Order of a reaction
Introduction:
The objective of this experiment is to determine the order of a reaction. For this we will look at the
redox reaction between Fe3+ ions and I- ions. The equation for the reaction is represented as follows.
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2Fe(NO3)3 + 2KI  2Fe(NO3)2 + 2KNO3 + I2
2Fe3+(aq) + 2I-(aq)  2Fe2+(aq) + I2
The I2 produced in this reaction imparts a color to this solution; hence we can study the rate of
formation of Iodine using a colorimeter and measure the change in absorbance using the Beer Lamberts
law.
Beer Lamberts Law
𝐴 = 𝑙𝑜𝑔10
𝐼0
𝐼
A = lc
‘A’ represents absorbance, which we will be measuring. ‘’ is molar absorptivity. ‘I0’ is the intensity of
light at zero absorbance or the blank. ‘I’ represents the absorbance of the coloured solution or intensity
of light passing through the sample solution. If I < I0 some light has been absorbed by the solution. ‘l’
represents the length of the light path through the solution and ‘c’ represents the concentration of the
solution.
𝐴 = 𝑙𝑜𝑔10
𝐼0
𝐼
The Experiment
Objective of the experiment:
The objective of this experiment is to determine the order of each of the reactants and the overall order
of the reaction.
2Fe3+(aq) + 2I-(aq)  2Fe2+(aq) + I2
Rate = k[Fe3+]x [I-]y
In order to determine the value of ‘x’ and ‘y’, we will use the Colorimeter and a Labquest
Principle:
The initial rate method is being adopted to determine the order of the reaction. For that we will study
the effect of changing concentration on the rates of the reactants.
The initial rates are determined using the colorimeter, which determines the changing rate
instantaneously; using this data the order of the reactants will be determined, and hence the order of
the reaction.
Reagents Required:
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0.02 M Fe3+ from Fe(NO3)3.9H2O. The iron(III)nitrate is prepared in 0.1 M HNO3 solution
0.02 M I-(aq) solution. The aqueous solution is prepared by dissolving solid KI in distilled water.
Distilled water or deionized water
0.1 M HNO3 for preparing the iron (III) solution
1 liter of each of the reagents should be sufficient for 10 groups of students, for rinsing and
about 5 trials.
Equipment’s and Glassware
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Labquest
Colorimeter
Cuvettes
Wash bottle with distilled water
Graduated pipettes
Beral pipettes
100 mL Volumetric flask
Three way bulbs for measuring and transferring reagents
50 mL beakers
Lint free tissue or least Kleenex to wipe the cuvettes
Stop Watch
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Procedure
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Turn on the lab-quest and connect the colorimeter – Leave the colorimeter turned on at least 5
minutes before you start data collection.
Lab quest set up.
o Go to > Sensors > Data Collection >
o Time bases > Rates = 1 sample/s > Interval = 1 s / sample > Length = 200 seconds
o OK
Colorimeter
o Select the wavelength 470 nm using < or >
o Prepare a blank: In this case the blank is the Fe3+ ion solution. Fill the cuvette 3/4th with
the blank. Close the cuvette.
o Place it in the colorimeter; the clear side of the cuvette should face the white triangle.
Close the colorimeter.
o Press calibrate. It is a one button calibration.
o When the red light stops blinking and the absorbance on the Lab quest is 000 the
instrument is calibrated.
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Reagents required:
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Fe3+ 0.02 M in 100 mL = Ci
0.02 M KI in 100 mL = Ci
Distilled water
Mix the volumes needed according to the table provided (at least 3 trials; 1,2 &3)
NOTE: Do not mix the two solutions until you are ready to record the readings.
DILUTION
Table 1 DILUTION
Trial
FeCl3 (mL)
KI (mL)
H2O (mL)
1
2
10.0
10.0
10.0
5.0
3
5.0
10.0
0.0
5.0
5.0
DATA TABLE
This table now gives you values which can be used to solve for the order of the reaction using initial rate
method.
TABLE 2 RATE OF REACTION
Trial
[Fe3+]
𝐶𝑖 𝑉𝑖
𝐶𝑓 =
𝑉𝑓
[I-]
𝐶𝑓 =
𝐶𝑖 𝑉𝑖
𝑉𝑓
1
0.01M
0.01M
2
0.01M
0.005M
3
0.005M
0.01M
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Initial rate (s–1)
Value of ‘m’ from
Labquest
Data collection:
Note: There should be consistency for the time taken between mixing the two solutions and data
collection for all the three trials, use a stop watch.
Trial 1
1. Pipette 10 mL of Fe3+ (Iron(III) solution) into a 50 mL clean dry beaker followed by 10 mL of I(Potassium Iodide) solution into the same beaker. It would be a good idea to start a stop watch
to maintain consistency in time between mixing the reagents and start of data collection. After
mixing the solutions properly using a glass rod, transfer the mixture to a clean cuvette using an
eye dropper or a beral pipette, close the cuvette.
2. Place the solution in the colorimeter that has been calibrated, close the colorimeter
3. Press the play button to start the data collection
4. Wait for 200 seconds for the collection to be complete. Data collection stops after 200 seconds.
5. Dispose the solutions appropriately as instructed.
Trial 2 and Trial 3
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4.
Prepare the solutions for trial 2 and 3 separately using the table 1 data.
Press the file cabinet icon on the Labquest
Repeat steps 2 to 5 under Trial 1
Repeat all these steps for trial 3.
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Data Analysis on the Labquest:
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Open Run 1
Select a segment of the graph for 30 seconds (50 seconds to 80 seconds as shown in the video)
Select analyze
Choose curve fit
Absorbance
Linear
Now write down the value of ‘m’ or slope of line in the data table
Next select OK
Choose Run 2 and repeat the process until you find the slopes for all the three runs. Record the
data.
Once you have the values you can calculate the order of the reaction
Note: When the slope is determined after collection of data for all three runs, you get the slope for the
same time segment for all three runs. This will work positively if the time between start of experiment
and data collection (pressing the play button for data collection) are the same.
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Calculation:
The order can be determined using initial rate method calculation
𝑇𝑟𝑖𝑎𝑙 1
𝑇𝑟𝑖𝑎𝑙 2
Should give you the order of iodide ions
𝑅𝑎𝑡𝑒 1
[0.01]𝑥
=
𝑅𝑎𝑡𝑒 2 [0.005]𝑥
[
[
]
[0.01]𝑥
=
] [0.005]𝑥
Solve for ‘x’
‘x’ is the order of Iodide ions
Similarly solve for ‘y’
𝑇𝑟𝑖𝑎𝑙 1
𝑇𝑟𝑖𝑎𝑙 3
Should give you the order of Fe3+ ions
𝑅𝑎𝑡𝑒 1
[0.01]𝑦
=
𝑅𝑎𝑡𝑒 3 [0.005]𝑦
[
[
]
[0.01]𝑦
=
] [0.005]𝑦
Solve for ‘y’
‘y’ is the order of Fe3+ ions
Rate = k[Fe3+]y [I-]x
Repeat calculation for the different trials performed to complete the lab.
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Pre lab Questions: Order of a Reaction
(Student has to submit before the experiment starts)
1. Define order of a reaction
2. Write the units for zero, first and second order reactions.
3. If the concentration of a reactant is doubled, and the rate of the reaction increases 8 fold, what is
the order of the reactant?
4. Calculate the mass of Fe(NO3)3.9H2O required to prepare 250 mL of 0.02 Molar solution.
5. Why do we add 0,1 M HNO3 solution while preparing the Iron(III)nitrate solution?
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Discussion questions:
To be submitted after performing the lab along with a lab report (Discussion questions may be part of
the lab report if writing a lab report)
1. What is the order for each of the reactant in the lab you just performed, explain your rationale?
2. Writ the rate law for the experiment that you performed.
3. Is the rate law and law of mass action the same? Does it suggest a possibility for the mechanism for
this reaction, explain.
