Subject: Chemistry
Class XII
Time Allowed: 3 hours
Max. Marks: 70
General Instructions:
Read the following instructions carefully.
a) There are 35 questions in this question paper with internal choice.
b) SECTION A consists of 18 multiple-choice questions carrying 1 mark each.
c) SECTION B consists of 7 very short answer questions carrying 2 marks each.
d) SECTION C consists of 5 short answer questions carrying 3 marks each.
e) SECTION D consists of 2 case- based questions carrying 4 marks each.
f) SECTION E consists of 3 long answer questions carrying 5 marks each.
g) All questions are compulsory.
h) Use of log tables and calculators is not allowed.
SECTION-A
The following questions are multiple-choice questions with one correct answer. Each question carries 1 mark. There is no internal choice in this section
1. The charge required for the reduction of 1 mol of MnO4– to MnO2 is
(a) 1 F
(b) 3 F
(c) 5 F
(d) 6 F
2. The rate constant of a reaction A → B is 0.6 × 103 mole per second. If the concentration of [A] is 5 M, then what will be concentration of [B] after 20 months?
(a) 0.36 M
(b) 0.72 M
(c) 1.08 M
(d) 3.60 M
3. If the initial concentration of reactant is doubled, t1/2 is also doubled, the order of reaction is
(a) zero
(b) 1
(c) 2
(d) 3
4. Consider Fig. and mark the correct option.
(а) Activation energy of forward reaction is E1 + E2 and product is less stable than reactant.
(b) Activation energy of forward reaction is E1 + E2 and product is more stable than reactant.
(c) Activation energy of both forward and backward reaction is E1 + E2 and reactant is more stable than product.
(d) Activation energy of backward reaction is E1 and product is more stable than reactant.
5. Which of the following has magnetic moment value of 5.9?
(a) Fe2+
(b) Fe3+
(c) Ni2+
(d) Cu2+
6. Which of the following can make a stable compound with a metal ion
(a) oxalate
(b) ethan1, 2 -diammine
(c) EDTA
(d) SCN–
7. The complex ions [Co(NH3)5(NO2)]2+ and [Co(NH3)5 (ONO)]2+ are called
(a) Ionization isomers
(b) Linkage isomers
(c) Co-ordination isomers
(d) Geometrical isomers
8. Which of the following are arranged in the decreasing order of dipole moment?
(a) CH3Cl, CH3Br, CH2F
(b) CH3Cl, CH3F, CH3Br
(c) CH3Br, CH3Cl, CH3F
(d) CH3Br, CH3F, CH3Cl
9. Phenol reacts with bromine in CS2 at low temperature to give
(a) m-bromophenol
(b) p-bromophenol
(c) o-and p-bromophenol
(d) 2,4,6-tribromophenol
10. The alcohol which does not react with Lucas reagent is
a) isobutyl alcohol
b) tert-butyl alcohol
c) sec-butyl alcohol
d) n-butanol
11. The addition of HCN to carbonyl compounds is an example of
(a) nucleophilic addition
(b) electrophilic addition
(c) free radical addition
(d) electromeric addition
12. Formaldehyde react with Grignard’s reagent to give addition products which on hydrolysis give
(a) tertiary alcohols
(b) secondary alcohols
(c) primary alcohols
(d) carboxylic acids
13. Which of the following: when heated with a mixture of ethanamine and alcoholic potash gives ethyl isocyanide?
(a) 2-chloropropane
(b) 2,2-dichloropropane
(c) trichloromethane
(d) tetrachloromethane
14. Amine that cannot be prepared by Gabriel-Phthalmidie synthesis is
(a) aniline
(b) benzyl amine
(c) methyl amine
(d) iso-butylamine
15. Given below are two statements labelled as Assertion (A) and Reason (R)
Assertion (A): Cu and Zn are not considered as transition metal.
Reason (R): Cu and Zn do not have their last electron in d orbital.
Select the most appropriate answer from the options given below:
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true
16. Given below are two statements labelled as Assertion (A) and Reason (R)
Assertion (A): Assertion: With HI, anisole gives iodo benzene and methyl alcohol.
Reason: Iodide ion combines with smaller group to avoid steric hindrance
Select the most appropriate answer from the options given below:
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.
17. Given below are two statements labelled as Assertion (A) and Reason (R)
Assertion: Hoffmann’s bromamide reaction is given by primary amines.
