Download Worksheet 8 - Plymouth State University

Freezing Point of a Solution
Materials:
two 250-mL beakers
ice to fill the beakers
50 mL water
50 mL ~20% salt solution (10 g NaCl(s) in 50 mL water)
two stout wooden stirrers
temperature probes and interface to a laptop computer and projector (or other devices) for
displaying and projecting the output of the probes. Alternatively, pairs of students can use
thermometers and stir, read, and record (announce) the temperatures of the mixtures in the
beakers every minute or so.
Procedure:
Fill each beaker with ice, insert a temperature probe in each, and stir each vigorously while
measuring the temperatures. Add the water to the ice in one beaker and the salt solution to the ice
in the other beaker. Stir vigorously for 2-3 minutes while measuring the temperatures of the
mixtures until the temperatures have become reasonably constant.
Observations:
What is the temperature of the stirred ice-water mixture?
What is the temperature of the stirred ice-salt solution mixture?
Analyis:
(1) At Tm, the melting point of a pure solid (or the freezing point of the pure liquid), the solid and
liquid phases are in equilibrium. When a solution of a non-volatile solute freezes, the solid
“product” consists of pure solvent, but the freezing/melting point of the solution is not the
same as the freezing/melting point of the pure solvent. Explain clearly how the mixtures in
the procedure and observations above exemplify these statements.
(2) How (directionally) is the freezing/melting point of a solution related to the freezing/melting
point of the pure solvent? How do you think entropy changes for these processes might be
related to this result?
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Pure Solid Solvent
Pure Liquid Solvent
Solution
(3) The entropy diagram below represents the entropies of the pure solid solvent, the pure liquid
solvent, and a solution of a non-volatile solid dissolved in the liquid solvent shown
schematically above. Based on what you know about entropy, label the three lines as Ssolid,
Sliquid, and Ssolution. Explain your reasoning. Draw an arrow (labeled Sm,solvent) representing
the entropy change when pure solvent freezes. Draw another arrow (labeled Sm,solution)
representing the entropy change when the solution freezes.
S
(4) How does the magnitude of Sm,solvent compare to the magnitude of Sm,solution? Explain.
(5) What is the value of G at the melting/freezing point? How do you know? What is the
mathematical relationship among Sm, Hm, and Tm? Explain.
(6) Since it is only the solvent that freezes in both mixtures, Hm,solvent = Hm,solution. Use this
information and your responses in (4) and (5) to show whether this entropy analysis predicts
that Tm,solution (the freezing point of a solution) is less than, equal to, or greater than Tm,solvent
(the freezing point of the pure solvent).
(7) Is your prediction in (6) consistent with your experimental observation? Explain.
Assessment:
Construct an entropy analysis analogous to the one above to predict how the boiling point of a
solution of a non-volatile solute should compare to the boiling point of pure solvent. Use
appropriate graphics and entropy diagrams to illustrate your analysis. Is your prediction
consistent with what you know from experience or previous study?See Chemistry, Section 8.12,
pp. 558–564.
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Rubber Band Thermodynamics
A rubber band can exist in two states: relaxed (R) and stretched (S). In this activity, you will
observe what happens when a rubber band is stretched (and then allowed to relax) and use these
observations to determine the signs of H, G, and S for the process. You will use these results
to predict what will happen when a partially stretched rubber band is heated and then test the
prediction.
(1) Hold a relaxed rubber band against your forehead or upper lip and quickly stretch it.
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(a) What is(are) your observation(s) for stretching the rubber band (that is, for R  S)?
(b) Based on your observation, what is the sign of Hsystem for the process R  S, where the
system refers to the rubber band? Explain how you know.
(c) What is the sign of Gsystem for the process R  S. Explain how you know.
(d) What is the sign of Ssystem for the process R  S? Explain how you know.
(2) Hold a stretched rubber band against your forehead or upper lip and quickly let it relax (but
don’t let go of the ends).
(a) What is(are) your observation(s) for the process S  R?
(b) Based on your observation, what is the sign of Hsystem for the process S  R, where the
system refers to the rubber band? Explain how you know.
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(c) What is the sign of Gsystem for the process S  R. Explain how you know.
(d) What is the sign of Ssystem for the process S  R? Explain how you know.
(3) Do your results for S make sense to you? Explain why or why not. What molecular model
can you think of that will explain these values?
(4) If a stretched rubber band is heated will it get shorter, get longer, or stay the same length?
Use the results from above to explain/justify your answer. Show that the explanation is
consistent with Le Chatelier’s principle.
(5) Test your prediction. Hang a weight on a rubber band, heat the rubber band with a hot-air
hair dryer, and observe the motion of the weight. On the back of the sheet, describe your
experiment and what you observe. Was your prediction correct?
[See Chemistry, Section 8.16, pp. 570-572.]
