_____University of Puget Sound
Department of Chemistry
Chem 230
EXP. 3–RATES OF REACTIONS:
BROMINATION OF ACETONE
LABORATORY OBJECTIVES AND ASSESSMENTS
1. Understand the proper use of a spectrophotometer.
a. Determine absorption spectrum and determine lambda max, λmax, of a solute.
b. Calculate molar absorptivity ε for a solute with a known concentration.
c. Demonstrate how to calibrate a spectrophotometer at a specific λ using a “blank”.
2. Understand how to use the absorbance of a reactant or product to measure the rate of
a reaction.
a. Demonstrate how to calculate rates of reaction using absorbance vs. time plots.
b. Identify sources of random error that contribute to the uncertainty in absorbance
measurement.
c. Identify sources of random error that contribute to the uncertainty in reaction rate
measurement.
d. Understand why the method of initial rates gives a method to determine Br2
concentration vs. time.
3. Understand components of a rate law.
a. Determine the order of a reaction from rate vs. concentration data.
b. Determine the rate constant of a reaction from rate vs. concentration data.
c. Calculate the order of this reaction in Br2, C3H6O, and H+.
4. Understand chemical hazards and proper waste disposal procedures.
a. Prepare a complete table of reagents with correct waste disposal guidelines.
b. Demonstrate safe laboratory technique to contain organic solvent vapors.
INTRODUCTION
Consider a general chemical reaction equation
aA
+
bB
cC (products)
(1)
where A, B and C are chemical species and a, b and c are the stoichiometric coefficients.
The rate of the reaction is measured as the change in concentration of any reactant or
product with time divided by the equation coefficients:
(2)
The Rate Law is an equation which expresses the rate in terms of the initial
concentration of reactants, and would be written:
Rate =k[A]m [B]n
(3)
where k is a proportionality constant, known as the specific rate constant, characteristic of
each reaction. The bracketed quantities [A] and [B] represent the molar concentrations of the
reactants, A and B. The sum of the exponents m and n define the overall order of the
reaction. The orders of the individual reactants (m and n, respectively) are experimentally
determined quantities and cannot be deduced from the overall balanced equation for the
reaction.
This experiment involves the study of the rate properties, or chemical kinetics, of the
following acid-catalyzed reaction between acetone (C3H6O) and bromine (Br2).
This reaction proceeds reasonably slowly at room temperature with its rate
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
1
depending on the concentrations of the Br2, C3H6O and H+ ion according to the rate law,
which for this reaction is:
rate = k[Br2]m[C3H6O] n[H+]p
(5)
The main purposes of the experiment will be to evaluate the rate constant k and the
reaction orders m, n, and p, for this reaction.
THE MECHANISM OF THE REACTION
The reaction as written in equation (4) is an overall reaction, which may be the sum of
several elementary reactions. A series of elementary reactions involving single steps in a
transformation is called a mechanism. One of the most important applications of kinetics
studies is for the testing of proposed mechanisms. In the case of the bromination of
acetone a reasonable hypothetical mechanism is the formation of a reactive intermediate
containing a structure called an “enol” featuring a carbon-carbon double bond. These
double bonds are well known to be very reactive to bromine. The reactions of this
proposed mechanism is shown in Figure 1.
Figure 1. Proposed mechanism for the bromination of acetone.
If this is the correct mechanism, the experimentally observed rate law must reflect the
slowest step, called the rate-determining step (rds). Because the first step is an acid-base
reaction between two species which are colorless, this step can be visualized by adding
an indicator, phenolphthalein. An indicator is a substance with a dramatic difference in
color between its acidic form and its basic form. By adding a very tiny amount of this to a
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
2
solution it is possible to follow the acid-base reaction of a colorless analyte by analogy;
when the indicator is converted to one form then the analyte has also been converted,
because it is in greater abundance than the indicator.
CATALYSTS IN RATE LAWS
You will notice that in equation (4) the H+ is put above the arrow and is not balanced
in the overall chemical equation. However, in equation (5) the [H+] appears as a
possibility in the overall rate law. How can this be? The overall reaction is made up of
elementary steps which determine the rate law and there is a possibility that the catalyst
is used at or before the rate-determining step before being regenerated. The catalyst’s
concentration would then determine the concentration of the intermediate that is formed.
If this is true, then the overall rate law will be affected by the catalyst’s concentration. In
the mechanism below, note where the catalyst is included. If the rate-determining step is
Steps 1 or 2 then the catalyst has not been regenerated and will influence the overall rate
law. If the rate-determining step is in Steps 3 or 4 then H+ has been regenerated and will
not be part of the overall rate law.
