Applied Practice in. Thermodynamics

Transcription

1 Applied Practice in Thermodynamics AP* Chemistry Series RESOURCE GUIDE Volume 11 *AP is a registered trademark of the College Entrance Examination Board, which was not involved in the production of, and does not endorse, this product. Pre-AP is a trademark owned by the College Entrance Examination Board Applied Practice, Ltd., Dallas, TX. All rights reserved.

2 Copyright 2009 by Applied Practice, Ltd All rights reserved. No part except the Student Practices portion of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. The Student Practices portion of this publication may be reproduced in quantities limited to the size of an individual teacher s classroom. It is not permissible for multiple teachers to share a single Resource Guide. Printed in the United States of America.

3 APPLIED PRACTICE Resource Guide Thermodynamics Teacher Notes and Strategies A Note for Teachers... 5 Teaching Strategies... 8 Glossary of Terms Student Practices Multiple-Choice Questions Entropy Free Energy Calculations Spontaneity Free Energy and Equilibrium Descriptive Chemistry and Laboratory Free-Response Questions Answer Key and Explanations Multiple-Choice Answer Key Multiple-Choice Answer Explanations Free-Response Answers and Scoring Guides *AP is a registered trademark of the College Entrance Examination Board, which was not involved in the production of, and does not endorse, this product by Applied Practice, Ltd., Dallas, TX. All rights reserved.

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5 Teacher Notes and Strategies for Thermodynamics

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7 A NOTE FOR TEACHERS The Applied Practice in AP Chemistry series was designed for use by teachers as an instructional supplement to major units in the AP Chemistry curriculum. This series was also conceived as a resource for teachers in preparing students for the AP Chemistry Exam. As you teach each unit, your students will have the opportunity to practice and to develop those skills required on the exams. Each book in the series includes: Teaching notes and strategies Glossary of terms 75 multiple-choice questions replicating Section I of the AP Chemistry Exam Multiple-choice answer keys and answer explanations 6 free-response questions replicating Section II of the AP Chemistry Exam Free-response answer keys and scoring guide We offer a few suggestions and explanations to help you receive the maximum benefit from our materials: 1. Applied Practice booklets do not purport to duplicate exactly an Advanced Placement Examination. However, questions are modeled on those typically encountered on these exams. Thus, students using these materials will become familiar and comfortable with the format, question types, and terminology of Advanced Placement Examinations. 2. Each Applied Practice booklet focuses on one topic within the AP Chemistry curriculum. These booklets are excellent resources for teachers and their students. Their unique format includes questions designed for use during the initial teaching of the required topics. Other questions are exceptional for the review phase of the course, as students pull the entire year together leading up to the AP Chemistry Exam. The AP exam often will require knowledge in multiple content areas on the same question. 3. You have the option of using the Applied Practice booklets for your own lesson and test preparation or, if you so choose, students may work through an Applied Practice test booklet on their own as they progress though the course. The students can check their own answers with the answer key and read the answer explanations provided in the teacher edition, conferring with the teacher as needed. 4. The order of topics in the Applied Practice booklets has been organized to follow a logical progression that is similar to the sequence in many of the most widely selected AP chemistry textbooks. You will find that they can easily be adapted to whatever sequence you find most productive at your school by Applied Practice, Ltd., Dallas, TX. All rights reserved. 5

8 5. The free response questions in each topic were created to provide practice questions similar to both those given in part A of the AP Chemistry Exam, which allows use of a calculator, and those given in part B, in which no calculator is allowed. In a few cases, the specific content is best assessed with a combination of both types. 6. Due to the emphasis on laboratory experience in the College Board s AP Chemistry program, the Applied Practice booklets in AP Chemistry frequently include laboratory-based questions appropriate to the subtopic addressed. A required laboratory-based question does appear on the AP Chemistry Exam. While most Applied Practice booklets in the AP Chemistry series do contain laboratory-based free-response questions, some topics do not lend themselves to the College Board-recommended laboratory experiments. However, each Applied Practice booklet does contain multiple-choice questions related to both laboratory and descriptive chemistry. Only one of the six free-response questions included on the AP Chemistry Exam is laboratory based. 7. Each booklet includes a glossary of terms that applies to the vocabulary of that particular topic. 8. If the teacher wishes to replicate the conditions under which students will take the actual AP Chemistry Exam, he or she should understand the following about multiple-choice versus free-response questions when using Applied Practice booklets: When answering multiple-choice questions (AP Exam, Section I) students are not allowed the use of a calculator, and the only reference information available to them is a periodic table (with only symbol, mass number, atomic number) and a small table of abbreviations/symbols used in the questions. When answering free-response questions (AP Exam, Section II), much more information is available to the student. In addition to the periodic table, a table of standard reduction potentials in aqueous solutions and a relatively complete list of equations, constants, and abbreviations/symbols are provided by Applied Practice, Ltd., Dallas, TX. All rights reserved.

