CBSE EXAMINATION PAPER-2023

The correct option is 'a dehydrohalogenation reaction' because the context states that alkyl halides react with alcoholic KOH to eliminate one molecule of halogen acid, forming alkenes. This process is specifically termed dehydrohalogenation, which involves the removal of halide and hydrogen, leading to the formation of a double bond in the alkene.

The correct answer is -k/2.303. In a first order reaction, the natural logarithm of the concentration of the reactant [R] versus time yields a straight line where the slope is equal to -k, the rate constant. The factor 2.303 is used to convert the natural logarithm to logarithm base 10, which is standard in reaction kinetics.

The correct answer is Polypeptides. An α-helix is a common secondary structure found in proteins (which are made of polypeptides). It is characterized by a right-handed coil, stabilized by hydrogen bonds between the backbone of the polypeptide chain.

Racemisation occurs in Sₙ1 reaction because this reaction involves the formation of a carbocation intermediate, which can lead to the production of both enantiomers, resulting in racemisation. In contrast, Sₙ2 reactions do not involve intermediates and therefore do not lead to racemisation.

The correct answer is 'increases with decrease in temperature.' Henry's law states that the solubility of a gas in a liquid is inversely related to temperature. Therefore, as temperature decreases, more gas can dissolve in the liquid, resulting in an increase in Henry’s constant.

The correct answer is 'Both (a) and (b)'. Both reactions involve the reaction between alkyl halides and sodium alkoxide, which are feasible under appropriate conditions. (CH₃)₃C–Cl can react with Na⁺ O–CH₂CH₃ to form an ether, while CH₃CH₂Br can react with Na⁺ O–C(CH₃)₃ to also form an ether. Both reactions are classified as nucleophilic substitutions, which are feasible in organic chemistry.

HCHO (formaldehyde) is the most reactive in nucleophilic addition reactions due to its structure, which has a highly polarized carbonyl group. The carbon atom in formaldehyde is more electrophilic and thus more susceptible to attack by nucleophiles compared to the other options, which have steric hindrance or less polarization in their carbonyl groups.

The correct option is –(2/3)d[A]/dt. This is because the stoichiometry of the reaction shows that for every 3 moles of A that react, 2 moles of B are produced. Hence, the rate of formation of B is related to the rate of consumption of A by the ratio of their coefficients in the balanced equation: d[B]/dt = -(2/3)d[A]/dt.

The correct option is 'Variable oxidation states'. Transition metals exhibit variable oxidation states, which allow them to easily gain or lose electrons during reactions. This characteristic is essential for their role as catalysts in various chemical processes, as they can facilitate the conversion of reactants to products by providing alternate pathways with lower activation energies.

[Pt Cl₂(en)₂](NO₃)₂ is correct because it indicates that the platinum is in the +4 oxidation state, has 2 chloride ligands (Cl₂), 2 ethane-1,2-diamine ligands (en), and is coordinated with 2 nitrate ions (NO₃) as counterions, matching the IUPAC nomenclature for the complex.

Both (A) and (R) are true, but (R) is not the correct explanation of (A). Osmotic pressure is indeed a colligative property, which depends on the number of solute particles in a solution rather than their identity. However, while osmotic pressure is related to the concentration of solute molecules, it is commonly expressed in terms of molarity rather than molality, thus making (R) inaccurate in directly explaining (A).

(A) is false, but (R) is true. Copper is actually a transition element as it is found in the d-block of the periodic table. Although it has a stable electronic configuration of 3d10 4s1, the d-orbitals are not completely filled in a traditional sense, thus making (R) true since it discusses the configuration correctly, but (A) incorrect.