Ap Chemistry Frq 2017 Answers

Deconstructing the 2017 AP Chemistry Free Response Questions: A complete walkthrough

The AP Chemistry exam is a significant hurdle for many high school students aiming for college science programs. Worth adding: a crucial part of the exam is the Free Response Questions (FRQs), which test your ability to apply your knowledge to complex chemical scenarios. This article provides a detailed breakdown of the 2017 AP Chemistry FRQs, offering not only the answers but also a thorough explanation of the underlying concepts and problem-solving strategies. Because of that, understanding these questions and their solutions will significantly enhance your preparation for future AP Chemistry exams. We’ll explore each question in detail, emphasizing the key concepts and providing insights into how to approach similar problems.

Question 1: Equilibrium and Solubility

This question focused on the equilibrium principles governing the solubility of a sparingly soluble salt, lead(II) iodide (PbI₂). It involved multiple parts requiring different approaches to equilibrium calculations.

Part (a): Writing Equilibrium Expressions

This part tested your understanding of writing equilibrium expressions, specifically the solubility product constant, K_sp. You were asked to write the K_sp expression for PbI₂.

Answer: The balanced dissolution equation for PbI₂ is:

PbI₂(s) ⇌ Pb²⁺(aq) + 2I⁻(aq)

That's why, the K_sp expression is:

K_sp = [Pb²⁺][I⁻]²

Part (b): Calculating Solubility

Given a K_sp value, you were required to calculate the molar solubility of PbI₂ in pure water.

Answer: Let 's' represent the molar solubility of PbI₂. Then, [Pb²⁺] = s and [I⁻] = 2s. Substituting into the K_sp expression:

K_sp = (s)(2s)² = 4s³

Solving for 's' using the provided K_sp value will give the molar solubility of PbI₂.

Part (c): Common Ion Effect

This part examined your understanding of the common ion effect. You were asked to determine whether the molar solubility of PbI₂ would be higher, lower, or the same in a solution containing a common ion (e.g., I⁻).

Answer: The presence of a common ion (I⁻) will decrease the solubility of PbI₂. This is due to Le Chatelier's principle; the addition of I⁻ shifts the equilibrium to the left, decreasing the concentration of dissolved Pb²⁺ and thus reducing the solubility.

Part (d): Solubility in Different Solutions

This part compared the solubility of PbI₂ in different solutions. A quantitative comparison would be needed, involving calculations similar to part (b), considering the presence of other ions affecting the ionic strength of the solution.

Question 2: Acid-Base Chemistry and Titration

This question explored various aspects of acid-base chemistry, including titration curves, pH calculations, and buffer solutions.

Part (a): Sketching a Titration Curve

You were provided information about a weak acid titration with a strong base and asked to sketch the titration curve.

Answer: The curve should show a gradual pH increase initially, a relatively steep rise near the equivalence point, and then a leveling off at higher pH values. Key features to include are the initial pH, the pH at the half-equivalence point (where pH = pKa), and the pH at the equivalence point.

Part (b): pH Calculation at Different Points

This part required calculating the pH at specific points during the titration (e., before the addition of any base, at the half-equivalence point, and at the equivalence point). g.Different calculation methods would be needed for each point, reflecting the changing chemical composition of the solution.

Answer: Before any base is added, the pH is calculated using the K_a of the weak acid. At the half-equivalence point, pH = pKa. At the equivalence point, the solution contains the conjugate base of the weak acid, and the pH is calculated considering the hydrolysis of this base.

Part (c): Buffer Region Explanation

This part focused on the buffer region of the titration curve. You needed to explain why this region shows minimal pH change upon the addition of small amounts of strong base.

Answer: The buffer region exists when significant amounts of both the weak acid and its conjugate base are present. This mixture resists changes in pH because the weak acid neutralizes added base, and the conjugate base neutralizes added acid.

Question 3: Thermodynamics and Electrochemistry

This question integrated thermodynamics and electrochemistry, focusing on the spontaneity of redox reactions and Gibbs Free Energy changes.