9 COPYRIGHT NOTICE The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material. Reproduction of individual worksheets from this booklet, excluding content intended solely for teacher use, is permissible by an individual teacher for use by his or her students in his or her own classroom. Content intended solely for teacher use may not be reproduced, stored in a retrieval system, or transmitted in any way or by any means (electronic, mechanical, photocopying, or otherwise) without prior written permission from Applied Practice. Reproduction of any portion of this booklet for use by more than one teacher or for an entire grade level, school, or school system, is strictly prohibited. By using this booklet, you hereby agree to be bound by these copyright restrictions and acknowledge that by violating these restrictions, you may be liable for copyright infringement and/or subject to criminal prosecution by Applied Practice, Ltd., Dallas, TX. All rights reserved. 7

10 TEACHING STRATEGIES Following are some suggestions for using Applied Practice materials as you work to help your students develop mastery in answering AP-style questions. GENERAL The AP Chemistry Exam will be a challenge for all students. In this section, we have provided strategies for answering the questions themselves as well as some suggestions for teachers to keep in mind when presenting the material. 1. A certain amount of rote memorization is required. We offer the following suggestions to help students prepare for this: Mnemonic devices are excellent tools for memorizing hard facts. For example, in Kings Play Chess On Funny Green Squares, the first letter of each word represents the divisions of the classification system in binomial nomenclature: kingdom, phylum, class, order, family, genus, and species. Students are also encouraged to create flashcards; a tried and true method that can help them be successful. Items that students are required to know for the AP Chemistry Exam include: names and formulas of polyatomic ions names and formulas of strong acids and bases, as well as common acids and bases the metric prefixes from pico to giga the Greek prefixes in molecular nomenclature: mono, di, tri, tetra, penta, etc. the solubility rules for ionic compounds the identity of the seven diatomic elements the phases of all elements at room temperature (25 o C) the SI quantities and their units rules for significant figures rules for assigning oxidation numbers to atoms in compounds 2. Students will see many graphs in both the multiple-choice and free-response questions. In a few instances, they may be required to produce the graph from given information. Teachers should incorporate interpretation of scientific data written in graphs during instruction both in the classroom and the laboratory. The more graphs that students are required to produce during the year, the easier the questions containing this type of information will be for them on the exam by Applied Practice, Ltd., Dallas, TX. All rights reserved.

11 MULTIPLE-CHOICE QUESTIONS (MC): 1. The questions in these booklets are valuable for both the initial teaching of the concepts of chemistry and for preparing students for the style of questions and the level of difficulty encountered on the AP Chemistry Exam. In particular, some subtopics are more fundamental than others. While these subtopics are required in order for students to be able to comprehend more advanced material later on, they would not necessarily appear as is in a question on the AP exam. However, we have included multiple-choice questions over this material to support instruction, to provide an expanded test bank of questions, and to familiarize students with this type of assessment. 2. The question most commonly asked by students preparing for the AP exam is: How many multiple-choice questions should I answer in Section I? To answer this question, we will consider what is required for a student to receive a score of 3 (which is labeled as qualified by the College Board and considered passing ; thus, this score is a reasonable goal for most students.) Judging by released past exams for which full data is available, a student typically must score approximately points out of the160 possible on Sections I and II combined in order to receive a 3. Thus, it is reasonable to assume that to pass the AP Chemistry Exam, students need to earn points or more from Section I. For each question answered incorrectly, a 0.25 point penalty will be subtracted from the score. Typically, if students are answering more questions correctly than incorrectly, they should continue to increase the number of questions they answer, for it takes four incorrect answers to cancel one correct answer. Most students reach a point at which answering additional questions merely lowers their score. Successful students over the years typically answer the following numbers of questions corresponding to these eventual scores: questions: questions: questions: 5 This formula by no means guarantees a particular score; each student is unique in his or her approach to answering questions. This is simply offered as a guideline; an average number of questions a student may choose to answer out of the 75 total questions in an effort to attain a particular achievement level. 3. Another question related to multiple-choice questions on Section I of the AP Chemistry Exam is, Should I guess? Because of the penalty for wrong answers (-0.25) and the fact that there is no penalty for questions not answered (just no points earned), it is advisable not to answer questions if the student has little or no knowledge of the information addressed in the question. However, if students can eliminate one or more of the answer choices, it may be a good idea to select an answer, as statistically the odds are then in their favor overall for those questions by Applied Practice, Ltd., Dallas, TX. All rights reserved. 9

12 4. A general strategy for multiple-choice questions involves time management. Since the exam limits students time to 90 minutes for Section I, they will need to learn to be efficient. While some students simply work more quickly than others, virtually all students find that time is a limiting factor on the AP Chemistry Exam. The ability to read and comprehend quickly is a tremendous advantage. Another skill that will help students overcome the time factor is reading and assessing the level of difficulty of each multiple-choice question quickly. Those questions that students believe they can answer but will take quite a bit of time to analyze should be skipped over initially. Students can mark those questions and return to them later, provided there is enough time. The vast majority of questions answered correctly by students are those for which they can determine the answer almost immediately. Those questions that consume the most time are also those that are most likely to be answered incorrectly. 5. As students approach AP-level question sets, teachers should prepare students to be satisfied with a lower percentage of correct answers than is typical for them up to this point in their educational experience. Most schools around the country consider 90% an A grade, and many AP chemistry students are disappointed when earning test scores in the 70% or 80% range. However, the AP Chemistry Exam is scored in such a way that a successful passing score is between 34% and 50%, and a 5 the highest score possible is achieved with any point total above approximately 63%. Thus, it will take some time for students to adjust psychologically. Students should understand that a typical passing rate nationwide on the AP Chemistry Exam is approximately 56% of those taking the exam. It is definitely an achievement for a high school student to find success with a 3, 4, or 5 on one of the most challenging AP exams. One way to help students cope with a lower percentage of correctly-answered questions is to point out that identifying one s weakest areas is actually very beneficial when taking the exam. Students who can identify these questions quickly and spend little time on them can accumulate precious minutes to use on other questions, thus raising their score. 6. The percentage scores on AP exams noted above refer to data published by the College Board on their released questions and will vary from year to year, as individual students and classes of students vary in their abilities from year to year. Also keep in mind is that when students are being assessed on tests in class during the school year, they are often assessed on a concentrated, narrow amount of information helpful for formative assessment and preparation, but not necessarily characteristic of a final AP exam administered after course has been completed. While students may achieve a higher degree of success on AP-level multiple-choice questions when they are tested immediately at the end of a unit; teachers are encouraged to factor in the level of difficulty and drastically reduced raw score that is regarded as highly successful by the College Board when assessing the students efforts by Applied Practice, Ltd., Dallas, TX. All rights reserved.

