Sample Paper Chemistry Class 12

Chemistry Unsolved Sample Paper Solutions ISC Class 12

Section-A

Answer 1.

  • A. (i) Donor, acceptor
  • (ii) dehydration, alkenes
  • (iii) rochelle salt
  • (iv) isoelectric point
  • B. (i) (c) Nearly same atomic size
  • (ii) (d) Tetrahedral and diamagnetic
  • (iii) (b) Unpaired electrons
  • (iv) (a) Carboxylic acid
  • C. (i) (c)
  • (ii) (a)
  • (iii) (b)
  • (iv) (d)
D. (i) (b) Both assertion and reason are true but reason is not the correct explanation for assertion.
 Explanation :

The unit of rate constant for second order reaction is mol–1 L s–1 and the rate of second order reaction depends on initial concentration of reactant. Thus, both assertion and reason are true but reason is not the correct explanation for assertion.

(ii) (a) Both assertion and reason are true and reason is the correct explanation of assertion.

 Explanation :

Liquids and solids exhibit practically no change of solubility with changes in pressure. Gases as might be expected, increase in solubility with an increase in pressure. Thus both assertion and reason are true and reason is the correct explanation of assertion.

Section-B

Answer 2.

  • Given:
  • Vapour pressure of pure CCl4 at 25°C (PCCl4 ° ) = 115.0 torr
  • Vapour pressure of pure SnCl4 at 25°C (PSnCl4 ° ) = 238.0 torr
  • Molar mass of CCl4 [M(CCl4)] = 154 g mol–1
  • Molar mass of SnCl4 [M(SnCl4)] = 170 g mol–1
  • Mass of CCl4 in solution (wCCl4) = 10 g
  • Mass of SnCl4 in solution (wSnCl4) = 15 g
  • From Dalton's law of partial pressure:
114
115

Answer 3.

The general electronic configuration of d-block elements can be written as (n – 1)d1–10 ns1–2. Where n is the
valence shell and (n – 1) is penultimate shell.
Transition elements show variable state oxidation in their compounds because their valence electrons are
in two different sets of orbitals, i.e., (n – 1)d and ns and there is a very small energy difference in between
(n – 1)d and ns orbitals. As a result, electrons of (n – 1)d orbitals as well as ns-orbitals take part in bond
formation. Thus, transition elements have variable oxidation states.

Answer 4.

116

Answer 5.

  • (i) Potassium cyanide (KCN).
  • (ii) 4-Nitroaniline < Aniline < 4-Methyl aniline.

Answer 6.

118

Answer 7.

119
  • Since there are no unpaired electrons,
  • complex is diamagnetic.
120
  • The complex has two unpaired electrons, therefore, it will be paramagnetic.

Answer 8.

  • (i) (a) When sodium phenoxide is heated with CO2 at 400 K and at 4-7 atm pressure, sodium salicylate is formed as the major product, which on acidification yields salicylic acid. This reaction is known as Kolbe’s reaction.
  • (b) The preparation of acetophenone from phenol is a two-step process. In the first step, phenol is converted into benzene in the presence of zinc under heating. In the second step, the benzene formed in the first step reacts with acetic anhydride in the presence of aluminium chloride and forms the desired product acetophenone.
122

OR

  • (ii) Alcohols undergo dehydration (removal of a molecule of water) on treating them with protonic acids(conc. H2SO4 or H3PO4) or catalysts such as anhydrous zinc chloride or alumina to form alkenes. For example, tert-butanol undergoes dehydration when it is treated with 20% H3PO4 at 358 K :
  • The relative case of dehydration of alcohols follows the order: Tertiary alcohol > Secondary alcohol > Primary alcohol.

Answer 9.

  • For the first order reaction,
125

Answer 10.

  • (i) Ethanal and propanal undergo cross aldol condensation when heated in presence of sodium hydroxide :
126
  • (ii) FCH2COOH < CHCl2COOH < NC–CH2COOH < CF3COOH

Answer 11.

