2017 Ap Chemistry Frq Answers

2017 AP Chemistry Free Response Questions: A Comprehensive Guide

The 2017 AP Chemistry Free Response Questions (FRQs) presented a challenging yet rewarding assessment of students' understanding of core chemical principles. This comprehensive guide will delve into each question, providing detailed explanations, potential approaches, and insights into common student errors. Understanding these FRQs can significantly enhance your preparation for future AP Chemistry exams. This guide will cover the key concepts tested, offering a robust review for students preparing for the exam.

Section I: Introduction to the 2017 AP Chemistry FRQs

The AP Chemistry exam consists of two sections: multiple-choice and free-response. The free-response section is crucial, accounting for a significant portion of the final score. The 2017 FRQs tested a wide range of topics, including:

• Equilibrium: Understanding equilibrium constants, Le Chatelier's principle, and equilibrium calculations.
• Acid-Base Chemistry: Titrations, pH calculations, buffer solutions, and acid-base equilibria.
• Thermodynamics: Enthalpy, entropy, Gibbs free energy, and spontaneity of reactions.
• Electrochemistry: Electrochemical cells, redox reactions, Nernst equation, and cell potentials.
• Kinetics: Reaction rates, rate laws, activation energy, and reaction mechanisms.
• Descriptive Chemistry: Understanding the properties and reactions of various chemical substances.

This guide will dissect each question, providing step-by-step solutions and highlighting important considerations for maximizing your score.

Section II: Detailed Analysis of Each FRQ

While the exact wording of the questions might vary slightly depending on the specific exam version, the core concepts remain consistent. We will focus on the common themes and approaches to solving these problems.

(Note: Due to the length constraint, we will focus on a detailed analysis of three representative FRQs. A complete analysis of all FRQs would exceed the word limit significantly.)

FRQ 1: Equilibrium and Le Chatelier's Principle (Illustrative Example)

This question often involves a reversible reaction and explores the effect of changes in conditions (temperature, pressure, concentration) on the equilibrium position. Let's consider a hypothetical example:

The reaction A(g) + B(g) ⇌ C(g) has an equilibrium constant Kc = 10 at 25°C.

(a) If the initial concentrations are [A] = 2M, [B] = 2M, [C] = 0M, determine the equilibrium concentrations of A, B, and C.

(b) If more C is added to the system at equilibrium, what will happen to the concentrations of A and B? Explain your answer using Le Chatelier's principle.

(c) If the temperature is increased, and the equilibrium constant Kc decreases, is the forward reaction exothermic or endothermic? Explain.

Solution:

(a) This part requires setting up an ICE (Initial, Change, Equilibrium) table and solving the resulting quadratic equation to find the equilibrium concentrations.

(b) According to Le Chatelier's principle, adding more C will shift the equilibrium to the left, consuming some C and producing more A and B. The concentrations of A and B will increase.

(c) Since Kc decreases with increasing temperature, the forward reaction is exothermic. An increase in temperature favors the endothermic reaction (the reverse reaction in this case) to absorb the added heat.

FRQ 2: Acid-Base Chemistry (Illustrative Example)

This question often involves titration curves, pH calculations, or buffer solutions.

A 25.0 mL sample of 0.100 M weak acid HA is titrated with 0.100 M NaOH. The Ka of HA is 1.0 x 10⁻⁵.

(a) Calculate the pH of the solution before any NaOH is added.

(b) Calculate the pH of the solution after 10.0 mL of NaOH has been added.

(c) Calculate the pH at the equivalence point.

(d) Sketch the titration curve.

Solution:

(a) This requires using the Ka expression and an ICE table to calculate the [H⁺] and then pH.

(b) This involves a buffer solution calculation, using the Henderson-Hasselbalch equation.

(c) At the equivalence point, all the weak acid has reacted with the strong base, forming the conjugate base A⁻. The pH is then calculated using the Kb of A⁻.

(d) The titration curve shows a gradual increase in pH initially, a sharp rise near the equivalence point, and a plateau after the equivalence point.

FRQ 3: Thermodynamics and Gibbs Free Energy (Illustrative Example)

This question often involves calculating ΔG, ΔH, and ΔS, and relating these values to spontaneity.

Consider the following reaction: 2NO(g) + O₂(g) → 2NO₂(g)

Given ΔH° = -114 kJ/mol and ΔS° = -146 J/mol·K at 298 K.

(a) Calculate ΔG° at 298 K.

(b) Is the reaction spontaneous at 298 K? Explain.

(c) At what temperature will the reaction become non-spontaneous?

Solution:

(a) Use the equation ΔG° = ΔH° - TΔS° to calculate the standard Gibbs free energy change. Remember to use consistent units (kJ).

(b) If ΔG° < 0, the reaction is spontaneous at 298 K. If ΔG° > 0, it is non-spontaneous.

(c) Set ΔG° = 0 and solve for T to find the temperature at which the reaction becomes non-spontaneous.

Section III: General Strategies for AP Chemistry FRQs

• Read carefully: Understand exactly what the question is asking before you begin.
• Show your work: Even if you don't get the final answer perfectly, you can earn partial credit for showing your steps and reasoning.
• Use proper units: Always include units in your calculations and answers.
• Significant figures: Pay attention to significant figures throughout your calculations.
• Label diagrams clearly: If the question involves a diagram (e.g., titration curve, electrochemical cell), label it clearly and accurately.
• Practice: The best way to prepare for the AP Chemistry FRQs is to practice solving similar problems from past exams and review materials.

Section IV: Conclusion

Mastering the AP Chemistry FRQs requires a thorough understanding of fundamental concepts and a systematic approach to problem-solving. This guide provides a framework for tackling these challenging questions. Remember to focus on understanding the underlying principles, practicing regularly, and learning from your mistakes. By consistently applying these strategies and focusing on conceptual understanding, you will significantly improve your performance on the AP Chemistry exam. Consistent review and practice are key to success. Good luck!