CBSE Previous Year Solved Papers Class 12 Chemistry Delhi 2011

CBSE Previous Year Solved  Papers  Class 12 Chemistry Delhi 2011

Time allowed: 3 hours                                                                                           Maximum Marks: 70

General Instructions:

1.  All questions are compulsory. 
2.  Questions number 1 to 5 are very short-answer questions and carry 1 mark each.
3.  Questions number 6 to 10 are short-answer questions and carry 2 marks each.
4.  Questions number 11 to 22 are also short-answer questions and carry 3 marks each.
5.  Questions number 23 is a value based question and carry 4 marks.
6.  Questions number 24 to 26 are long-answer questions and carry 5 marks each. 
7.  Use log tables, if necessary. Use of calculators is not allowed.

SET I

Note: Except for the following questions, all the remaining questions have been asked in previous sets.

Question.1.‘Crystalline solids are anisotropic in nature’. What does this statement mean?
Answer : It means that some of the physical properties of crystalline solids such as refractive index, electrical conductivity, thermal expansion, etc. show different values when measured along different directions in the same crystal.

Question.2.Express the relation between conductivity and molar conductivity of a solution held in a cell.
Answer : The molar conductivity of a solution is related to conductivity of that solution as :

Question.3.Define ‘Electrophoresis’.
Answer: Electrophoresis is the phenomenon of movement of colloidal particles under the applied electric field.

Question.4.What is the structure of XeF₂ molecule? Draw it.
Answer:

Question.5.Write the IUPAC name of the following compound : (CH₃)₃CCH₂Br
Answer: l-bromo-2, 2-dimethylpropane

Question.6.Draw the structure of 3-methylbutanal.
Answer:

Question.7.Arrange the following compounds in an increasing order of their solubility in water:

Question.8.Biodegradable polymers.
Answer : Bio-degradable polymers are natural polymers that disintegrated themselves over a period of time by enzymatic hydrolysis, e.g. starch, cellulose, etc.

Question.9.The chemistry of corrosion of iron is essentially an electro-chemical phenomenon. Explain the reactions occurring during the corrosion of iron in the atmosphere.
Answer : According to electrochemical theory of rusting, the impure iron surface behaves like small electrochemical cell in the presence of water containing dissolved oxygen or carbon dioxide. In this cell pure iron acts as an anode and impure iron surface acts as cathode. Moisture having dissolved CO₂ or O₂  acts as an electrolyte. The reactions at cathode and anode are as follows :

Question.10. Determine the values of equilibrium constant (K_c) and ΔG° and for the following reaction :

Question.11. Distinguish between ‘rate expression’ and ‘rate constant’ of a reaction.
Answer : Rate constant is the rate of reaction when the concentration of each reactant is taken as unity.
Rate expression expresses the rate fef reaction in terms of molar concentrations of the reactants with each term raised to their power, which may or may not be same as the stoichiometric coefficient of that reactant in the balanced chemical equation.

Question.12. Give reason for :

1.  The N-O bond in NO^–₂is shorter than the N-O bond in NO^–₃
2. SF₆ is kinetically an inert substance.

OR
State reasons for each of the following:

1. All the P-Cl bonds in PCl₅ molecules are not equivalent.
2. Sulphur has greater tendency for catenation than oxygen.

Answer :

1. The resonating structure of NO^–₂shows that NO₂ two bonds are sharing double bond while in NO^–₃ three bonds are sharing a double bond. Thus, NO^–₂ has shorter bond than that of NO^–₂ .
   Resonating structure of NO^–₂ and NO^–₃.
   
2. SF₆ is kinetically inert due to high oxidizing power and electronegativity of fluorine atom which causes steric hindrance and it unable to react further with any other atom.

OR

1.  It is because all bonds are not in same plane. Three P-Cl equatorial bonds are in one plane whereas two axial bonds are in vertical plane which are longer than equatorial bonds.
   
2. Sulphur has a much greater tendency for catenation than oxygen because of its bigger size and low electronegativity due to which the S-S bond is stronger than 0-0 bond and there is more interelectronic repulsion in 0-0 than in S-S bond.

