Question

Group 16 elements are called chalcogens i.e., ore forming elements (oxygen, sulphur, selenium etc.) because most of the ores are oxides and sulphides. Oxygen is gas where as other elements of group 16 are solids. Oxygen shows anomalous behaviour. Oxygen is diatomic where is sulphur exists as $S_8$ which has crown shaped structure. It shows allotropy. Sulphur is present in onion and garlic that is why they have pungent smell. Sulphur is used for manufacture of sulphuric acid which is called ‘King of chemicals’, used in fertilizer, detergents, dyes and drugs. The following questions are multiple choice questions. Choose the most appropriate answer. 1) Group 16 elements are also known as a) Noble elements b) Halogens c) Pnictogens d) Chalcogens 2) Acidic character of hydrides of group 16 elements is in the order a) $H_2$O < $H_2$S < $H_2$Se < $H_2$Te b) $H_2$S < $H_2$Se < $H_2$Te < $H_2$O c) $H_2$O < $H_2$Se < $H_2$Te < $H_2$S d) $H_2$O < $H_2$S < $H_2$Te < $H_2$Se 3) Hybridisation of S in $SF_4$ and geometry of $SF_4$ are respectively a) $sp^3$d, trigonal pyramidal b) $sp^3$d, see saw c) $sp^3$, tetrahedral d) $dsp^2$, square planner 4) Which is not an acidic oxide? a) $CO_2$ b) $SO_2$ c) $Na_2$O d) $Cl_2$$O_7$ 5) Which is not correct about allotropes of sulphur a) The stable form at room temperature is rhombic sulphur b) Monoclinic sulphur is stable above 369 K and transforms into rhombic sulphur below it c) At 369 K both the forms are stable d) Monoclinic sulphur is soluble in $CS_2$ while rhombic sulphur not

Read the passage given below and answer the following questions: Nucleophilic substitution reactions are of two types; substitution nucleophilic bimolecular (SN2) and substitution nucleophilic unimolecular (SN1) depending on molecules taking part in determining the rate of reaction. Reactivity of alkyl halide towards SN1 and SN2 reactions depends on various factors such as steric hindrance, stability of intermediate or transition state and polarity of solvent. SN2 reaction mechanism is favoured mostly by primary alkyl halide or transition state and polarity of solvent, SN2 reaction mechanism is favoured mostly by primary alkyl halide then secondary and then tertiary. This order is reversed in case of SN1 reactions. (i) Which of the following is most reactive towards nucleophilic substitution reaction? (a) $C_6$$H_5$Cl (b) $CH_2$=CHCl (c) $ClCH_2$$CH=CH_2$ (d) $CH_3$CH=CHCl (ii) Isopropyl chloride undergoes hydrolysis by (a) SN1 mechanism (b) SN2 mechanism (c) SN1 and SN2 mechanism (d) neither SN1 nor SN2 mechanism (iii) Tertiary alkyl halides are practically inert to substitution by SN2 mechanism because of (a) insolubility (b) instability (c) inductive effect (d) steric hindrance (iv) Which of the following is the correct order of decreasing SN2 reactivity? (a) $RCH_2$X > $R_2$CHX > $R_3$CX (b) $R_3$CX > $R_2$CHX > $RCH_2$X (c) $R_2$CHX > $R_3$CX > $RCH_2$X (d) $RCH_2$X > $R_3$CX > $R_2$CHX (v) An organic molecule necessarily shows optical activity if it- a) contains asymmetric carbon atoms b) is non-polar c) is non-superimposable on its mirror image d) is superimposable on its mirror image

