Important Questions for CBSE Board Exams (Chemistry)

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Q.1) Which point defect in crystals does not alter the density of the relevant solid?

Answer: Frenkel defect.


Q.2) Define the term ‘Tyndall effect’.

Answer: The scattering of light by colloidal particles is known as Tyndall effect.


Q.3) Why is the froth floatation method selected for the concentration of sulphide ores?

Answer: As only sulphide ore particles are wetted by oil while gangue particles are wet by water.


Q.4) Why is Bi (V) a stronger oxidant than Sb (V)?

Answer: Because Bi (V) is more stable than Sb (V) due to inert pair effect.


Q.5) Write the structure of 3-oxopentanal.

Answer:

Picture1.png




Q.6) Why is an alkyl amine more basic than ammonia?

Answer: Alkyl amine is more basic than ammonia because the + I effect or electron donating nature of alkyl group increases electron density on ‘N’ atom in alkyl amine.


Q.7) Give an example of elastomers.

Answer: Buna–S, neoprene


Q.8) 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?

Answer:

Let the rate law be,

            14.PNG

(i) If [A] is doubled than rate, 

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(ii)    If [A] is reduced to half then rate, Picture2.png


Q.9) Explain the role of

    (i) Cryolite in the electrolytic reduction of alumina.

   (ii)Carbon mono-oxide in the purification of nickel.

Answer: (i) Role of cryolite

  •        It lowers the melting point of the mixture.
  •        It makes alumina a good conductor of electricity.

(ii) When nickel is heated with carbon monoxide it forms a volatile complex nickel tetra carbonyl which on further heating at higher temperature decomposes to give pure nickel.

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Q.10) Draw the structures of the following molecules:

         (i) XeF4         (ii) BrF3

Picture4.png

Picture6.png           Picture5.png


Q.11) Complete the following chemical reaction equations:

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Answer:

Picture8.png


Q.12) Differentiate between molality and molarity of a solution. What is the effect of change in temperature of a solution on its molality and molarity?

Answer:

*Molality is the number of moles of solute per thousand grams of solvent whereas molarity is the number of moles of solute dissolved in one litre of solution.

*Molality is independent of temperature whereas molarity changes with change in temperature as volume changes with temperature.


Q.13) Which ones in the following pairs of substances undergoes SN2 substitution reaction faster and why?

Picture9.png

Answer: (i) It is primary halide therefore undergoes SN2 reaction faster.


Q.14) Explain what is meant by

         (i) a peptide linkage       (ii) a glycosidic linkage

Answer: (i) The      Picture10.png    bond formed between two amino acid molecules with loss of water in a polypeptide is called peptide linkage.

(ii) The linkage between two mono-saccharides molecules through oxygen atom in a disaccharide or polysaccharide is known as glycosidic linkage.


Q.15) Name two water soluble vitamins, their sources and the diseases caused due to their deficiency in diet.

Answer: B group vitamins and vitamins C are soluble in water.Picture11.png


Q.16) Draw the structures of the monomers of the following polymers:

                (i)Teflon           (ii) Polythene

Answer:

Picture12.png


Q.17) Iron has a body-centred cubic unit cell with a cell edge of 286.65 pm. The density of iron is 7.87g cm–3. Use this information to calculate Avogadro’s number (At. mass of Fe = 56g mol–1).

Answer: 

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Q.18) Account for the following:

(i)NH3 is a stronger base than PH3.

(ii)Sulphur has a greater tendency for catenation than oxygen.

Answer: (i) NH3 is stronger base than PH3. This is because the lone pair of electrons on N atom in NH3 is directed and not diffused as it is in PH3 due to larger size of phosphorus and hence more available for donation.

(ii) Sulphur has a greater tendency for catenation than oxygen because S-S bond is stronger than  O – O bond due to less interelectronic repulsions.


Q.19) Which point defect in its crystal units alters the density of a solid?

Answer: Schottky defect


Q.20) What is the difference between multimolecular and macromolecular colloids? Give one example of each. How are associated colloids different from these two types of colloids?

Answer: Multimolecular colloids: In this type of colloids, colloidal particles are aggregates of atoms or molecules each having size less than 1nm, e.g., sulphur sol, gold sol.

Macromolecular colloids: In this type of colloids, colloidal particles are themselves large molecules of colloidal dimensions, e.g., starch, proteins, polyethene, etc.

Associated colloids: There are certain substances which at low concentrations behave as normal electrolyte, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. Such colloids are known as associated colloids, e.g., soaps and detergents.


Q.21) Explain the following observations:

Fluorine does not exhibit any positive oxidation state.

Answer: Fluorine does not exhibit any positive oxidation state as it is the most electro-negative element and does not have d-orbitals in its valance shell.


Q.22) Explain the following observations.

      (i) The majority of known noble gas compounds are those of Xenon.

     (ii) Phosphorus is much more reactive than nitrogen.

Answer: (i) This is due to low ionization enthalpy of xenon.

(ii) Phosphorus is much more reactive than nitrogen as the P–P single bond (213 kJ mol-1) is much weaker than N-N triple bond (941.3 kJ mol-1).


Q.23) How do antiseptics differ from disinfectants? Give one example of each type.

Answer:

*Antiseptics are chemical substances which prevent the growth of micro-organisms and may even kill them but are not harmful to living tissues.

*Antiseptics are generally applied to living tissues such as wounds, cuts, ulcers and diseased skin surfaces.

           e.g., Dettol, furacine, soframicine are antiseptics.

*Disinfectants are chemical substances which kill micro-organisms or stop their growth but are harmful to human tissues.

*Disinfectants are applied to inanimate objects such as floor, drainage system, instrument, etc.

          e.g., Chlorine in the concentration of 0.2 to 0.4 ppm in aqueous solution and SO2 in very low concentration are disinfectants.


Q.24

 

 

 


Q.25) Which point defect in its crystal units increases the density of a solid?

Answer: Interstitial defect increases the density of a solid.


Q.26) What does the part ‘6, 6’ mean in the name nylon-6,6?

Answer: It means the two monomers combine to make nylon 6,6, contain six carbon atoms each.


Q.27) What are the following substances? Give one example of each type.

                       (i) Antacid     (ii) Non-ionic detergents    (iii) Antiseptics

Answer: (i) Antacids: Chemical substances which removes the excess acid in the stomach & raise the pH to appropriate level, e.g., sodium hydrogen carbonate, a mixture of aluminium and magnesium hydroxide, ranitidine, etc.

(ii) Non-ionic Detergents: Non-ionic detergents do not contain any ion in their constitution. One such detergent is formed when stearic acid reacts with polyethylene glycol.

(iii) Antiseptics: These are the chemical substances which prevent the growth of micro-organisms and may even kill them but are not harmful to living tissues. Antiseptics are applied to living tissues such as wounds, cuts, ulcers. Dettol, soframicine are antiseptics.


 

 


 

 


Q.30) How are the following colloids different from each other in respect of dispersion medium and dispersed phase? Give one example of each type.

(i) An aerosol (ii) A hydrosol  (iii) An emulsion

Answer:

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Q.31) Bond dissociation energy of F2 is less than that of Cl2. Explain.

Answer: Bond dissociation energy of F2 is less than Cl2 this is due to relatively large electron – electron repulsion among the lone pairs in F2 molecule where they are much closer to each other than in case of Cl2.


 

 

 

 

 

 

 

 

 

 

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