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# Board Paper of Class 12-Science 2020 Chemistry Delhi(Set 2) - 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
Out of  , which will undergo SN1 reaction faster with OH? VIEW SOLUTION

• Question 8
What type of linkage is present in polysaccharides? VIEW SOLUTION

• Question 9
Name an artificial sweetener whose use is limited to cold drinks. VIEW SOLUTION

• Question 10
Name the polymer which is used for making non-stick utensils. VIEW SOLUTION

• Question 11
Kohlrausch given the following relation for strong electrolytes:
$\wedge ={\wedge }_{0}-\mathrm{A}\sqrt{\mathrm{C}}$
Which of the following equality holds?
(a)
(b)
(c)
(d) VIEW SOLUTION

• Question 12
In an electrochemical process, a salt bridge is used
(a) as a reducing agent.
(b) as an oxidizing agent.
(c) to complete the circuit so that current can flow.
(d) None of these VIEW SOLUTION

• Question 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
(a) 1
(b) 0
(c) 2
(d) 1/2 VIEW SOLUTION

• Question 14
Which of the following will give a white precipitate upon reacting with AgNO3?
(a) K2[Pt(en)2Cl2]
(b) [CO(NH3)3Cl3]
(c) [Cr(H2O)6]Cl3
(d) [Fe(H2O)3 Cl3] VIEW SOLUTION

• Question 15
Copper matte contains
(a) Cu2S, Cu2O and silica
(b) Cu2S, CuO and silica
(c) Cu2S, FeO and silica
(d) Cu2S, FeS and silica VIEW SOLUTION

• Question 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, KCI dissociates to produce more number of particles.
(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): 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 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.
(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) : 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 20
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 21
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 22
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 23
Draw the structures of the following:
(i) H2S2O7
(ii) BrF5 VIEW SOLUTION

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

• Question 25
Identify the monomers in the following polymers:
VIEW SOLUTION

• Question 26
Discuss the nature of bonding in metal carbonyls. VIEW SOLUTION

• Question 27
How do antiseptics differ from disinfectants? Name a substance which can be used as a disinfectant as well as an antiseptic. VIEW SOLUTION

• Question 28
Identify A, B, C, D, E and F in the following:
VIEW SOLUTION

• Question 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] VIEW SOLUTION

• Question 30
Define the following terms with a suitable example in each:
(a) Polysaccharides
(b) Denatured protein
(c) Fibrous protein VIEW SOLUTION

• Question 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] VIEW SOLUTION

• Question 32
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 33
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 34
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 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.

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

• Question 36
(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 37
(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
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