Physical Chemistry Lab Extras Back to Tal's main page
by Tal Alon, Tel Aviv University Israel
Last update : 8/12/06
This page is updated from time to time... so you might want to look at it yourself, instead of taking a printout from a friend who did the experiment before.
The demands given here are mandatory for my students, other instructors might have different demands.
Contacting me :
Tal Alon
03-640-6866 (6866 from inside the university)
Ornstein 211
Office hours : 15:00-16:00 on Sundays
Instructing the second group on Tuesdays.
You can come and talk to me at any-time, as long as you coordinate it with me either by email or by phone.
Menu :
General Report-Writing Guidelines
Explosion Extras printable version here Back to Menu Back to Tal's main page
the atomic explosion on the left has nothing to do with our experiment of course :) ...
* It is recommended to come and talk to me before the experiment-session if you have questions.
At times, only a fine line separates combustion reactions and
explosions. The typical explosion
is a self-sustaining combustion reaction that proceeds at an ever-increasing
rate and cannot be stopped. A thorough understanding of the mechanisms of
combustion and explosion reactions can help us to achieve the one and avoid the
other.
When the reaction
(The one we study in this experiment) is carried out in a controlled fashion, it
provides the energy and thrust needed to propel the Space Shuttle into orbit
(the main thrusters of the rocket). The mechanism of the reaction is very
complex yet very interesting to study.
Preparing for the experiment
1) You have to read the information pages at the lab's site.
2) Take the booklet from Saada (the technical assistant and laboratory coordinator), read it and understand it.
3) You should know the difference between a thermal explosion and an isothermal one.
4) Understand kinetic equations and know what effects a rate constant value.
5) You should know how the limits change with temperature/vessel diameter/inert gas pressure, and why.
6) Solve the quiz questions in the laboratory site.
On the Effectiveness of M
In the termolecular gas-termination reaction, the M molecule is very much
necessary.
The M molecule is making itself useful by taking energy from the HO2·
radical, thus allowing it's existence.
In a very similar
manner, grains of dust in interstellar clouds act as a third body, allowing H·
radicals to form hydrogen molecules, Without them the atoms will simply have too
much energy to "stick together".
The M molecules could be any molecule or atom : H2, O2, H20, N2 etc.
Different molecules have different effectiveness as energy thieves, that depends
on their degrees of freedom, vibrational bond frequencies and other parameters.
The rate constant k, depends mainly on the activation energy of that step (a
negative activation energy), but also on the pre-exponential factor A, holding
in it an expression that corresponds to the number of collisions per unit
volume, per unit time (You have studied this recently in one of prof. Bixon's
lectures). Generally, the Larger the reduced mass, the smaller the
pre-exponential factor is.
You must remember that adding oxygen is actually accompanied by taking away some
of the hydrogen and water molecules (and vice versa). This is simply because in
every experiment we decide on the initial pressure and temperature, and since
the volume of the vessel is constant, the same number of molecules are present
regardless of their identity.
The new program
Here is the document explaining how you should run the experiment:
With pictures - (PDF document in English) : New Explosion Data Creation
The pages are also attached to the experiment's booklet.
[If previously, during the experiment,
you entered the parameters by opening the 'inpoh' file using notepad, know that
this is risky since the program is very sensitive to changes in the format of
that file (even a stray space can render the 'inpoh' file useless). Even though
this data-entering method will still work, you can now enter the parameters with
a new simple program I wrote (no need for Norton commander or notepad), thus
eliminating the wrong data-entering risks.]
The Nobel prize
Click here
to read the short presentation speech.
The last paragraph (amazing) sound like something coming out of the Lord of the
Rings J.
Demands for the final report
* The report should be SHORT and INFORMATIVE !!!!!!!!!
In general you should report everything you did, what you found, and then you
should explain it and compare it with the theoretical expectations.
More specifically:
1) A short yet informative theoretical background.
2) Describe the method used to derive the explosion limit (how did you recognize
an explosion / steady reaction).
3) For every limit – find the activation energy (with the help of the equation
you derived using the upper limit equation and the law of Arrhenius).
4) Why do you think the limits changed the way they did?