PHYSICAL CHEMISTRY- I (FALL 2008)
CH 3450-01
Instructor:
Dr. Aparna Waghe
Office: Boyd 118A
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Phone: 603-535-3251
Email: [email protected]
Office Hours: MTWF 11:00am - noon or by appointment
Class Time &
Room:
MWF 1:25pm – 2:15 pm, Boyd 239
Lab Time & Room: Thursday 9:30am – 12:15pm, Boyd 209
Pre-requisites:
CH 2130, CH 2140, MA 2550 & MA 2560
Course Description: Physical chemistry I (CH 3450) is a four credit course with a lab
component (see lab syllabus). The main focus of this course is to
understand the bulk properties of matter. The emphasis will be on the
thermodynamics, solutions, electrochemistry, and chemical kinetics. In
brief, the students in this course will
 study the properties of ideal gas, how do these properties differ for real
gases and be able to construct an equation of state that describes these
properties.
 develop the concepts of physical and chemical change in terms of
thermodynamics.
 be able to apply the knowledge of thermodynamics, in particular
enthalpy and entropy to make predictions for chemical systems.
 develop the concepts of chemical potential and how to use it to
account for the equilibrium composition of chemical reactions.
 explore the reaction rates by considering the motion of molecules.
 study rate laws; be able to construct a rate law from proposed
mechanisms.
 be able to use critical thinking and problem solving skills to solve
chemical problems.
 use calculus, computer spreadsheets and graphics for problem solving
Course Text:
Required: Atkins’ Physical Chemistry by Peter Atkins and Julio de Paula,
8th Edition (W. H. Freeman & Co., New York, 2006)
Recommended: Physical Chemistry - student solution manual by P.
Atkins
Web site:
The web site that accompanies the textbook is available at
www.whfreeman.com/pchem8 It offers links to many useful websites,
interactive tools such as calculators, plotters, periodic table etc.
Calculator:
You should bring a simple scientific calculator to every class and exams.
No sharing of calculators allowed during exams and quizzes.
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Course Format:
The material will be presented in a lecture format. Powerpoint
presentation will be used to discuss various figures, graphs, data etc.
Problem solving is an essential part of physical chemistry course.
Discussion/question and problem solving will be part every class. All
assignments, announcements and grades will be posted on Blackboard, so
be sure to check it regularly.
Attendance:
Regular class attendance and active participation in class discussions,
questions and problem solving is expected. It is your responsibility to be
aware of all announcements made during the class.
Reading:
You must read the assigned chapter of the text book before coming to the
class, as well as after the material is taught.
Course Grade:
All students learn at different pace. My goal as an instructor is to
encourage good study habits and help you learn. You will have various
opportunities to demonstrate your understanding of the course material.
Course assessment will be done as follows:
Three exams: 15%* each (drop lowest scoring exam)
Final Exam: 20%
Quizzes:
15%
Problem Set: 10%
Lab:
25%
A ten point scale will be used to assign letter grades, i.e. 90-100% = A,
80-89% = B, etc. Borderline cases will be evaluated by the instructor.
Exams:
These are designed to assess the understanding of the concepts learned in
the course and ability to apply them to solve chemical problems. There
will be three exams during the semester. These will be held on the dates
given in the class schedule. The lowest scoring exam of the three will be
dropped from the final course grade. Final exam will be cumulative and
the grade will not be dropped. The date and time of final exam will be as
per the university exam schedule.
No make-up exams will be given, unless you have a legitimate reason for
missing the exam. You need to bring a documentation verifying reason for
missing an exam (e.g. doctor’s note). If you cannot take a test, you must
contact instructor as soon as possible. You will score zero for any missed
exams due to unexcused absence.
Homework:
End of the chapter problems will be assigned daily for homework. These
are assigned to help you understand the material taught in class
effectively. The homework will not be graded. However, these problems
may be used for quizzes and exams with some modifications. Working in
study groups is encouraged but try to attempt problems on your own first.
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Quizzes:
These will be given twice a week to encourage regular study habits.
Quizzes will also help assess the strengths and weaknesses of students and
instructor in any areas. You are expected to do assigned reading and work
out the assigned problems.
Problem Set:
As a future professional chemist, you will need to have good problem
solving skills. You should also be able to use spreadsheet for data analysis.
Tables and graphs are important part of analysis. Problem sets will give
you opportunity to practice problem solving and critical thinking to
explain various concepts. These problems will be more involved, and may
need the use of spreadsheet. You are encouraged to work with your
classmates. However, the answers should be prepared independently and
not copied from others. Problem sets will usually be given on Wednesday
and will be due on the following Wednesday at the beginning of the class.
They will be graded and answer keys will be posted on Blackboard. No
late problem set will be accepted.
Problem Solving:
The solution to each problem in exams, quizzes and problem sets must be
written out clearly. It is easy to provide a feedback when work is shown
clearly. Be sure to show all work leading to the final answer. Problems
without full work will not be given any credit. Final answer must have
correct unit. Points will be deducted for answers without correct unit.
Regular problem solving and reading is essential for the success in this course.
Cramming at last minute will not help.
Tentative Class Schedule
Date
Sept 3 – Sept 24
Topic
Chapter
The properties of gases
1
The First Law of Thermodynamics
2
Exam I – Thursday, Sept. 25
Sept 26 - Oct 22
The Second Law of Thermodynamics
3
Physical transformations of pure substances
4
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Exam II – Thursday, Oct. 23
Oct 24 – Nov 19
Simple mixtures
5
Phase diagrams
6
Chemical equilibrium
- Electrochemistry
7
Exam III – Thursday, Nov. 20
Nov 21- Dec 12
Molecules in motion
21
The rates of chemical reactions
22
The kinetics of complex reactions
23
Final Exam (cumulative) - Friday, Dec. 19
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