USING BEER’S LAW TO DETERMINE [Br2]
All of the reagents in reaction (4) are colorless except bromine, which is orange in
dilute solution, with the maximum absorbance near 400 nm. To measure the
concentration of bromine we can use Beer’s Law
(6)
Where A is absorbance, b is the path length (usually 1 cm), ε, epsilon, is the molar
absorptivity, and c is concentration (Harris, p. 398-408). You will first determine ε, which
allows you to calculate concentration of bromine at any time during a reaction.
RATE LAW BY INITIAL RATE METHOD
For measuring the rate of reaction (4) we will use the ”method of initial rates”. While
a plot of concentration vs. time is usually a curve, the slope for the initial portion of this
curve is often reasonably straight, and so can be used to compare trials of different initial
starting conditions. The Ocean Optics spectrometers in conjunction with LoggerPro will
allow selection of a common initial rate for all the experiments. For example, if the initial
rates all seem fairly linear for the first 100 seconds, then ∆[Br2]/∆t can be set to
∆[Br2]/100s for all the trials; the negative of this will be the rate.
Experimentally, the observed slope of the reaction will be the change in Absorbance
with time. This slope can be converted to the absolute rate simply by dividing by epsilon
and the pathlength.
The study of reaction (4) will be conducted at room temperature using the
concentrations listed in Table 1. In all of the experiments the concentrations of one
reagent is varied while all others are held constant. Thus, the order of reaction (4) with
respect to each reactant can be determined. For example, if while holding C3H6O and H+
constant, the Br2 concentration is doubled and the rate doubles, then it may be concluded
that reaction (4) is first order with respect to Br2, i.e., m = 1. On the other hand, if doubling
the concentration of Br2 produces no effect on the rate, then it may be concluded that m =
0. Or, if doubling Br2 results in the rate being quadrupled, the reaction is second order
with respect to Br2 , i.e., m = 2. Similar conclusions may be drawn about the
concentration-time dependency of C3H6O and its order (n), as well as for H+.
When the reaction order is not an integer value, it may be necessary to do a more
detailed calculation.
If, for example, in two trials the concentration of Br2 is doubled while C3H6O and H+ is
held constant, and if we define the initial concentrations of Br2 as x and 2x, the initial
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
3
concentration of C3H6O as y, and H+ as z, the corresponding rates are then R1 (using x, y
and z) and R2 (using 2x, y and z).
The corresponding rate expressions become
R1= k[x]m[y]n[z]p
(7)
R2= k[2x]m[y]n[z]p
(8)
Dividing equation (8) by equation (7) yields
=
[2x]m
[x]m
(9)
Equation (9) may be solved by taking the natural logarithm of both sides:
ln
= ln
[2x]m
= m ln 2
[x]m
(10)
Solving for m we have
m=
(11)
An equation similar to (11) may be derived for n and p. These equations should be
used whenever R1 is not an integral multiple of R2 (within 5%).
Once we know the order for each reactant we can determine the rate constant k for the
reaction by simply substituting in the known values of rate, concentration and order.
EXPERIMENTAL PROCEDURE
WORK WITH A PARTNER
GENERAL INSTRUCTION FOR USING LOGGERPRO
When you open the program the spectrometer may not be connected, so you select from the menu
bar “Experiment>connect interface>spectrometer>spectrometer 1. The light in the spectrometer
should go on.
In Part 1- - Calibration of Spectrophotometer–Using a cuvette of DI water for the BLANK, select
from the menu “experiment>calibrate” and follow the instructions for calibration. A dialog box
should appear telling you to wait; then you accept calibration and click OK. The screen will
display a visible spectrum with the plot of Abs. vs. Wavelength.
Determination of λ max of Br2–You will replace the cuvette contents with the working bromine
solution and click “Collect”. A full spectrum of Absorbance vs. Wavelength will appear with a
large hump on the left. Click “Stop”. Record the λ max and the absorbance of the λ max. Next select
“Experiment>save latest run”. Click on the rainbow icon at the top bar. A dialog box appears;
select the bubble “Absorbance vs. Time”. Be sure the correct λmax is checked in the column on the
right-hand side. When the OK is selected a blank graph with time as the independent axis should
appear.
In Part 2 – When you are ready to start a run, click “Collect” at the very same instant your partner
injects the bromine into the Acetone/H+/Water mixture. After the trial is finished, click “Stop”.
Then select “Experiment>save latest run”. When you click “Collect” for the next trial the first plot
should remain on the graph. Make sure you save every run, and save the data to your flash drive
or email at the end of the experiment for further annotation, rescaling if needed and printing.