13 7. AP chemistry students typically do struggle with AP-level questions early in the course. AP-level questions require a broad background in the overall chemistry curriculum, and are also more difficult than those most students have encountered up to this point. It is advisable to provide answers to practice questions for students, especially early in the school year. They will find it helpful to read the question, work out an answer, and then get immediate feedback. Providing answer keys allows students to work backward from the answer and develops their understanding of how to attack multiple-choice questions at this level of difficulty. The answer explanations provided in the Applied Practice booklets are particularly useful in this regard. In addition to allowing some access to answer keys (unless, of course, you will be using these questions for assessment), teachers should plan to include time in class for guided practice of these questions, working them out with the students. Collaboration among students in small learning groups is also a useful technique. Chemistry is much easier to learn when students are able to talk about the content and reason out answers together. 8. A simple but useful tip for students is to underline, or in some way highlight, key terms in the questions. Many times, a student will choose an incorrect answer because he or she simply misread the question. FREE-RESPONSE QUESTIONS (FRQs): 1. One of the most common reasons for producing an unsatisfactory response to a freeresponse question on an AP Chemistry Exam is that the student did not answer the specific question that was asked. Often, students write information that is factually correct but does not apply to the question, or their response relates to information in the question without actually answering the question. Free-response questions often present a great deal of information when only small bits of that information are actually needed to answer each of many different parts of the question. Students should read the entire question before they begin to write. They should underline, or in some way highlight, exactly what is being asked in each part of the question. After writing their response, students should proofread their work to be sure they answered the question specifically. Often, students will look back over their answers when their papers are returned and realize their answers make little sense, often responding, But I meant to say. They will likely experience greater success when they take time to read over their answers to verify they actually wrote what they intended to say. 2. Students should be specific in their answers and should understand the difference between a definition and information that justifies or explains their answer. In most cases, students should use complete sentences. A few questions have parts that merely require a list of items, and others ask for additions to diagrams, but for the most part, 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 11

14 students will need to write coherently. A one-word answer is not sufficient to earn credit. 3. Students should be sure they understand the difference between an answer that justifies their point and one that merely cites a correlation. A common example on past tests is a question about what determines the relative strength of London dispersion forces between molecules. Larger-sized atoms and molecules have more electrons and are more easily ionizable ; thus, they are more likely to form dispersion forces. As atoms and molecules get larger, their nuclei get heavier, and thus the atomic and molecular weights increase along with dispersion forces. However, while increasing mass correlates to increasing strength of attraction between molecules, it does not explain or justify the increase. An analogy illustrating this concept would be a graph of children s mass with their level of intelligence. As they age, their brains develop at the same time their bodies grow and become heavier. Thus, older children are both smarter and heavier than younger children. But it would be incorrect to say that the children are becoming smarter because they are becoming heavier. 4. When teaching concepts in chemistry, it is tempting to make atoms, ions, and molecules anthropomorphic. Humans tend to more easily understand concepts that are explained in a way that is relevant to the way we think and behave. Teachers must be careful not to instruct the students about what the atoms want or need. Often, students will write answers about an atom that is happy because it got the electron it needed. Even though students may have come to an appropriate level of comprehension on this concept, they will not receive credit when citing the emotions of a piece of matter on the AP Chemistry Exam. 5. Students should write out words fully, unless the abbreviations are universally accepted. While their teachers may have come to understand a particular student s shorthand, students are likely to lose out when the AP exams are graded. 6. Students should develop the habit of avoiding pronouns in their written answers, such as beginning their first sentence with it. In many questions, there are dozens of things to which it could refer, and students will receive no credit for their answer if they use vague pronouns. 7. Using the free-response questions in the Applied Practice books is a good way to develop students writing skills. Answers, explanations, and scoring guides for each of the six questions in an Applied Practice booklet are provided. The more writing students do, the better they will become at answering the questions in Section II of the AP exam. Although it takes time to grade students writing, the time will be well worth it in terms of their improvement and eventual success. Free-response questions are of three general types: large calculation questions that require good problem solving skills, equation writing, and essay types that require good writing skills by Applied Practice, Ltd., Dallas, TX. All rights reserved.