The solubility of gases increases with increase of pressure. William Henry made a systematic investigation of the solubility of a gas in a liquid. According to Henry’s law “the mass of a gas dissolved per unit volume of the solvent at constant temperature is directly proportional to the pressure of the gas in equilibrium with the solution”. Dalton during the same period also concluded independently that the solubility of a gas in a liquid solution depends upon the partial pressure of the gas. If we use the mole fraction of gas in the solution as a measure of its solubility, then Henry’s law can be modified as “the partial pressure of the gas in the vapour phase is directly proportional to the mole fraction of the gas in the solution”. Answer the following MCQs by choosing the most appropriate options: (i) Henry’s law constant for the solubility of methane in benzene at 298 K is 4.27 × $10^5$ mm Hg. The solubility of methane in benzene at 298 K under 760 mm Hg is (a) 4.27 × $10^{-5}$ (b) 1.78 × $10^{-3}$ (c) 4.27 × $10^{-3}$ (d) 1.78 × $10^{-3}$ (ii) The partial pressure of ethane over a saturated solution containing 6.56 × $10^{-2}$ g of ethane is 1 bar. If the solution contains 5.00 × $10^{-2}$ g of ethane then what will be the partial pressure (in bar) of the gas? (a) 0.762 (b) 1.312 (c) 3.81 (d) 5.0 (iii) KH (K bar) values for Ar(g), $CO_2$(g), HCHO(g) and $CH_4$(g) are 40.39, 1.67, 1.83 × $10^{-5}$ and 0.413 respectively. Arrange these gases in the order of their increasing solubility. (a) HCHO < $CH_4$ < $CO_2$ < Ar (b) HCHO < $CO_2$ < $CH_4$ < Ar (c) Ar < $CO_2$ < $CH_4$ < HCHO (d) Ar < $CH_4$ < $CO_2$ < HCHO (iv) When a gas is bubbled through water at 298 K, a very dilute solution of the gas is obtained. Henry’s law constant for the gas at 298 K is 150 K bar. If the gas exerts a partial pressure of 2 bar, the number of millimoles of the gas dissolved in 1 L of water is (a) 0.55 (b) 0.87 (c) 0.37 (d) 0.66

Molecular Nitrogen $N_2$ comprises about 78% by volume of Earth’s atmosphere. It occurs as Sodium nitrate, $NaNO_3$(chile saltpeter) & Potassium nitrate, $KNO_3$(Indian altpeter) in earth’s crust. Since nitrate are very soluble in water so these are not wide spread in the earth’s crust. Nitrogen is also an important constituent of amino acids, protein & nucleic acids in plants & animals. Nitrogen shows anomalous behavior from rest of the elements due to following reasons; Smaller size, high ionization enthalpy, high electronegativity & absence of d-orbital. It has unique ability to form p∏-p∏ multiple bonds with itself & with small size atoms like C & O as they have small size & high electronegativity. Heavier elements of this group do not form p∏-p∏ bonds as their atomic orbitals are so large & diffuse that they can’t have effective overlapping. Thus Nitrogen exists as diatomic molecules $N_2$ with a triple bond. Consequently, its bond enthalpy (941.4 KJ $mol^{-1}$) is very high. P, As & Sb form only single bonds as P-P, As-As & Sb-Sb. Due to much bond enthalpy N is much less reactive than P. Single N-N bond is weaker than single P-P bond due to high interelectronic repulsion of the non bonding electrons, owing to small bond length. As a result, the catenation tendency is weaker in nitrogen. Hence nitrogen exists as gas while phosphorus exists as solid. Nitrogen can’t form d∏- d∏ bond due to absence of d- orbitals so it can’t expand its covalency beyond four as heavier members can. The following questions are multiple choice questions. choose the most appropriate answer. 1) Among group 15 elements which exists as gas at room temperature a) Arsenic b) Bismuth c) Nitrogen d) Phosphorous 2) The stability of +5 oxidation state decreases and that of +3 state increases down the group in group 15 elements due to a) inert pair effect b) decrease in ionisation enthalpy c) increase in size d) shielding effect 3) Nitrogen is restricted to a maximum covalency of 4 because of a) absence of d-orbitals b) presence of d-orbitals c) absence of s and p-orbitals d) none of the above 4) Extra pure $N_2$ can be obtained by heating a) $NH_3$ with CuO b) $NH_4NO_3$ c) $\left(NH_4\right)_2Cr_2O_7$ d) $Ba\left(N_3\right)_2$ 5) Catenation tendency is weaker in nitrogen, because of a) single N–N bond is weaker b) single N–N bond is stronger c) ability to form pi bonds by N atoms d) none of the above

Read the passage given below and answer the following questions: The replacement of hydrogen atom in a hydrocarbon, aliphatic or aromatic results in the formation of haloalkanes and haloarenes respectively. Haloalkanes contain halogen atom attached to $sp^3$ hybridised carbon atom of an alkyl group whereas haloarenes contain halogen atom attached to $sp^2$ hybridised carbon atom of an aryl group. Haloalkanes and haloarenes may be classified on the basis of number of halogen atoms in their structures as mono, di or poly halogen compounds and also on the basis of the state of hybridisation of carbon atom to which the halogen atom is bonded. (i) Which of the following halide is 2°? (a) Isopropyl chloride (b) Isobutyl chloride (c) n-propyl chloride (d) n-butyl chloride (ii) Which of the following is a Gem-dibromide is : (a) $CH_3$$CH(Br)CH_2$(Br) (b) $CH_3$$CBr_2$$CH_3$ (c) $CH_2$$(Br)CH_2$$CH_2$ (d) $CH_2$$BrCH_2$Br (iii) IUPAC name of (${\\(CH_3\\)}_3$)CCl is: (a) 3-Chlorobutane (b) 2-Chloro-2-methylpropane (c) t-butyl chloride (d) n-butyl chloride (iv) Which of the following is a primary halide? (a) Isopropyl iodide (b) Secondary butyl iodide (c) Tertiarybutylbromide (d) Neohexylchloride (v) Which one of the following is not an allylic halide? (a) 4-Bromopent-2-ene (b) 3-Bromo-2-methylbut-1-ene (c) 1-Bromobut-2-ene (d) 4-Bromobut-1-ene