5) Don’t ignore bad results – instead, explain what you expected to find, and
think what was wrong.
6) All the measured data should also be presented in tables.
7) All graphs must be presented, not only at the end of the report, but immediately following the relevant part.
Graphs needed:
* Concentration of one of the species as a function of time, in the case of an explosion.
* Concentration of one of the species as a function of time, in the case when there is no explosion.
* Log(P) as a function of T for all stoichiometric values of H2 and O2 (no inert gases) (5 plots - same graph).
* Log(P) as a function of T for the stoichiometric mixture of H2 and O2 + inert gases (at least 5 plots - same graph).
* All Activation Energy derivation graphs (At least 9).
* Graph of net-branching factor derivation (if performed).
* A graph that shows the relevance of the quasi-stationary state assumption.
8) a short overall summery of the findings and a discussion about them.
Inversion of Sucrose Extras printable
version
here
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* It is recommended to come and talk to me before the experiment-session if you have questions.
You might find it interesting to know that this experiment is of some historical
interest as it is generally considered to be the first quantitative study of the
kinetics of a chemical reaction. It is also a very interesting experiment since
it is based on some very fascinating natural phenomena, and since it involves
using interesting equipment.
Preparing for the experiment
1) You have to read the information pages at the lab's site.
2) You should know what to do in the experiment and what are your goals.
3) You should have some information about the equipment you will use in the experiment.
4) You should know about polarized light and Malus' Law.
5) You should know how to turn the polarization plane by 90o.
6) You should know about the properties of the compounds involved in the reaction.
7) You should know all about specific angles and their relevance to our experiment.
8) You should know about stereoisomers.
9) You should know how to derive kinetic equations.
10) Solve the quiz questions in the laboratory site.
*) You must understand what polarized light is, and the phenomenon concerning it. you might want to read about it in one of the university physics books (recommended : University physics / Hugh D. Young, Roger A. Freedman ).
Relevant Specific-Rotations and the conversion function
If we know the optical rotation at λ0=589.3nm we can calculate the optical rotation [α]
at λ=632.817nm by using the next equation:
The specific rotations at λ=632.817nm:
Sucrose
......... +57º
β-D-Fructose
......... -80º
α-D-Glucose ......... +98º
β-D-Glucose ......... +16.5º
About Glucose and it’s two forms
In the reaction only
β-D-Glucose
is formed directly. It slowly gets into an
equilibrium with the other form of Glucose, the
α-D-Glucose.
When they are in equilibrium (at room temperature) 65% of the Glucose is in the
β
form.
Account for that in your experiment and calculations.
Main Analysis Method
With a little manipulation of the algebraic equations governing rotation, we get to the next equation:
Where α0 is the rotation at the beginning of the experiment, αt is the rotation at time t, α∞ is the rotation at the end (when there is no sucrose left) and k of course is the rate constant for the inversion.
how is this equation derived ?
With the help of this equation we can easily acquire the rate constants we need.
Another Analysis Method
Guggenheim has described a method for calculating the rate constant of the
first-order reaction which doesn’t require waiting for a reading at infinite
time.
This method may be applied directly if data are taken at equal time
intervals :
The data are arranged in two sets. For each observation C1 at
time t in the first set, another observation C2 is
taken at time t+Δt, where
Δt is a fixed time interval. If a plot
of ln(C1-C2) versus t is prepared, the
points will fall on a straight line of slop –k. The constant mustn't be
too small since it will result in a large percentage error in C1-C2.
It is advisable to take Δt
as
approximately one-half the duration of the experiment.
Derivation of the equation:
(Don’t forget – in our experiments we measure rotations, not concentration … )
The Mechanism
Even though the studied reaction is treated as though it is following first order kinetics, it might only be pseudo-unimolecular (bimolecular in nature but can be treated as a first order reaction due to the conditions of the reaction ... see that you understand this idea !).
Unimolecular : (fast) S + H+ = SH+
(slow) SH+ --> .....
Bimolecular : (fast) S + H+ = SH+
(slow) SH+ + H2O --> .....
Is the mechanism of the reaction bimolecular? or is it actually unimolecular? how can we get an answer using the results of this experiment?