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
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To get slope and intercept data from the application, click and hold the pad while moving the
cursor across the portion of the line which is reasonably smooth and linear. It should be
highlighted in blue. Then select “Analyze> Linear fit” A box appears on the graph. Record the
slope and intercept. You will use the intercept as Absorbance of the bromine at the start time; also
record the time and absorbance at a later point in the reaction. (The slope is not the rate; it is
∆A/∆t) .
Preparation of Reaction solutions
4.0 M Acetone. Your professor will prepare the solution of aqueous acetone as a demonstration of nonadditive volumes. Record the results of the demonstration in your notebook. This is now a 4.0M acetone
solution. It has a high vapor pressure, so keep it stoppered to prevent evaporation and headaches. In a clean,
dry, and labeled test-tube (13x100) obtain about 5.0 mL of this 4.0 M acetone from a dispenser in the hood.
Unused acetone will be collected in the Acetone Only waste container.
1.0 M HCl
In a clean, dry, and labeled test-tube (13x100) obtain about 5.0 mL of 1.0 M HCl from a
dispenser in the hood.
Unused HCl can be disposed down the sink and flushed with running water.
0.004 M Bromine In a clean, dry, and labeled test-tube obtain about 4-5 mL of 0.02 M Br2 from a
dispenser in the hood. Using a 1.00-mL syringe and a 10.0-mL graduated cylinder, carefully measure 1.0 mL
of the bromine then fill to 5.0 mL with DI water. Transfer this to a clean, labeled test tube and keep it
stoppered. This is the 0.004 M working bromine solution.
Unused excess bromine should be disposed of in the Bromine Only Waste in the hood
immediately because of its vapors.
At your workstation designate a 250-mL Erlenmeyer flask with stopper as a Mixed Waste
container exclusively for solutions containing both acetone and bromine. Keep it stoppered.
ALL WASTE AND LEFTOVER REAGENTS CONTAINING ACETONE
OR BROMINE MUST BE DISPOSED INTO THE APPROPRIATELY
LABELED WASTE CONTAINERS IN THE HOODS.
DO NOT MIX BROMINE WASTE WITH ACETONE WASTE.
PART 1. OBTAINING λ max AND DETERMINING ε
Open LoggerPro, turn on the spectrometer and allow it to warm up. Obtain the designated number of glass
cuvettes.
Calibrate the spectrometer with DI water.
Use a clean and dry cuvette (dry with paper towel or Kimwipe) for each trial. By looking at the position of the
light beam you can tell how much to fill the cuvette. Always position the cuvette in the same way by the mark
on the side.
Use a syringe to add ~2-3 mL of the working bromine solution to the cuvette. To take the spectrum of the
working bromine solution click “Collect”. When the spectrum appears, save the run, record the wavelength
of maximum absorbance, λmax, and the absorbance at the λmax and SAVE this absorbance spectrum. A
printed and annotated plot of Abs. vs. λ will be required for lab report. Change to Absorbance vs. time mode
by clicking on icon with the rainbow. Selecting the bubble button for Absorbance vs. time. LoggerPro
automatically selects the wavelength of the maximum absorbance. Be sure to check that this is the best
wavelength, i.e., λmax.
Calculate ε and record it at the top of Table 2. You will use this to calculate the concentration of the bromine
in your solutions. Save your results to your flash drive.
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
5
PART 2. DETERMINING RATE LAW - Dependence of Reaction Rate on Concentration
A) Instructor Demonstration of Rate of Acid-base Equilibration
Using the 1.00-mL syringe measure out 1.50 mL water, 0.50 mL acetone and one drop of the indicator
phenolphthalein into the cuvette and swirl to mix.
Note the color and time for change.
Using the same syringe measure 0.50 ml of the HCl and quickly add the acid to this mixture and note the
time needed for the color to change. Record your observations in your notebook (see below).
Observations of rate of acid-base equilibration
+
4.0 M C3H6O (mL)
1.0 M H (mL)
Instr. Demo
0.5
0.5
Observations
Water (mL)
1.5
0.0040 M Br2 (mL)
NA
Discard the contents in the Acetone Only waste container in the Hood area, rinse and dry cuvet.
B) Rate Constant and Rate Orders
The volumes to be used in the Trials are as detailed in Table 1. Label a 1.00-mL syringe “Syringe 1” for the
acetone, acid and water and label another “Syringe 2” for the working bromine solution.
Perform trials 1 thru 4 at room temperature. Record the temperature of the solutions to 0.2°C. Record
temperature of solutions in Table 2.
All wastes from Trials and any excess reagents must be collected for proper disposal (see
disposal rules above).