15 The first type of question will be seen on Section A of Part II of the exam, and the last two types will be found on Section B of Part II. 8. When answering free-response questions that require calculations (section II, part A, questions 1, 2, and 3), students should develop these good habits from the beginning of the year: Always list any mathematical formula to be used (i.e. PV=nRT) at the beginning of the question. Never put numerical information in an equation without first writing that equation down first. The vast majority of all mathematical formulas to be used are listed in the reference sheets provided on the AP Chemistry Exam in section II. Some knowledge of mathematical formulas will be required on section I (multiplechoice), so it is good for students to know them and not always rely on the reference sheets. While students work must be shown, typically much less work is required than that required by most high school math teachers. If the student lists the mathematical formula, substitutes the correct numerical values from the question into the proper location, and clearly marks the answer, the student does not need to write out each of the various algebraic steps. However, there is no disadvantage in scoring if students are more confident showing their work. They simply need to be coached in the amount of time to use on each calculation question. Some students will need to write fewer steps if time becomes a factor overall. Once the students calculate and write a final answer, they should do the following each time: Double check the number of significant figures in the answer. They must be within one significant figure up or down from the correct number. Double check the calculations. Make sure they have labeled their answer as to which quantity it is (do not leave answer as x=0.050 ). A label helps ensure they are answering the question correctly. Always include units (except for a few that do not use units, such as specific gravity and the equilibrium constant, K eq, etc). Box in the final answer. This not only helps the grader, but it provides an easy way for the students to check their work to ensure they have finished each part of each question. Also, answers from previous sections are often used on subsequent parts of the question. It is easier to find this information if it is boxed by Applied Practice, Ltd., Dallas, TX. All rights reserved. 13

16 GLOSSARY OF TERMS cuvette transparent tubular laboratory vessel or dish for holding a liquid endothermic a chemical reaction in which heat is absorbed enthalpy (H) the amount of energy contained in the substances in a reaction (system) available for transfer between itself and the environment (surroundings) entropy (S) a measure of the randomness or disorder of a system (substances being studied); thermodynamic quantity that applies to the second law of thermodynamics equilibrium constant (K a, K b, K w, etc.) the constant value that expresses the relationship between the concentration of products and reactants in a reversible chemical reaction at equilibrium exothermic a chemical reaction that produces heat Gibbs free energy (G) a measure of the capacity of a system to do work, such as the likelihood of a chemical reaction to form products; a thermodynamic quantity defined by the equation: G = H TS Henderson-Hasselbalch equation used to determine the ph of a buffer system; ph = pk a + log [conjugatebase] [conjugate acid] momentum the product of mass and velocity, usually refers to linear momentum that includes direction and magnitude of motion precipitation the formation of an insoluble solid compound during a chemical reaction in a solution spontaneous a chemical or physical change that occurs by itself thermodynamics the study of the relationship between matter and heat or other forms of energy, involved in a chemical or physical reaction by Applied Practice, Ltd., Dallas, TX. All rights reserved.

17 Student Practices for Thermodynamics

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19 Multiple-Choice Questions 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 17

20 Entropy The following answer choices can be used in questions 1-3. Each answer may be used once, more than once, or not at all. (A) CO (g) + H 2 O (g) CO 2(g) + H 2(g) (B) Na 2 CO 3(s) Na 2 O (s) + CO 2(g) (C) N 2(g) + 3H 2(g) 2NH 3(g) (D) KCl (s) K + (aq) + Cl - (aq) (E) H 2 O (s) H 2 O (l) 1. In which reaction would the entropy change be closest to zero? 2. In which reaction would S have the most positive value? 3. In which reaction would S have the most negative value? 4. Reactions where entropy decreases include I. (NH 4 ) 2 CO 3(s) 2NH 3(g) + CO 2(g) +H 2 O (g) II. NaCl (s) Na + (aq) + Cl - (aq) III. SO 2(g) + CaO (s) CaSO 3(s) (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 5. Which of the following correctly lists the substances in terms of increasing standard molar entropy? I. H 2 O (s) < H 2 O (l) < H 2 O (g) II. O 2(g) < H 2 O (l) < CaCl 2(s) III. Cl 2(g) < PCl 3(g) < PCl 5(g) (A) I only (B) III only (C) I and II only (D) I and III only (E) I, II, and III by Applied Practice, Ltd., Dallas, TX. All rights reserved.

21 6. The entropy of a pure crystalline substance at absolute zero, is zero, is a statement of which of the following? (A) The 1st law of thermodynamics (B) The 2nd law of thermodynamics (C) The 3rd law of thermodynamics (D) The law of conservation of energy (E) The law of conservation of matter 7. A typical unit for entropy is (A) J. (B) J -1 K -1. (C) J mol K. (D) J mol -1. (E) J mol -1 K Which of the following thermodynamic quantities defines the degree of disorder? (A) Enthalpy (B) Entropy (C) H (D) Gibbs free energy (E) G 9. Which of the following processes involves an increase in entropy? (A) The precipitation of lead(ii) chloride when mixing solutions of lead(ii) nitrate and sodium chloride (B) The freezing of ethanol (C) The condensation of water vapor (D) The decomposition of a solid, group 2 carbonate to yield carbon dioxide gas when heated (E) The formation of solid calcium sulfate when gaseous SO 3 is passed over heated, solid CaO 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 19