Part (a): Standard Reduction Potentials

This part tested your ability to use standard reduction potentials to determine the overall cell potential (E°cell) for a given redox reaction.

Answer: The E°cell is calculated by subtracting the standard reduction potential of the anode (oxidation half-reaction) from the standard reduction potential of the cathode (reduction half-reaction). A positive E°cell indicates a spontaneous reaction under standard conditions.

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Part (b): Gibbs Free Energy (ΔG°) Calculation

This part required you to calculate the standard Gibbs Free Energy change (ΔG°) for the redox reaction using the calculated E°cell.

Answer: The relationship between ΔG° and E°cell is given by: ΔG° = -nFE°cell, where 'n' is the number of moles of electrons transferred and 'F' is Faraday's constant. A negative ΔG° indicates a spontaneous reaction.

Part (c): Equilibrium Constant (K) Calculation

This section tested your understanding of the relationship between ΔG°, the equilibrium constant (K), and temperature (T).

Answer: The relationship between ΔG°, K, and T is given by: ΔG° = -RTlnK, where 'R' is the ideal gas constant and 'T' is the temperature in Kelvin. Solving this equation allows you to calculate K.

Part (d): Effect of Temperature and Concentration

This part inquired about the effect of changing temperature and reactant concentrations on the spontaneity of the redox reaction.

Answer: The effect of temperature on spontaneity depends on the sign of ΔS° (standard entropy change). Increasing the concentration of reactants will generally favor the forward reaction, making it more spontaneous.

Question 4: Kinetics

This question focused on chemical kinetics, exploring reaction rates, rate laws, and activation energy.

Part (a): Determining Rate Law

You were provided experimental data and asked to determine the rate law for a given reaction.

Answer: Analyze the experimental data to determine the order of the reaction with respect to each reactant. The order is determined by observing how the rate changes when the concentration of a specific reactant is changed while holding others constant.

Part (b): Calculating Rate Constant

Once the rate law is determined, you needed to calculate the rate constant (k).

Answer: Use one set of experimental data (concentration and rate) and the determined rate law to solve for the rate constant (k).

Part (c): Activation Energy Calculation

This part involved using the Arrhenius equation to determine the activation energy (Ea) of the reaction given data at two different temperatures.

Answer: The Arrhenius equation is: k = Ae^(-Ea/RT). Using data at two temperatures, you can set up two equations and solve for Ea.

Part (d): Reaction Mechanism

This section might involve proposing a possible reaction mechanism consistent with the experimental rate law. This requires a good understanding of elementary steps and rate-determining steps in a reaction mechanism.

Question 5: Descriptive Chemistry

This question often involves a variety of chemical concepts and focuses on your understanding of descriptive inorganic chemistry. It could include questions about properties of elements and compounds, trends in the periodic table, and reactions of different substances. Specific content will vary from year to year but often includes:

• Periodic trends: Explaining trends in properties like ionization energy, electronegativity, and atomic radius across periods and down groups.
• Chemical bonding: Describing the types of bonding (ionic, covalent, metallic) and their properties.
• Reactions: Predicting the products of reactions and writing balanced chemical equations.
• Structure and properties: Relating the structure of a compound to its properties (e.g., solubility, melting point, boiling point).

Strategies for Answering AP Chemistry FRQs

To succeed on the AP Chemistry FRQs, consider these strategies:

• Read carefully: Understand exactly what the question is asking before you begin.
• Show your work: Clearly show all your steps and calculations. Even if your final answer is incorrect, you may receive partial credit for showing correct steps.
• Use units: Always include units in your calculations and answers.
• Organize your work: Present your answers in a clear and organized manner.
• Practice: The key to success is practice. Work through as many past AP Chemistry FRQs as possible.

This complete walkthrough provides a detailed look into the 2017 AP Chemistry FRQs. So naturally, remember that thorough understanding of the underlying principles, consistent practice with diverse problem types, and a clear approach to problem-solving are crucial for success on the AP Chemistry exam. By mastering these concepts and strategies, you'll significantly increase your confidence and performance on this challenging but rewarding exam.