Board Paper of Class 12-Science 2013 Chemistry (SET 3) - Solutions

What is the difference between multi-molecular and macromolecular colloids? Give one example of each. 

(a) Which metal in the first transition series (3d series) exhibits + 1 oxidation state most frequently and why? 

(b) Which of the following cations are coloured in aqueous solutions and why ?

Sc³⁺, V³⁺, Ti⁴⁺, Mn²⁺ (At. Nos. Sc = 21, V = 23, Ti = 22, Mn = 25)

What happens when 

(i) PCl₅ is heated ?

(ii) H₃PO₃ is heated ?

Write the reactions involved.

18 g of glucose, C₆H₁₂O₆ (Molar Mass = 180 g mol⁻¹) is dissolved in 1 kg of water in a sauce pan. At what temperature will this solution boil? 

(K_b for water = 0.52 K kg mol⁻¹, boiling point of pure water = 373.15 K)

Explain the mechanism of the following reaction: 

(a) Give an example of zone refining of metals.

(b) What is the role of cryolite in the metallurgy of aluminium? 

Write the dispersed phase and dispersion medium of the following colloidal systems: 

(i) Smoke

(ii) Milk

OR

What are lyophilic and lyophobic colloids ? Which of these sols can be easily coagulated on the addition of small amounts of electrolytes?

The conductivity of 0.20 M solution of KCl at 298 K is 0.025 S cm⁻¹. Calculate its molar conductivity. 

Account for the following:

(i) The C−Cl bond length in chlorobenzene is shorter than that in CH₃ − Cl.

(ii) Chloroform is stored in closed dark brown bottles. 

How will you convert: 

(i) Propene to Propan-1-ol?

(ii) Ethanal to Propan-2-ol?

After watching a programme on TV about the adverse effects of junk food and soft drinks on the health of school children, Sonali, a student of Class XII, discussed the issue with the school principal. Principal immediately instructed the canteen contractor to replace the fast food with the fibre and vitamins rich food like sprouts, salad, fruits etc. This decision was welcomed by the parents and the students. (3)

After reading the above passage, answer the following questions:

(a) What value are expressed by Sonali and the Principal of the school?

(b) Give two examples of water-soluble vitamins.

How would you account for the following? 

(i) Transition metals exhibit variable oxidation states.

(ii) Zr (Z = 40) and Hf (Z = 72) have almost identical radii.

(iii) Transition metals and their compounds act as catalyst.

OR

Complete the following chemical equations:

(i)

(ii)

(iii)

(a) Which one of the following is a food preservative? (3)

Equanil, Morphine, Sodium benzoate

(b) Why is bithional added to soap?

(c) Which class of drugs is used in sleeping pills?

(a) What type of semiconductor is obtained when silicon is doped with boron? 

(b) What type of magnetism is shown in the following alignment of magnetic moments?

(c) What type of point defect is produced when AgCl is doped with CdCl₂?

Give the structures of products A, B and C in the following reactions:

(i)

(ii) (3)

Write the IUPAC names of the following coordination compounds: 

(i) [Cr(NH₃)₃Cl₃]

(ii) K₃[Fe(CN)₆]

(iii) [CoBr₂(en)₂]⁺, (en = ethylenediamine)

Determine the osmotic pressure of a solution prepared by dissolving 2.5 × 10⁻² g of K₂SO₄ in 2L of water at 25°C, assuming that it is completely dissociated. 

(R = 0.0821 L atm K⁻¹ mol⁻¹, Molar mass of K₂SO₄ = 174 g mol⁻¹)

Calculate the emf of the following cell at 298 K: 

Write the names and structures of the monomers of the following polymers:

(i) Bakelite

(ii) Nylon-6

(ii) Polythene 

(a) Give reasons for the following:

(i) Bond enthalpy of F₂ is lower than that of Cl₂.

(ii) PH₃ has lower boiling point than NH₃.

(b) Draw the structures of the following molecules:

(i) BrF₃

(ii) (HPO₃)₃

(iii) XeF₄ 

OR

(a) Account for the following:

(i) Helium is used in diving apparatus.

(ii) Fluorine does not exhibit positive oxidation state.

(iii) Oxygen shows catenation behavior less than sulphur.

(b) Draw the structures of the following molecules:

(i) XeF₂

(ii) H₂S₂O₈

(a) Although phenoxide ion has more number of resonating structures than Carboxylate ion, Carboxylic acid is a stronger acid than phenol. Give two reasons.

(b) How will you bring about the following conversions?

(i) Propanone to propane

(ii) Benzoyl chloride to benzaldehyde

(iii) Ethanal to but-2-enal 

OR

(a) Complete the following reactions:

(i)

(ii)

(iii)

(b) Give simple chemical tests to distinguish between the following pairs of compounds:

(i) Ethanal and Propanal

(ii) Benzoic acid and Phenol

(a) A reaction is second order in A and first order in B.

(i) Write the differential rate equation.

(ii) How is the rate affected on increasing the concentration of A three times?

(iii) How is the rate affected when the concentrations of both A and B are doubled?

(b) A first order reaction takes 40 minutes for 30% decomposition. Calculate t_1/2 for this reaction. (Given log 1.428 = 0.1548) (5)

OR

(a) For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction.

(b) Rate constant ‘k’ of a reaction varies with temperature ‘T’ according to the equation:

Where E_a is the activation energy. When a graph is plotted for a straight line with a slope of −4250 K is obtained. Calculate ‘E_a’ for the reaction.

(R = 8.314 JK⁻¹ mol⁻¹)