22 10. Which of the following statements is NOT correct? (A) Solids have smaller entropy values than gases. (B) Gases have higher entropy values than liquids. (C) Liquids have higher entropy values than solids. (D) Particles in solution have lower entropy values than those in solids. (E) Gases have higher entropy values than solids. 11. Which of the following is a direct consequence of this statement: The entropy of a pure crystalline substance at absolute zero, is zero? (A) All substances will have values for standard molar entropy that are greater than zero. (B) A substance at a temperature of 300 K will have a lower absolute entropy than the same substance at a temperature of 273 K. (C) Entropy can be measured in units of kj mol -1 K -1. (D) Absolute zero is equal to 0 K. (E) Entropy and enthalpy are directly proportional. 12. All of the following are true statements in relation to entropy EXCEPT (A) the change in entropy during a reaction is given the symbol S (B) positive values of S are associated with increases in disorder (C) entropy is a measure of the randomness or disorder of a system (D) S can be calculated using the formula, S = S (products) S (reactants) (E) a positive value for S is essential for a spontaneous reaction 13. Which of the following substances will have the largest value for standard molar enthalpy? (A) H 2 O (l) at 298K (B) H 2 O (l) at 300K (C) H 2 O (g) at 380K (D) H 2 O (g) at 385 K (E) H 2 O (g) at 400 K by Applied Practice, Ltd., Dallas, TX. All rights reserved.

23 14. Which of the following is a statement of the 2nd law of thermodynamics? (A) Energy is conserved in a chemical reaction. (B) Mass is conserved in a chemical reaction. (C) The entropy of a pure crystalline substance at absolute zero, is zero. (D) The total entropy of the universe tends to increase. (E) Momentum is conserved in a collision. 15. Which of the following is the correct set-up for the calculation of the entropy change in the reaction below? 2NO 2(g) N 2 O 4(g) Standard molar entropies at 298 K: NO 2(g) = 240 J mol -1 K -1, N 2 O 4(g) = 304 J mol -1 K -1 (A) (B) (C) 304 ((2)(240)) (D) 304 ((298)(240)) (E) 304 ((298)(2)(240)) 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 21

24 Free Energy Calculations The following answer choices can be used in questions Each answer may be used once, more than once, or not at all. (A) S = S (products) S (reactants) (B) G o o o = G f (products) G f (reactants) (C) H o o o = ΔH f (products) ΔH f reactants) (D) G o = - RT lnk (E) G o = H o - T S o 16. Can be used directly to calculate the entropy change at a particular temperature, in a reaction where the value for Gibbs free energy change and enthalpy change are known 17. Can be used directly to calculate the Gibbs free energy change at a particular temperature, for a reaction where values for enthalpy and entropy changes are known 18. Can be used directly to calculate enthalpy change of a reaction where the entropy change and Gibbs free energy change for the reaction at a particular temperature are known 19. For a reaction at a particular temperature, T, it is found that G o = -10 kj mol -1, H o = -20 kj mol -1, and S o = +0.1 kj mol -1 K -1. Which of the following is true? I. The reaction will be spontaneous at all temperatures. II. T = 1000 K III. The reaction will be spontaneous at the temperature studied. (A) I only (B) II only (C) III only (D) I and III only (E) I, II, and III by Applied Practice, Ltd., Dallas, TX. All rights reserved.

25 20. In the expression G o = - RT lnk, in order to calculate a value for G o in units of J mol -1 I. T must be recorded in units of K II. R must have a value of III. R must have units of J mol -1 K -1 (A) I only (B) III only (C) I and II only (D) I and III only (E) I, II, and III 21. The standard Gibbs free energy of formation of ethene, hydrogen, and ethane are 68.1, 0.00, and kj mol -1, respectively. Which is the correct combination of calculated G o value and statement regarding the spontaneity of the reaction? (A) +101 kj mol -1, spontaneous (B) -101 kj mol -1, spontaneous (C) kj mol -1, nonspontaneous (D) kj mol -1, spontaneous (E) kj mol -1, spontaneous C 2 H 4 + H 2 C 2 H 6 Questions refer to the following information. The Haber process can be summarized by the reaction below, and data associated with each of the substances is shown in the table. N 2(g) + 3H 2(g) 2NH 3(g) Substance ΔH o f in kj mol -1 S o in J mol -1 K -1 N 2(g) H 2(g) NH 3(g) What is the value of H o (in kj) for the forward reaction as written? (A) +92 (B) +46 (C) 0 (D) -46 (E) by Applied Practice, Ltd., Dallas, TX. All rights reserved. 23

26 23. What is the value for S o for the forward reaction? (A) -199 J mol -1 K -1 (B) 0 J mol -1 K -1 (C) +199 J mol -1 K -1 (D) +386 J mol -1 K -1 (E) +484 J mol -1 K For the reverse reaction G o is found to be kj at 298 K. What is the value for the equilibrium constant for the forward reaction? (A) 6.64 x 10 5 (B) 6.64 x 10-5 (C) 3.32 x 10-4 (D) 1.30 x 10-3 (E) 3.32 x The single displacement reaction between zinc metal and copper(ii) ions in solution is summarized below. What can be said of the equilibrium constant and the spontaneity of this reaction at this temperature? Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) G = -212 kj mol -1 (A) K > 1, and the reaction is nonspontaneous. (B) K = 1, and the reaction is nonspontaneous. (C) K > 1, and the reaction is spontaneous. (D) K < 1, and the reaction is spontaneous. (E) K = 0, and the reaction is spontaneous. 26. For a reaction that is known to be spontaneous at a particular temperature, it is also found that the S o value is negative. What can be deduced about the reaction from this information? (A) H is negative. (B) H is positive. (C) H = 0. (D) H is positive at high temperatures and negative at low temperatures. (E) Nothing can be deduced about H from this information by Applied Practice, Ltd., Dallas, TX. All rights reserved.

