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Answers and Review for Multiple Choice Practice on Applications of Thermodynamics
⛔STOP ! ⛔ Before you look at the answers, make sure you gave this practice quiz a try so you can assess your understanding of the concepts covered in Unit 9. Click here for the practice questions: AP Chemistry Unit 9 Multiple Choice Questions.
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Facts about the test: The AP Chemistry exam has 60 multiple choice questions and you will be given 1 hour 30 minutes to complete the section. That means it should take you around 15 minutes to complete 10 questions.
*The following questions were not written by College Board and although they cover information outlined in the AP Chemistry Course and Exam Description the formatting on the exam may be different.
1. Which of the following will have a positive change in entropy?
Answer: The combustion reaction is the only option that has a positive entropy change because the reaction starts with 5 moles of gaseous molecules and ends with 7 moles of gaseous particles.
📄 Study AP Chemistry, Unit 9.1: Introduction to Entropy
2. A reaction has the following thermodynamic data. Which of the following statements is true?
ΔH° = -75.2 kJ/mol
ΔS° = 53.4 J/mol · K
A. The reaction is never thermodynamically favorable.
B. The reaction is thermodynamically favorable at high temperatures.
C. The reaction is thermodynamically favorable at all temperatures.
D. The reaction is thermodynamically favorable at low temperatures.
Answer: When enthalpy is negative and entropy is positive, the reaction is thermodynamically favorable regardless of the temperature.
📄 Study AP Chemistry, Unit 9.3: Gibbs Free Energy and Thermodynamic Favorability
3. What is the value for ΔG and thermodynamic favorability of the following reaction at 450 K?
A. ΔG = 170. kJ/mol; The reaction is thermodynamically favorable.
B. ΔG = 10.9 kJ/mol; The reaction is NOT thermodynamically favorable.
C. ΔG = 10.9 kJ/mol; The reaction is thermodynamically favorable.
D. ΔG = 170. kJ/mol; The reaction is NOT thermodynamically favorable.
Answer: Using ΔG = ΔH - TΔS, one finds that ΔG = 170 kJ/mol. Since ΔG is a positive number, this indicates that the reaction is not thermodynamically favorable.
📄 Study AP Chemistry, Unit 9.3: Gibbs Free Energy and Thermodynamic Favorability
4. What is the value for the equilibrium constant (K) at 305 K given the following information?
A. 1.01
B. 5.05 x 10^5
C. 0.987
D. 1.98 x 10^-5
📄 Study AP Chemistry, Unit 9.5: Free Energy and Equilibrium
5. Given the following redox reaction, which half-reaction will occur at the cathode?
Answer: The reduction occurs at the cathode. In the original reaction, the copper ion was being reduced, therefore the half-reaction involving the reduction of copper occurs at the cathode.
📄 Study AP Chemistry, Unit 9.7: Galvanic (Voltaic) and Electrolytic Cells
6. A galvanic cell is set up such that a redox reaction occurs between silver and aluminum. Given the following data, determine the cell potential.
A. 4.06 V
B. 0.86 V
C. -0.86 V
D. 2.46 V
Answer: A galvanic cell happens spontaneously, therefore the aluminum is oxidized while the silver is reduced. Since cell potentials are intensive properties, multiplying the reaction by a coefficient DOES NOT change the value for the standard reduction potential. Therefore, the sum of the standard reduction potentials equals the cell potential.
📄 Study AP Chemistry, Unit 9.8: Cell Potential and Free Energy
7. Given that the cell potential for a reaction is 0.52 V at 298 K, what is the sign for free energy and thermodynamic favorability of the following reaction?
A. ΔG is positive and the reaction is thermodynamically favorable.
B. ΔG is positive and the reaction is NOT thermodynamically favorable.
C. ΔG is negative and the reaction is thermodynamically favorable.
D. ΔG is negative and the reaction is NOT thermodynamically favorable.
Answer: Since the cell potential is positive, the value for ΔG must be negative. When ΔG is negative, the reaction is thermodynamically favorable.
📄 Study AP Chemistry, Unit 9.3: Gibbs Free Energy and Thermodynamic Favorability
8. Which of the following statements regarding the following reaction is true? The cell potential for the reaction is 0.46 V.
A. This reaction can be studied in a galvanic cell because the reaction is thermodynamically favorable.
B. This reaction can be studied in a galvanic cell because the reaction is not thermodynamically favorable.
C. This reaction can be studied in an electrolytic cell because the reaction is thermodynamically favorable.
D. This reaction can be studied in an electrolytic cell because the reaction is not thermodynamically favorable.
Answer: Galvanic cells are used to study thermodynamically favorable reactions. This reaction is thermodynamically favorable because the cell potential is positive.
📄 Study AP Chemistry, Unit 9.8: Cell Potential and Free Energy
9. Which of the following reasons correctly justifies why this thermodynamically favorable reaction does not occur unless a flame is brought to the reactant?
A. This reaction is endothermic and therefore requires an input of energy.
B. This reaction requires enough energy to overcome the activation energy.
C. This reaction requires a catalyst.
D. The flame provides a reactant that was not initially present in the reaction.
Answer: Most combustion reactions are thermodynamically favorable. They require an input of energy in order to overcome their activation energy.
📄 Study AP Chemistry, Unit 9.4: Thermodynamic and Kinetic Control
10. Which of the following statements regarding the following reaction is true? The free energy for the reaction is 59.9 kJ/mol
A. The reaction is reactant favored because K>1
B. The reaction is reactant favored because K<1
C. The reaction is product favored because K<1
D. The reaction is product favored because K>1
Answer: The reaction is reactant favored because ΔG is positive. This means that K, the equilibrium constant, is less than 1.
📄 Study AP Chemistry, Unit 9.0: Unit 9 Overview
11. Consider the following data. Which of the following statements is true?
A. This reaction is never thermodynamically favorable.
B. This reaction is always thermodynamically favorable.
C. This reaction is thermodynamically favorable at low temperatures
D. This reaction is thermodynamically favorable at high temperatures
Answer: When enthalpy is positive and entropy is positive, the reaction is thermodynamically favorable at high temperatures.
📄 Study AP Chemistry, Unit 9.3: Gibbs Free Energy and Thermodynamic Favorability
12. What is the entropy change for the reaction?
A. -48 J/K
B. 48 J/K
C. -161 J/K
D. 161 J/K
Answer: ΔS = [4(70.0)+2(214)] - [2(127)+3[205]] = -161
📄 Study AP Chemistry, Unit 9.2: Absolute Entropy and Entropy Change
13. What is the standard reduction potential for Lead given E°cell = 0.87 and the following information in the image below?
A. -0.13 V
B. 0.13 V
C. 1.61 V
D. -1.61 V
📄 Study AP Chemistry, Unit 9.8: Cell Potential and Free Energy
14. At 500 K, what is the enthalpy of the reaction given the following data?
A. -197 kJ/mol
B. 197 kJ/mol
C. 101 kJ/mol
D. 101,000 kJ/mol
📄 Study AP Chemistry, Unit 9.3: Gibbs Free Energy and Thermodynamic Favorability
15. Which of the following reactions has a negative value for the change in entropy?
Answer: When a substance condenses the substance's entropy decreases. This correlates to a negative entropy.
📄 Study AP Chemistry, Unit 9.1: Introduction to Entropy
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