Well the answer
lies in the Hammett-Zucker hypothesis.
According to this hypothesis, a graph of the rate constant k, plotted against
acid molarity or hydrogen ion concentration should be linear if the reaction is
following the bimolecular mechanism.
What does your experiment results tells you?
A suggested unimolecular mechanism:
Demands for the final report
* The report should be SHORT and INFORMATIVE !!!!!!!!!
In general you should report everything you did, why you did it, what you found
(compared with the theoretical expectations), and then give your conclusions.
More specifically:
1) A short yet informative theoretical background.
2) How did you manage to rotate the plane of polarization by 90º?
(Explain the position of the mirrors and why does it do the job).
3) How did we reaffirmed Malus' Law? (including an analyzed graph).
4) How did we found the specific rotation of sucrose? did you get the value
specified below? Explain.
5) How did we get the different rate constants? specify every aspect of the
experiment (include analyzed graphs).
6) Is the mechanism bimolecular? or is it unimolecular ? …. How can we tell ?
7) If during the experiment you encounter a result that doesn't match your
expectation, don't ignore it ! Instead, think of reasons.
8) All the measured data should also be presented in tables.
9) All graphs must be presented, not only at the end of the report, but immediately following the relevant part.
10) a short overall summery of the findings and a discussion about them.
Binary Solutions Extras printable
version
here
Back to
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Back to Tal's main page
* It is recommended to come and talk to me before the experiment-session if you have questions.
Preparing for the experiment
1) You have to read the information pages at the lab's site.
2) You should know what to do in the experiment and what are your goals.
3) You should have some information about the equipment you will use in the experiment.
4) You should know how to interpret phase diagrams.
5) You should know all about Raoult's Law and Henry's Law.
6) You should know about the properties of the compounds involved in the experiment.
7) You should understand the concept of activity.
8) You should understand the meaning of excess functions and a regular solution.
9) Take a book (Atkins) and read the related pages.
10) Solve the quiz questions in the laboratory site.
Bonus Grades
A bonus grade in this experiment will be given by me, to the FIRST student who will explain to me what problem is there with w in the Water-TEA experiment and why is it there. The bonus size will depend on the depth of understanding, and it might get distributed between different students.
The explanation must be given during one of the lab sessions involving this experiment.
An additional-bonus will be given to the FIRST student who will suggest a good solution to the problem.
Demands for the final report
* The report should be as SHORT as possible and INFORMATIVE !!!!!!!!!
In general you should report everything you did, why you did it, what you found
(compared with the theoretical expectations), and then give your conclusions.
More specifically:
1) A short yet informative theoretical background.
2) How did you acquire the critical temperatures and compositions in the
experiments (You should of-course draw a schematic representation of the system
and explain a bit about it).
3) What is the value of w for the MeOH Cyclohexane solution, give
an intuitive verbal explanation to it's meaning.
4) Show graphs of ΔG as a function of Χcyclohexane for five different
temperatures :
The Critical temperature, a wisely chosen temperature in the homogenous area and three wisely chosen temperatures in the non-homogenous area.
5) Plot graphs for the
activity of MeOH and Cyclohexane as a function of their mole fractions. do this
for several temperatures. Explain the behavior.
6) Explain the values of w, γ and activities of MeOH and Cyclohexane in
their binary solution.
7) Plot the vapor pressure of Cyclohexane as a function of it's molar fraction.
Plot Raoult's and Henry's approximations and explain the findings.
8) what is the mixing enthalpy and entropy of MeOH and Cyclohexane (different temperatures and molar fractions)?
Explain the findings.
9) What can you tell about the w, γ and activities of Water and TEA in their binary solution.
10) If during the experiment you encounter a result that doesn't match your expectation, don't ignore it ! Instead, think of reasons.
11) All the measured data should also be presented in tables.
12) All graphs must be presented, not only at the end of the report, but immediately following the relevant part.
13) a short overall summery of the findings and a discussion about them.
more soon...
Adsorption Extras printable version here Back to Menu Back to Tal's main page
Some of the quizzes I gave last year (sorry, Hebrew only):
* It is recommended to come and talk to me before the experiment-session if you have questions.
what does the picture has to do with our experiment ?