27 Questions refer to the following information. A reaction is known to have H o = +190 kj, S o = +50 J K -1, and G o = +90 kj. Assuming that H o and S o are fixed, answer the questions that follow. 27. Calculate the value of T in Kelvin for the reaction at this temperature. (A) 100 K (B) 200 K (C) 500 K (D) 1000 K (E) 2000 K 28. Which expression shows the correct set up for finding the Kelvin temperature at which the reaction becomes spontaneous? (A) +290 = +190 T(+50) (B) +290 = +190 T(+50/1000) (C) 0 = +190 T(+50) (D) 0 = +190 T(+50/1000) (E) 0 = +50 T(+190/1000) 29. A reaction that occurs in the gaseous phase can be summarized by the equation below. X (g) Y (g) + Z (g) K p is found to be 3.0 x 10 2 at 273 K. If a mixture of gases with the partial pressures shown in the table below is introduced into a flask at 273 K, what will be true at this instantaneous point in time? Gas Partial Pressures X 10 Y 10 Z 50 (A) The reaction is spontaneous in the reverse direction. (B) The reaction is spontaneous in the forward direction. (C) The reaction is at equilibrium. (D) G o for the forward reaction is positive. (E) G o = by Applied Practice, Ltd., Dallas, TX. All rights reserved. 25

28 30. The equilibrium constant for a single displacement reaction between a metal and a solution of another metal s ions is found to be 4.0 x When equilibrium has been established, which of the following correctly describes the free energy change and the relative amounts of reactants and products found in the reaction mixture? (A) The free energy change is positive, and large amounts of products are found. (B) The free energy change is negative, and large amounts of products are found. (C) The free energy change is positive, and large amounts of reactants are found. (D) The free energy change is zero, and large amounts of reactants are found. (E) The free energy change is zero, and large amounts of products are found by Applied Practice, Ltd., Dallas, TX. All rights reserved.

29 Spontaneity The following answer choices can be used in questions Each answer may be used once, more than once, or not at all. (A) G o = -, H o = -, S o = + (B) G o = -, H o = +, S o = + (C) G o = -, H o = -, S o = - (D) G o = +, H o = +, S o = - (E) G o = +, H o = -, S o = Reactions with these characteristics are spontaneous at all temperatures. 32. Reactions with these characteristics are never spontaneous at any temperature. 33. Reactions with these characteristics are spontaneous because of their favorable entropies rather than their non-favorable enthalpies 34. An exothermic reaction where the number of moles of gas increases is I. One that has a negative H o. II. One that has a positive S o. III. One that is spontaneous at all temperatures. (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 35. Which of the following is true for the ΔH f o, ΔS f o, and ΔG f o for any element? I. ΔH o f = 0 II. ΔS o f = 0 III. ΔG o f 0 (A) I only (B) III only (C) I and II only (D) II and III only (E) I, II, and III 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 27

30 36. When water vapor condenses, the process involves which change combination below? (A) H o = +, S o = + (B) H o = +, S o = - (C) H o = -, S o = + (D) H o = -, S o = - (E) H o = +, S o = Which combination of H o, S o, and T S o leads to a nonspontaneous reaction at all temperatures? (A) H o = +, S o = -, T S o = + (B) H o = -, S o = +, T S o = - (C) H o = +, S o = +, T S o = + (D) H o = -, S o = -, T S o = + (E) H o = +, S o = -, T S o = If G o for the forward reaction in an equilibrium process is greater than 0, all of the following statements are true EXCEPT (A) the reaction will have a negative G o for the reverse process (B) the forward reaction is nonspontaneous (C) G o = -RT lnk (D) K > 1 (E) the reverse reaction is spontaneous 39. For a spontaneous reaction, (A) G o = -, K > 1, lnk = + (B) G o = -, K > 1, lnk = - (C) G o = -, K < 1, lnk = + (D) G o = +, K < 1, lnk = + (E) G o = +, K > 1, lnk = by Applied Practice, Ltd., Dallas, TX. All rights reserved.

31 40. For a reaction with a positive S o and a positive H o, what can be said of the process? (A) S o favors spontaneity, but it can only be spontaneous if temperatures are relatively high. (B) S o favors spontaneity, but it can only be spontaneous if temperatures are relatively low. (C) S o does not favor spontaneity, but it can be spontaneous if temperatures are relatively high. (D) S o does not favor spontaneity, but it can be spontaneous if temperatures are relatively low. (E) S o favors spontaneity, and it will be spontaneous at all temperatures. 41. For the reaction shown below, which statement correctly predicts the change in the value of G o and associated change in spontaneity as temperature increases? (Assume that H o and S o are temperature independent.) C (s) + O 2(g) CO 2(g) H = -393 kj, S = 3.3 J mol -1 K -1 (A) G o is increasingly negative, and fewer products will be present at equilibrium. (B) G o is increasingly positive, and fewer products will be present at equilibrium. (C) G o is increasingly negative, and more products will be present at equilibrium. (D) G o is increasingly positive, and more products will be present at equilibrium. (E) G o does not change, and the amount of products at equilibrium will not change. 42. A spontaneous reaction that produces products from a position where only reactants are present initially definitely has (A) H o = + (B) H o = - (C) G o = + (D) G o = - (E) S o = For a reaction that is nonspontaneous at any temperature, what is true? (A) The reaction must be exothermic, and disorder increases. (B) The reaction must be endothermic, and disorder increases. (C) The reaction must be exothermic, and disorder decreases. (D) The reaction must be endothermic, and disorder decreases. (E) The reaction could be endothermic or exothermic, but disorder must increase by Applied Practice, Ltd., Dallas, TX. All rights reserved. 29