Reason: Primary amines are more basic than secondary amines
Select the most appropriate answer from the options given below:
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.
Given below are two statements labelled as Assertion (A) and Reason (R)
Assertion (A): Glucose produces n-hexane when reduced in presence of red phosphorus
Reason: Glucose have a ketone group
Select the most appropriate answer from the options given below:
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is not the correct explanation of A.
c) A is true but R is false.
d) A is false but R is true.
SECTION: B
This section contains 7 questions with internal choice in two questions. The following questions are very short answer type and carry 2 marks each.
19. Define conductivity and molar conductivity for the solution of an electrolyte. Discuss their Variation with change in temperature.
Ans. Conductivity: Conductivity of a solution is defined as the conductance of a solution of 1 cm length and having 1 sq. cm as the area of cross-section. It is represented by K. Its unit is S cm-1
Molar conductivity: Molar conductivity of a solution at a dilution V is the conductance of all the ions produced from one mole of the electrolyte dissolved in V cm3 of the solution when the electrodes are 1 cm apart and the area of the electrodes is so large that the whole of the solution is contained between them. It is represented by Λm. Its unit is S cm2 mol-1
Conductivity and molar conductivity of electrolytes increase with increasing temperature.
20. a) Define rate of reaction
Ans. a) The change in concentration of reactant or product per unit time is called rate of reaction
b) Express the rate of the following reaction in terms of the formation of ammonia:
N2(g) + 3H2(g) → 2NH3(g)
Ans.
21. Show that in a first order reaction, time required for completion of 99.9% is 10 times of half-life (t1/2) of the reaction.
Ans. When reaction is completed 99.9%, [R]n = [R]0 – 0.999[R]0
k =2.303 /t . log[R0]/[R]
t = 6.909/k For half-life of the reaction t 1/2 = 0.693/k t / t1/2= 10times
OR
A reaction is of second order with respect to a reactant. How will the rate of reaction be affected if the concentration of this reactant is
(i) Doubled, (ii) reduced to half?
Ans. Since Rate = K[A]2
For second order reaction Let [A] = a, then Rate = Ka2
(i) If [A] = 2a then Rate = K (2a)2 = 4 Ka2
∴Rate of reaction becomes 4 times
(ii) If [A] = a/2 then Rate = K (a/2)2=Ka2/4
∴ Rate of reaction will be 1/4 th
22. a) Write IUPAC name for the compound: [CoCl2 (en)2]Cl
b) Out of the following two coordination entities which is chiral (optically active) and Why?
(1) cis-[CrCl2 (ox)2 ] 3–
(2) trans-[CrCl2 (ox)2 ] 3–
Ans. a) Dichloridobis (ethane-1,2-diamine)cobalt (III) chloride
b) cis – [CrCl2 (ox)2 ] 3-is chiral (optically active).
23. a) Arrange the following in increasing order of boiling point:
(i) CH3CH2CH2CH2Br
(ii) (CH3)3.Br
(iii) (CH3)2C.Br
b) Convert Propene to 1- nitro propane
Ans. a) (CH3)2C.Br < (CH3)2.CHCH2.Br < CH3CH2CH2CH2Br
b) CH3-CH=CH2 + HBr (Peroxide) -> CH3CH2CH2 Br+AgNO2 -> CH3CH2CH2NO2
OR
Give reasons:
a) R-X reacts with KCN to give cyanides as major product and iso cyanides as major product with AgCN.
Ans. a) CN– is ambidentate nucleophile
b) Chloroform is preserved in dark coloured bottles.
Ans. b) Chloroform get oxidized to phosgene in sunlight. So dark coloured bottles used to prevent reaction with sunlight.
24. a) Arrange the following compounds in an increasing order of their reactivity in nucleophilic addition reactions:
Ethanal, Propanal, Propanone, and Butanone.
Ans. a) Butanone < Propanone < Propanal < Ethanal
b) Give a chemical test to distinguish between Ethanal and Propanal.
Ans. b) Ethanal on heating with I2 in NaOH gives a yellow ppt of iodoform but propanal does not respond to this test.
25. a) Write a difference between a nucleoside and a nucleotide
Ans. a) any one difference (The chemical composition of nucleotide consists of a phosphate group, a sugar and a nitrogenous base.
