11: (Marks 1)

Which of the following is refined by the zone refining process ?

(A) Cu

(B) Zn

(D) Sn

Answer :

Question 12: (Marks 1)

Racemisation occurs in

(A) S_N2 reaction

(B) S_N1 reaction

(D) S_N2 reaction as well as S_N1 reaction

Answer :

Question 13: (Marks 1)

on heating with CHCl3 and alcoholic KOH gives foul

Answer :

Question 14: (Marks 1)

One mole of CrCl₃. 6H₂O compound reacts with excess AgNO₃ solution to yield two moles of AgCl (s). The structural formula of the compound is

(A) [Cr(H₂O)₅ Cl] Cl₂. H₂O

(B) [Cr(H₂O)₃ Cl₃] . 3H₂O

(D) [Cr(H₂O)₆] Cl₃

Answer :

(A) [Cr(H₂O)₅ Cl] Cl₂. H₂O

Question 15: (Marks 1)

Peptide linkage is present in

(A) Carbohydrates

(B) Vitamins

(D) Rubber

Answer :

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) : Conductivity of an electrolyte decreases with decrease in concentration.

Reason (R) : Number of ions per unit volume increases on dilution.

Answer :

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

Question 17: (Marks 1)

Assertion (A) : The C – O – H bond angle in alcohols is slightly less than the tetrahedral angle.

Reason (R) : This is due to the repulsive interaction between the two lone electron pairs on oxygen.

Answer :

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

Question 18: (Marks 1)

Assertion (A) : [Pt(en)₂ Cl₂]²⁺ complex is less stable than [Pt(NH₃)₄Cl₂]²⁺ complex.

Reason (R) : [Pt(en)₂Cl₂]²⁺ complex shows chelate effect.

Answer :

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

Question 19: (Marks 1)

Assertion (A) : Osmotic pressure is a colligative property.

Reason (R) : Osmotic pressure is directly proportional to molarity.

Answer :

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

Question 20: (Marks 1)

Assertion (A) : Reactivity of ketones is more than aldehydes.

Reason (R) : The carbonyl carbon of ketones is less electrophilic as compared to aldehydes.

Answer :

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

SECTION B

Question 21: (Marks 2)

In the given reaction

A + 3B → 2C,

the rate of formation of C is 2·5 x 10^–4 mol L^–1 s^–1.

Calculate the

(i) rate of reaction, and

(ii) rate of disappearance of B.

Answer :

In the given reaction

Question 22: (Marks 2)

Write the role of the following :

(i) NaAlF₄ in the extraction of Aluminium

(ii) CO in the refining of Ni

Write the chemical equations involved in the leaching of bauxite ore to prepare pure alumina.

Answer :

i) Acts as solvent / lowers the melting point of the mixture / Increases the conductivity.

ii) It forms a volatile compound Ni(CO)₄, which decomposes at higher temperature to give pure Nickel.

Write the role of the following

Question 23: (Marks 2)

Write two differences between physisorption and chemisorption.

Define the following terms with a suitable example of each :

(i) Associated colloids

(ii) O/W emulsion

Answer :

Write two differences between physisorption and chemisorption.

Question 24: (Marks 2)

(a) Write the IUPAC name and hybridisation of the complex [CoF₆]^3–. (Given : Atomic number of Co = 27)

(b) What type of isomerism is shown by the complex [Co(en)₂Cl₂]²⁺? Name the structure of an isomer of this complex which is optically active.

Answer :

a) Hexafluoridocobaltate(III)

sp³d²

b) Isomerism – Geometrical / optical cis isomer is optically active

Question 25: (Marks 2)

Give reasons :

(i) Shaving soaps contain glycerol.

(ii) Antacids should not be used for longer time.

Answer :

(i) To prevent rapid drying.

(ii) Antacids will make the stomach alkaline and trigger the production of more acid.

Question 26: (Marks 2)

Define the following terms :

(i) Oligosaccharides

(ii) Invert sugar

Answer :

i) Carbohydrates that yield two to ten monosaccharides units on hydrolysis.

ii) Hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (-) and the product is named as invert sugar.

Question 27: (Marks 2)

Write the products of the following reaction :

6 XeF₄ + 12 H₂O →

Is this reaction a disproportionation reaction ? Give reasons in support of your answer.

