CBSE Previous Year Solved Papers Class 12 Chemistry Outside Delhi 2010
Time allowed: 3 hours Maximum Marks: 70
- All questions are compulsory.
- Questions number 1 to 5 are very short-answer questions and carry 1 mark each.
- Questions number 6 to 10 are short-answer questions and carry 2 marks each.
- Questions number 11 to 22 are also short-answer questions and carry 3 marks each.
- Questions number 23 is a value based question and carry 4 marks.
- Questions number 24 to 26 are long-answer questions and carry 5 marks each.
- Use log tables, if necessary. Use of calculators is not allowed.
Question.1.What type of interactions hold the molecules together in a polar molecular solids?
Answer : The molecules of polar molecular solid are held together by strong dipole-dipole interactions.
Question.2.What is meant by ‘limiting molar conductivity7?
Answer: Molar conductivity at infinite dilution is known as limiting molar conductivity,
Question.3.Fluorine does not exhibit any positive oxidation state. Why ?
Answer : Fluorine is the most electronegative element therefore, does not exhibit any positive oxidation state.
Question.4.Given the IUPAC name of the following
Question.5.Write the . structure of the molecule of a compound whose IUPAC name is l-phenylpropan-2-ol.
Question.6.What is Tollen’s reagent ? Write one use of this reagent.
Answer : Tollen’s reagent is an ammonical solution of silver
nitrate. It is used for the silver mirror test of aldehydes (Both aromatic and aliphatic aldehydes)
Question.7.What is meant by ‘reducing sugars’?
Answer : Sugars that can reduce Tollen’s or Fehlings solution are called reducing sugars. Reducing sugars contain tree aldehyde group and gets reduced in a chemical reaction to -OH group.
Question.8.What does the designation, 6, 6, mean in the name nylon-6,6?
Answer : The monomeric unit of Nylon-6, 6 polymer is derived from the two monomers hexamethylene diamine and adipic acid [NH-(CH2)6-NH-CO(CH2)4-CO] in which 6-6 represents, the number of carbon atoms in the two.
Question.9.Define the terms, ‘osmosis’ and ‘osmotic pressure’. What is the advantage of using osmotic pressure as compared to other colligative properties for the determination of molar masses of solutes in solutions?
Answer : The flow of solvent from solution of lower concentration to higher concentration of solution through a semi-permeable membrane is called osmosis. Minimum external pressure that prevents osmosis is called osmotic pressure. Osmotic pressure can be measured at room temperature and for very dilute solutions. Its magnitude is large even for very dilute solution.
Question.10. Express the relation among the cell constant, the resistance of the solution in the cell and the conductivity of the solution. How in the conductivity of a solution related to its molar conductivity.
Question.11. Given that the standard electrode potentials (E°) of metals are:
The half reactions of an electrochemical cell are given below :
Construct the redox reaction equation from the two half-reactions and calculate the cell potential from the standard potentials and predict if the reaction is reactant or product favoured.
Question.12. Describe the following:
- When strong beam of light is passed through a colloidal solution it gets scattered and path of light beam gets illuminated by a bluish light. This phenomenon is known as Tyndall effect.
- The catalytic action that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is known as shape-selective catalyst, e.g. zeolites.
Question.13. What is meant by coagulation of a colloidal solution? Name any method by which coagulation of lyophobic sols can be carried out.
Answer : The bringing together of colloidal particles such that they either float on the surface or precipitate by adding suitable electrolyte is called coagulation of colloidal solutions. Methods : addition of electrolyte, electrophoresis.
Question.14. Complete the following reactions equation :
Question.15. Draw the structural formulae of the following compounds :
Question.16. Give the chemical tests to distinguish between the following pairs of compounds :
(i) Ethylamine and Aniline
(ii)Aniline and Benzylamine
Question.17. Identify A and B in each of the following processes :
Question.18. Draw the molecular structure of the monomers of
Question.19. The density of copper metal is 8.95 g cm-3. The radius of copper is 127.8 pm. Is the copper unit cell simple cubic, body-centred cubic or face-centred cubic?
(Given : atomic mass of Cu = 63.54 g mol-1 and NA = 6.02 x 1023mol-1)
Question.20. What mass of NaCl (molar mass = 58.5 g mol-1) be dissolved in 65 g of water to lower the freezing point by 7.5°C ? The freezing point depression constant,kf for water is 1.86 kg mol-1. Assume van’t Hoff factor for NaCl is 1.87.
Question.21. Describe the role of the following:
(i) NaCN in the extraction of silver from a silver ore
(ii)Iodine in the refining of titanium.
(iii)Cryolite in the metallurgy of aluminium.
Describe the principle involved in each of the following processes of metallurgy :
(i) Froth floatation method
(ii)Electrolytic refining of metals
(iii)Zone refining of metals
(i) NaCN is Used to leach the silver ore in the presence of air. Pure silver is obtained by replacement in the process of extraction of silver.
(i) In this method, the ore particles are wetted with oil and come on the surface as froth and gangue particles get wetted by water and settle down.