32 44. All of the following statements about a reaction that is spontaneous at all temperatures are true EXCEPT (A) it is exothermic (B) it has a positive S o (C) it has a negative G o at all temperatures (D) it will have a T S o term that is positive (E) it will have products that are less disordered than reactants 45. Which of the following combinations is impossible? (A) H o = +, S o = -, T S o = - (B) H o = -, S o = +, T S o = + (C) H o = +, S o = +, T S o = + (D) H o = -, S o = -, T S o = - (E) H o = +, S o = -, T S o = by Applied Practice, Ltd., Dallas, TX. All rights reserved.

33 Free Energy and Equilibrium The following answer choices can be used in questions Each answer may be used once, more than once, or not at all. (A) K > 1 (B) K = 1 (C) K < 1 (D) K = 0 (E) K < All spontaneous reactions have this value for K. 47. The value of K when G o is a negative number. 48. When K has this value, the value for G o is Which of the following is a mathematical consequence of G o = - RT lnk? I. K > 1 means G o is negative. II. lnk for a spontaneous reaction must be positive. III. R, the gas constant, in this expression must have the value 8.31 if G o is to be measured in J mol -1. (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 50. For a system where G = G o, which of the following statements is true? I. Equilibrium has been established. II. Q = K III. The term RT lnq = 1 (A) I only (B) III only (C) I and II only (D) II and III only (E) I, II, and III 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 31

34 51. When equilibrium is established, what can be said of the free energy present in the system? (A) It is at a maximum. (B) It is at a minimum. (C) G o is positive. (D) G o is negative. (E) It is between the maximum and minimum values. 52. The rusting of iron at 298 K has a G o value approximately equal to -1.5 x 10 6 J. What does this value alone tell us about the chemical reaction involved in the rusting of iron? (A) The reaction is fast. (B) The reaction is nonspontaneous. (C) The reaction will definitely have a positive S o. (D) The reaction will definitely have a positive H o. (E) The reaction will have a value for K that is greater than When equilibrium in a system has been established, all of the following are true EXCEPT (A) the formation of the reactants and the products are equally favored (B) the term RT lnk = 0 (C) Q = K (D) G o = 0 (E) free energy has reached a maximum 54. For a spontaneous reaction, the expression G o = - RT lnk tells us that as conditions are changed to make K larger, the reaction will also (A) create a more likely forward reaction (B) become less spontaneous (C) yield a lnk term that is smaller (D) produce an increasingly positive G o (E) create more reactants by Applied Practice, Ltd., Dallas, TX. All rights reserved.

35 55. Mathematically, the correct expression for the calculation of K, the equilibrium constant, is (A) K = - G o RT (B) K = ln G o RT ΔG ( RT (C) K = e ) (D) K = ln G o (E) K = e RT ΔG 56. The G o value for the self-ionization of water at 298 K is kj mol -1. What is the approximate value of K at this temperature? (A) 1.0 x (B) 1.0 x (C) 1.0 x 10 7 (D) 1.0 (E) 1 x The reaction below has a K value that is approximately equal to 4.0 x What does this value suggest about the reaction? Fe 3+ (aq) + 3OH - (aq) Fe(OH) 3(s) (A) That the K value for the reverse reaction will also be very large (B) That the reaction is unlikely to occur (C) That the G o for the reaction will be negative (D) That iron(iii) hydroxide will not precipitate (E) That the reaction is nonspontaneous 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 33

36 58. Consider the equilibrium that is set up between water and steam as in the equation below. It is found that the equilibrium is established at a temperature of 373 K. Which of the following statements is false? H 2 O (l) H 2 O (g) (A) Water boils at 373 K. (B) G o for the forward reaction at 298 K will be positive. (C) G o for the forward reaction at 400 K will be negative. (D) G o for the forward reaction at 373 K will be positive. (E) At equilibrium, G o is zero. 59. The reaction below has a value for lnk that is equal to 110 at 298 K. What does this suggest about the reaction? 2CO (g) + O 2(g) 2CO 2(g) (A) That K is less than 1 (B) That the reaction is nonspontaneous (C) That G o for the reaction is a negative number (D) That many more reactants than products are present in the equilibrium mixture (E) That ΔG f o (products) ΔG f o (reactants) = a positive number 60. An extremely large negative value for G o suggests all of the following EXCEPT (A) K is a very large, positive number (B) large quantities of products are formed (C) the reaction is essentially irreversible (D) the reverse reaction is likely (E) the reaction is spontaneous by Applied Practice, Ltd., Dallas, TX. All rights reserved.

