Rate of a Chemical Reaction using Colorimetry
Adapted by Tom Russo from MicroChemistry, distributed by Theta Technologies, 203 Bluegrass Ave., Suite 179H, South Gate, KY 41071, (606) 441-4768.
PURPOSE
In this laboratory activity you will investigate the effect of changes in concentration of the reactants on the rate of a
reaction. Specifically, this experiment will use colorimetry to monitor the colour change of dichromate ions to
chromium (III) ions as a secondary alcohol, propan-2-ol (isopropyl alcohol) is oxidized.
THEORY
All chemical reactions occur at specific rates. The rate of a chemical reaction depends on several
physical and chemical factors. These factors are:
1. The concentration of the reactants
2. The temperature of the reaction
3. The pressure on the reaction
4. The presence of a catalyst
This experiment will be used to determine the effect of changes in concentration of the reactants on the rate of the
chemical reaction. The reaction chosen for this experiment is the oxidation-reduction reaction of acidified potassium
dichromate and propan-2-ol, a secondary alcohol, that produces chromoium (III) ions and a ketone, propane
(acetone).
The equation for the reaction is:
2Cr2O72-(aq) + 3 CH3CH2CH2OH (aq) + 16H+(aq) → 4Cr3+ (aq)+ 3CH3CH2COOH (aq) + 11H2O(ℓ)
Dichromate solutions are bright yellow that turn bright orange in the presence of concentrated sulfuric acid.
Chromium (III) solutions are blue-green in colour. The other chemical species are all clear and colourless.
The reaction rate of this reaction can be monitored, not simply because of a colour change, but because of the
change in the intensity of each colour as the concentration of Cr2O72- or Cr3+ ions change over time. The intensity of
the colour is directly related to the concentration of the chemical species that produces that colour. The wavelength
of the colour observed corresponds to the wavelength of the light transmitted; more of that colour is allowed through
the solution. Conversely, less of that wavelength of light is absorbed by the solution.
As the orange reaction mixture turns to the blue products. If the orange light (630nm) is monitored, the transmission
of orange should decrease over time as the concentration of orange decreases. Likewise, the absorption of orange
light should increase as the concentration of blue (orange absorbing) Cr3+ ions increases. If blue light (460 nnm) is
monitored, the transmission of blue should increase over time as the concentration of blue increases. Likewise, the
absorption of blue light should decrease as the concentration of orange (blue absorbing) Cr2O72- ions decreases.
The rate of this chemical reaction is given by the equation:
Rate = k [CH3CH2CH2OH]a [Cr2O72-]b [H+]c
The letters a, b, and c seen as exponents are numerals which can only be determined experimentally. Each reactant
must be varied separately while the other is kept constant. The effect on the rate of the reaction is noted and the
value of the exponent is determined in this way:
• If a change in concentration of one of the reactants has no effect, the exponent is 0.
• If doubling the concentration doubles the rate, the exponent is 1.
• If doubling the concentration quadruples the rate, the exponent is 2.
The over all order of the reaction is determined by adding a + b + c.
EQUIPMENT
• Science Workshop™ Interface
• graduated cylinder, 10 mL (2)
• colorimeter
• cuvette (included with colorimeter)
• apron and safety goggles
CHEMICALS
• distilled water
• potassium chromate, K2CrO4
• sulfuric acid, H2SO4,18 mol dm-3
SAFETY PROCEDURES
Follow all safety directives given by your teacher.
• propan-1-ol (n-propanol)
PROCEDURE
For this activity, the colorimeter measures the change in absorbance of light by a solution during a chemical reaction. The rate of
change of absorbance is measured for different concentrations of the chemicals in the reaction. The Science Workshop program
records and displays data.
PART I: Computer Setup
1. Connect the interface to the computer, turn on the interface, and turn on the computer.
2. Connect the DIN plug of the colorimeter to Analog Channel A on the interface.
3. Open the Science Workshop file titled as shown; Windows: C14_REA1.SWS
• The document has a Graph display with a plot of the Absorbance of light (A) versus Time (min).
4. The "Sampling Options…" for this experiment are: Periodic Samples = Fast at 10 Hz.
5. The vertical axis (absorbance) on the Graph is scaled from 0 to 2.0. The horizontal axis (time) on the Graph is scaled from 0 to 3
minutes.
PART II: Sensor Calibration and Equipment Setup
About the Colorimeter
The colorimeter is designed to analyze monochromatic (“one-color”) light that passes through a solution.
The solution is put into a rectangular container called a cuvette, which is then placed inside the
colorimeter. The measure of the amount of light that passes through a solution is called “transmittance”.
Transmittance is a ratio of the intensity of the transmitted light to the intensity of the original light, and is
usually expressed as a percentage.
Absorbance is related to transmittance. The light absorbed by a solution depends on the absorbing
ability of the solution, the distance traveled by the light through the solution, and the concentration of the
solution. The relationship of absorbance to transmittance is:A = 2 - log % T
The Science Workshop program automatically calculates absorbance.
Calibration
We recommend you do a new calibration any time you perform a new colorimetry experiment or
change the wavelength within an experiment.
The general method for doing a 2-point calibration is as follows: A zero percent calibration is done with
no light passing through the cuvette (included with the colorimeter). The wavelength knob on the
colorimeter is turned to “0% T” (see figure below). In this position the computer can read data from the
colorimeter, but the light source is turned off. Since the light is off, it makes no difference if a cuvette is in
the cuvette slot. A 100% calibration is done with the wavelength knob turned to select one of the three
light emitting diode (LED) wavelengths. This turns on the red, green, or blue LED. (For this experiment
you will use the RED wavelength.) A blank is placed in the cuvette slot. The blank is a cuvette
containing the solvent used in the solution being studied, usually distilled water. The blank acts as a
control by taking into account the small amount of light absorbed by the solvent and by the walls of the
cuvette.
Follow these steps to calibrate the colorimeter.
PASCO scientific 4 Chemistry Experiment Library: 06020
Science Workshop C14 Reaction Rate I
tr © 1996, PASCO scientific C14 - 4
1. Prepare a blank by filling a cuvette 3/4 full with distilled water. To correctly use a colorimeter
cuvette, remember:
• All cuvettes should be wiped clean and dry on the outside with a tissue.
• Handle cuvettes only by the top edge of the ribbed sides.
• All solutions should be free of bubbles.
• Always position the cuvette with its reference mark facing toward the white reference mark at the
right of the cuvette slot on the colorimeter. The light beam will pass through the smooth sides of
the cuvette.
2. In the Experiment Setup window, double-click on the colorimeter icon.
• This opens the Analog Sensor setup dialog box.
3. Turn the wavelength knob on the top of the colorimeter to the “0% T” position. Close the lid of
the cuvette slot (with or without the “blank” cuvette in place). When the voltage value shown as
“Cur Value:” in the Analog Sensor setup dialog box stabilizes, click on the “Read” button for
“Low Value:”, and enter “0” (for 0% transmittance).
4. To do a 100% calibration, place the blank in the cuvette slot and close the lid of the colorimeter.
5. For this experiment, turn the wavelength knob of the colorimeter to the RED (635 nm) LED
color.
6. When the voltage value stabilizes, click on the “Read” button for “High Value:”, and enter a
transmittance value of “100” (for 100% transmittance).
• The colorimeter is now calibrated.
Equipment Setup
When sodium thiosulfate and hydrochloric acid are mixed, the solution gradually becomes darker. The
solution absorbs more and more light (absorbance goes up).