(ii)In this method, impure metal is made to act as anode and a stripe of pure metal is made as cathode. Electrolyte is used in the cell to conduct electricity. When current is passed, metal ions from the electrolyte are deposited at cathode as pure metal and the impure metal from the anode dissolves into the electrolyte in the form of ions.
(iii)Zone refining is used to obtain high purity metal using the principle that impurities are more soluble in the molten state than in the solid state of metal,
Question.22. Explain the following cases giving appropriate reasons:
- Nickel does not form low spin octahedral complexes.
- The πcomplexes are known for the transition metals only.
- CO2+ is easily oxidized to CO3+ in the presence of a strong ligand.
- The low spin octahedral complexes involve d2sp3 hybridisation which is not possible for nickel due its configuration [Ar] 3d84s2 therefore, Ni does not form low spin octahedral complexes.
- Transition elements have vacant and partially filled d-orbitals leading to variable valency and hence π-complex formation.
- CO2+ ions are easily oxidized to CO3+ in presence of strong ligand because crystal field stabilization energy of CO3+ion is higher than CO2+ ion as CO3+has d 6 configuration and CO2+ has d7 configuration.
Question.23. How would you differentiate betweenSN1 and SN2 mech-anisms and give examples of each.
Answer: SN1generally involves a weak nucleophile in aqueous solution to form carbocation intermediate. The rate of reaction depends upon the concentration of only one reactant.
Question.24. How would you convert the following:
(i) Phenol to benzoquinone
(ii)Propanone to 2-Methypropan-2-ol
(iii)Propene to propan-2-ol Answer:
Question.25. How would you account for the following :
- NCl3 is an endothermic compound while NF3 is an exothermic one.
- XeF2 is linear molecule without a bend.
- The electron gain enthalpy with negative sign for fluorine is less than that for chlorine; still fluorine is a stronger oxidizing agent than chlorine.
- This is because bond dissociation energy of F2 is lower than Cl2. Morever, fluorine forms stronger bond with nitrogen due to comparable size.
- XeF2has sp3d hybridization with 3 lone pairs in equatorial position and (F) attached at axial positions.
- Fluorine has smaller size compared to chloride and has low enthalpy of dissociation of F-F bond. Therefore it is a stronger .oxidizing agent than chlorine.
Question.26. Amino acids may be acidic, alkaline or neutral. How does this happen? ‘What are essential and non-essential amino acids? Name one of each type.
Answer : Amino acids can be acidic, basic on neutral depending up on the relative number of amine and carboxyl group in their molecule. Equal number of amine and carboxyl group make it neutral; more no. of amine (—NH2) than carboxyl groups make it basic and more carboxyl group as compared to amine group make it acidic.
The amino acids which the body is not able to synthesise are taken through diet are called essential amino acids. E.g. valine, while the amino acids which the body can synthesise are called non-essential amino acids, e.g Glycine.
Question.27. Explain the following terms with one example in each case:
- Food preservatives
- Food preservatives are chemical substances used to prevent microbial growth enhancing their appeal in food without changing the nutritive vklue. e.g. Sodium benzoate, sodium metasuphite.
- Enzymes are biocatalysts of body and are made up of amino acids. The enzyme is so built that it binds to the substrate in specific manner. pH as well as temperature specific, e.g. E. Coli
- Detergents are sodium salts of long chain alkylbenzene sulphonic acids. They do not form scum with hard water. e.g. Sodium laurylsulphonate, sodium n-dodecyl benzene sulphonote, etc.
Question.28. (a) Explain the following term:
- Rate of a reaction
- Activation energy of a reaction
(b) The decomposition of phosphine, PH3, proceeds according to the following equation :
It is found that the reaction follows the following rate equations:
It is found that the reaction follows the following rate equations:
The half-life of PH3 is 37.9 s at 120°C
(i) How much time is required for 3/4th of PH3 to decompose ?
(ii)What fraction of the original sample of PH3remains behind after 1 minute ?
Explain the following term :
(i) Order of a reaction
(ii) Molecularity of a reaction
(b) The rate of a reaction increases four times when the temperature changes from 300 K to 320 K. Calculate the energy of activation of the reaction, assuming that it does not change with temperature. (R = 8.314 J K-1mol-1)
(a) (i) The rate of decrease (or increase) in the concentration of reactants (or product) per unit time is called rate of reaction.
(ii) The minimum energy required to be given to the reactant to start a reaction is called activation energy.
(b) (i) Hence t1/2 = 37.9 s
(a) (i) The sum of powers of the concentrations of the reactants in the rate law expression is called the order of that chemical reaction.
(ii) The number of components i.e. atoms, ions or molecules taking part in elementary reaction and collide with one another to start the reaction is. called molecularity of a reaction.
Question.29. (a) Complete the following chemical equations :
(b)Explain the following observations :
(i)La3+ (Z = 57) and Lu3+ (Z = 71) do not show any colour in solutions.
(ii)Among the bivalent cations in the first series of transition elements, manganese exhibits the maximum paramagnetism.
