Tiêu đề 4. Chemical Bonding and Molecular Structure.pdf ✅

Chemical Bonding and Molecular Structure Topic 1 Electrovalent, Covalent and Co-ordinate Bonding 1. Given below are two statements Assertion (A): Ionic compounds are formed by nondirec- tional bonds Reasoning (R): They are soluble in nonpolar solvents The correct answer is [AP/May 17, 2023 (II)] (a) Both (A) and (R) are correct and (R) is the correct explanation of (A) (b) Both (A) and (R) are correct and (R) is not the cor- rect explanation of (A) (c) (A) is correct but (R) is not correct (d) (A) is not correct but (R) is correct Ans: (c) Ionic bonds are electrostatic forces of attraction that are non-directional in nature. Thus, (A) is correct. Ionic compounds are soluble in polar solvents that solv- ate them via ion-dipole interactions. Thus, (R) is incorrect. 2. Which of the following is not the property of co- valent substances? [AP/May 15, 2023 (II)] (a) have definite shape (b) have low melting points (c) good conductors of electricity (d) soluble in non-polar solvents Ans: (c) Covalent solids are bad conductors of electric- ity due to the absence of free electrons like in metallic solids. 3. The order of the average bond length of the given bonds is [TS/Aug. 6, 2021 (I)] (a) C = O < C = N < C ≡ C < N − O (b) C ≡ C < C = O < C = N < N − O (c) C ≡ C < C = O < N − O < C = N (d) C = N < C = O < N − O < C ≡ C Ans: (c) The bond length is inversly proportional to the bond strength. The bond strength of C ≡ C will be maximum among the given options hence it will, show minimum bond length value. 4. How many lone pairs of electrons are present in a hydroxyl ion? [AP/Sept. 17, 2020 (I)] (a) Two pairs (b) Three pairs (c) One pair (d) Four pairs Ans: (b) In hydroxyl ion, 3 lone pairs of electrons are present on oxygen atom of hydroxyl ion. Topic 2 Octet Rule, Resonance and Hydrogen Bonding 5. The correct order of C − O bond length in CO, CO3 2−, CO2 is [AP/May 16, 2023 (II); Similar to TS/July 18, 2022 (I)] (a) CO < CO3 2− < CO2 (b) CO3 2− < CO2 < CO (c) CO < CO2 < CO3 2− (d) CO2 < CO3 2− < CO Ans: (a)
The bond length is smallest for the C ≡ O bond due to strong triple bond. Among CO3 2− and CO2, CO3 2− has the smaller C − O bond length due to resonance stabilization that makes the bond stronger. 6. Identify incorrectly matched set from the follow- ing [AP/May 15, 2023 (I); Similar to AP/July 4, 2022 (II)] (a) Molecules with incomplete octet- BeH2, BCl3 (b) Polar molecules- BF3, CCl4 (c) Molecules with expanded octet- PCl5, SF6 (d) Odd electron molecules- NO,NO2 Ans: (b) BF3 and CCl4 are non-polar molecules due to symmetrical arrangement of the bond pairs that result in the cancellation of the individual bond dipoles (μ = 0). 7. The incomplete Lewis representation of CO3 2− is given below. The formal charge on atoms marked as a, b, and c, respectively, are [TS/July 19, 2022 (I)] (a) a :0b :0 c:-1 (b) a : 0 b : −2c: 0 (c) a: −2 b: 0c: 0 (d) a :0b :-1 c:-1 Ans: (a) The Lewis structure of CO3 2− is Formal charge (F.C.) = Valence e − −lone pair e − − 1 2 shared e −. F.C. on C(a) = 4 − 0 − 1 2 (8) = 0 F.C. on O(b) = 6 − 4 − 1 2 (4) = 0 F.C. on O(c) = 6 − 6 − 1 2 (2) = −1 8. In which of the following hydrogen bonding is strongest? [TS/May 3, 2019 (I)] (a) O − H ⋯ ⋅ N (b) O − H ⋯ O (c) O − H ⋯ ⋅ F (d) F − H ⋯ ⋅ F Ans: (d) More be the electronegativity difference be- tween H-atom and the atom bonded with H-atom, more stronger is the H-bonding. Since, electronegativity difference between H-atom and F-atom is maximum. Therefore, option (d) is the correct answer. Topic 3 Dipole Moment and Bond Polarity 9. What are the formal charges on terminal ox- ygens of ozone molecule? [AP/May 19, 2023 (I)] (a) +1, −1 (b) +1, +1 (c) −1, −1 (d) 0, −1 Ans: (d) Formal charges on terminal oxygens = 0 and -1 .
10. Match the following List – I (Molecules) A. H2O B. BF3 C. NH3 D. NF3 List – II (Dipolemoment μ, D) I. 0 II. 0.23 III. 1.47 IV. 1.85 The correct answer is [AP/May 18, 2023 (I)] (a) A - IV, B - I, C - II, D - III (b) A - IV, B - I, C - III, D - II (c) A - IV, B - III, C - I, D - II (d) A - III, B -IV, C - II, D – I Ans: (b) μ = 1.85 μ = 1.47 (bond dipoles added) μ = 0 (Symmetrical) μ = 0.23 (bond dipoles cancel out) 11. Identify the option in which the molecules are arranged in the correct order of their dipole mo- ments. [TS/May 12, 2023 (II); Similar to TS/May 4, 2019 (II)] (a) CO2 < H2O < H2 S (b) HF < HCl < HBr (c) BF3 < NF3 < NH3 (d) CH4 < CHCl3 < CCl4