Answer :

6XeF₄ + 12 H₂O → 4Xe + 2XeO₃ + 24HF + 3O₂

Yes

Xe⁺⁴ changes to Xe o and Xe +6 / Xe gets oxidized as well as reduced in the same reaction.

SECTION C

Question 28: (Marks 3)

Calculate the maximum work and log Kc

Answer :

Calculate the maximum work and log Kc

Question 29: (Marks 3)

A first order reaction is 40% complete in 80 minutes. Calculate the value of rate constant (k). In what time will the reaction be 90% completed ?

[Given : log 2 = 0·3010, log 3 = 0·4771, log 4 = 0·6021, log 5 = 0·6771, log 6 = 0·7782]

Answer :

A first order reaction is 40% complete in 80 minutes. Calculate the value

Question 30: (Marks 3)

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

(i) Buna-S

(ii) Nylon-6,6

(iii) Bakelite

Answer :

Write the names and structures of the monomers in the following

Question 31: (Marks 3)

Write the major product(s) of the following reactions

Answer :

Write the major product(s) of the following reactions

Question 32: (Marks 3)

Write the mechanism of the following reaction

Answer :

Write the mechanism of the following reaction

Question 33: (Marks 3)

Write the products formed when (CH₃)₃ C – CHO reacts with the following reagents :

(i) CH₃COCH₃ in the presence of dilute NaOH

(ii) HCN

(iii) Conc. NaOH

Answer :

i) (CH₃)₃CCH(OH)CH₂COCH₃

ii) (CH₃)₃CCH(OH)CN

iii) (CH₃)₃CCOONa + (CH₃)₃CCH₂OH

Question 34: (Marks 3)

Define Lyophobic and Lyophilic sol with a suitable example of each. Why is coagulation of Lyophilic sol difficult as compared to Lyophobic sol ?

Define the following terms :

(i) Shape-selective catalysis

(ii) Kraft temperature

(iii) Peptization

Answer :

Lyophobic sol is liquid or water repelling.

Example: Fe(OH)₃ sol / gold sol (or any other suitable example)

Lyophilic sol is liquid or water attracting. Example: starch sol / rubber sol (or any other suitable example)

Due to the charge and solvation of the colloidal particles.

i) The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules.

ii) The formation of micelles takes place only above a particular temperature is called Kraft temperature (Tk)

iii) Process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte.

SECTION D

Question 35: (Marks 5)

(a) Give reasons :

(i) Helium does not form compounds like Xenon.

(ii) HClO₄ is a stronger acid than HOCl.

(iii) Sulphur is a polyatomic solid whereas Oxygen is a diatomic gas.

(b) Write one reaction as an example of each, to show that conc. H₂SO₄ acts as

(i) an oxidising agent, and

(ii) a dehydrating agent.

(a) Account for the following :

(i) Hydration enthalpy of F^– ion is more than Cl^– ion.

(ii) SO₂ is a reducing agent, whereas TeO₂ is an oxidising agent in group-16 oxides.

(b) Write the reaction of F₂ with water. Why does I₂ not react with water ?

Answer :

Give reasons

Question 36: (Marks 5)

Give reasons

Answer :

Give reasons

Question 37: (Marks 5)

(a) A solution contains 5·85 g NaCl (Molar mass = 58·5 g mol^–1) per litre of solution. It has an osmotic pressure of 4·75 atm at 27°C. Calculate the degree of dissociation of NaCl in this solution.

(Given : R = 0·082 L atm K^–1 mol^–1)

(b) State Henry’s law. Why is air diluted with helium in the tanks used by scuba divers ?

(a) When 19·5 g of F – CH₂ – COOH (Molar mass = 78 g mol^–1) is dissolved in 500 g of water, the depression in freezing point is observed to be 1°C. Calculate the degree of dissociation of F – CH₂ – COOH.

[Given : Kf for water = 1·86 K kg mol^–1]

(b) Give reasons :

(i) 0·1 M KCl has higher boiling point than 0·1 M Glucose.

(ii) Meat is preserved for a longer time by salting.

Answer :

A solution contains 5·85 g NaCl (Molar mass = 58·5 g mol–1