Board Paper of Class 12-Science 2008 Chemistry (SET 1) - Solutions

Explain as to why haloarenes are much less reactive than haloalkanes towards nucleophilic substitution reactions.

OR

Which compound in each of the following pairs will react faster in S_N2 reaction with −OH? Why?

(i) CH₃Br or CH₃I

(ii) (CH₃)₃CCl or CH₃Cl

(a) State the IUPAC name of the following compound:

(b) Complete the following chemical equation:

CH₃CH₂CH=CH₂ + HBr

State Henry’s law correlating the pressure of a gas and its solubility in a solvent and mention two applications for the law.

A first order decomposition reaction takes 40 minutes for 30% decomposition. Calculate its value.

What is meant by the ‘rate constant, k’ of a reaction? If the concentration be expressed in mol L⁻¹ units and time in seconds, what would be the units for k (i) for a zero-order reaction and (ii) for a first-order reaction?

Define the following terms in relation to proteins:

(i) Peptide linkage

(ii) Denaturation

List the reactions of glucose which cannot be explained by its open-chain structure.

Assign a reason for each of the following statements:

(i) Ammonia is a stronger base than phosphine.

(ii) Sulphur in vapour state exhibits a paramagnetic behaviour.

Draw the structure of the following molecules:

(i) SF₄

(ii) XeF₄

What are biodegradable and non-biodegradable detergents? Give one example of each class.

What is semiconductor? Describe the two main types of semiconductors and explain mechanisms for their conduction.

Calculate the temperature at which a solution containing 54 g of glucose, (C₆H₁₂O₆), in 250 g of water will freeze.

(K_f for water = 1.86 K mol⁻¹ kg)

What are lyophilic and lyophobic sols? Give one example of each type. Which one of these two types of sols is easily coagulated and why?

State briefly the principles which serve as basis for the following operations in metallurgy:

(i) Froth floatation process

(ii) Zone refining

(iii) Refining by liquation

Write chemical equations for the following processes:

(i) Chlorine reacts with a hot concentrated solution of sodium hydroxide

(ii) Orthophosphorous acid is heated

(iii) PtF₆ and xenon are mixed together

OR

Complete the following chemical equations:

(i) Ca₃P₂(s) + H₂O(l) →……..

(iii) F₂(g) + H₂O(l) →……..

(a) What is a ligand? Give an example of a bidentate ligand.

(b) Explain as to how the two complexes of nickel, [Ni(CN)₄]²⁻ and Ni(CO)₄, have different structures but do not differ in their magnetic behaviour. (Ni = 28)

Name the reagents which are used in the following conversions:

(i) A primary alcohol to an aldehyde

(ii) Butan-2-one to butan-2-ol

(iii) Phenol to 2, 4, 6-tribromophenol

Account for the following observations:

(i) pK_b for aniline is more than that for methylamine.

(ii) Methylamine solution in water reacts with ferric chloride solution to give a precipitate of ferric hydroxide.

(iii) Aniline does not undergo Friedel-Crafts reaction.

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

(i) Buna-S

(ii) Neoprene

(iii) Nylon-6

Conductivity of 0.00241 M acetic acid solution is 7.896 × 10⁻⁵ S cm⁻¹. Calculate its molar conductivity in this solution. If for acetic acid be 390.5 S cm² mol⁻¹, what would be its dissociation constant?

OR

Three electrolytic cells A, B and C containing solutions of zinc sulphate, silver nitrate and copper sulphate respectively, are connected in series. A steady current of 1.5 ampere was passed through them until 1.45 g of silver was deposited at the cathode of cell B. How long did the current flow? What mass of copper and what mass of zinc were deposited in the concerned cells?

(Atomic masses of Ag = 108, Zn = 65.4, Cu = 63.5)

Assign reasons for the following:

(i) The enthalpies of atomisation of transition elements are high.

(ii) The transition metals and many of their compounds act as good catalyst.

(iii) From element to element the actinoid contraction is greater than the lanthanoid contraction.

(iv) The E⁰ value for the Mn³⁺/Mn²⁺ couple is much more positive than that for Cr³⁺/Cr²⁺.

(v) Scandium (Z = 21) does not exhibit variable oxidation states and yet it is regarded as a transition element.

OR

(a) What may be the possible oxidation states of the transition metals with the following d electronic configurations in the ground state of their atoms:

3d³4s², 3d⁵4s² and 3d⁶4s²

Indicate relative stability of oxidation states in each case.

(b) Write steps involved in the preparation of (i) Na₂CrO₄ from chromite ore and (ii) K₂MnO₄ from pyrolusite ore.

(a) Complete the following reaction statements by giving the missing starting material, reagent or product as required:

(i)

(ii)

(iii)

(b) Describe the following reactions:

(i) Cannizaro reaction

(ii) Cross aldol condensation

OR

(a) How would you account for the following:

(i) Aldehydes are more reactive than ketones towards nucleophiles.

(ii) The boiling points of aldehydes and ketones are lower than those of the corresponding acids.

1. The aldehydes and ketones undergo a number of addition reactions.

(b) Give chemical tests to distinguish between:

(i) Acetaldehyde and benzaldehyde

(ii) Propanone and propanol