Question.13. Assign reasons for the following:

1. Copper (I) ion is not known in aqueous solution.
2.  Actinoids exhibit greater range of oxidation states than lanthanoids.

Answer:

1.  In aqueous solution Cu+ undergoes disproportionation to form a more stable Cu²⁺ ion.
   2Cu⁺(aq) → Cu²⁺(aq) + Cu(s)
   The higher stability of Cu²⁺ ion is aqueous solution may be attributed to its greater negative  Δ_hydH than that of Cu⁺ion. It compensates the second ionisation enthalpy of Cu involved in the formation of Cu²⁺ions.
2. Actinoids exhibit greater range of oxidation states than lanthanoids. This is because there is less energy difference between 5d and 6d orbitals belonging to actinoids than the energy difference between 4d and 5d orbitals in case of lanthanoids.

Question.14.Explain the following giving one example for each :

1. Reimer-Tieman reaction.
2. Friedel Craft’s acetylation of anisole.

Answer :

1.  When phenol is treated with chloroform in presence of aqueous NaOH at 340 K followed by hydrolysis gives salicylaldehyde.
   
2.  When anisole is treated with acetylchloride in presence of anhydrous AlCl₃, 2-methoxy acetophenone is formed.
   

Question.15. How would you obtain :
(i) Picric acid (2,4, 6-Trinitrophenol) from phenol.
(ii)2-Methylpropene from 2-methylpropanol?
Answer:

Question.16. What is essentially the difference between a-form of glucose and p-form of glucose? Explain.
Answer : In a-form of glucose OH group lies at Cl towards right but in P-form of glucose it is towards left. This is because OH group at Cl in glucose is chiral.

Question.17. Describe what you understand by primary structure and secondary structure of proteins.
Answer : Primary structure of proteins refer to the*sequence in which amino acids are joined together by peptide linkage. The sequence of amino acids in primary structure is very specific. Any change in the sequence of amino acids creates a different protein with different biological activity.
Secondary structure of proteins refers to the conformation which arises due to the coiling of polypeptide chain due to intramolecular hydrogen bonding between carbonyl and —NH groups. Depending upon the size of -R group, there are two secondry structures of pfotein, i.e. a -Helix and p -Pleated sheet like structure.

Question.18. Mention two important uses of the following

1.  Bakelite
2. Nylon-6

Answer: Bakelite: It is used in the manufacture of electrical switches, protective coatings, barrels, etc.
Nylon-6 : It is used in making Fabrics, tyre cords, mountaineering ropes etc.

Question.19. Silver crystallizes in face-centered cubic unit cell. Each side of this unit cell has a length of 400 pm. Calculate the radius of the silver atom. (Assume the atoms just touch each other on the diagonal across the face of the unit cell. That is each face atom is touching the four comer atoms).

Question.20. Nitrogen pentoxide decomposes according to equation :

(i) Calculate the rate constant. Include units with your answer.
(ii)What will be the concentration of N₂O₅ after 100 minutes?
(iii)Calculate the initial rate of reaction.
Answer. (i) We Know that

Question.21. Explain how the phenomenon of adsorption finds application in each of the following processes :

1. Production of vacuum
2. Heterogeneous catalysis
3. Froth Floatation Process

OR
Define each of the following terms :

1.  Micelles
2. Peptization
3. Desorption

Answer :

1. In vacuum flasks, activated charcoal is placed between the walls of the flask so that any gas which enters into annular space either due to glass imperfection or diffusion through glass is adsorbed and create a vacuum.
2. If the catalysts and reactants are present in different phase, the process of catalysis is called as heterogenous catalysis. For example, manufacture of NH₃ from N₃ and H₂ by Haber’s process using iron as catalyst
   
   In this process, the-reactants are in gaseous phase whereas catalyst is in solid phase. *
3. This method is used for removing gangue from sulphide ores. In this powdered ore is mixed with collectors (e.g. pine oil, fatty acids etc.) and froth stabilizers (e.g. cresols, aniline) which enhance non-wet ability of the mineral particles and froth stabilisation respectively. As a result of which ore comes with froth and gangue remain in the solution.
   