  • With the successive filling of the inner orbitals, 4f, there is a gradual decrease in the atomic and the ionic sizes of inner transition elements along the series. This is known as ‘lanthanoid contraction’. This phenomenon can be attributed to the imperfect shielding of one electron by another in the same sub-shell.
  • Causes of lanthanide contraction: The lanthanide contraction is due to the imperfect shielding of one 4f electron by another in the same sub shell. As we move along the lanthanide series, the nuclear charge and the number of 4f electrons increase by one unit at each step. However, due to imperfect shielding, the effective nuclear charge increases causing a contraction in electron cloud of 4f-subshell.

Section-C

Answer 12.

127
128

Answer 13.

The products will be as follows:

130

Answer 14.

  • (i) (a) D-glucose reacts with hydroxylamine to form oxime.
132
  • (b) D-glucose reacts with acetic anhydride to give penta-acetate.
133
  • (ii)
Globular Protein Fibrous Protein
1. Globular proteins have almost spheroidal shape due to folding of the polypeptide chain. Polypeptide chains of fibrous proteins consist of thread like molecules which tend to lie side by side to form fibres.
2. Globular proteins are soluble in water. Fibrous proteins are insoluble in water.
3. Globular proteins are sensitive to small changes of temperature and pH. Therefore, they undergo denaturation on heating or on treatment with acids/bases. Fibrous proteins are stable to moderate changes of temperature and pH.
4. They possess biological activity thats why they act as enzymes. Example: Maltase, invertase, etc., hormones (insulin) antibodies, transport agents. They do not have any biological activity but serve as chief structural material of animal tissues. Example: Keratin in skin, hair, nails and wool.

OR

  • The following reactions cannot be explained by the open chain structure of glucose.
  • 1. Despite of having aldehyde group, glucose does not gives 2, 4-DNP test, Schiff’s test and it does not forms the hydrogen sulphite addition product with NaHSO3.
  • 2. The pentacetate of glucose does not reacts with hydroxyl amine indicating the absence of free – CHO group.
  • 3. D-glucose on treatment with methyl alcohol in the presence of dry HCl gas gives two isomers, methyl α–D glucoside and methyl β-D glucoside. These glucosides do not reduces Fehling’s solution and also do not react with hydrogen cyanide indicating the absence of free-CHO group. A ring structure called pyranose structure (α- and β) is proposed for glucose molecule.

Answer 15.

  • The plot of ln k vs 1/T gives a straight line according to the equation given below :
  • ln k = – Ea/RT + ln A
  • Activation energy can be calculated by the value of slope (= – Ea/R) of the graph.
  • Given, Slope = – 5841 K and R = 8.314 JK– 1 mol–1
  • Putting the values :
  • Slope = – Ea/R
  • – Ea = – 5841 K × 8.314 JK– 1 mol–1
  • = 48562 J mol–1
  • Hence, the energy of activation for the reaction is 48562 J mol–1.

Answer 16.

  • (i)
134
  • (ii) The resonating structures of phenol can be shown as :
135
(iii) Acidic nature of carboxylic acids:
  • 1. The carboxyl group (—COOH) imparts an acidic character to carboxylic acids.
  • 2. A carboxyl group is made of —OH group bonded to a carbonyl group.
  • 3. In aqueous solution, the H atom in OH of carboxyl group dissociates as proton and carboxdylate ion is formed as the conjugate base.
136
  • 4. Carboxylate ion is resonance stablised by two equivalent resonance structures as shown below:
138
  • 5. Carboxylate ion has two resonance structures (i) and (ii) and both of them are equivalent to each other.
  • 6. This gives good resonance stabilisation to carboxylate ion, which in turn gives an acidic character to carboxylic acids.

Answer 17.

139

Answer 18.

140
  • (iii) Since, the reaction is first order reaction and the rate of first order reaction is directly proportional to the concentration of reactants the rate of reaction will also double.

Section-D

Answer 19.