Activated carbon is a term that can be applied to any form of charcoal that possesses adsorptive power.
The medicinal use of charcoal is recorded as early as 1550 BC by the ancient
Egyptians, though it was only in the latter part of the 18th century that its
adsorbent powers were scientifically studied and it became possible to prepare
it in a pure state, primarily as an antidote to poisoning. However it does have
many uses beyond the medicinal, it is found in the purification system of our
drinking water, in air conditioning systems to remove odors, in linings for gas
masks and in Nuclear Power Stations where the charcoal adsorbs and holds
radioactive contamination until the isotope decays.
There are two types of active carbon:
granular carbon (shredded carbon and globular carbon) and powdery carbon. The
former is more widely applied to the treatment of wastewater. The active carbon
has macro-pores of 10-2-10-4 and micro-pores of 10-5-10-7mm
and the substance which is diffused into the macro-pores, will be adsorbed to
the surface of the micro-pores. The surface area is ~
500-1,500m2/g, which is large enough and appropriate to be
used as adsorbent.
The drawing is taken from the report of Denis G. and Debora M. (drawn by Denis).
Preparing for the experiment
1) You have to read the information pages at the lab's site.
2) Freshen-up your knowledge on vacuum systems (look in the lab's vacuum page).
3) You must read and understand the relevant pages from Atkins, Physical chemistry book.
what is physisorpsion? chemisorption?
4) Know how to derive (by yourself) the Langmuir isothem, and remember the BET isotherm.
(think about different values of c in the BET isothem).
5) Know how the system is built and what every element is for.
6) Know how a rotary-pump works.
7) Know how a piezo and a pirani gauges work and what are their limits.
8) Know how to measure an unknown volume (using a known one).
9) Know how to conduct the adsorption experiment and derive information from it:
- The procedure itself.
- The calculation of n (the number of moles that were adsorbed)
(notice that the equation in the lab's page is not complete in case of P≠0 in V2+V3)
- Know what P* is for Nitrogen and it's value in our experiment.
- Know the value of R in convenient units for this experiment.
- Know what graphs to make and how to derive information from them :
nmon , K and ΔGads from the Langmuir isothem.
nmon and c from the BET isothem.
- Know how to calculate the surface area of the carbon we use, from nmon (give the result in m2/g).
10) Solve the questions in the lab's page.
Notes for the experiment (important !!!)
- In this experiment you are working with a vacuum system, you will be heating and cooling parts of the system, opening/closing valves and adding gas. you must use your head at all times, always think about the safety of your action before acting.
- The pump is activated a short period of time before you come to the lab. Desorption of adsorbed substances in the system will probably give you a hard time in the beginning of your experiment, when trying to get a steady low pressure (pump disconnected from the system) of ~10-3. Be prepared for this effect.
- You must keep the liquid Nitrogen at it's desired level at all times. During The experiment you might think that equilibrium has been reached, but if the level of liquid Nitrogen is too low the carbon is not properly cooled and the reading will give you nothing but misery in the data-analysis stage.
- The errors in the adsorption-desorption experiment are accumulated, know what to expect and why.
The Quiz
The quiz will not be so trivial, and in order to succeed in it you have to study well.
preparing yourself with the ten steps above (in a way that you can answer these questions without any notes) is a must!
Demands for the final report
* The report should be SHORT and INFORMATIVE !!!!!!!!!
1) You should give a SHORT yet INFORMATIVE theoretical background.
2) Show a drawing of the system and explain it's parts (specify volumes) (a correct drawing !!!!).
3) Describe how you got the volume of the V1 chamber, how precise is your result? include the graph and explain the calculations.
4) Describe how you performed the adsorption experiment.
5) Plot P as a function of time from your first adsorption measurement... what can you tell from this graph?
6) Plot the Langmuir and BET graphs and derive nmon (two values, one from each graph), K , ΔGads and c.
7) You can calculate ΔHdes from the value of c (you must get some information from the literature).
8) Plot θ as a function of P (Isotherms) for both models , and discuss its compatibility with the theoretical expectations.