A nucleoside has a chemical composition that consists of a sugar and a base without the phosphate group)
b) Vitamin C must be taken regularly in diet. Why?
Ans. b) due to solubility in water it will intake regularly.
(Water soluble vitamins must be supplied regularly in diet because they are readily excreted in urine and cannot be stored (except vitamin B12) in our body)
SECTION: C
This section contains 5 questions with internal choice in two questions. The following questions are short answer type and carry 3 marks each.
26. a) State Henry’s law.
Ans. a) henry law expression or statement- Henry’s law is a gas law which states that at the amount of gas that is dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid when the temperature is kept constant
b) The vapour pressure of pure benzene at a certain temperature is 0.850 bar. A non-volatile, non-electrolyte solid weighing 0.5 g when added to 39.0 g of benzene (molar mass 78 g mol-1). Vapour pressure of the solution, then, is 0.845 bar. What is the molar mass of the solid substance?
Ans. b) P10 = 0.850 bar; p = 0.845 bar; M1 = 78 g mol–1; w2 = 0.5 g; w1 = 39 g
P10 -P1/ P10 = W2 X M 1 / M2 X W1
0.850 bar – 0.845 bar /0.850 bar
0.5 g × 78 g mol /M 2× 39 g
M2 = 170 g mol–1
27. a) The Complex [Ti (H2O) 6] 3+ is a coloured compound. Justify.
Ans. a) due to (d-d transition)
b) Explain: [Co (NH3)6] 3+ is an inner orbital complex whereas [CoF6]3- is an outer orbital complex.
Ans. b) Proper VBT diagram and magnetism (refer ncert page no 255)
6, +2
Outer electronic configuration of nickel (Z = 28) in ground state is 3d84s2. Nickel in this complex is in + 2 oxidation state. It achieves + 2 oxidation state by the loss of the two 4s-electrons. The resulting Ni2+ ion has outer electronic configuration of 3d8. Since CN– ion is a strong field, under its attacking influence, two unpaired electrons in the 3d orbitals pair up.
Outer electronic configuration of nickel (Z = 28) in ground state is 3d84s2 Nickel in this complex is in + 2 oxidation state. Nickel achieves + 2 oxidation state by the loss of two 4s-electrons. The resulting Ni2+ ion has outer electronic configuration of 3d8. Since CP ion is a weak field ligand, it is not in a position to cause electron pairing
c) Write the coordination number and oxidation number for Fe in the coordination entity [Fe(CN)6 ] 4-
OR
a) Explain on the basis of valence bond theory that [Ni(CN)4]2- ion with square planar structure is diamagnetic and [NiCl4]2- ion with tetrahedral geometry is paramagnetic.
Ans. a) C6H5 CH2Cl will undergo SN1reaction faster.
The carbocation formed by C6 H5 CH2Cl gets stabilized through resonance.
Greater the stability of carbocation, greater will be its ease of formation from the respective halide
b) FeSO4 solution mixed with (NH4)2SO4 solution in 1: 1 molar ratio gives the test of Fe2+ ion but CuSO4 solution mixed with aqueous ammonia in 1 : 4 molar ratio does not give the test of Cu2+ ion. Explain why?
Ans. When FeSO4 and (NH4)2SO4 solutions are mixed in 1 : 1 molar ratio, a double salt known as Mohr’s salt is formed. It has the formula FeSO4.(NH4)2SO4.6H2O.
But in CuSO4, the Cu is part of coordination entity and cannot be ionized
28. a) Which one of the following compounds will undergo faster hydrolysis reaction by SN1 mechanism? Justify your answer.
C6H5CH2Cl or CH3CH2CH2Cl
Ans. a) C6H5 CH2Cl will undergo SN1reaction faster.
The carbocation formed by C6 H5 CH2Cl gets stabilized through resonance.
Greater the stability of carbocation, greater will be its ease of formation from the respective halide
b) What happens when chlorobenzene reacts with Sodium hydroxide at 623K and 300 atm pressure?
Ans. b) chloro benzene converts into phenol (with reaction
29. What happens when (Attempt any three)
i) Propanone is treated with methyl magnesium bromide and the product is hydrolysed.
ii) Two moles of Benzaldehyde are heated with concentrated NaOH.
iii) 2,4 DNP is added to acetone.
iv) Tert.butyl alcohol is heated with copper at 573K.
