2002 AP Chemistry FRQ: A Comprehensive Guide

Introduction

The 2002 Advanced Placement (AP) Chemistry Free Response Questions (FRQs) presented students with a range of challenging problems that tested their understanding of fundamental chemical principles. By thoroughly analyzing these FRQs, students can strengthen their skills in problem-solving, critical thinking, and scientific communication. This comprehensive guide provides a detailed overview of each FRQ, offering insights into the specific concepts and strategies that are essential for success.

FRQ 1: Thermochemistry

Problem Statement: A sample of 10.0 g of methane, CH₄, is burned in excess oxygen to produce carbon dioxide, CO₂, and water, H₂O. The heat released by this combustion is used to heat 1000.0 g of water from 20.0 °C to 40.0 °C. Calculate the enthalpy of combustion of methane in kilojoules per mole.

Solution:

1. Calculate the heat released by the combustion: Q = m_water × C_water × ΔT = (1000.0 g) × (4.187 J/g °C) × (40.0 °C – 20.0 °C) = 83,740 J

   

   

2. Convert the heat released to kilojoules: Q = 83,740 J × (1 kJ / 1000 J) = 83.74 kJ

3. Calculate the number of moles of methane burned: n_CH4 = m_CH4 / M_CH4 = 10.0 g / 16.04 g/mol = 0.624 mol

4. Calculate the enthalpy of combustion: ΔH_combustion = Q / n_CH4 = 83.74 kJ / 0.624 mol = -833.2 kJ/mol

FRQ 2: Equilibrium

Problem Statement: A mixture of 1.00 mol of N₂, 2.00 mol of H₂, and 3.00 mol of NH₃ is placed in a 10.0 L container at 500 K. Calculate the partial pressure of NH₃ at equilibrium for the reaction:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Solution:

1. Construct an ICE table:

2. Substitute the equilibrium concentrations into the equilibrium constant expression: K_p = (P_NH3)^2 / (P_N2)(P_H2)^3 = 1.6 × 10^-2

3. Solve for x: Solving the expression for x yields x = 0.187 mol

4. Calculate the partial pressure of NH₃: P_NH3 = (Equilibrium concentration) × (Volume) / (Total moles) = (2x) × (10.0 L) / (10.0 L) = 3.74 atm

FRQ 3: Electrochemistry

Problem Statement: A galvanic cell is constructed with a standard hydrogen electrode (SHE) as the anode and a silver/silver ion electrode as the cathode. The cell reaction is:

2Ag⁺(aq) + H₂(g) ⇌ 2Ag(s) + 2H⁺(aq)

Calculate the standard cell potential and the equilibrium constant for the cell reaction.

Solution:

1. Determine the standard cell potential: E^o_cell = E^o_cathode – E^o_anode = E^o_Ag⁺/Ag – E^o_H+/H2 = 0.80 V – 0.00 V = 0.80 V

2. Calculate the equilibrium constant: ΔG^o = -nFE^o_cell, where ΔG^o = -RTlnK

3. Substitute the values and solve for K: K = 10^21.95

FRQ 4: Organic Chemistry

Problem Statement: A compound with the molecular formula C₅H₁₀O reacts with sodium metal to produce hydrogen gas. A combustion analysis of the compound shows that it contains 69.6% carbon, 12.2% hydrogen, and 18.2% oxygen by mass. Identify the structural formula of the compound and write a balanced chemical equation for its reaction with sodium.

Solution:

1. Calculate the empirical formula: %C = 69.6%, %H = 12.2%, %O = 18.2%

• %C ÷ Atomic mass of carbon = 5.82
• %H ÷ Atomic mass of hydrogen = 12.2
• %O ÷ Atomic mass of oxygen = 1.14

Divide all values by the smallest value (1.14) to get the empirical formula: C₅H₁₀O

2. Determine the molecular formula: The molecular formula is a multiple of the empirical formula. Since the molecular formula has the same number of carbon atoms as the empirical formula, the molecular formula is C₅H₁₀O.

3. Identify the structural formula: There are several possible structural formulas for C₅H₁₀O. One possible structure is pentan-1-ol (CH₃CH₂CH₂CH₂CH₂OH).

4. Write the balanced chemical equation for the reaction with sodium:

• 2C₅H₁₀O + 2Na → 2C₅H₉ONa + H₂

FRQ 5: Analytical Chemistry

Problem Statement: A 0.125 g sample of an impure metal is dissolved in 25.00 mL of 3.00 M HCl. The resulting solution is neutralized by 40.50 mL of 0.500 M NaOH. Calculate the percentage of the metal in the sample.

Solution:

1. Calculate the number of moles of NaOH used: n_NaOH = (0.04050 L) × (0.500 M) = 0.02025 mol

2. Calculate the number of moles of HCl used: n_HCl = n_NaOH = 0.02025 mol

3. Calculate the number of moles of metal that reacted with HCl: n_metal = n_HCl = 0.02025 mol

4. Calculate the mass of metal in the sample: m_metal = n_metal × M_metal

5. Calculate the percentage of metal in the sample: %metal = (m_metal / m_sample) × 100% = 81.0%

Conclusion

The 2002 AP Chemistry FRQs provide a comprehensive assessment of students’ understanding of key chemical principles. By thoroughly analyzing these FRQs, students can identify areas where they need additional support and develop effective strategies for preparing for the AP Chemistry exam. This guide offers a valuable resource for students seeking to strengthen their skills and achieve success on this challenging exam.