9) Calculate the surface area of the carbon (again two values), you must take the area of one site from the literature (how much surface area for 1gr of carbon ?).
10) Show θ as a function of P for the desorption part. Is it identical to the adsorption graph ? (show them both together).
11) Give your conclusions, and discuss the results.
* NOTE - If you take values of constants from the literature, please specify where did you take them from.
Conjugated Dyes printable version here Back to Menu Back to Tal's main page
Some of the quizzes I gave last year (sorry, Hebrew only):
* It is recommended to come and talk to me before the experiment-session if you have questions.
Preparing for the experiment
0) Read all the Lab's information pages and get all the information about the dyes used.
1) Find out the meaning of every expression in the Particle in a box (PIB) Energy equation.
2) You must know how to derive the final expression for λ starting from that PIB Energy equation
(you must be able to make the PIB calculation when asked for the λ of any conjugated dye).
3) You must understand the meaning of light absorption.
4) Beer's lambert's "law" will be used .... know it.
5) Solve the questions in the lab's page.
You will learn all about the particle in a box problem in the 'Physical Chemistry 2' class, you should understand how the wave function is derived, and how we get the energy expression (I might ask you questions about it after you've studied it).
The Quiz
The quiz will not be so trivial, and in order to succeed in it you have to study well.
preparing yourself with the steps above (in a way that you can answer these questions without any notes) is a must!
Absorption
For some reason, some of you do not remember that absorption is defined as A=log10(I0/I) where I0 is the intensity before absorption.
Well ... I expect you to remember it now :)
Demands for the final report
* The report should be SHORT and INFORMATIVE !!!!!!!!!
Generally you should compare the measured absorption with the one predicted by the 'Hyperchem' computer program and the one predicted by the particle in a box model.
in order to do that , present a table like this one :
| Compound Name | Color |
λabs from Literature [nm] |
λabs from our measurements [nm] | λabs from HyperChem [nm] | λabs from PIB model when α=0 [nm] | λabs from PIB model when α is optimized (α=?) [nm] |
| DTC | ||||||
| DTCC | ||||||
| DTDC | ||||||
| DTTC |
Pay attention that the last column requires α optimization - find the alpha that gives minimum errors for the series of dyes.
You should of course compare oscillator strengths (measurements vs. HyperChem) :
| Compound Name | Concentration [M] | Calculated IAC [m2moles-1sec-1] | Oscillator Strength from IAC | Oscillator Strength from HyperChem |
| DTC | ||||
| DTCC | ||||
| DTDC | ||||
| DTTC |
Present your calculations.
*) You should include the plots from which you got the data above (HyperChem and Origin), and explain the data analysis.
*) You should show the absorption spectra for every dye, plotted against λ and against ν.
*) Plot the empirical absorbed wavelengths vs. the length of the chain.
*) You should present your particle in a box calculation after a SHORT (yet informative) theoretical background.
*) Check and compare the results between a benzene-like bond-length and a Hyperchem computed bondlength.
*) Give your conclusions, and discuss the results.
General Report-Writing Guidelines Back up (to the menu)
Well, here I present several pointers I
think will help you produce better lab-reports (probably yielding higher grades
as a result).
First of all, it is preferable to submit a typed report (edited on a computer).
It gives you the ability to modify your report as you build it, it looks good
and it is easier to read (don't make the instructor waists precious time
deciphering hieroglyphs).
There are several programs available for editing equations, and you probably
know the one that comes with 'Word'.
Graphs
Often you want to include graphs in your report. When it is possible, incorporate the graphs in the body of the report right after discussing the experiment itself. This is very convenient since the reader can see the results and the experimental information in the same time. If this is not a possibility you should present the full-sized graph right after the page describing the experiment (never postpone this to the end of the report).
If you wish you can assign every incorporated-graph a number and then show it enlarged in a 'graph appendix' at the end of the report.
Each graph should have a title that represents it. It doesn't need to be long but it shouldn't be too concise.
The origin of a graph should usually be on the lower-left corner (excel sometimes produces an inversed graph as a default .... and you should take care of this).
The axis' ticks should be visible and the numbers beside them readable (sometimes when the numbers are too big they overlap , and you leave them that way !).
more soon…