(iii)Cu+ ions is not known in aqueous solutions.
Question.30.(a) Illustrate the following name reactions giving a chemical equation in each case :
- Clemmensen reaction
- Cannizzaro’s reaction
(b) Describe how the following conversion can be brought about:
(i) Cyclohexanol to cyclohexan 1-one
(ii)Ethylbenzene to benzoic acid
(iii)Bromobenzene to benzoic acid
(a) Illustrate the following name reactions :
(i) Hell-Volhard-Zelinsky Reaction (ii) Wolfif-Kishner reduction reaction
(b) How arc the following conversions carried out:
(i) Ethylcyanide to ethanoic acid .
(ii)Butan-l-ol to butanoic acid
(iii)Methylbenzene to benzoic acid
Note : Except for the following questions, all the remaining question have been asked in previous set.
Question.1. Which type of semi-conductor is obtained when silicon is doped with arsenic ?
Question.2. Nitrogen is relatively inert as compared to phosphorus. Why?
Answer: Since P-P single bond is much weaker than N = N bond. Therefore, P is more reactive and N is inert.
Question.6. What are monosaccharides?
Answer : Monosaccharides are the carbohydrates that can’t be further hydrolysed to simpler smaller units, e. g. Glucose, fructose.
Question.8. What is meant by‘copolymerisation’?
Answer : The process of polymerisation from a mixture of two or more unsaturated monomer units is called copolymerization e. g. Buna-S, Nylon-6,6.
Question.13. Define the following:
- Reversible sols
- The process of converting 4 precipitate into a colloidal solution by shaking in dispersion medium and adding suitable electrolyte is called peptization.
- Reversible sols are lyophil’ic sols that can be remade by simple mixing dispersed phase with the dispersing medium.
Question.15. Complete the following chemical reaction equation:
Question.17. Give the chemical tests to distinguish between the following pairs of compounds :
(i) Methylamine and Dimethylamine
(ii)Aniline and N-Methylamine
Answer: (i) Methylamine gives carbylamine test when heated with alcoholic solution of KOH and chloroform and gives foul smell of methyl isocyanide. Dimethylamine does not give carbylamines test.
Question.18. Draw the structure of the monomers of the following
Question.19. Silver crystallizes in face centred cubic unit cell. Each side of the unit cell is of length 409 pm. What is the radius of t silver atoms?
Question.20. What mass of ethylene glycol (molar mass = 62.0 g mol1) must be added to 5.50 kg of water to lower the freezing point of water from 0° C to – 10.0°C? ( Kf for water = 1.86 K kg mol-1)
Question.27. What are analgesic drugs? How are they classified and when are they usually recommended for use?
Answer: Analgesics drugs : The class of drugs used to relieve pain and inflammation are called analgesic drugs.
They are classified as :
- Narcotic analgesics
- Non-narcotic analgesics
Non-narcotic drugs are used in prevention of heart attacks like aspirin and Narcotic drugs are required for relief of cardiac pain, post operative pain. e. g. Morphine, heroin, etc.
Note: Except for the following questions, all the remaining question have been asked in previous sets.
Question.1.Write a distinguishing feature of metallic solids.
Answer : Metallic solids have metallic bonds. They have positive metal centres and mobile electrons due to which they have high electrical and thermal conductivity.
Question.3.Differentiate between molarity and molality of a solution.
Answer: Molarity is the number of moles of solute dissolved per litre of the solution
Question.8.What are the products of hydrolysis of sucrose?
Answer : Dextrorotatory glucose and laevorotatory fructose
Question.20. 15 g of an unknown molecular substance was dissolved in 450 g of water. The resulting solution freezes at -0.34°C. What is the molar mass of the substance? (Kf for water = 1.6 Kkg mol-1 )
Question.22. How would you account for the following:
- The electron gain enthalpy with negative sign is less for oxygen than- that for sulphur.
- Phosphorus shows greater tendency for catenation than nitrogen.
- Fluorine never acts as the central atom in polyatomic interhalogen compounds.
- Due to high electronegativity and small size of oxygen compared to sulphur.
- Due to low electronegativity, large size and presence of d-orbitals in phosphorus.
- Fluorine has small size therefore, it cannot accommodate larger sized halogen atoms around it, d-orbitals are absent therefore, it does not show positive oxidation state of +3, +5 and +7 hence does not act as a central atom in polyatomic inter-halogen compounds.
Question.25. Write the name, the state of hybridization, the shape and the magnetic behaviour of the following complexes :
Question.26. Differentiate between fibrous proteins and globular proteins. What is meant by the denaturation of a protein?
The breaking down of proteins into amino acids due to high temperature is known as denaturation.
Question.27. Explain the following terms with an example for each :
- Antibiotics are substances obtained from micro¬organisms to inhibit the growth or kill microorganisms, e. g. penicillin, ampicillin, etc.
- Antiseptics are antimicrobial substances that are applied
to living tissues present microbial growth and also kill them. e. g. dettol, iodine, etc.
- Analgesics are substances that are used to get relief from pain. e.g. paracetamol aspirin, etc.