Ans. a) Correct chemical reaction.
b) Correct chemical reaction.
c) Correct chemical reaction.
d) Correct chemical reaction.
30. a) Arrange the following in decreasing order of their basic strength:
C6H5NH2, C2H5NH2 , (C2H5 ) 2NH, NH3
b) Why Aniline does not undergo Friedel-Crafts reaction?
c) What happen when C6H5 (NH) CH3 reacts with CHCl3 and KOH?
Ans. a) (C2H5 ) 2NH > C2H5NH2 > NH3 > C6H5NH2
b) Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.
c) No reaction occurs.
SECTION: D
The following questions are case-based questions. Each question has an internal choice and carries 4 (1+1+2) marks each. Read the passage carefully and answer the questions that follow:
31. When a solution does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution. The vapour pressure of such a solution is either higher or lower than that predicted by Raoult’s law. If it is higher, the solution exhibits positive deviation and if it is lower, it exhibits negative deviation from Raoult’s law.
The osmotic pressure of a solution is the excess pressure that must be applied to a solution to prevent osmosis, i.e., to stop the passage of solvent molecules through a Semipermeable membrane into the solution. Osmotic pressure is colligative property as it depends on the number of solute molecules and not on their identity. For dilute solutions, it has been found experimentally that osmotic pressure is proportional to the molarity, C of the Solution at a given temperature T. Thus: Π = C R T Here Π is the osmotic pressure and R is the gas constant .Π = (n2 /V) RT
a) Define ideal solution.
b) What kind of deviation is found in solution of alcohol in water?
c) 200 cm3 of an aqueous solution of a protein contains 1.26 g of the protein. The osmotic pressure of such a solution at 300 K is found to be 2.57 × 10-3 bar. Calculate the molar mass of the protein.
Ans. a) solution that follow raoult’s law at all concentrations , is ideal solution .
b) positive deviation
C) The various quantities known to us are as follows: Π = 2.57 × 10–3 bar,
V = 200 cm3 = 0.200 litre
T = 300 K
R = 0.083 L bar mol-1 K-1
Substituting these values in equation we get
M2= w2RT/ Π V
M2 =1 .26 g × 0.083 L bar K-1 mol -1 × 300 K/ 2.57×10 -3 bar × 0.200 L
= 61,022 g mol-1
OR
Why Osmotic Pressure is used to measure the molar mass of biomolecules?
Ans. Correct Reasons
32. The carbohydrates may also be classified as either reducing or nonreducing sugars. All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars. All monosaccharides whether aldose or ketose are reducing sugars. Fructose also has the molecular formula C6H12O6 and on the basis of its reactions it was found to contain a ketonic functional group at carbon number 2 and six carbons in straight chain as in the case of glucose. It belongs to D-series and is a laevorotatory compound. It is appropriately written as D-(–)-fructose. Its open chain structure is as shown. Polysaccharides contain a large number of monosaccharide units joined together by glycosidic linkages. These are the most commonly encountered carbohydrates in nature. They mainly act as the food storage or structural materials. Protein found in a biological system with a unique three-dimensional structure and biological activity is called a native protein. When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed.
a) Sucrose cannot reduce the Tollen’s reagent. why?
b) The optical activity of sucrose is changed to from dextro to leavo after some time. Explain it.
c) What is denaturation of protein and which structure of protein remains intact during denaturation?
Ans. a) sucrose do not have free aldehyde group.
b) Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (–) and the product is named as invert sugar
c) When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein.
OR
What is the significance of D and L and + and – sign in sugars.
Ans. The D-L system corresponds to the configuration of the molecule: spatial arrangement of its atoms around the chirality center.plus minus indicate dextro and laevo.
Ans. The D-L system corresponds to the configuration of the molecule: spatial arrangement of its atoms around the chirality center.plus minus indicate dextro and laevo.
SECTION: E
The following questions are long answer type and carry 5 marks each. Two questions have an internal choice.
33. (a) A cell is prepared by dipping a zinc rod in 1M zinc sulphate solution and a silver electrode in 1M silver nitrate solution. The standard electrode potential given:
E0Zn2+ / Zn = – 0.76 V, E0Ag+ / Ag = + 0.80 V
What is the effect of increase in concentration of Zn2+ on the Ecell?
(b) Write the products of electrolysis of aqueous solution of NaCl with platinum electrodes.
