AP Chemistry Unit 9 Progress Check Free Response Guide

AP Chemistry Unit 9 Progress Check Free Response Guide

AP Chemistry Unit 9 Progress Check focuses on free-response questions that assess students' understanding of thermodynamics, entropy, and redox reactions. This guide provides detailed explanations and calculations related to standard entropy changes, thermodynamic favorability, and oxidation-reduction processes. Designed for AP Chemistry students preparing for the exam, it includes practice problems and scoring criteria. The content aligns with the 2024 AP Chemistry curriculum, ensuring relevance and applicability for exam preparation.

Key Points

  • Analyzes thermodynamic favorability using Gibbs free energy calculations.
  • Explains entropy changes in phase transitions of water, including sublimation and vaporization.
  • Covers redox reactions with oxidation state changes and half-reaction methods.
  • Includes scoring criteria for free-response questions to aid in exam preparation.
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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
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FAQs of AP Chemistry Unit 9 Progress Check Free Response Guide

What is the significance of Gibbs free energy in thermodynamics?
Gibbs free energy (ΔG) is a crucial thermodynamic quantity that indicates the spontaneity of a reaction. A negative ΔG value suggests that a reaction can occur spontaneously, while a positive ΔG indicates non-favorability. Understanding ΔG helps students predict whether a chemical process will proceed under given conditions, which is essential for mastering AP Chemistry concepts.
How does entropy change during phase transitions of water?
Entropy, a measure of disorder, changes significantly during phase transitions of water. For instance, when ice sublimates directly to vapor, the entropy increases more than when liquid water evaporates. This is due to the greater freedom of movement and spatial configurations available to gas molecules compared to those in solid or liquid states. Understanding these changes in entropy is vital for AP Chemistry students.
What are the key characteristics of redox reactions?
Redox reactions involve the transfer of electrons between species, resulting in changes in oxidation states. In these reactions, one species is oxidized (loses electrons) while another is reduced (gains electrons). Identifying oxidation states and understanding the flow of electrons is essential for analyzing chemical reactions in AP Chemistry, particularly in electrochemistry.
How can students prepare for the AP Chemistry free-response section?
Students can prepare for the AP Chemistry free-response section by practicing with past exam questions and understanding the scoring criteria. Familiarity with key concepts such as thermodynamics, kinetics, and equilibrium is essential. Additionally, showing clear calculations and logical reasoning in their answers can help maximize their scores. Utilizing study guides that focus on free-response formats can also be beneficial.

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