  • (i) (a) Freons are chlorofluorocarbon compounds of methane and ethane.
  • (b) Freon 12 (CCl2F2).
  • c) Freon’s chemical components degrade the ozone layer as it passes through the air. Depletion of the ozone layer increases the amount of ultraviolet radiation reaching the earth’s surface, providing a serious health risk to people.
  • (ii) (a) Glucose is also known as dextrose and is an aldohexose.
  • (b) The secondary structure of protein refers to the shape in which a long polypeptide chain can exist. They are found to exist in two different types of structures namely, a-helix and b-pleated sheet structure.

Answer 20.

  • (i) (a) Both shows coordination isomerism because both cationic and anionic entities and isomers differ in the distribution of ligands in the coordination entity of cationic and anionic part.
  • (b) In [Ni(H2O)6]2+ Ni is in +2 oxidation state with electronic configuration 3d8. In the presence of weak ligand H2O the two unpaired electrons do not pair up and hence the complex has two unpaired electrons. Therefore, it is coloured and shows d-d transitions which absorbs red light and emits green complimentary light. In case of [Ni)CN)4]2– Ni also shows +2 oxidation state but CN ligand is strong ligand and two unpaired electrons undergo pairing, as a result, no d-d occurs. So, it is colourless.
  • (c) Pentaamminecarbonatocobalt (III) chloride.
  • (ii) (a) Hexaamminecobalt(III) ion
  • (b) In [Co(NH3)6]3+ Co is in +3 state and has configuration 3d6. In the presence of NH3, 3d electrons pair up leaving two d-orbitals empty. Therefore, the hybridisation of the complex is d2sp3. Hence, it is inner orbital complex.
  • (c) Primary valency is zero because there is no ion outside coordination sphere and secondary valency is 6 because there are six monodentate ligand attached with cobalt metal ion.
  • (d) Octahedral geometry.

Answer 21.

  • (i) The cell can be represented as :
142
  • (a) When an external opposite potential is applied less than 2.71 V the direction of flow of current would remain same.
  • (b) When an external opposite potential is applied more than 2.71 V the direction of flow of current would be reversed.
  • (ii) Reactions which will occur in the solution are:
143
  • This reaction occurs at the anode due to over-potential of O2 oxidation of Cl° is preferred. Hence, Cu will deposit at cathode and Cl2 gas will be liberated at anode.

OR

  • (i) (a) Molar conductivity is defined as the conducting power of all the ions produced by dissolving one mole of an electrolyte in solution.
  • Λm = K/C
  • (b) Secondary batteries are those batteries which can be recharged by passing electric current through them and hence can be used over again e.g., Lead storage battery.
  • (ii) (a) The cell can be represented as:
144
  • Hence, the emf of the cell is 1.42 V.
(b) For strong electrolytes the molar conductivity is increased only slightly on dilution. A strong electrolyte is completely dissociated in solution and thus, furnishes all ions for conductance. However, at higher concentrations, the dissociated ions are close to each other and thus, the interionic attractions are greater. These forces retard the motion of the ions and thus, conductivity is low. With decrease in concentration (dilution), the ions move away from each other thereby feeling less attractive forces from the counter ions. This results in an increase in molar conductivity with dilution. The molar conductivity approaches a maximum limiting value at infinite dilution designated as Λm 0. In case of weak electrolytes as the solution of a weak electrolyte is diluted, its ionisation is increased. This results in more number of ions in solution and thus, there is an increase in molar conductivity, also there is a large increase in the value of molar conductivity with dilution, especially near infinite dilution. However, the conductance of a weak electrolyte never approaches a limiting value. Or in other words it is not possible to find conductance at infinite dilution (zero concentration). So, limiting molar conductivity for weak electrolytes is obtained by using Kohlrausch law, from the limiting molar conductivities of individual ions (λ°). Kohlrausch law of independent migration of ions states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual ontributions of the anion and the cation of the electrolyte.

ISC 36 Sample Question Papers

All Subjects Combined for Class 12 Exam 2023

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