 

1. When soaps and detergents are added to water, a cluster of charged particle is formed by the aggregation of variety of molecules. Thus formed is called micelle.
2. The process of converting a fresh precipitate into colloidal particles by shaking it with the dispersion medium in the presence of a small amount of a suitable electrolyte is called peptization.
3. The process of removal of adsorbed substance from the surface of a solid or a liquid by heating or by reducing pressure is called desorption.

Question.22. Describe the principle behind each of the following
processes:

1.  Vapour phase refining of a metal.
2. Electrolytic refining of a metal
3. Recovery of silver after silver ore was leached with NaCN.

Answer :

1. Vapour Phase refining: The impure metal is first converted to its unstable volatile compound which is evaporated and then decomposed by heating at higher temperature to give pure metal, leaving behind the impurities.
2. In this method impure metal is made anode and a thin sheet of pure metal is made cathode, and are put in a suitable electrolyte containing soluble salt of same metal. On passing current the more basic metal remains in the solution and the less basic one go to the anode and gets deposited as anode mud.
3. NaCN acts as a leaching agent or oxidizing agent, thus oxidize Ag to Ag⁺  which then combine with CN^–  ions to form respective soluble complex.
   

Question.23.Complete the following chemical equations :

Question.24. Write the name, stereochemistry and magnetic behaviour of the following:

Question.25. Answer the following:
(i) Haloalkanes easily dissolves in organic solvents, why?
(ii)What is known as a racemic mixture? Give an example.
(iii)Of the two bromoderivatives, C₆H₅CH(CH₃)Br and C₆H₅CH( C₆H₅)Br, which one is more reactive in SNI substitution reaction and why?
Answer :
(i) Haloalkanes dissolve in organic solvents because the new intermolecular attraction between haloalkanes and organic solvent molecules have the same strength as the one being broken in the separate haloalkanes and solvent molecules.
(ii) Racemic mixture is an equimolar mixture of two enantiomers and is hence optically inactive, e.g. (± butan-2-ol)

Question.26. (a) Explain why an alkylamine is more basic than ammonia?
(b) How would you convert:
(i) Aniline to nitrobenzene
(ii)Aniline to iodobenzene
Answer :
(a) Alkyl groups are electron donating groups and increase the electron density on nitrogen in alkylamine making them more basic than ammonia.

Question.27. Describe the following giving one example for each :

1. Detergents
2. Food preservatives
3. Antacids

Answer :

1. Detergents are soluble salts of sodium potassium sulphonic acids unlike soaps they are non-biodegradble but they can be conveniently used both with soft and hard water. e.g. Sodium alkylbenzene sulphonate.
2. Food preservatives are chemicals used to preserve food by preventing microbial growth e.g. Sodium benzoate, Table salt, etc.
3. The substances which are taken to neutralize the excess acid and maintaining the pH to an appropriate level in stomach are called antacids. There are two types of anatacids systemic antacids, e.g, NaHCO₃ and non-systemic e.g. Milk of magnesia.

Question.28. (a) Differentiate between molality and molarity of a solution. How does a change in temperature influence their values?
(b) Calculate the freezing point of an aqueous solution containing 10.50 g of MgBr₂  in 200 g of water. (Molar mass of MgBr₂ = 184 g) (K_ƒ for water = 1.86 K kgmol^-1
OR
(a) Define the terms osmosis and osmotic pressure. Is the osmotic pressure of a solution a colligative property? Explain
(b) Calculate the boiling point of a solution prepared by adding 15.00 g of NaCl to 250.0 g of water. (K_b for water = 0.512 K kg mol^-1, Molar mass of NaCl = 58.44 g)
Answer:
(a) Molarity is the number of moles of solute dissolved in 1 litre of solution. It is temperature dependent.

Molality is the number of moles of solute dissolved per 1 kg of the solvent. It is temperature independent.