4. List all possible instrumentation errors.
5. List all possible human errors that you may have encountered?
6. List all the errors that could have been avoided in this lab.
7. What are some possible modifications that you would suggest for this experiment?
8. Write a conclusion for you experiment
9. Include data sheet along with calculation.
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Data sheet – Submit to teacher after lab
To be submitted to the teacher on the day of the lab immediately after data collection.
This table now gives you values which can be used to solve for the order of the reaction using initial rate
method.
TABLE 2 RATE OF REACTION
Trial
[Fe3+]
𝐶𝑖 𝑉𝑖
𝐶𝑓 =
𝑉𝑓
1
2
3
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
[I-]
𝐶𝑓 =
𝐶𝑖 𝑉𝑖
𝑉𝑓
Initial rate (s–1)
Value of ‘m’ from
Labquest
Quick Setup And Short Lab Guide
1.
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8.
Wear goggles at all times
If you come in contact with any solution please wash it off using cold tap water
Inform you teacher ASAP
Connect the colorimeter to the Labquest
Turn on the Labquest
Set the wavelength to 470 nm using the <> arrows on the colorimeter
Allow the colorimeter to warm up for 5 minutes
Sensor set up
a. Select sensor > Data Collection> Mode> Time Based>
b. Rate = 1 sample/ second
c. Interval = 1 sample / second
d. Duration = 200 seconds
9. Colorimeter
a. Prepare a blank: In this case the blank is the Fe3+ ion solution. Fill the cuvette 3/4th with the
blank. Close the cuvette.
b. Place it in the colorimeter; the clear side of the cuvette should face the white triangle of the
colorimeter. Close the colorimeter.
c. Press calibrate. It is a one button calibration.
d. When the red light stops blinking and the absorbance on the Lab quest is 000 the
instrument is calibrated.
Table 1 Dilution guideline
10. Now prepare solutions for one trail at a
time and perform data collection for all the
three trials, use Table 1 data. Use 50 mL
beakers and measuring pipettes.
Trial
FeCl3 (mL)
KI (mL)
H2O (mL)
1
2
3
10.0
10.0
5.0
10.0
5.0
10.0
0.0
5.0
5.0
11. Data Analysis on the Labquest:
 Open Run 1
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Select a segment of the graph for 30 seconds (50 seconds to 80 seconds as shown in the video)
Select analyze
Trial
[Fe3+] M
[I-] M
Initial rate (s–1) = m
Choose curve fit
1
0.01
0.01
Absorbance
2
0.01
0.005
Linear
3
0.005
0.01
Now write down the value of ‘m’ or slope of line in the data table
Next select OK
Choose Run 2 and repeat the process until you find the slopes for all the three runs. Record the
data.
 Once you have the values you can calculate the order of the reaction using the calculation page
12. Dispose the reagents you have mixed in the cans placed in the fume cupboard. Wash glassware and
return to appropriate locations.
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Teacher Notes
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The order of the reaction is 1 with respect the iodide ions [I-] and zero with respect to Fe3+
The rate law. Rate = k[Fe3+]0[I-]
Prepare 0.02 M Fe3+ by dissolving 8.0808 grams of Fe(NO3)3.9H2O in 0.1 M HNO3 or you can
dissolve the compound in water and add 100 mL of 1 M HNO3 before making up the solution.
(This has to be done by the teacher)
Prepare 0.02 M KI solution by dissolving 3.3201 g KI in 1 liter using distilled water.
Note: When the slope is determined after collection of data for all three runs, you get the slope for the
same time segment for all three runs. This will work positively if the time between start of experiment
and data collection (pressing the play button for data collection) are the same. Use a stops watch if need
be.
©Stan Vincent Eureka 2015 – Order of a reaction SCH4U
Resources for Sharing and Collaboration
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©Stan Vincent Eureka 2015 – Order of a reaction SCH4U