(c) Calculate e.m.f. of the following cell at 298 K:
Ni(s) / Ni2+ (0.01 M) // Cu2+ (0.1M) / Cu (s)
[ Given E0 Ni2+/ Ni = – 0.25 V , E0 Cu2+/Cu = + 0.34 V )Write the overall cell reaction.
Ans. a) Ecell decreases.
b) Anode: Cl2 ↑ Cathode : H2
c) E0cell = 0.59V
NERNST EQUATION FOR CELL
Ecell = 0.6195V
OR
a) What is the role of zinc chloride in dry cell?
b) /\m ° for NaCl, HCl and NaAc are 126.4, 425.9 and 91.0 S cm2 /mol respectively. Calculate Λo for HAc.
c) Write the chemical reactions taking place at the electrodes during discharging of lead storage battery.
Ans. a) Complex formation.
b) (425.9 + 91.0 – 126.4 ) S cm2 mol –1 = 390.5 S cm2 mol–1 .
c) Anode: Pb(s) + SO4 2–(aq) → PbSO4 (s) + 2e–
Cathode: PbO2 (s) + SO4 2–(aq) + 4H+ (aq) + 2e– → PbSO4 (s) + 2H2O (l )
Overall reaction
Pb(s) + PbO2 (s) + 2H2 SO4 (aq) → 2PbSO4 (s) + 2H2O(l)
34. a) Assign reasons for the following:
(i) Copper (I) ion is not known in aqueous solution.
(ii) Actinoids exhibit greater range of oxidation states than lanthanoids
(iii) Cr2+ is reducing in nature while with the same d-orbital configuration (d4) Mn3+ is an oxidising agent.
b) Complete the following chemical equations:
(i) MnO4– (aq) + S2O32- (aq) + H2O (1) →
(ii) Cr2O72- (aq) + Fe2+ (aq) + H+ (aq) →
Ans. (i) Cu2+(aq) is much more stable than Cu+(aq). This is because although second ionization enthalpy of copper is large but Δhyd (hydration enthalpy) for Cu2+(aq) is much more negative than that for Cu+(aq) and hence it more than compensates for the second ionization enthalpy of copper. Therefore, many copper (I) compounds are unstable in aqueous solution and undergo disproportionation as follows: 2Cu+ → Cu2+ + Cu
(ii) Because of very small energy gap between 5f, 6d and 7s subshells all their electrons can take part in bonding and shows variable oxidation states.
Ans. (ii) Because of very small energy gap between 5f, 6d and 7s subshells all their electrons can take part in bonding and shows variable oxidation states.
(iii) Cr2+ has the configuration 3d4 which easily changes to d3 due to stable half filled t2g orbitals. Therefore Cr2+ is reducing agent. While Mn2+ has stable half filled d5 configuration. Hence Mn3+ easily changes to Mn2+ and acts as oxidising agent.
Ans. (iii) Cr2+ has the configuration 3d4 which easily changes to d3 due to stable half filled t2g orbitals. Therefore Cr2+ is reducing agent. While Mn2+ has stable half filled d5 configuration. Hence Mn3+ easily changes to Mn2+ and acts as oxidising agent.
35. a) Arrange the following compounds in increasing order of their boiling points:
CH3CHO, CH3CH2OH, CH3OCH3, CH3CH2CH3
Ans. a) CH3CH2CH3 < CH3CHO< CH3OCH3 < CH3CH2OH,
b) Would you expect Benzaldehyde to be more reactive or less reactive in nucleophilic addition reactions than propanal? Explain your answer.
Ans. b) The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance and hence it is less reactive than propanal.
c) 4 Nitro benzoic acid is more acidic than 4- methoxy benzoic acid. Give reason.
Ans. c) electron withdrawing group NO2 attached
d) Explain the following reaction
1) Aldol condensation 2) Etard reaction
Ans. d) correct chemical reactions
OR
- Arrange the following compounds in increasing order of their acidity:
benzoic acid, nitro benzoic acid, methyl benzoic acid
Ans. methyl benzoic acid < benzoic acid < nitro benzoic acid
b) What happens when Phenyl magnesium bromide react with dry ice
Ans. Correct equation
c) Write the reactions involved in the following:
(i) Hell-Volhard Zelinsky reaction.
(ii) Decarboxylation reaction.
(iii) Wollf-Kishner reduction.
(iii) correct equation