Ap Chemistry 2017 Frq Answers

AP Chemistry 2017 FRQ Answers: A practical guide

The 2017 AP Chemistry Free Response Questions (FRQs) tested students' understanding of various core concepts in chemistry. And this complete walkthrough provides detailed explanations and answers for each question, aiming to clarify the reasoning behind each solution and highlight common pitfalls to avoid. Understanding the 2017 FRQs is crucial for current AP Chemistry students preparing for their exam, providing invaluable insight into question structure and scoring expectations. This guide will dig into each question, offering step-by-step solutions, incorporating relevant equations, and emphasizing the importance of clear communication in AP Chemistry.

Question 1: Thermodynamics and Equilibrium

This question focused on the thermodynamics of a chemical reaction and its equilibrium position. The reaction involved the formation of an iron(III) thiocyanate complex ion, [Fe(SCN)]²⁺, from iron(III) ions and thiocyanate ions.

(a) Writing the Equilibrium Expression

The question asked to write the equilibrium constant expression, K_c, for the formation of [Fe(SCN)]²⁺. This required understanding the relationship between the equilibrium constant and the concentrations of reactants and products.

Answer: K_c = [[Fe(SCN)]²⁺] / ([Fe³⁺][SCN⁻])

(b) Determining the Equilibrium Constant

Given initial concentrations and the equilibrium concentration of [Fe(SCN)]²⁺, this section tested the ability to calculate the equilibrium constant using an ICE (Initial, Change, Equilibrium) table.

Answer: Setting up the ICE table with the given information allows for the calculation of equilibrium concentrations of Fe³⁺ and SCN⁻. These are then substituted into the equilibrium constant expression determined in part (a) to calculate K_c. The exact values depend on the specific data provided in the original question. Students should show their work clearly, including the ICE table and the calculations.

(c) Predicting the Shift in Equilibrium

This part explored Le Chatelier's principle. Students were asked to predict the shift in equilibrium upon adding more thiocyanate ions.

Answer: According to Le Chatelier's principle, adding more SCN⁻ (a reactant) will shift the equilibrium to the right, favoring the formation of more [Fe(SCN)]²⁺. This increases the concentration of the product to relieve the stress on the system.

(d) Calculating the Standard Free Energy Change

This section required understanding the relationship between the equilibrium constant and the standard free energy change (ΔG°) using the equation ΔG° = -RTlnK, where R is the gas constant and T is the temperature in Kelvin.

Answer: Using the calculated K_c from part (b) and the given temperature (in Kelvin), students needed to plug the values into the equation ΔG° = -RTlnK to calculate the standard free energy change. A negative ΔG° indicates a spontaneous reaction under standard conditions.

(e) Determining the Effect of Temperature Change on Equilibrium

The final part examined the effect of increasing temperature on the equilibrium position. This required understanding the enthalpy change (ΔH) of the reaction. Information about the reaction's enthalpy was likely provided in the original question.

Answer: If the reaction is exothermic (ΔH < 0), increasing the temperature will shift the equilibrium to the left, favoring the reactants. If the reaction is endothermic (ΔH > 0), increasing the temperature will shift the equilibrium to the right, favoring the products. The response should clearly state the relationship between ΔH and the effect of temperature on equilibrium.

Question 2: Acid-Base Chemistry and Titrations

This question often involves a titration problem, covering concepts such as pH calculations, buffer solutions, and equivalence points. A specific weak acid or base and its titration with a strong acid or base are usually presented.

(a) Calculating pH of a Weak Acid Solution

Students would be given the concentration and K_a of a weak acid and asked to calculate the pH.

Answer: This requires the use of an ICE table and the equilibrium expression for the weak acid dissociation. Solving the resulting quadratic equation or using the approximation (if appropriate) will provide the hydronium ion concentration ([H₃O⁺]), and pH = -log[H₃O⁺].

(b) Calculating pH at the Equivalence Point

This involves calculating the pH after the addition of a certain volume of strong acid/base to neutralize the weak acid/base.

Answer: At the equivalence point, the weak acid/base is completely neutralized, resulting in the conjugate base/acid. The pH is determined by the hydrolysis of the conjugate. This often involves calculating the concentration of the conjugate and using the K_b (K_a for the conjugate acid of a weak base) to find the hydroxide/hydronium ion concentration.

(c) Calculating pH at a Specific Point in the Titration

This section often requires calculating the pH after adding a specific volume of titrant before reaching the equivalence point, creating a buffer solution.

Answer: This involves using the Henderson-Hasselbalch equation: pH = pK_a + log([A⁻]/[HA]). The concentrations of the weak acid (HA) and its conjugate base (A⁻) are calculated considering the moles reacted and moles remaining after the addition of a specific volume of the strong acid/base.

(d) Sketching the Titration Curve

This part requires drawing a titration curve based on the pH calculations from the previous parts.

