ECAT — Engineering College Admission Test (UET)
Take the ECAT Chemistry — Chemical Bonding (Set 6): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer review. Covers 30…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. In the Lewis structure N≡N–O (the central nitrogen has a triple bond to the terminal N and a single bond to O, with the O carrying three lone pairs and the terminal N one lone pair), the formal charge on the central nitrogen is:
Answer: C. +1
Explanation: Formal charge = valence electrons − non-bonding electrons − ½(bonding electrons) = 5 − 0 − ½(8) = +1 (central N has four bonds and no lone pair).
Q2. Nitric oxide (NO), the nitrosonium ion (NO⁺) and the NO⁻ ion are compared. Which one has the shortest N–O bond and why?
Answer: D. NO⁺, with bond order 3
Explanation: Valence electrons: NO⁺ 10, NO 11, NO⁻ 12. Bond orders (MO theory) are 3, 2.5 and 2. The highest bond order gives the shortest bond: NO⁺.
Q3. According to molecular orbital theory, which two species have the same bond order?
Answer: C. O₂⁺ and NO
Explanation: O₂⁺ (11 valence electrons): (10 − 5)/2 = 2.5; NO (11 valence electrons): 2.5. The other pairs differ: O₂ 2 vs N₂ 3; O₂⁻ 1.5 vs NO⁺ 3; O₂²⁻ 1 vs CO 3.
Q4. The shape of the BrF₅ molecule (5 bond pairs and 1 lone pair on bromine) is:
Answer: D. square pyramidal
Explanation: Six electron pairs give an octahedral arrangement; one position is a lone pair, so the five F atoms form a square pyramid.
Q5. Each O–H bond in a certain angular molecule has a bond dipole of 1.50 D, and the H–O–H angle is 104.5°. The resultant dipole moment of the molecule is:
Answer: B. 1.84 D
Explanation: μ = 2μ_bond cos(θ/2) = 2 × 1.50 × cos 52.25° = 1.84 D.
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Q6. The observed dipole moment of HBr is 0.80 D and the H–Br bond length is 1.41 Å. Taking 1 D = 0.2082 e Å (so the dipole for a full unit charge separation is 4.803 D per Å), the percentage ionic character is:
Q7. From the data (kJ mol⁻¹) ΔH_f(KCl) = −437, sublimation of K = +89, ionisation energy of K = +419, ½ bond dissociation of Cl₂ = +122, electron affinity of Cl = −349, the lattice energy of KCl (energy released when gaseous ions form the solid) is:
Q8. The number of σ and π bonds in a molecule of propyne, CH₃–C≡C–H, is:
Q9. Naphthalene, C₁₀H₈, consists of two fused benzene rings (11 C–C connections including the shared bond, 8 C–H bonds). The total number of σ and π bonds in it is, respectively:
Q10. In the acetate ion, CH₃COO⁻, the two C–O bonds are identical because of resonance. The C–O bond order is:
Q11. The H–P–H bond angle in PH₃ (about 93°) is much smaller than the H–N–H angle in NH₃ (about 107°). The best explanation is that:
Q12. Which of these liquids cannot form hydrogen bonds between its own molecules, even though its molecules contain an oxygen atom with lone pairs?
Q13. Using mean bond enthalpies (kJ mol⁻¹): C=C 614, O–H 463, C–C 348, C–H 413, C–O 358, estimate ΔH for CH₂=CH₂ + H₂O → CH₃CH₂OH, in which one C=C, one O–H break and one C–C, one C–H and one C–O form.
Q14. As the temperature of a metallic conductor is raised, its electrical conductivity:
Q15. Which of the following ions is pyramidal rather than planar?
Q16. Which homonuclear diatomic molecule is paramagnetic and has a bond order of 1 according to molecular orbital theory?
Q17. The σ bond in the F₂ molecule is formed by:
Q18. The hybridisation of the central atom is sp² in:
Q19. Which of the following species has an angular (bent) shape?
Q20. The total number of lone pairs of electrons in a hydroxylamine molecule, NH₂OH, is:
Q21. Lattice energy is roughly proportional to q₊q₋/(r₊ + r₋). Using ionic radii Mg²⁺ 72 pm, O²⁻ 140 pm, Na⁺ 102 pm, F⁻ 133 pm, the lattice energy of MgO is about how many times that of NaF?
Q22. Which hydrogen bond is the strongest?
Q23. In the MO diagram of N₂ (but not of O₂), the σ₂p orbital lies above the π₂p orbitals. The reason is:
Q24. Which species has a bond order of 2.5 and is paramagnetic?
Q25. Of the three isomeric trichlorobenzenes, which has zero net dipole moment?
Q26. Ethanol (C₂H₅OH) boils at 78 °C but its isomer dimethyl ether (CH₃OCH₃) boils at −24 °C. The difference arises because:
Q27. According to MO theory, the species He₂⁺ has:
Q28. The enthalpy of hydrogenation of cyclohexene is −120 kJ mol⁻¹, and that of benzene to cyclohexane is −208 kJ mol⁻¹. Taking benzene as three isolated C=C bonds as the reference, the resonance stabilisation energy of benzene is:
Q29. The white phosphorus molecule P₄ is tetrahedral. The P–P–P bond angle in it, and the number of P–P bonds, are:
Q30. In allene, CH₂=C=CH₂, the planes of the two terminal CH₂ groups are: