CHEMICAL KINETICS
The objective of this experiment is to determine the rate law for the reaction
between the dichromate ion and the acetaldehyde molecule in a dilute
solution of nitric acid.
Cr2O72– + CH3CHO + H+  Cr3+ + CH3CO2H (unbalanced)
(1)
The initial rate of the reaction will be measured at room temperature. It's
dependence on the concentration of dichromate and acetaldehyde, CH3CHO,
will be determined by conducting two series of experiments which you will
design. In the first, you will vary the concentration of dichromate while the
concentrations of all the other components are held constant; in the second,
the acetaldehyde concentration is varied while holding all other
concentrations constant. From these data, the rate law with respect to
dichromate and acetaldehyde can be determined. (The dependence of the
rate law on the hydronium ion concentration will not be determined in this
experiment.)
PRINCIPLES
An important step in determining the mechanism of a chemical reaction is the
empirical rate law. A rate law specifies how the rate of appearance of products and the
rate of disappearance of reactants depend on concentration. For the generalized reaction:
aA + bB xX + yY
(2)
kineticists define the rate of the reaction as
1 d[A]
1 d[B] 1 d[X]
1 d[Y]
Rate = –a dt = –b dt = x dt = –y dt
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(3)
Frequently, though not always, a rate law takes on the following simple form:
Rate = [A]q[B]r[C]s
(4)
The values of the exponents provide important clues in the elucidation of the mechanism.
Note that the exponents q and r are not necessarily equal to the stoichiometric coefficients
a and b. Furthermore, a species C which does not appear in the net stoichiometric
equation can influence the reaction rate. Such a species is called a catalyst. Not all
species which appear in the net equation have corresponding terms in the rate law.
Rate laws can be determined from observing the concentration dependence of the
initial rate, the rate at short times where there is no appreciable change in concentration of
any species, or from determining the integrated rate law, a function relating the
concentration of a species to time. An example of an integrated rate law was encountered
in the Neutron Activation Analysis experiment, where N = N0e–kt, and N is the number of
radioactive copper atoms remaining at time t. In this experiment, you will use the former
method, as the mathematical analysis is straightforward and the results are not
complicated by the potential production of side products.
EXPERIMENTAL PROCEDURE
The preparation of solutions and the data collection for this experiment will be
made in teams of 2 or 3 depending on the lab size, since there are only 6 usable
spectrophotometers. You will prepare your own solutions by mixing appropriate volumes
of reverse osmosis water and the following stock solutions: 6.8 M HNO3, 0.39 M
CH3CHO, and 0.27 M Cr2O72–. The concentration of the CH3CHO solution might vary
from one lab day to the next, since acetaldehyde is quite volatile and difficult to prepare
accurately. Use 0.39 M to determine what volume to use in your samples and to arrive at
a reasonable approximation for the molarity of each solution. However, all of your
measurements will be made in one day from the same set of stock solutions. Since your
goal is to determine the rate law for the reaction and not the rate constant, small
variations in the concentrations of the stock solutions from one lab period to the next will
have no effect on the results of your experiment. We can not guarantee that stock
solutions prepared on different days will have exactly the same concentrations; therefore,
data collected over a period of more than one lab day can not be mixed.
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Prepare a table in your lab notebook as shown in Table 1 (for both series of
experiments) indicating the volumes of reverse osmosis water and each reactant
solution that constitutes a sample before coming to lab to expedite the experiment.
You will want 5 samples of varying molarity for each series of experiments. Since the
cuvettes that are used in the Lambda 12 Spectrophotometer have the standard size of 1
cm2, only a few milliliters of the mixed final solution will be required. The best data are
collected when the dichromate concentration ranges from 0.014 – 0.108 M, and the
acetaldehyde concentration is in the range 0.017 – 0.136 M. If the concentrations are too
low, the reaction is much slower, and the time required to obtain accurate changes in
concentration is longer than recommended. If the concentrations are too high, the
reaction proceeds so quickly that the plot of absorbance versus time is non-linear, and
special numerical techniques are required to calculate initial rates. Your first sample
solution, which will be used as a reference point, should have the following reactant
concentrations: [H+] = 1.36 M, [CH3CHO] = 0.078 M, and [Cr2O72–] = 0.054 M.
Assume that the volumes add. Therefore, if 1.0 ml of nitric acid stock solution, 1.0 ml of
acetaldehyde stock solution, and 1.0 ml of dichromate stock solution are mixed with 2.0
ml of reverse osmosis water, this reference solution is prepared. Each subsequent
solution will be mixed in a similar manner with a total volume of 5 ml such that their
final concentrations are within the ranges indicated above. The calculations to determine
the proper volumes of each solution for the preparation of the remaining samples is left to
you, and should be determined before coming to lab. (Note: the automatic pipets you
will use can deliver volumes in 0.005mL increments from 0.200-1.000 mL. Volumes in
excess of 1.000 mL require multiple additions.)
Table 1: The protocol for the first sample when the CH3CHO solution is varied.
The concentration of each species in the stock solution is given in parentheses.
Sample
Name
ALBWES01
Species
CH3CHO
(0.39 M)
Volume
1.0 ml
Final concentration 0.078 M
H2O
Cr2O72–
HNO3
Total
2.0 ml
––
(0.27 M)
1.0 ml
0.054 M
(6.8 M)
1.0 ml
1.36 M
Volume
5.0 ml
––
You will need to show your table to the lab instructor or T.A. before you will be allowed
to start the experiment. You may wish to revise your plan and make additional
measurements after you have completed the experiments outlined in your table.
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USING THE LAMBDA 12 SPECTROPHOTOMETER
1. The instruments will be configured and prepared for your use in advance. If at
any time during this experiment you are unsure about using the spectrophotometer, ask
your instructor or T. A. for assistance.
2. Check to make sure the correct parameters are loaded into the computer
by using the mouse to click on the following folders in the METHOD file using the "tabs"
on the respective folders at the bottom of the screen.
a) the Timed. folder
: 580 nm; Tot. Time: 60 s; Time Int.: 1 s; Autosave: ON;
Autoprint: OFF; Ordinate Min.: 0; Ordinate Max.: 1; Display:
serial.
b) the Inst. folder
Ordinate Mode: A; Lamp UV: OFF; Lamp Vis: ON;
Response: 0.1
c) the Sample folder
Calculation Factor: factor
Label the Sample. The sample folder contains a spreadsheet in which you provide a name
and a comment for each run. You have two options. You can enter a single entry prior to
each run or you can enter information for a series of runs, one line per run. In the former
case, you can go directly to the data analysis step after each run; in the latter case, you
must complete all the runs entered in the spreadsheet before analyzing the data. In either
case, you want to enter a unique name, the Sample Identity, for each run, otherwise the
results from a previous run will be erased when you start another run with the same name.
The Sample Identity is limited to 8 characters as this name also becomes the file name for
the run. We recommend the following format for the 8 characters of the Sample Identity:
initials of each user and a two-digit number, e.g. ALBWES01. We also encourage you to
enter information about the run, e.g. the reactant concentrations, in the optional Sample
Information column.
3. You will use the following procedure in each run. Using a pipetman, add all
the reagents except for the last solution to a 10 mL beaker. Be sure to change the tips for
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each solution. The last solution should be either the dichromate or the acetaldehyde,
since these are the two reacting species; it is most convenient to add the solution for the
component whose concentration is being held constant last. Check to see if the
instrument is ready. If you are conducting the experiment one run (one row in the
spreadsheet) at a time or if you are beginning the first run of a series, the start/stop button
should be green and read “start”. If you are in the middle of a series of runs, the prompt
to change samples should be displayed. When you are ready, add the last reagent to the
10 mL beaker. To mix the solution and load the cuvette, quickly pour a small amount of
the solution into a clean, dry cuvette and then pour it back into the beaker before filling
the cuvette approximately two thirds full. Clean the sides of the cuvette with a Kimwipe,
and place it in the sample cell holder; click on OK to begin the run. The speed with
which you perform the forgoing steps is critical to the success of the experiment—we
want to obtain data from the first part of the reaction and not the middle or end.