37 Descriptive Chemistry and Laboratory The following answer choices can be used in questions Each answer may be used once, more than once, or not at all. (A) K a (B) K b (C) K c (D) K p (E) K sp 61. The specific equilibrium constant that could be determined directly by performing an analysis of data collected in a titration between aqueous sodium hydroxide and an organic acid. 62. The equilibrium constant that could be determined directly by the analysis of the partial pressures of NH 3, H 2, and N 2 in an equilibrium mixture. 63. The equilibrium constant that could be determined by the analysis of an equilibrium mixture of barium ions, sulfate ions, and barium sulfate. 64. In order to avoid death, living systems need I. a source of energy II. a molecular system to allow energy to be transferred into useful work III. to avoid a state of thermodynamic equilibrium (A) I only (B) II only (C) III only (D) I and II only (E) I, II, and III 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 35

38 65. In a procedure to determine the equilibrium constant for the reaction below that uses a colorimeter, what steps are necessary during the experiment? Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) I. Calibration of the colorimeter with solutions of known concentration II. Ensuring that the cuvette is clean and free from fingerprints III. Plotting a graph in order to determine concentrations (A) I only (B) III only (C) I and II only (D) I and III only (E) I, II, and III Questions refer to the following information. The K sp value for a relatively insoluble salt such as PbI 2 can be determined by performing an experiment that measures the concentration of ions present in a saturated solution of the salt. 66. Which expression allows the correct determination of K sp? (A) Ksp = [Pb 2+ ][I - ] (B) Ksp = [Pb 2+ ][I - ] 2 (C) Ksp = [Pb 2+ ] 2 [I - ] 2 [Pb 2+ ][I ] 2 (D) K sp = [PbI 2 ] 2+ [ Pb ][ I ] (E) Ksp = [ PbI ] by Applied Practice, Ltd., Dallas, TX. All rights reserved.

39 67. One method of measuring the concentration of iodide ions present in the saturated solution is to convert them to I 2 which is colored, and then to use a colorimeter to determine the concentration of I 2 present. Which of the following would be suitable to convert iodide ions to I 2? (A) A reagent that causes precipitation (B) An oxidizing agent (C) A reducing agent (D) A reagent that causes a gas to be produced (E) A reagent that increases the solubility of PbI Which of the following salts could be analyzed in a way similar to that in question #67 in order to determine a meaningful K sp value? (A) MgCl 2 (B) NaI (C) NaBr (D) PbBr 2 (E) Pb(NO 3 ) 2 Questions refer to the following information. A weak acid, CH 3 COOH, is titrated with a strong base, KOH, and it is found that ml of M acid requires ml of base for complete neutralization. 69. At what point in the titration will the concentration of salt produced in the neutralization reaction be equal to the concentration of the acid remaining in the mixture? (A) When 0.00 ml of KOH has been added (B) When 7.50 ml of KOH has been added (C) When ml of KOH has been added (D) When ml of KOH has been added (E) Never 2009 by Applied Practice, Ltd., Dallas, TX. All rights reserved. 37

40 70. When the concentration of the salt produced in the neutralization reaction is equal to the concentration of the acid remaining in the mixture, which of the following relationships is true? (A) pk a = 1 x 10-7 (B) ph = -log pk a (C) ph = K a (D) ph = pk a (E) ph = A similar experiment is conducted, this time using a different acid, HCl, but the same base. It was not possible to determine a K a value. Which of the following is a feasible reason for the failure of this experiment? (A) No suitable indicator can be found for this titration. (B) It is impossible to plot a titration curve for this titration. (C) HCl is a strong acid and as such no end point can be determined with KOH. (D) HCl is a strong acid and as such cannot be neutralized by KOH. (E) HCl is a strong acid and as such has no meaningful value for K a. Questions refer to the following information. In an experiment to determine the K c for an esterification reaction, a known number of moles of ethanoic acid are brought together with an equal number of moles of ethanol and a known, small amount of sulfuric acid at a known temperature. There is no ester or water present initially. The reaction mixture is allowed to reach equilibrium. CH 3 COOH + C 2 H 5 OH CH 3 CO 2 C 2 H 5 + H 2 O The equilibrium mixture is then titrated with NaOH solution. 72. What is the role of the sulfuric acid? (A) A reactant (B) An intermediate (C) A product (D) A catalyst (E) An oxidizing agent by Applied Practice, Ltd., Dallas, TX. All rights reserved.

41 73. What is the purpose of titrating the mixture with NaOH? (A) To determine only the moles of CH 3 COOH present (B) To determine only the moles of C 2 H 5 OH present (C) To determine only the moles of sulfuric acid present (D) To determine only the moles of CH 3 CO 2 C 2 H 5 present (E) To determine the total moles of CH 3 COOH and sulfuric acid present 74. Once the number of moles of CH 3 COOH at equilibrium has been established, what can also be established? (A) Only the moles of ethanol at equilibrium (B) Only the moles of ester at equilibrium (C) The moles of ethanol and water but not the moles of ester at equilibrium (D) The moles of ester and water but not the moles of ethanol at equilibrium (E) The moles of ethanol, ester, and water at equilibrium 75. The experiment yields a K c value = 3.9. The experiment is then repeated using larger initial quantities of reactants at the same temperature. What is the consequence for the value of K c in this new trial? (A) It will be larger than 3.9. (B) It will be smaller than 3.9. (C) It will be equal to 3.9 because the temperature has not changed. (D) It will be equal to 3.9 since it is a constant and has the same magnitude at all temperatures. (E) It is not possible to tell without more data by Applied Practice, Ltd., Dallas, TX. All rights reserved. 39