Rate of a Chemical Reaction I (Colorimeter)

PASCO scientific 1 Chemistry Experiment Library: 06020 

Science Workshop 

C14 Reaction Rate I 

Concept: rate of reactions 

Time: 30 m 

SW Interface: 300, 500 & 700 

Macintosh® file: C14 Reaction Rate 1 

Windows® file: C14_REA1.SWS 

Rate of a Chemical Reaction I 

(Colorimeter) 

Adapted by Tom Russo from MicroChemistry, distributed by Theta Technologies, 203 Bluegrass Ave., Suite 179H, 

South Gate, KY 41071, (606) 441-4768. 

EQUIPMENT NEEDED 

• Science Workshop Interface 

• colorimeter 

• cuvette (included with colorimeter) 

• graduated cylinder, 10 mL (2) 

• apron and safety goggles 

CHEMICALS AND CONSUMABLES 

• distilled water 

• hydrochloric acid, 6 Molar 

• sodium thiosulfate, 0.2 Molar 

PURPOSE 

In this laboratory activity you will investigate the effect of changes in concentration of the reactants on 

the rate of a reaction. 

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 

In this experiment you will 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 acidic reduction of the thiosulfate 

ion to sulfur and sulfur dioxide. 

tr © 1996, PASCO scientific C14 - 1




The equation for the reaction is: 

S 2 O 3 

2- (aq) + 2 H + (aq) =========> SO2 (g) + S (s) + H 2 O 

In order to determine the effect of concentration on the rate of the reaction, you will follow the reaction 

by using the colorimeter to monitor the formation of the solid sulfur generated. The solid sulfur will block 

the light in the colorimeter and the amount of blockage is directly proportional to the amount of sulfur in 

suspension. 

The rate of this chemical reaction is given by the equation: 

Rate = k [thiosulfate] a [acid] b 

The letters a and b 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. 

SAFETY PROCEDURES 

Follow all safety directives given by your teacher. 

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; 

Macintosh: C14 Reaction Rate 1 

Windows: C14_REA1.SWS 





• The document has a Graph display with a plot of the Absorbance of light (A) versus Time (min). 

• Note: For quick reference, see the Experiment Notes window. To bring a display to the top, click 

on its window or select the name of the display from the list at the end of the Display menu. 

Change the Experiment Setup window by clicking on the “Zoom” box or the Restore button in the 

upper right hand corner of that window. 

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. 





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 





#3 1 mL of 2 M sodium thiosulfate and 1 

mL of distilled water 

#4 0.5 mL of 2 M sodium thiosulfate and 

1.5 mL of distilled water 

2 mL of 6 M hydrochloric acid 

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. 

DIN plug 

Wavelength selection knob 

Pivoting lid 

Light source Cuvette Photodetector 

Colorimeter: Cut-away view 

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. 





• “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: 

Y 2 − Y 1 

X 2 − X 1 

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) 





#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?

Rate of a Chemical Reaction I (Colorimeter)

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