Table 1. Reaction mixtures at Room Temperature
Trial
1
2
3
4
Contents of Syringe 1
+
4.0 M C3H6O (mL)
1.0 M H (mL)
0.5
0.5
0.5
0.5
1.0
0.5
0.5
1.0
Water (mL)
1.0
0.5
0.5
0.5
Contents of Syringe 2
0.0040 M Br2 (mL)
0.5
1.0
0.5
0.5
Prepare all Trials 1 to 4 from Table 1 as follows:
Using one 1.00-mL syringe labeled Syringe 1 measure out the water, the HCl and the acetone into the
cuvette and swirl to mix.
Using the other 1.00-mL syringe labeled Syringe 2 measure out the bromine. NOTE: Do not add the
bromine to the reaction cuvet until ready to begin timing with LoggerPro.
Run the reaction - Trials 1 to 4 as follows:
Using the syringe, inject the bromine into the cuvette quickly and simultaneously start data collection.
Promptly insert and position cuvette in spectrophotometer. The timing of the Loggerpro data collection is
started (click “Collect”) upon addition of the bromine. When the trial is completed, discard the contents in
the Mixed Waste container at your work area, rinse and dry cuvet.
Under the Experiment menu select Store Latest Run. Repeat for all four trials. Plots of all four trials should
be overlaid on one plot also and sketched in your notebook. Save your data to your flash drive for further
analysis and printing.
NOTE: Discard the contents in the Mixed Waste container at your work area.
At the completion of the experiment collect all your local wastes into the appropriate COMMUNITY
WASTE in the Hood area.
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
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DATA AND CALCULATIONS:
NOTE:
Even though examples of tables and calculations are shown in this section, all data
must be entered directly into your laboratory notebook.
CALCULATIONS FOR PART 1.
DETERMINATION OF ε
From the results of the absorption spectrum form Part 1, determine epsilon using:
ε=
(12)
where A is the absorbance at the maximum, b = 1 cm and c is the known concentration of the working
bromine solution, [Br2].
CALCULATIONS FOR PART 2. ORDERS OF THE REACTION AND RATE CONSTANT
DETERMINATION
Make a table, in the data section of your lab notebook, similar to, but larger than the
example below, first calculating the reaction rate for each mixture.
Table 2. Results of Reactions at Room Temperature. ε =_______ T(°C)_____
Trial
[Br2]
[C3H6O]
[H+]
Slope
rate
(M/s)
k
1
2
3
4
The reactant concentrations in the Trial are not those of the stock solutions, since the reagents were diluted
by the other solutions. The final volume of the Trial is 2.50 mL in all cases. Since the number of moles of
reactant does not change on dilution we can say, for example, for acetone, that
moles C3H6O = [C3H6O]stock X Vstock = [C3H6O]mixture X Vmixture
For Trial 1, [C3H6O]stock = 4.0M, Vstock = 0.50 mL, Vmixture = 2.50 mL
Therefore,
[C3H6O]mixture =
= 0.80 M
Using epsilon, from Part 1 calculations, and the results of trial 1, the average rate is found by
⎛ slope ⎞
rate = − ⎜
⎟
⎝ epsilon × b ⎠
(13)
Repeat this process for each trial.
Determination of the Orders of the Reaction and k
€ Trial 1 in the concentration of only one species (see table). This means that
Each mixture 2 to 4 differs from
for any pair of mixtures that includes Trial 1, there is only one concentration that changes. From the ratio of
the rates for such a pair of mixtures we can find the order for the reactant whose concentration was changed.
The following describes the method for determining the order of the reaction with respect to the reactants.
In the data or calculation section of your notebook, write Equation (5) for Trials 1 and 2, substituting the
rates and the concentrations of Br2, C3H6O, and H+ ion from the table you have just completed
rate 2 =
= k[ ]m[ ]n[ ]p
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
7
rate 1 =
=k[ ]m[ ]n[ ]p
Divide the first equation by the second, noting that nearly all the terms cancel out! The result is simply
=
If you have done this properly, you will have an equation involving only m as an unknown. Solve this
equation for m, the order of the reaction with respect to bromine.
m=
(nearest integer)
Applying the same approach to Trials 1 and 3, find the value of n, the order of the reaction with respect to
C3H6O.
rate 3 = _______ =k[ ]m[ ]n[ ]p
rate 1 = _______ = k[ ]m[ ]n[ ]p
Dividing one equation by the other in a manner similar to the method shown above results in the value
for n.
Now that you have the idea, apply the method once again, this time to Trials 1 and 4, and find p, the
order with respect to H+ ion.