Molality is independent of temperature, whereas molarity is a function of temperature because volume depends on temperature and mass does not.
(b) Since MgBr₂ is an isotonic compound. Hence undergoes complete dissociation

(a) Osmosis is the phenomenon of flow of solvent through a semi-permeable membrane from the region of higher concentration to the region of lower concentration.
The osmotic pressure of a solution is the excess pressure that must be applied to the solution to prevent the passage of solvent molecule through a semi-permeable membrane into the solution.
Yes, osmotic pressure is a colligative property as it depends only on the amount of solute present in the solution.
(b) Initial moles after dissociation we know that,

Question.29. Give chemical tests to distinguish between:
(i) Propanal and propanone
(ii)Benzaldehyde and acetophenone (b) How would obtain :
(i) But-2-enal from ethane
(ii)Butanoic acid from butanol
(iii)Benzoic acid from ethylbenzene
OR
(a) Describe the following giving linked chemical equations
(i) Cannizzaro reaction
(ii)Decarboxylation
(b) Complete the following chemical equations :

Question.30. (a) Explain the foUowing:
(i) NF₃ is an exothermic compound whereas NCl₃ is not.
(ii)F₂is most reactive of all the four common halogens,
(b) Complete the following chemical equations:

(a) Account for the following :
(i) The acidic strength decreases in the order HCl > H₂S > PH₃
(ii)Tendency to form pentahalides decreases down the group in group 15 of the periodic table.
(b) Complete the following chemical equations :

Answer : (a) (i) Due to smaller size of F as compared to Cl, the N-F bond is much stronger than N-Cl bond while bond dissociation energy of F₂ is much lower than that of Cl₂. Therefore, energy related during the formation of NF₃ molecule is more than the energy needed to break N₂ and F₂ molecules into individual atoms. In other words, formation of NF₃ is an exothermic reaction.
The energy released during the formation of NCl₃ molecule is less than the energy needed to break N₂ and Cl₂ molecule into individual atoms. Thus, formation of NCl₃ is an endothermic reaction.
(ii) Fluorine is most reactive of all the four common halogens because of is low bond dissociation energy due to which it readily dissociates into atoms and reacts with other substances readily

(a) (i) Because of decrease in electronegativity from chlorine to phosphorus, the dissociation enthalpy from HCl to H-P increases, and their tendency to release H⁺ ion decreases and thus acidic strength decreases.
(ii) This is due to inert pair effect. The stability of +5 oxidation state decreases down the group in group 15.

SET II

Note : Except for the following questions, all the remaining question have been asked in previous set.

Question.1. Which stoichiometric defect in crystals decreases the density of a solid?
Answer : Schottky defect.

Question.3. What is meant by shape-selective catalysis of reactions?
Answer: The reaction that depend for shape selective catalysts uses zeolites as catalyts for reaction on the shape and size of pores and of reactants and products.

Question.4. Draw the structure of XeF₄ molecule
Answer:

Question.9. Explain what is meant by

1.  a peptide linkage,
2. a glycosidic linkage?

Answer :

1.  Peptide linkage is present in proteins to bind together amino acids. The linkage involves the carboxyl group of one amino acid and amine group of another amino acids,
2. Glycosidic linkage is the (-C-O-C-) linkage present between two molecules of a monosaccharide to form a disaccharide.

Question.10. Name the bases present in RNA. Which one of these is not present in DNA.
Answer : Four bases present in RNA are adenine, guanine, Cytosine and Uracil.

Question.22. Explain the role of each the following in the extraction of metals from their ores :
(i) CO in the extraction of nickel.
(ii) Zinc in the extraction of silver.
(iii) Silica in the extraction of copper.

Question.24. For the complex [Fe(en)₂Cl₂], identify the following:

1. Oxidation number of iron.
2. Hybrid orbitals and shape of the complex
3. Magnetic behaviour of the complex
4. Number of its geometrical isomers.
5.  Whether there may be optical isomer also.
6. Name of the complex.