Answer: A correctly drawn titration curve should show the initial pH of the weak acid/base, the buffer region, a sharp increase/decrease in pH around the equivalence point, and the final pH after the equivalence point.

(e) Identifying an Indicator

The final part often requires selecting an appropriate indicator based on the pH at the equivalence point.

Answer: The indicator's pK_a should be close to the pH at the equivalence point. The color change of the indicator should ideally occur within the sharp pH change region of the titration curve.

Question 3: Kinetics and Reaction Mechanisms

This question typically focuses on the rates of chemical reactions and reaction mechanisms.

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(a) Determining Rate Law

Given experimental data on initial rates, students are required to determine the rate law for a reaction.

Answer: This involves analyzing the change in initial rates with respect to changes in reactant concentrations. The exponents in the rate law (orders of reaction) are determined by comparing the changes in concentration to the changes in rate. As an example, if doubling the concentration of a reactant doubles the rate, the reaction is first order with respect to that reactant.

(b) Calculating the Rate Constant

After determining the rate law, the rate constant (k) can be calculated using the data from one of the experiments.

Answer: Substitute the values from one experiment (concentrations and the observed rate) into the rate law, and solve for k.

(c) Proposing a Reaction Mechanism

This part involves proposing a plausible reaction mechanism consistent with the experimentally determined rate law.

Answer: The slow (rate-determining) step of the proposed mechanism must match the experimentally determined rate law. The elementary steps should add up to the overall balanced equation.

(d) Identifying Intermediates and Catalysts

The proposed mechanism might contain intermediates (species that are formed and consumed in the reaction) and catalysts (species that speed up the reaction without being consumed).

Answer: Intermediates appear in the mechanism but not in the overall reaction equation. Catalysts appear in the mechanism and are regenerated at the end.

(e) Explaining the Effect of a Catalyst

The final part might ask about how the addition of a catalyst will affect the reaction rate and the activation energy.

Answer: A catalyst lowers the activation energy, increasing the rate of the reaction without being consumed itself. It provides an alternative reaction pathway with a lower activation energy.

Question 4: Electrochemistry

This question typically focuses on electrochemical cells (galvanic and electrolytic), redox reactions, and electrode potentials.

(a) Writing Balanced Half-Reactions

Given a redox reaction, students need to write balanced half-reactions for oxidation and reduction.

Answer: This involves balancing the number of atoms and charges in each half-reaction separately. The number of electrons gained in reduction must equal the number of electrons lost in oxidation. Worth keeping that in mind.

(b) Calculating Standard Cell Potential

Using standard reduction potentials, students need to calculate the standard cell potential (E°) for a galvanic cell.

Answer: E°_cell = E°_reduction - E°_oxidation. The standard reduction potentials are typically provided in a table.

(c) Calculating Gibbs Free Energy Change

Using the standard cell potential, students can calculate the standard Gibbs free energy change (ΔG°) for the reaction.

Answer: ΔG° = -nFE°, where n is the number of moles of electrons transferred and F is Faraday's constant.

(d) Determining the Effect of Concentration

This section might explore the effect of changing reactant concentrations on the cell potential using the Nernst equation.

Answer: The Nernst equation accounts for the effect of non-standard concentrations on the cell potential. This involves plugging the concentrations into the Nernst equation and calculating the cell potential under non-standard conditions.

(e) Describing an Electrolytic Cell

This might involve describing the operation of an electrolytic cell and identifying the anode and cathode.

Answer: An electrolytic cell uses electrical energy to drive a non-spontaneous redox reaction. The anode is the electrode where oxidation occurs, and the cathode is the electrode where reduction occurs. The direction of electron flow is opposite to a galvanic cell.

Question 5: Descriptive Chemistry and Qualitative Analysis

This question tests the knowledge of descriptive chemistry, including properties of elements and compounds, and qualitative analysis techniques. This might include questions related to solubility, precipitation, complex ion formation, etc. The specific details of the question vary greatly from year to year.

Question 6: Organic Chemistry (Occasionally Included)

Sometimes, the AP Chemistry exam includes an organic chemistry question. This could cover various aspects, such as:

• Nomenclature: Naming organic compounds based on their structures.
• Isomerism: Identifying different types of isomers (structural, geometric, etc.).
• Reactions: Predicting the products of common organic reactions.
• Spectroscopy: Interpreting simple NMR or IR spectra.

Conclusion: Mastering the 2017 AP Chemistry FRQs

The 2017 AP Chemistry FRQs provide a valuable resource for students preparing for the exam. By carefully reviewing these questions and understanding the solution strategies, students can improve their problem-solving skills, learn to communicate their reasoning clearly, and gain confidence in tackling diverse chemistry concepts. So this detailed analysis of the 2017 FRQs should serve as a strong foundation for this preparation. But remember that consistent practice with a variety of FRQs, coupled with a strong understanding of the underlying principles, is crucial for success on the AP Chemistry exam. Remember to consult your textbook and class notes for additional clarification and practice problems.