You will test four solutions as follows:
Solution Component A Component B
#1 2 mL of 2 M sodium thiosulfate 2 mL of 6 M hydrochloric acid
#2 2 mL of 2 M sodium thiosulfate 1 mL of 6 M hydrochloric acid and 1
mL of distilled water
PASCO scientific 5 Chemistry Experiment Library: 06020
Science Workshop C14 Reaction Rate I
tr © 1996, PASCO scientific C14 - 5
#3 1 mL of 2 M sodium thiosulfate and 1
mL of distilled water
2 mL of 6 M hydrochloric acid
#4 0.5 mL of 2 M sodium thiosulfate and
1.5 mL of distilled water
2 mL of 6 M hydrochloric acid
The general procedure is as follows:
1. Measure the liquids needed for Component A into one graduated cylinder.
2. Measure the liquids needed for Component B into a second graduated cylinder.
3. Put Component B into a cuvette.
4. Add Component A to the same cuvette and put a cap on the cuvette.
5. Quickly invert the cuvette to mix the components.
6. Quickly put the cuvette into the colorimeter.
7. Record data.
PART III: Data Recording
1. Turn the wavelength selection knob on the colorimeter to RED. When you are ready to begin data
recording, place Component B for the first solution in the cuvette. Add Component A for the first
solution to the cuvette. Cap the cuvette, invert it to mix the components, and put the cuvette into
the colorimeter.
Colorimeter: Cut-away view
Light source Cuvette Photodetector
Pivoting lid
Wavelength selection knob
DIN plug
Rate of Reaction: Effect of Change in Concentration of Reactants
2. Click the “REC” button to begin recording data.
3. Record data for 3 minutes.
4. Click on the “STOP” button to end data recording.
PASCO scientific 6 Chemistry Experiment Library: 06020
Science Workshop C14 Reaction Rate I
tr © 1996, PASCO scientific C14 - 6
• “Run #1” will appear in the Data list in the Experiment Setup window.
5. Repeat the procedure for solutions 2, 3, and 4. There will be four runs of data at the end of the
data recording.
ANALYZING THE DATA
1. Click on the Graph window to bring it to the front. Save your data by selecting “Save As…” from
the File menu.
2. Determine the rate of reaction for Solution #1 The procedure for doing this is as follows:
3. Use the DATA menu button to select Run #1.
4. Click the “Smart Cursor” button. The cursor changes to a crosshair when you move it into the
Graph data display area.
5. Move the Smart Cursor to the beginning of the plot of absorbance versus time (at approximately
0.25 in this example).
6. Record the Absorbance value and the Time value as “Y1” and “X1” for Solution #1.
7. Move the Smart Cursor to a position on the plot where the absorbance is a convenient higher
value, such as 0.65, for example.
8. Record the Absorbance value and the Time value as “Y2” and “X2” for Solution #1.
9. Calculate the rate of reaction as the ratio of the change in absorbance over time:
Y2-Y1
X2-X1
10. Repeat the process to determine the rate of reaction for Solutions 2, 3, and 4. For Solution #2,
select Data Run #2 from the DATA menu. For Solution #3, select Data Run #3. For Solution #4,
select Data Run #4.
DATA TABLE
Solution Y1 (A) X1 (Min) Y2 (A) X2 (Min) RATE
#1 (2 mL thiosulfate, 2 mL
HCl)
#2 (2 mL thiosulfate, 1 mL
HCl + 1 mL water)
PASCO scientific 7 Chemistry Experiment Library: 06020
Science Workshop C14 Reaction Rate I
tr © 1996, PASCO scientific C14 - 7
#3 (1 mL thiosulfate + 1 mL
water, 2 mL HCl)
#4 (0.5 mL thiosulfate + 1.5
mL water, 2 mL HCl)
QUESTIONS
1. What is the effect on the rate of the reaction by halving the concentration of thiosulfate?
2. What is the order of the reaction due to thiosulfate?
3. What was the effect on the rate of reaction by halving the concentration of acid?
4. What is the order of the reaction due to acid?
5. What is the over all order of the reaction?