Answer:

1. +3
2. Sp³d²,octahedral shape.
3. paramagnetic
4. 2 geometrical isomers, cis and trans.
5.  Only cis-isomers shows optical isomerisrrt
6. Dichloridobis (ethylenediamine) iron(III) chloride

Question.27. Explain the following terms with one suitable example for each:

1. A sweetening agent for diabetic patients
2. Enzymes
3. Analgesics

Answer :

1.  Artificial sweeting agent are chemicals that sweeten food. However, unlike natural sweetners, they do not add calories to our body, not harmful to diabtic patients, e.g. Saccharin, aspartame.
2. Enzymes are biocatalysts which are structurally globular proteins. They are sensitive to substrate, pH and temperature changes, e.g. Trypsin
3. Analgesics are chemical substances which reduces pain
   without causing impairment of consciousness, mental confusion, in coordination of paralysis or some other disturbance of nervous system, e.g. Aspirin, (non-narcotic analgesic) and morphine (narcotic analgesic)

Question.28. (a) State the following:

1.  Henry’s law about partial pressure of a gas in a mixture.
2. Raoult’s law in its general form in reference to solutions.

(b) A solution prepared by dissolving 8.95 mg of a gene fragment in 35.0 mL of water has an osmotic pressure of 0.335 torr at 25°C. Assuming the gene fragment is non-electrolyte, find its molar mass.
OR
(a) Difference between molarity and molality in a solution. WTiat is the effect of temperature change on molarity and molality in a solution?
(b) What would be the molar mass of a compound if 6.21 g of it dissolved in 24.0 g of chloroform form a solution that has a boiling point of 68.04°C. The boiling point of
pure chloroform is 61.7°C and the boiling point elevation constant, for chloroform is 3.63°C/m.
Answer : (a)

1. Henry’s law states that the partial pressure of gas in vapour phase is directly proportional to its mole fraction in the solution,
2.  Raoults law states that for a solution with volatile components, the partial vapour pressure of each component present in the solution is proportional to its mole fraction in the solution.

OR
(a) Molarity is defined as the number of moles of solute present in 1 litre of solution, while molality is defined as the number of moles of solute present in 1 kg of solvent. Molality does not have any effect of change in temperature because mass does not change with temperature, whereas molarity changes with temperature.

SET III

Note : Except for the following questions, all the remaining question have been asked in previous sets.

Question.4. Draw the structure of BrF₃ molecule

Question.8. In nylon 6, 6 what does the designation ‘6,6’ mean?
Answer: 6,6-refers to the number of carbon atoms in each of its monomer, adipic acid and hexamethyldiamine

Question.9.What type of a battery is lead storage battery? Write the
anode and the cathode reactions and the overall reactions occurring in a lead storage battery.
Answer : The lead storage battery is a secondary cell which is rechargeable. During discharging, the electrode reaction occurs as follows :
At anode:

Question.10. The half-reactions of an electrochemical cell are given below:

Question.13. Assign reasons for each of the following:

1.  Transition metals generally form coloured compounds.
2. Manganese exhibits the highest oxidation state of +7 among the 3d series of transition elements.

Answer :

1. Due to presence of unpaired electrons and d-d transition, the transition metals are generally coloured.
2.  Manganese (z = 25), has the maximum number of unpaired electrons. Thus, it shows oxidation states from +2 to +7 which is maximum in number as compared to other elements of transition series.

Question.18. Name the sub-groups into which polymers are classified on the basis of magnitude intermolecular forces.
Answer : Elastomers : They have weakest intermolecular forces of attraction.
Fibres : They have strong intermolecular forces of attraction among its molecules.
Thermoplastics Polymers : They are semifluid substances having low molecular weight.
Thermosetting polymers – They have intermolecular. forces intermediate between those of elastomers and fibres.

Question.19. The density of lead is 11.35 g cm^-3 and the metal crystallizes with fee unit cell. Estimate the radius of lead atom. (At.mass of lead = 207 g mol^-1 and N_A = 6.02 x 10²³ mol^-1)

Question.26. Complete the following chemical equations :

Answer:

Question.27. Answer the following questions :

1. Why do soaps not work in hard water ?
2. What are the main constituents of dettol?
3. How do antiseptics differ from disinfectants?

Answer :

1. Hard water contains insoluble chloride of calcium and magnesium which form insoluble ppt. (scum) with soap and thus cannot be rinsed off easily.
2. The main constituents of dettol are chloroxylenol and a-terpine.
3. Antiseptics : These are chemical substances which either kill or prevent the growth of micro-organism but do not cause harm to the living tissues.
   Disinfectants : These are chemical substances which kill the  microbes. They are toxic in nature and thus cause harm to the tissues of the skin