2017 Ap Chem 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 key concepts. This comprehensive guide will delve into each question, providing detailed answers, explanations, and insights into the underlying chemistry principles. This guide aims to be a valuable resource for students preparing for future AP Chemistry exams, providing a deeper understanding beyond simply memorizing answers. Understanding the why behind the answer is crucial for true mastery of the subject.

Introduction: Understanding the AP Chemistry FRQ Structure

The AP Chemistry exam consists of two sections: multiple-choice and free-response. The free-response section is arguably more challenging, requiring not only knowledge recall but also the ability to apply that knowledge to novel scenarios, explain chemical processes, and communicate scientific reasoning clearly and concisely. The 2017 FRQs tested a wide array of topics, including:

• Thermodynamics: Enthalpy, entropy, Gibbs free energy, and spontaneity.
• Equilibrium: Equilibrium constants, Le Chatelier's principle, and calculations involving equilibrium concentrations.
• Acid-Base Chemistry: pH calculations, titration curves, and buffer solutions.
• Electrochemistry: Cell potentials, redox reactions, and electrolytic cells.
• Kinetics: Reaction rates, rate laws, and activation energy.
• Descriptive Chemistry: Properties of elements and compounds.

Let's now examine each of the 2017 FRQs in detail. Note that while specific numerical answers are provided, the emphasis will be on the methodology and underlying chemical principles. Understanding the process is far more important than simply memorizing numbers.

Question 1: Thermodynamics and Equilibrium

This question typically involved calculations related to enthalpy, entropy, and Gibbs free energy, often in the context of a chemical reaction. It might also include questions about the spontaneity of a reaction at different temperatures. A strong answer would show a clear understanding of the relationships between these thermodynamic parameters and their application in predicting reaction spontaneity.

Example: A reaction is given, along with its enthalpy change (ΔH) and entropy change (ΔS). Students might be asked to:

• Calculate the Gibbs free energy change (ΔG) at a specific temperature. This requires applying the equation ΔG = ΔH - TΔS.
• Determine the temperature at which the reaction becomes spontaneous. This requires solving for T when ΔG = 0.
• Explain the factors contributing to the sign and magnitude of ΔH and ΔS.

Detailed Answer (Illustrative): A thorough answer wouldn't just plug numbers into a formula. It would explain the reasoning behind each step, such as discussing the significance of a positive or negative ΔH and ΔS in relation to the spontaneity of the reaction. It would also include correct units and significant figures.

Question 2: Acid-Base Chemistry and Titration

This section usually features a titration problem, requiring calculations of pH at different points in the titration curve. Students may need to write balanced chemical equations, identify the equivalence point, and calculate the pH at the equivalence point and at points before and after the equivalence point. A strong response demonstrates understanding of acid-base equilibria and buffer solutions.

Example: A weak acid is titrated with a strong base. Students might be asked to:

• Write a balanced net ionic equation for the reaction.
• Calculate the pH of the solution before any base is added.
• Calculate the pH at the halfway point of the titration.
• Calculate the pH at the equivalence point.
• Sketch the titration curve.

Detailed Answer (Illustrative): The answer should not simply provide the pH values. It should explain the calculations performed, showing the use of appropriate equilibrium expressions (e.g., Ka for weak acids, Kb for weak bases). It should also illustrate understanding of the changing chemical environment during the titration. The sketch of the titration curve should be appropriately labeled, showing the equivalence point and the buffer region.

Question 3: Kinetics and Reaction Mechanisms

This often involves interpreting kinetic data, determining rate laws, and discussing reaction mechanisms. It requires a solid understanding of rate constants, activation energy, and the factors that influence reaction rates.

Example: Experimental data on the rate of a reaction is provided. Students might be asked to:

• Determine the rate law for the reaction.
• Determine the value of the rate constant.
• Propose a plausible reaction mechanism consistent with the rate law.
• Discuss the effect of temperature on the reaction rate.

Detailed Answer (Illustrative): A strong response would involve correctly interpreting the data to determine the order of the reaction with respect to each reactant and then use this information to write the rate law. It would then explain the method used to calculate the rate constant, showing the calculations clearly. The proposed reaction mechanism should be consistent with the experimentally determined rate law, and the explanation should address the effect of temperature on the rate constant using the Arrhenius equation.

Question 4: Electrochemistry and Redox Reactions

This question usually focuses on electrochemical cells, including galvanic cells and electrolytic cells. Students might be asked to calculate cell potentials, write balanced redox reactions, and explain the function of various components of the cell.

Example: A galvanic cell is described. Students may be asked to:

• Write the balanced half-reactions and the overall cell reaction.
• Calculate the standard cell potential (E°cell).
• Determine the direction of electron flow.
• Explain the function of the salt bridge.
• Predict the effect of changing the concentration of a reactant.

Detailed Answer (Illustrative): This would involve writing correctly balanced half-reactions, correctly assigning oxidation states, using the standard reduction potentials to calculate E°cell, and accurately identifying the anode and cathode. The answer should clearly explain the function of the salt bridge in maintaining electrical neutrality and the impact of concentration changes on the cell potential using the Nernst equation.

Question 5: Descriptive Chemistry and Chemical Bonding

This question often tests students' knowledge of the properties and behavior of elements and compounds. It may involve explaining trends in the periodic table, predicting the shapes of molecules using VSEPR theory, or discussing the types of chemical bonds present in different compounds.

Example: The question might focus on the properties of a group of elements or compounds. Students might be asked to:

• Explain the trends in a particular property (e.g., ionization energy, electronegativity) down a group or across a period.
• Predict the geometry of a molecule using VSEPR theory.
• Describe the bonding in a compound (e.g., ionic, covalent, metallic).
• Explain the relationship between molecular structure and properties.

Detailed Answer (Illustrative): A successful response would accurately describe the trends in periodic properties, provide a clear explanation for the observed trends based on atomic structure and electron configuration. It would correctly predict the molecular geometry and explain its justification using VSEPR theory. The description of bonding should include accurate representations of electron distribution and bonding types.

Conclusion: Mastering the AP Chemistry FRQs

The 2017 AP Chemistry FRQs demonstrate the importance of not just memorizing facts but understanding the underlying principles and applying them to new situations. A strong answer requires clear communication, accurate calculations, and a comprehensive understanding of the relevant chemical concepts. By reviewing these questions and their detailed answers, students can develop a deeper understanding of the material and improve their performance on future AP Chemistry exams. Remember, practice is key to success! Work through numerous practice problems, focusing on clearly articulating your reasoning and demonstrating a thorough grasp of the fundamental principles. This approach will not only improve your score but also enhance your appreciation for the beauty and power of chemistry.