Class 12 Chemistry · 27 questions · 27 with answers
Which of the following statements is not true for hexagonal close packing? (1)
(iii) Octahedral voids of second layer are covered by spheres of the third layer.
Brine is electrolysed using inert electrodes. The reaction at anode is _____. (1) – 1 V
(i) Cl (aq.) → Cl (g) + e– ; ECell = 1.36V 2 2 + V
In a qualitative analysis when H2S is passed through the solution of a salt acidified with HCl, a black precipitate is obtained. On boiling the precipitate with dil. HNO3, it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution will give ________. (1)
(ii) Deep blue solution of [Cu (NH3)4] .
What is the IUPAC name of the compound ? (1)
(ii) N, N-Dimethylbutan-1-amine
E Cell for some half cell reactions are given below. On the basis of these mark the correct answer. (2) + 1 V (a) H (aq.) + e– → H (g) ; E Cell = 0.00V 2 2 + V (b) 2H2O (l) → O2 (g) + 4H (aq.) + 4e– ; E Cell = 1.23V 2– 2– V (c) 2SO4 (aq.) → S2O8 (aq.) + 2e– ; E Cell = 1.96 V
(i) In dilute sulphuric acid solution, hydrogen will be reduced at cathode.
(iv) In dilute sulphuric acid solution, water will be oxidised at anode.
What happens when a lyophilic sol is added to a lyophobic sol? (2)
(i) Lyophobic sol is protected.
(iii) Film of lyophilic sol is formed over lyophobic sol.
How do emulsifying agents stabilise emulsion? (1)
Emulsifying agent forms an interfacial film between suspended particles and the particles of dispersion medium. (1)
On what principle is the zone refining based? (1)
Zone refining is based on the principle that impurities are more soluble in melt than in the solid state of metals. (1)
Why cross links are required in rubber to have practical applications? (1)
Cross links bind the polymer chains. These help the polymer to come to the original position after the stretching force is released. Thus, increase its elastomeric properties. (1)
Name an artificial sweetener which has dipeptide linkage between two aminoacids. (1)
Aspartame (1)
Why does electrical conductivity of semiconductors increase with rise in temperature? (2) – 2+
In semiconductors, the gap between the valence band and the conduction band is small. On increasing temperature, more electrons can jump from valence band to conduction band and conductivity increases. (2) – 2+ + 3+
In the ring test of NO3 ion, Fe ion reduces nitrate ion to nitric oxide, which 2+ combines with Fe (aq.) ions to form brown complex. Write reactions involved in the formation of brown ring. (2) 257 Model Question Paper-II
NO3 + 3Fe + 4H → NO + 3Fe + 2H2O 2+ 2+ [Fe(H2O)6] + NO → [Fe(H2O)5(NO)] + H2O Distribution of marks • 1 mark for each equation (1 ×2 = 2 marks)
Arrange the following complex ions in increasing order of crystal field splitting energy ∆0. [Cr(Cl)6]3–, [Cr(CN)6]3–, [Cr(NH3)6]3+ (2)
Increasing order is [Cr(Cl)6]3–< [Cr(NH3)6]3+ < [Cr(CN)6]3– Distribution of marks • Correct order (2 marks)
Explain why allyl chloride is hydrolysed more readily than n-propylchloride? (2)
Allyl chloride shows high reactivity as the carbocation formed by hydrolysis is stabilised by resonance where as no resonance stabilisation of carbocation formed by n-propyl chloride is possible. (2 makrs)
Write name(s) of starting materials for the following polymer and identify its monomer unit. (2)
261 Model Question Paper-II Distribution of marks • Monomer unit (1 mark) • Starting material melamine and formaldehyde (½ ×2 = 1 mark)
What is the advantage of using antihistamines instead of antacids in the treatment of hyperacidity. (2)
When 1 mol of NaCl is added to 1 litre of water, the boiling point of water increases. On the other hand, addition of 1 mol of methyl alcohol to one litre of water decreases the boiling point of water. Explain why does this happen. (3) –
Value of standard electrode potential for the oxidation of Cl ion is more positive than that of water, even then in the electrolysis of aqueous sodium chloride – solution, why is Cl oxidised at anode instead of water? (3)
