Chemistry Practice Problems
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SN2 Reaction Mechanism
Consider the reaction of (S)-2-bromobutane with NaOH in DMSO solvent. (a) Predict the product and its stereochemistry. (b) Write the rate law for this reaction. (c) What would happen to the rate if the concentration of NaOH is doubled?
Grignard Reagent with Carbonyl Compounds
Predict the major organic product(s) for each of the following reactions: (a) CH3MgBr + formaldehyde (HCHO), followed by H3O+. (b) CH3CH2MgBr + acetone (CH3COCH3), followed by H3O+. (c) PhenylMgBr + CO2, followed by H3O+. Explain the pattern in terms of the relationship between the carbonyl compound and the resulting alcohol.
EAS Directing Effects
Starting from benzene, propose a synthesis of 2-bromo-4-nitrotoluene. (a) What is the correct order of reactions? (b) Explain why the reverse order would give the wrong product. (c) Draw all intermediates and explain the directing effects at each step.
Crystal Field Splitting in Octahedral Complexes
For the octahedral complex [Fe(H2O)6]2+: (a) Determine the d-electron count. (b) Draw the crystal field splitting diagram. (c) Predict whether this complex is high-spin or low-spin. (d) Calculate the crystal field stabilization energy (CFSE) in terms of ฮo.
Isomerism in Coordination Compounds
For the complex [Co(en)2Cl2]+ (en = ethylenediamine): (a) Draw all possible geometric isomers. (b) Identify which isomers are optically active. (c) Name each isomer using IUPAC nomenclature. (d) Predict which isomer would have a larger crystal field splitting and explain why.
Molecular Orbital Theory for Transition Metal Complexes
Using ligand field theory (MO approach), explain why CO is a much stronger field ligand than H2O in the spectrochemical series. (a) Draw the relevant MO interactions for both ligands with a transition metal. (b) Explain the role of ฯ-backbonding. (c) How does this affect the magnitude of ฮo? (d) Predict the effect on the CO stretching frequency when CO coordinates to a metal with many d-electrons versus few d-electrons.
Gibbs Free Energy and Spontaneity
For the reaction N2(g) + 3H2(g) โ 2NH3(g) at 298 K, given ฮHยฐ = -92.4 kJ/mol and ฮSยฐ = -198.3 J/(molยทK): (a) Calculate ฮGยฐ at 298 K. (b) Is the reaction spontaneous at 298 K? (c) At what temperature does the reaction become non-spontaneous? (d) Explain why this reaction is favored at low temperatures but not high temperatures.
Chemical Kinetics - Rate Laws
The gas-phase reaction 2NO(g) + O2(g) โ 2NO2(g) has been studied experimentally. The following initial rate data were obtained at 25ยฐC: Run 1: [NO]โ = 0.010 M, [O2]โ = 0.010 M, Rate = 2.5 ร 10โปยณ M/s Run 2: [NO]โ = 0.020 M, [O2]โ = 0.010 M, Rate = 1.0 ร 10โปยฒ M/s Run 3: [NO]โ = 0.010 M, [O2]โ = 0.020 M, Rate = 5.0 ร 10โปยณ M/s (a) Determine the rate law. (b) Calculate the rate constant k. (c) Propose a mechanism consistent with this rate law.
Particle in a Box - Quantum Mechanics
Consider the conjugated molecule 1,3,5-hexatriene as a one-dimensional particle in a box model for its ฯ electrons. The box length can be approximated as L = 5 ร 1.40 ร = 7.00 ร (5 C-C bonds ร bond length). (a) Calculate the energy levels for n = 1 through n = 4. (b) Six ฯ electrons fill the levels according to the Pauli exclusion principle. Determine the HOMO-LUMO gap. (c) What wavelength of light would excite an electron from HOMO to LUMO? (d) How does this compare with the experimental absorption maximum (~258 nm)?
UV-Vis Spectroscopy and Beer-Lambert Law
A solution of potassium permanganate (KMnO4) shows an absorbance of 0.750 at 525 nm when measured in a 1.00 cm cuvette. The molar absorptivity of KMnO4 at 525 nm is 2.40 ร 10ยณ L/(molยทcm). (a) Calculate the concentration of the solution. (b) What would the absorbance be if the concentration were doubled? (c) What percent of light is transmitted through the original solution?
Acid-Base Titration Curves
Calculate the pH at the following points during the titration of 50.0 mL of 0.100 M acetic acid (CH3COOH, Ka = 1.8 ร 10โปโต) with 0.100 M NaOH: (a) Before any NaOH is added. (b) After 25.0 mL of NaOH (half-equivalence point). (c) At the equivalence point (50.0 mL NaOH). (d) After 60.0 mL of NaOH (past equivalence point). (e) Sketch the titration curve and identify the buffer region.
HPLC Separation and Resolution
In a reverse-phase HPLC analysis, two compounds A and B have retention times of 12.5 min and 14.2 min respectively, with peak widths at base of 1.1 min and 1.3 min. The column dead time is 2.0 min. (a) Calculate the resolution (Rs) between peaks A and B. (b) Calculate the selectivity factor (ฮฑ). (c) Calculate the retention factor (k') for each compound. (d) If you need Rs โฅ 1.5 for baseline separation, what changes would you make to the mobile phase?
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