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# Board Paper of Class 12-Science 2020 Chemistry Delhi(Set 1) - Solutions

General Instructions:
(i) Question paper comprises four sections - A, B, C and D.
(ii) There are 37 questions in the questions paper. All questions are compulsory.
(iii) Section - A: Q. No. 1 to 20 are very short answer type questions carrying one mark each. Answer these questions in one word or one sentence.
(iv) Section - B: Q. No. 21 to 27 are short answer type questions carrying two marks each.
(v) Section - C: Q. No. 28 to 34 are long answer type-I questions carrying three marks each.
(vi) Section - D: Q. No. 35 to 37 are long answer type-II questions carrying five marks each.
(vii) There is NO overall choice in the question paper. However an internal choice has been provided in 2 questions of two marks, 2 questions of three marks and all the 3 questions of five marks. You have to attempt only one of the choices in such questions.
(viii) However, separate instructions are given with each section and question, wherever necessary.
(ix) Use of calculators and log tables is NOT permitted.
The halogens have the smallest atomic radii in their respective periods. The atomic radius of fluorine is extremely small. All halogens exhibit – 1 oxidation state. They are strong oxidising agents and have maximum negative electron gain enthalpy. Among halogens, fluroine shows anomalous behaviour in many properties. For example electro negativity and ionisation enthalpy are higher for fluorine than expected whereas bond dissociation enthalpy, m.p and b.p and electron gain enthalpy are quite lower than expected. Halogens react with hydrogen to give hydrogen halides (HX) and combine amongst themselves to form a number of compounds of the type XX’, XX'3, XX'5 and XX'7 called inter-halogens.

• Question 1
Why halogens have maximum negative electron gain enthalpy? VIEW SOLUTION
The halogens have the smallest atomic radii in their respective periods. The atomic radius of fluorine is extremely small. All halogens exhibit – 1 oxidation state. They are strong oxidising agents and have maximum negative electron gain enthalpy. Among halogens, fluroine shows anomalous behaviour in many properties. For example electro negativity and ionisation enthalpy are higher for fluorine than expected whereas bond dissociation enthalpy, m.p and b.p and electron gain enthalpy are quite lower than expected. Halogens react with hydrogen to give hydrogen halides (HX) and combine amongst themselves to form a number of compounds of the type XX’, XX'3, XX'5 and XX'7 called inter-halogens.

• Question 2
Why fluorine shows anomalous behaviour as compared to other halogens? VIEW SOLUTION
The halogens have the smallest atomic radii in their respective periods. The atomic radius of fluorine is extremely small. All halogens exhibit – 1 oxidation state. They are strong oxidising agents and have maximum negative electron gain enthalpy. Among halogens, fluroine shows anomalous behaviour in many properties. For example electro negativity and ionisation enthalpy are higher for fluorine than expected whereas bond dissociation enthalpy, m.p and b.p and electron gain enthalpy are quite lower than expected. Halogens react with hydrogen to give hydrogen halides (HX) and combine amongst themselves to form a number of compounds of the type XX’, XX'3, XX'5 and XX'7 called inter-halogens.

• Question 3
Arrange the hydrogen halides (HF to HI) in the decreasing order of their reducing character. VIEW SOLUTION
The halogens have the smallest atomic radii in their respective periods. The atomic radius of fluorine is extremely small. All halogens exhibit – 1 oxidation state. They are strong oxidising agents and have maximum negative electron gain enthalpy. Among halogens, fluroine shows anomalous behaviour in many properties. For example electro negativity and ionisation enthalpy are higher for fluorine than expected whereas bond dissociation enthalpy, m.p and b.p and electron gain enthalpy are quite lower than expected. Halogens react with hydrogen to give hydrogen halides (HX) and combine amongst themselves to form a number of compounds of the type XX’, XX'3, XX'5 and XX'7 called inter-halogens.

• Question 4
Why fluorine is a stronger oxidizing agent than chlorine? VIEW SOLUTION
The halogens have the smallest atomic radii in their respective periods. The atomic radius of fluorine is extremely small. All halogens exhibit – 1 oxidation state. They are strong oxidising agents and have maximum negative electron gain enthalpy. Among halogens, fluroine shows anomalous behaviour in many properties. For example electro negativity and ionisation enthalpy are higher for fluorine than expected whereas bond dissociation enthalpy, m.p and b.p and electron gain enthalpy are quite lower than expected. Halogens react with hydrogen to give hydrogen halides (HX) and combine amongst themselves to form a number of compounds of the type XX’, XX'3, XX'5 and XX'7 called inter-halogens.

