12 Hydrogen Peroxide Decomposition
Purpose
Perform a series of Iodide-Catalyzed hydrogen peroxide decomposition reactions. Determine the rate constant and activation energy of the reaction.
Background
Hydrogen peroxide (H2O2) in aqueous solution decomposes very slowly under ordinary conditions. The equation for the decomposition is:
2H2O2(aq) 2H2O + O2
A catalyst such as potassium iodide, manganese oxide, or catalyse enzyme may be used to increase the rate of reaction. Conducting a catalyzed decomposition of hydrogen peroxide in a closed vessel allows the determination of the reaction rate as a function of the pressure increase caused by the production of oxygen gas by the reaction*. By varying the initial molar concentration of H2O2 solution, the rate law for the reaction can be determined. By measuring the reaction rate at various temperatures, the activation energy, Ea, can be calculated.
*The initial rate of gas production can be converted to the rate of change of concentration using the ideal gas law.
Materials
Equipment• Xplorer GLX / • Rubber stopper, 1 hole
• Temperature probe / • Rubber stopper, solid
• PASPORT Chemistry Sensor / • 18 mm X 150 mm Test tube
• Plastic tubing and connectors (with sensor) / • Plastic Beral pipet
• Beaker, 1L / • Graduated cylinders, 10 mL (2)
Consumables
• 0.5 M Potassium iodide (KI) solution / • 3% Hydrogen peroxide solution
• Room temperature water
Safety Precautions
• Wear safety goggles and follow all standard laboratory safety procedures
Equipment Setup
1) Plug the temperature probe into one of the temperature ports () on the left side of the Xplorer GLX.
2) Connect a piece of plastic tubing to the one hole rubber stopper using a barbed connector. Put a quick coupling on the other end of the plastic tubing.
3) Attach the tubing to the pressure port on the Chemistry Sensor.
4) Plug the Chemistry Sensor into one of the ports on the top of the GLX.
Procedure
Part I 3% H2O2 decomposition with 0.5 M KI at room temperature
1) Set up a room temperature water bath using a 1L beaker.
2) Place the temperature probe attached to the GLX into the water bath.
3) Use a graduated cylinder to measure 4 mL of 3% H2O2 into the test tube and plug the tube with the solid rubber stopper. Immerse the tube in the water bath.
4) Draw 1 mL of 0.5 M KI solution into a Beral pipet. Invert the pipet and place the bulb end into the water bath.
5) Turn on the GLX and press to open the Digits display. When the display stabilizes, record the temperature in your data table for parts I through III.
6) Press on the GLX to open the Graph display.
7) Press on the GLX to begin data collection
8) Start the reaction and begin data collection
a Remove the solid stopper from the test tube.
b Remove the beral pipet from the water bath and transfer 1 mL of KI solution into the test tube. Swirl the contents of the test tube to mix the reagents.
c Seal the test tube with the one hole stopper that is connected to the Chemistry Sensor. Place the test tube back into the water bath. Be sure the test tube is immersed in the water bath far enough to cover the reaction solution.
9) Continue collecting data until the slope on the graph is consistent, then press to end data collection.
10) Remove the stopper from the test tube to release the pressure. Dispose of the test tube contents in the appropriate waste container.
11) Examine the graph of pressure versus time. Select a straight region of the graph after the pressure has begun to increase. Press and select Linear fit from the drop-down menu. Record the slope as the initial rate of reaction in your data table.
12) Clean and rinse the test tube for the next trial.
Part II H2O2 decomposition with 0.25 M KI at room temperature
1) Use a Beral pipet to measure 1 mL of distilled water to a 10 mL graduated cylinder. Add 1 mL of 0.5 M KI solution to the graduated solution. Swirl the contents of the graduated cylinder to mix the solution.
2) Draw 1 mL of the 0.25 M KI solution into the plastic Beral pipet. Invert the pipet and place it into the water bath.
3) Measure 4 mL of 3% H2O2 solution into the test tube. Plug the test tube with the solid rubber stopper and place the test tube into the water bath.
4) Repeat steps 6 through 12 of part I to complete this trial. Record the data in your data table.
Part III 1.5% H2O2 decomposition with 0.5 M KI at room temperature
1) Measure 2 mL of 3% H2O2 into a clean graduated cylinder. Add 2 ml of distilled water to the graduated cylinder and swirl the contents to mix the solution.
2) Transfer the 4 mL of 1.5% H2O2 solution from the graduated cylinder to the test tube. Plug the test with the solid rubber stopper. Place the test tube into the water bath.
3) Draw 1 mL of 0.5 M KI solution into a plastic Beral pipet. Invert the pipet and place the bulb end of the pipet into the water bath. Allow the test tube and pipet to remain in the water bath for at least two minutes to equilibrate.
4) Repeat steps 6 through 12 of part I to complete this trial. Record the data in your data table.
Part IV 3% H2O2 decomposition with 0.5 M KI at ~30°C
1) Place the water bath on a hot plate and heat until the temperature reaches about 30° C.
2) Repeat steps 6-12 of part I to complete this trial. Record your results.
Analyze
Copy the tables below and record calculations in your lab notebook as you complete your analysis.
Data Table
Part / Reactants / Temperature (°C) / Initial rate (kPa/s)I / 4 mL 3% H2O2 + 1 mL 0.5 M KI
II / 4 mL 3% H2O2 + 1 mL 0.25 M KI
III / 4 mL 1.5% H2O2 + 1 mL 0.5 M KI
IV / 4 mL 3% H2O2 + 1 mL 0.5 M KI
Analysis Table
Part / Initial Rate (mol/L/s) / [H2O2]after mixing / [I-]
after mixing / Rate constant
(k)
I
II
III
IV
Analysis Questions
1) Calculate the rate constant, k, and write the rate law expression for the catalyzed decomposition of hydrogen peroxide. Explain how you determined the order of the reaction H2O2 and KI.
2) The following mechanism has been proposed for this reaction:
H2O2 + I- IO- +H2O
H2O2 + IO- I- + H2O + O2
If this mechanism is correct, which step must be the rate-determining step: Explain.
3) Use the Arrhenius equation shown below to determine the activation energy, Ea, for this reaction.