Proteins are high molecular mass complex biomolecules of amino acid The important proteins required for our body are enzymes, hormones, antibodies, transport proteins, structural proteins, contractile proteins etc. Except for glycine, all o-amino acids have chiral carbon atom and most of them have L-configuration. The amino acids exists as dipolar ion called zwitter ion, in which a proton goes from the carboxyl group to the amino group. A large number of-amino acids are joined by peptide bonds forming polypeptides. The peptides having very large molecular mass (more than 10,000) are called proteins. The structure of proteins is described as primary structure giving sequence of linking of amino acids; secondary structure giving manner in which polypeptide chains are arranged and folded; tertiary structure giving folding, coiling or bonding polypeptide chains producing three dimensional structures and quaternary structure giving arrangement of sub- units in an aggregate protein molecule. In these questions (Q. No. i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices. a) Assertion and reason both are correct statements and reason is correct explanation for assertion. b) Assertion and reason both are correct statements but reason is not correct explanation for assertion. c) Assertion is correct statement but reason is wrong statement. d) Assertion is wrong statement but reason is correct statement. i) Assertion .- All amino acids are optimally active. Reason : Amino acids contain asymmetric carbon atoms. ii) Assertion .- In o-helix structure, intramolecular H-bonding takes place whereas in β-pleated structure,intermolecular H-bonding takes place. Reason : An egg contains a soluble globular protein called albumin which is present in the white part. iii) Assertion .- Secondary structure of protein refers to regular folding patterns of continuous portions of the polypeptide chain Reason : Out of 20 amino acids, only 12 amino acids can be synthesized by human body. iv) Assertion .- The. helical structure of protein is stabilized by intramolecular hydrogen bond between —NH and carbonyl oxygen. Reason : Sanger’s reagent is used for the identification of N-terminal amino acid of peptide chain.

The sequence of bases along the DNA and RNA chain establishes its primary structure which controls the specific properties of the nucleic acid. An RNA molecule is usually a single chain of ribose-containing nucleotide. On the basis of X-ray analysis of DNA, J.D., Watson and F.H.C. CYST (shared noble prize in 1962) proposed a three dimensional secondary structure for DNA. DNA molecule is a long and highly complex, spirally twisted, double helix, ladder like structure. The two polynucleotide chains or strands are linked up by hydrogen bonding between the nitrogenous base molecules of their nucleotide monomers. Adenine (purine) always links with thymine (pyrimidine) with the help of two hydrogen bonds and guanine (purine) with cytosine (pyrimidine) with the help of three hydrogen bonds. Hence, the two strands extend in opposite directions, i.e., are antiparallel and complimentary. In these questions (Q. No. i-iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices. a) Assertion and reason both are correct statements and reason is correct explanation for assertion. b) Assertion and reason both are correct statements but reason is not correct explanation for assertion. c) Assertion is correct statement but reason is wrong statement d) Assertion is wrong statement but reason is correct statement. i) Assertion - DNA molecules and RNA molecules are found in the nucleus of a cell. Reason : There are two types of nitrogenous bases, purines and pyrimidines. Adenine (A) and guanine (G)are substituted purines; cytokine (C), thymine (T) arid uracil (U) are substituted pyrimidines. ii) Assertion .- In both DNA and RNA, heterocyclic base and phosphate ester linkages are at C- 1’ and C-5’respectively of the sugar molecule. Reason : Nucleotides and nucleosides mainly differ from each other in presence of phosphate units. iii)Assertion .- The backbone of RNA molecule is a linear chain consisting of an alternating units of heterocylic base, D-ribose and a phosphate. Reason : The segment of RNA which acts as the instruction manual for the synthesis of protein is ribose. iv) Assertion.- In DNA, the complementary bases are, adenine and guanine; thymine and cytosine. Reason : The phenomenon of mutation is chemical change in DNA molecule.