ACS CP Organic, Inorganic & Physical Chemistry 3 — Questions and Answers
Question 1: What is the hybridization of the central boron atom in BF₃?
- sp
- sp² (Correct answer)
- sp³
- sp³d
Correct answer: sp²
Boron in BF₃ has three bonding pairs and no lone pairs, resulting in trigonal planar geometry and sp² hybridization.
Question 2: Which of the following correctly describes the rate law for an SN2 reaction?
- Rate = k[substrate]
- Rate = k[nucleophile]
- Rate = k[substrate][nucleophile] (Correct answer)
- Rate = k
Correct answer: Rate = k[substrate][nucleophile]
SN2 reactions are second-order overall, with the rate depending on both the substrate and the nucleophile concentrations.
Question 3: In the Born-Haber cycle for NaCl, which step involves the largest energy release?
- Ionization of Na(g)
- Electron affinity of Cl(g)
- Lattice energy formation (Correct answer)
- Sublimation of Na(s)
Correct answer: Lattice energy formation
Lattice energy formation releases the most energy in the Born-Haber cycle because of strong electrostatic interactions in the ionic crystal.
Question 4: According to the Arrhenius equation, a plot of ln(k) vs 1/T gives a straight line. What does the y-intercept represent?
- -Ea/R
- ln(A) (Correct answer)
- Ea/R
- -ln(A)
Correct answer: ln(A)
The Arrhenius equation ln(k) = ln(A) - Ea/R × (1/T) shows the y-intercept equals ln(A), where A is the pre-exponential factor.
Question 5: Which spectroscopic technique is most useful for determining the carbon skeleton connectivity in an unknown organic molecule?
- IR spectroscopy
- UV-Vis spectroscopy
- ¹³C NMR spectroscopy (Correct answer)
- Mass spectrometry
Correct answer: ¹³C NMR spectroscopy
¹³C NMR, especially DEPT and 2D techniques like HMBC, directly reveals carbon environments and connectivity in organic molecules.
Question 6: Which of the following transition metal complexes is expected to be diamagnetic?
- [Fe(H₂O)₆]³⁺ (high spin)
- [Ni(CN)₄]²⁻ (square planar) (Correct answer)
- [Cr(NH₃)₆]³⁺
- [MnCl₄]²⁻
Correct answer: [Ni(CN)₄]²⁻ (square planar)
Square planar [Ni(CN)₄]²⁻ has d⁸ Ni²⁺ with all electrons paired due to the strong-field CN⁻ ligands forcing a low-spin configuration.
Question 7: What is the van't Hoff factor (i) expected for a 0.1 m solution of MgCl₂ assuming complete dissociation?
- 1
- 2
- 3 (Correct answer)
- 4
Correct answer: 3
MgCl₂ dissociates into one Mg²⁺ and two Cl⁻ ions, giving i = 3 particles per formula unit.
ACS CP Organic, Inorganic & Physical Chemistry 3
What is the hybridization of the central boron atom in BF₃?