Note: when placing the cuvette into the sample holder, be sure that the two
clean/smooth faces of the cuvette face either side such that the light passes through them,
while the ridged sides face front and back.
To obtain the most accurate data while using a pipetman, adjust the volume of the
pipet by turning the setting ring so the numbers read a value past the desired volume, and
then return back to the proper position. Firmly attach a plastic tip to the pipet taking care
to keep the instrument and tip as free from contamination as possible. Be sure to change
the tips with each sample solution, but save the tips to use for that solution in each
subsequent run. To fill the pipet, press the control button down to the first stop, immerse
the tip vertically in the solution to be pipetted, and slowly let the control button return to
its initial position. Remove the tip from the solution by sliding it along the inside of the
container. If it is necessary, use a Kimwipe to clean off any droplets of solution on the
outside of the pipet tip, being careful not to touch the end of the tip with the tissue. When
emptying the pipet, hold the tip at an angle against the inside of the container (DO NOT
immerse the tip in the solution) and slowly press the button down to the first stop, then
pause. Press the button down to the second stop to completely empty the tip. Remove
the pipet away from the sample solution making sure that the control button remains
completely depressed. Never lay the pipet down when the tip contains liquid since
this has the potential to contaminate and damage the pipet mechanism.
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4. After the run is complete, expand the plot to fill the screen by clicking on the
appropriate icon in the upper right-hand corner of the window. Examine the quality of
the data.
5. Select Data Handling on the menu bar and choose Data Calculator. Set the
Algorithm (upper left of screen) to Slope and open your file on Dataset (bottom left of
screen). Moving across the bottom of the screen, choose the slope from 0 to 30 seconds,
and then click on Calculate. Record this information in your lab notebook being certain
to include the reactant concentrations of the sample. Be certain that the region in which
the slope was analyzed is linear. If curvature is apparent, choose a region with a shorter
time duration, or rerun the sample.
6. Close the Data Calculator by double-clicking on the horizontal bar in the upper
left-hand corner of the screen. Then click on the Sample folder tab in the bottom center
of the screen. At this point, the Sample Identity and Sample Information can be changed.
7. A new sample run can be performed by repeating steps 3-5.
CALCULATIONS
1. Write the balanced net ionic equation for the reaction.
2. Express the molar concentrations for each component of each sample solution.
3. Determine the rate law for the reaction. Assume that the rate law is of the
form: Rate = k[H+]m [CH3CHO]n [Cr2O72–]p. The following strategy can be used to
determine how the rate depends on [Cr2O72–], and a similar strategy can be applied to
[CH3CHO]. For the series of runs in which [H+] and [CH3CHO] are held constant, a plot
of slope versus [Cr2O72–]p is a straight line with zero intercept. That is, select the
“Constant is Zero” option in the Regression window of Excel. Taking the previously
calculated slope of the absorbance plot to be proportional to the initial rate, plot the
slopes versus [Cr2O72–]p, where the exponent p should be an integer or a half-integer. To
determine the best value of p, try several possibilities and see which gives the best fit.
For example, to fit a quadratic dependence, the slope is the dependent variable and the
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square of the dichromate ion concentration is the independent variable.
4. Express all your data and calculations to the correct number of significant
figures.
5. Be sure to show sample calculations and solve the problem on the report sheet.
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Name
Lab Section
Date Report Submitted
CHEMICAL KINETICS
1.
2.
3.
4.
Balanced net ionic equation of the reaction:
Rate law:
Rate =
Temperature:
Reactant molarities in the samples:
Part 1––Varying dichromate concentration:
[Cr2O72–]
[H+]
[CH3CHO]
Slope
Part 2––Varying acetaldehyde concentration:
[Cr2O72–]
[H+]
[CH3CHO]
Slope
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Show sample calculations on reverse. Include all Excel graphs and printouts.
5. Solve the following problem: A run is conducted in which the hydronium ion
concentration is kept constant and dichromate is in large excess. If the acetaldehyde
concentration, [CH3CHO], is measured over a significant period of time, what function of
[CH3CHO] versus what function of time yields a straight line?
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