Introduction
The 2008 Advanced Placement (AP) Chemistry Free-Response Questions (FRQs) are valuable resources for students preparing for the AP Chemistry exam. These questions test students’ understanding of key concepts and their ability to apply their knowledge to real-world scenarios. This comprehensive guide will provide a detailed analysis of each 2008 FRQ, including scoring rubrics and expert tips.
Question 1: Equilibrium and Reaction Rates
Question:
Consider the following reaction:
2NO(g) + 2H2(g) <=> N2(g) + 2H2O(g)
a) Calculate the equilibrium constant, Kp, for this reaction at 298 K, given that the partial pressures of the gases at equilibrium are:
- P(NO) = 0.102 atm
- P(H2) = 0.204 atm
- P(N2) = 0.048 atm
- P(H2O) = 0.146 atm
b) If the initial partial pressure of NO is 0.200 atm and the initial partial pressure of H2 is 0.300 atm, predict the partial pressure of NO at equilibrium.
Scoring Rubric for Question 1
a) Equilibrium Constant Calculation:
- 2 points: Correctly calculates the equilibrium constant using the given partial pressures and the equilibrium constant expression.
b) Prediction of NO Partial Pressure:
- 1 point: Correctly states that the reaction will shift towards the product side.
- 1 point: Correctly uses an ICE table to determine the equilibrium partial pressure of NO.
- 1 point: Obtains the correct numerical answer for the equilibrium partial pressure of NO.
Expert Tips for Question 1
- Familiarize yourself with the equilibrium constant expression (Kp = (PN2 * PH2O) / (PNO)^2 * (PH2)^2).
- Use the partial pressures at equilibrium to calculate Kp directly.
- For part b), calculate the change in partial pressure of NO and use the equilibrium constant to determine the equilibrium partial pressure.
Question 2: Thermodynamics
Question:
A 0.500-mol sample of an ideal gas undergoes a reversible isothermal expansion from an initial volume of 1.00 L to a final volume of 4.00 L at 298 K.
a) Calculate the work done by the gas during the expansion.
b) Calculate the change in entropy of the gas during the expansion.
Scoring Rubric for Question 2
a) Work Done Calculation:
- 2 points: Correctly applies the isothermal work equation (W = -nRTln(Vf/Vi)).
- 1 point: Obtains the correct numerical answer for the work done.
b) Entropy Change Calculation:
- 2 points: Correctly applies the entropy change equation for an isothermal process (DeltaS = nRln(Vf/Vi)).
- 1 point: Obtains the correct numerical answer for the entropy change.
Expert Tips for Question 2
- Understand the concept of isothermal processes and the relationship between work and entropy.
- Use the given values for the number of moles, initial volume, final volume, and temperature to directly calculate the work done.
- Apply the entropy change equation to determine the change in entropy.
Question 3: Acid-Base Equilibria
Question:
A solution is prepared by dissolving 0.200 mol of sodium acetate (CH3COONa) in 1.00 L of water. The pH of the solution is 8.90.
a) Calculate the concentration of acetic acid (CH3COOH) in the solution.
b) Calculate the pKa of acetic acid.
Scoring Rubric for Question 3
a) Acetic Acid Concentration Calculation:
- 2 points: Correctly applies the Henderson-Hasselbalch equation (pH = pKa + log([A-]/[HA])) to calculate the concentration of acetic acid.
- 1 point: Obtains the correct numerical answer for the acetic acid concentration.
b) pKa Calculation:
- 2 points: Correctly subtracts the logarithmic expression in the Henderson-Hasselbalch equation from pH to obtain pKa.
- 1 point: Obtains the correct numerical answer for the pKa of acetic acid.
Expert Tips for Question 3
- Be familiar with the Henderson-Hasselbalch equation and its application to buffer solutions.
- Use the given pH value and the concentration of sodium acetate to calculate the concentration of acetic acid.
- Determine the pKa of acetic acid by subtracting the logarithmic expression from the pH.
Question 4: Electrochemistry
Question:
A voltaic cell is constructed using a zinc electrode (Zn/Zn2+) and a copper electrode (Cu/Cu2+). The standard reduction potentials are:
- Zn2+ + 2e- -> Zn: E° = -0.76 V
- Cu2+ + 2e- -> Cu: E° = +0.34 V
a) Calculate the standard cell potential (E°cell) for the voltaic cell.
b) Calculate the cell potential (Ecell) when the concentrations of Zn2+ and Cu2+ are 0.100 M and 0.0100 M, respectively.
Scoring Rubric for Question 4
a) Standard Cell Potential Calculation:
- 2 points: Correctly calculates the standard cell potential using the equation E°cell = E°red(cathode) – E°red(anode).
- 1 point: Obtains the correct numerical answer for the standard cell potential.
b) Cell Potential Calculation:
- 2 points: Correctly applies the Nernst equation (Ecell = E°cell – (RT/nF)ln([Zn2+]/[Cu2+])) to calculate the cell potential.
- 1 point: Obtains the correct numerical answer for the cell potential.
Expert Tips for Question 4
- Understand the concept of standard cell potential and how it is related to the reduction potentials of the half-reactions.
- Use the given reduction potentials to calculate the standard cell potential.
- Apply the Nernst equation to determine the cell potential under non-standard conditions.
Question 5: Nuclear Chemistry
Question:
The radioactive isotope of iodine, 131I, has a half-life of 8.04 days. A sample of 131I contains 6.40 x 10^12 atoms.
a) Calculate the number of 131I atoms that will remain after 24.12 days.
b) Calculate the amount of time required for the number of 131I atoms to decrease to 1.00 x 10^10 atoms.
Scoring Rubric for Question 5
a) Number of Remaining Atoms Calculation:
- 2 points: Correctly applies the exponential decay law (N(t) = N0 * (1/2)^(t/t1/2)) to calculate the number of remaining atoms.
- 1 point: Obtains the correct numerical answer for the number of remaining atoms.
b) Time Required Calculation:
- 2 points: Correctly rearranges the exponential decay law to solve for time (t = t1/2 * log(N0/N)).
- 1 point: Obtains the correct numerical answer for the time required.
Expert Tips for Question 5
- Be familiar with the exponential decay law and its application to radioactive decay.
- Use the given half-life and the initial number of atoms to calculate the number of remaining atoms after a specific time.
- Rearrange the exponential decay law to determine the time required for a specific number of atoms to remain.
Conclusion
The 2008 AP Chemistry Free-Response Questions provide a comprehensive assessment of students’ understanding of key chemistry concepts. By analyzing these questions and understanding the scoring rubrics and expert tips, students can effectively prepare for the AP Chemistry exam and demonstrate their mastery of the subject.