Once the orders are known, Eq. 5 can be used with the respective concentrations of the reactants to
evaluate k for Trials 1 through 4.
QUESTIONS
Q.1. Why should k have nearly the same value for each of the above trials?
Q.2 In this reaction, the acid is not part of the overall balanced chemical equation.
Explain why the acid appears in the overall rate law?
Q.3. Does your experimental rate law correlate with the mechanism proposed in the
introduction? Which elementary step is rate-limiting?
WHAT TO DO
To Be Turned in From Exp. 2-Microscale Determination of Vitamin C:
Please hand in the following items in a packet STAPLED and in the following order:
a) Answers to the Questions-Q.1. through Q.6. Some questions require
multiple answers. (Always restate question and report answers to the
questions in complete sentences.)
b) A summary (on one page from Notebook) listing the following results;
1) Pipet Calibration-Mean True Volume ± std. dev.
2) Titration of Asc, Acid Standard-Mean C(mg/drop) ± 95% confidence
interval.
3) Titration of Vit. C in tablet- Mentire tab(mg/tablet) ± 95% confidence interval.
4) Titration of Vit. C in juice serving- M(mg/juice serving) ± 95% confidence
interval.
c) Completed spreadsheet printout with Sheet 1 on one page and Sheet 2 on one
separate page. See Handout on Moodle page for information on “Fit to Page”
printing or “Set Print Area” in Excel”. Put your name on both pages.
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
8
d) Sample calculations from your notebook of 1) your interpolation of the
density or “density factor” at the exp. temperature and 2) the calculation of
|µ − x|
tcalc , t calc =
.
s
n
e) Copies of the relevant tables and remaining pages from your notebook.
Be Sure To Put Your Name on Every Page.
€
For Experiment
3
Before Lab:
Read this experiment. Fill in the “Title bar” to include the experiment
#, title, your name, partner's name, date, lab section, and TA's name. This information
should be included on each page in your laboratory notebook for every experiment.
All prelab material should be written in your laboratory notebook; the copy pages
will be checked by your TA at the start of lab.
For this week’s experiment your Prelab write-up should include, in addition to the
completed title bars, the following sections.
NOTE: Tables in notebook should always be prepared larger than the
samples in the lab write-up. Always leave plenty of room for data entry
into the tables.
1.
Purpose
After carefully reading the experiment, briefly state what you understand
to be the purpose of this lab exercise.
2.
Table of Reagents
You will be working with various reagents, some of which are potentially
hazardous. It is good practice to know something about the properties of
these substances before you get in contact with them, so you need to
complete a Table of Reagents in your lab notebook, such as the one in the
Experiment 4. Note that a table always has a number and a descriptive title.
BRIEFLY INDICATE ANY SPECIAL WASTE DISPOSAL REQUIREMENTS FOR
SUBSTANCES INVOLVED IN THIS LAB. The reagents include; acetone,
bromine, 1-bromoacetone and hydrochloric acid.
3.
A table for summarizing the volumes of each reagent (see Table 1.)
4.
A table, prepared for summarizing the results of each reaction for part 2b of
the experiment (see Table 2.) with completed calculations of [Br2], [C3H6O],
and [H+] entered in the table.
You will have your TA record score for the copy pages of all these pages from your
notebook at the beginning of the lab period.
During the Lab: Carry out the procedure as written. Modify your procedure if you
find it necessary, but be sure to record the modifications. Record all observations and
measurements in your notebook as you do the experiment. Show sample calculations in
your laboratory notebook. Be sure to obtain the data for all reactions in the experiment.
To Be Turned In: NOTE: Specific pages and order of assignment to be turned in will
be state on the Moodle page for the next experiment.
The report for this experiment includes:
1. Orders of Reaction
Completed Table 2. Results of Reactions at Room Temperature
Write a summary of the Rate Law found by this experiment below clearly
explaining–
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
9
a) value for average k (with units).
b) calculated rate law.
c) rounded rate law.
Be sure to have sample calculations in your notebook, showing all work, for each
type of calculation, e.g., reactant concentrations in Table 2, orders of reactants and
specific rate constant, k, for a mixture in Part A and average k.
2. Questions
Always restate question and report answers to the questions in complete sentences.
Compile on one to two pages.
3. Two Figures (Copy plot only, paste into Word Doc., add appropriate Title, add
appropriate figure caption, print each figure on a full page, landscape, , etc.).
a) Figure 1- A printed and annotated plot of Abs. vs. λ including λmax.
b) Figure 2- A printed and annotated plot of all Abs. vs. time runs overlaid onto one
plot including linear fit determinations.
c230 Exp. 3 - Rates of Reactions: Bromination of Acetone
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