AP Unit 9 Progress Check: FRQ Chemistry Free-Response Guide

AP Unit 9 Progress Check: FRQ Chemistry Free-Response Guide

AP Unit 9 Progress Check focuses on free-response questions for AP Chemistry students preparing for the exam. It includes detailed explanations of thermodynamic principles, entropy changes, and redox reactions. The guide provides step-by-step calculations and justifications for various chemical processes, enhancing understanding of key concepts. Ideal for AP Chemistry students aiming to excel in their assessments, this resource covers essential topics and problem-solving techniques relevant to the curriculum.

Key Points

  • Explains thermodynamic favorability and entropy changes in chemical reactions.
  • Includes detailed calculations for standard entropy changes and cell reactions.
  • Covers oxidation-reduction reactions with oxidation number justifications.
  • Provides a structured approach to solving AP-style free-response questions.
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1.
For parts of the free-response question that require calculations, clearly show the method used and the steps
involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and
equations may be included in your answers where appropriate.
Answer the following questions related to
.
Substance
at
(a) Using the information in the table above, determine the value of at for the process represented by
the equation .
(b) Considering your answer to part (a), indicate whether the process is thermodynamically favorable at
.
Justify your answer.
(c) Considering your answer to part (b), explain why
has a measurable equilibrium vapor pressure at
.
Water vapor can be produced in two different processes, as represented below.
Process 1 Process 2
(d) In terms of concepts of entropy and the particle-level structure of the different phases of water, explain why
the change in entropy, , is greater for process 1 than for process 2.
Part (a)
Select a point value to view scoring criteria, solutions, and/or examples and to score the response.
0 1
The response includes both the following criteria:
Part (b)
AP CHEMISTRY Scoring Guide
Unit 9 Progress Check: FRQ
AP Chemistry
Page 1 of 7
Select a point value to view scoring criteria, solutions, and/or examples and to score the response. The answer to part (b)
must be consistent with part (a). If a student makes an error in a previous part of the question, the student can still earn
credit in a later part if the incorrect answer is used correctly.
0 1
The response indicates that the process is not thermodynamically favorable at
because is positive.(OR, if
part (a) indicates a negative value of
,the response indicates that the process is thermodynamically favorable
because
is negative.)
Part (c)
Select a point value to view scoring criteria, solutions, and/or examples and to score the response.
0 1
The response indicates that even though the process is not thermodynamically favorable, it does not follow that the
process does not occur at all. At equilibrium, the reactant (liquid water) is
favored over the product (water vapor). This
does not mean that some of the liquid water cannot evaporate to become water vapor.
Part (d)
Select a point value to view scoring criteria, solutions, and/or examples and to score the response.
0 1
The response indicates that the change in entropy between ice,
, and water vapor, , is greater because the
entropy of water in the solid phase, with molecules bound in fixed positions within the solid, is less than the entropy of
water in the liquid phase, with molecules able to move more freely and form a greater number of spatial configurations.
Water in the gas phase, with molecules much more dispersed and even more free to move, has the highest entropy of the
three phases. Therefore, process 1 (sublimation) involves a greater change in entropy than process 2 (vaporization of a
liquid) does.
Scoring Guide
Unit 9 Progress Check: FRQ
Page 2 of 7
AP Chemistry
2.
For parts of the free-response question that require calculations, clearly show the method used and the steps
involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and
equations may be included in your answers where appropriate.
Reaction 1:
A chemist wants to produce from a sample of pure according to reaction 1, represented by the
equation above.
(a) Using the data in the following table, calculate the value of the standard entropy change,
, for the
reaction.
Substance
Absolute Entropy at
33
205
43
(b) Given that for reaction 1 is positive , what must be true about the sign of for
the reaction? Justify your answer.
The decomposition of
into and is not a thermodynamically favored reaction. However, to
produce
from , the chemist decides to pass gas over the as it is heated strongly,
as represented below.
Reaction 2:
(c) Is reaction 2 an oxidation-reduction reaction? Justify your answer using oxidation numbers.
A third chemical reaction is represented below.
Reaction 3:
(d) Show how a combination of reaction 1 and reaction 3 can be used to produce reaction 2.
(e) Determine the value of
for reaction 2.
The chemist takes the
produced from reaction 2 and uses it to make an electrode in a galvanic cell, as
shown in the following diagram.
Scoring Guide
Unit 9 Progress Check: FRQ
AP Chemistry
Page 3 of 7
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End of Document
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FAQs of AP Unit 9 Progress Check: FRQ Chemistry Free-Response Guide

What are the key topics covered in the AP Unit 9 Progress Check?
The AP Unit 9 Progress Check covers essential topics such as thermodynamic favorability, entropy changes, and redox reactions. Students will learn to calculate standard entropy changes for various chemical reactions and understand the implications of these calculations on reaction feasibility. Additionally, the guide discusses the principles of oxidation and reduction, including how to identify changes in oxidation states. These concepts are crucial for mastering the AP Chemistry curriculum and performing well on the exam.
How does the guide help with understanding entropy changes?
The guide provides a comprehensive explanation of entropy changes in different chemical processes, particularly comparing sublimation and vaporization. It illustrates why the change in entropy is greater for sublimation, where solid transitions directly to gas, compared to vaporization, which involves a liquid phase. By analyzing the particle-level structure of water in different phases, students gain insight into the molecular behavior that influences entropy. This understanding is vital for answering related questions on the AP exam.
What is the significance of redox reactions in this guide?
Redox reactions are a critical component of the AP Chemistry curriculum, and this guide emphasizes their importance through detailed explanations and examples. It defines oxidation and reduction processes, helping students identify which species are oxidized and reduced based on changes in oxidation states. The guide also includes practical applications of redox reactions, such as in galvanic cells, where students learn to write balanced net-ionic equations. Understanding these concepts prepares students for both free-response and multiple-choice questions on the exam.
What types of calculations are included in the AP Unit 9 Progress Check?
The AP Unit 9 Progress Check includes various calculations related to thermodynamics and electrochemistry. Students will encounter problems requiring them to calculate standard entropy changes, Gibbs free energy, and cell potentials. The guide emphasizes the importance of showing work and justifying answers, which is essential for scoring well on free-response questions. By practicing these calculations, students enhance their problem-solving skills and gain confidence in their ability to tackle complex chemistry problems.

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