• Question 5
What are the sizes of X and X' in the interhalogen compounds? VIEW SOLUTION

• Question 6
Name the cell used in hearing aids and watches. VIEW SOLUTION

• Question 7
How much charge in terms of Faraday is required to reduce one mol of ${\mathrm{MnO}}_{4}^{-}$ to Mn2+? VIEW SOLUTION

• Question 8
Write the slope value obtained in the plot of log [Ro]/[R]. Vs. time for a first order reaction. VIEW SOLUTION

• Question 9
Name the sweetening agent used in the cooking of sweets for a diabetic patient. VIEW SOLUTION

• Question 10
Name the polymer which is used for making electrical switches and combs. VIEW SOLUTION

• Question 11
In the Mond's process the gas used for the refining of a metal is
(a) H2
(b) CO2
(c) CO
(d) N2 VIEW SOLUTION

• Question 12
The conversion of an alkyl halide into an alcohol by aqueous NaOH is classified as
(a) a dehydrohalogenation reaction
(b) a substitution reaction
(d) a dehydration reaction VIEW SOLUTION

• Question 13
CH3CONH2 on reaction with NaOH and Br2 in alcoholic medium gives
(a) CH3CH2NH2
(b) CH3CH2Br
(c) CH3NH2
(d) CH3COONa VIEW SOLUTION

• Question 14
The oxidation state of Ni in [Ni(CO)4] is
(a) 0
(b) 2
(c) 3
(d) 4 VIEW SOLUTION

• Question 15
Amino acids are
(a) acidic
(b) basic
(c) amphoteric
(d) neutral VIEW SOLUTION

• Question 16
Assertion (A): Conductivity of an electrolyte increases with decrease in concentration.
Reason (R) : Number of ions per unit volume decreases on dilution.
(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. VIEW SOLUTION

• Question 17
Assertion (A): The C-O-C bond angle in ethers is slightly less than tetrahedral angle.
Reason (R) : Due to the repulsive interaction between the two alkyl groups in ethers.
(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. VIEW SOLUTION

• Question 18
Assertion (A): Low spin tetrahedral complexes are rarely observed.
Reason (R): Crystal field splitting energy is less than pairing energy for tetrahedral complexes.
(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. VIEW SOLUTION

• Question 19
Assertion (A) : Elevation in boiling point is a colligative property.
Reason (R) : Elevation in boiling point is directly proportional to molarity.
(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. VIEW SOLUTION

• Question 20
Assertion (A) : Oxidation of ketones is easier than aldehydes.
Reason (R) : C-C bond of ketones is stronger than C-H bond of aldehydes.
(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. VIEW SOLUTION

• Question 21
State Raoult's law for a solution containing volatile components. What is the similarity between Raoult's law and Henry's law? VIEW SOLUTION

• Question 22
Write the role of
(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. VIEW SOLUTION

• Question 23
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 particles? How is it responsible for the stability of Colloidal Sol? VIEW SOLUTION

• Question 24
(a) Write the IUPAC name and hybridization of the complex [Fe(CN)6]3–.
(Given: Atomic number of Fe = 26)
(b) What is the difference between an ambidentate ligand and a chelating ligand? VIEW SOLUTION

• Question 25
How do antiseptics differ from disinfectants? Name a substance which can be used as a disinfectant as well as an antiseptic. VIEW SOLUTION

• Question 26
Identify the monomers in the following polymers:

(i)

(ii)

VIEW SOLUTION

• Question 27
Draw the structures of the following:
(i) H2S2O8
(ii) XeF6 VIEW SOLUTION

• Question 28
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] VIEW SOLUTION

• Question 29
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] VIEW SOLUTION

• Question 30
Differentiate between following:
(i) Amylose and Amylopectin
(ii) Globular protein and Fibrous protein
(ii) Nucleotide and Nucleoside VIEW SOLUTION

• Question 31
Identify A, B, C, D, E and F in the following:
VIEW SOLUTION

• Question 32
Give the structures of final products expected from the following reactions:

(i) Hydroboration of propene followed by oxidation with H2O2 in alkaline medium.

(ii) Dehydration of (CH3)3 C – OH by heating it with 20% H3PO4 at 358 K.

OR

How can you convert the following?

(i) Phenol to o-hydroxy benzaldehyde.

(ii) Methanal to ethanol

(iii) Phenol to phenyl ethanoate.

VIEW SOLUTION

• Question 33
Give reasons:
(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. VIEW SOLUTION

• Question 34
Write three differences between lyophobic sol and lyophilic sol.

OR

Define the following terms:
(i) Protective colloid
(ii) Zeta potential
(iii) Emulsifying agent VIEW SOLUTION

• Question 35
(a) Give reasons:
(i) Transition metals and their compounds show catalytic activities.
(i) 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:
(i) Ti3+ is coloured whereas Sc3+ is colourless in aqueous solution.
(ii) Cr2+ is a strong reducing agent.
(b) Write two similarities between chemistry of lanthanoids and actinoids.
(c) Complete the following ionic equation:
VIEW SOLUTION

• Question 36
(a) Write the products formed when benzaldehyde reacts with the following reagents:

(i) CH3CHO in presence of dilute NaOH
(ii)
(iii) Conc. NaOH

(b) Distinguish between following:

(i)

(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 nuclephilic addition reaction:

(c) Draw the structure of 2, 4 DNP derivative of acetaldehyde. VIEW SOLUTION

• Question 37
(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.

OR

(a) A first order reaction is 50% complete in 30 minutes at 300 K and in 10 minutes at 320 K. 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] VIEW SOLUTION
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