Under the conditions of electrolysis of aqueous sodium chloride, oxidation – of water at anode requires overpotential hence Cl is oxidised instead of water. Distribution of marks • Explanation (2 marks) • Reaction (1 mark)
How copper is extracted from low grade copper ores? (3)
Copper is extracted by hydrometallurgy from low grade copper ores. It is leached out using acid or bacteria. The solution containing Cu2+ is treated with scrap iron, Zn or H2. + Cu2+ (aq) + H2 (g) → Cu(s) + 2H (aq) Cu2+ + Fe(s) → Fe2+ (aq) + Cu(s) Distribution of marks • Reactions (1 ×2 = 2 marks) • Explanation (1 mark)
Calculate the volume of 0.1 M NaOH solution required to neutralise the products formed by dissolving 1.1 g of P4O6 in H2O. (3) n+
P4O6 + 6H2O → 4H3PO3 [H3PO3 + 2NaOH → Na2HPO3 + 2H2O] × 4 P4O6 + 8NaOH → 4Na2HPO4 + 2H2O 1mol 8 mol Product formed by 1 mol P4O6 is neutralised by 8 mol NaOH 1.1 1.1 ∴ Product formed by mol P4O6 will be neutralised by × 8 mol NaOH 220 220 Molarity of NaOH solution is 0.1M ⇒ 0.1 mol NaOH is present in 1 L solution 1.1 1.1 × 8 88 4 ∴ × 8 mol NaOH is present in L = L = L 220 220 × 0.1 220 10 = 0.4 L = 400 mL NaOH solution Distribution of marks • Correct chemical euqations (½ ×3 = 1½ mark) • Correct method of calculation (1 mark) • Correct answer (½ mark) n+
A complex of the type [M (AA)2 X2] is known to be optically active. What does this indicate about the structure of the complex? Give one example of such complex. (3)
Since complex of the type [M(AA)2 X2] is optically active it indicates that 2+ complex has cis-octahedral structure. e.g. cis-[Pt(en)2(Cl) 2] or + cis-[Cr(en)2(Cl)2] . Distribution of marks • Electronic configuration in the presence of weak field ligand (1 marks) • Electronic configuration in the presence of strong field ligand (1 mark) • Explanation (1 mark)
Predict the major product formed on adding HCl to isobutylene and write the IUPAC name of the product. Explain the mechanism of the reaction. (3)
The mechanism involved in this reaction is as follows : Step I : Step II: Distribution of marks • Structure of isobutylene (½ mark) 263 Model Question Paper-II • IUPAC name of the product (½ mark) • 2 steps of mechanism (2 marks)
Explain why rate of reaction of Lucas reagent with three classes of alcohols different? Give chemical equations wherever required. (3)
The reaction of alochol with Lucas reagent proceeds through carbocation formation. More stable is the carbocation, faster is the reaction. Carbocation formed by 1° alochol is least stable hence reaction is slow. Distribution of marks • Reaction (½ ×3 = 1½ mark) • Reason (1½ mark)
A primary amine, R—NH2 can be reacted with alkyl halide, RX, to get secondary amine, R2NH, but the only disadvantage is that 3° amine and quaternary ammonium salts are also obtained as side products. Can you suggest a method where CH3NH2 forms only 2° amine? (3)
Label the glucose and fructose units in the following disaccharide and identify anomeric carbon atoms in these units. Is the sugar reducing in nature? Explain. (3)
C1 of glucose unit and C2 of fructose unit are anomeric carbon atoms in this disaccharide. The disaccharide is non reducing sugar because —OH groups attached to anomeric carbon atoms are involved in the formation of glycoside bond. Distribution of marks • Recognising glucose and (½ mark) fructose units correctly • Identification of anomeric carbon (½ ×2 = 1 mark) • Proper explanation for non reducing nature (1½ marks)
Assertion (A): When NaCl is added to water a depression in freezing point is observed.
Reason (R): The lowering of vapour pressure of a solution causes depression in the freezing point. (2)
Assertion (A): Bond angle in ethers is slightly less than the tetrahedral angle.
Reason (R): There is repulsion between the two bulky (—R) groups. (2)
(v) (1)