Chemical Catalysis: Decomposition of H2O2 in Presence of MnO2

CHEMISTRY
Chemical Catalysis: Decomposition
of H 2 O 2 in Presence of MnO 2
Introduction
Catalysis is the increase in rate of a reaction as the result of addition of a substance
called - catalyst. A catalyst takes part in the reaction but is not consumed in it: The
catalyst enters the reaction in one step and is regenerated in another step.
A pure solution of H 2 O 2 is stable. But when a catalyst, like for example, MnO 2 ,
platinum metal or Fe +2 ions are added, it rapidly decomposes to water and molecular
oxygen.
MnO 2
H 2 O 2 H 2 O + O 2
In this experiment we follow the release of molecular oxygen resulting from the
decomposition of H 2 O 2 in presence of MnO 2, using pressure sensors .
Equipment
• Two 10 ml glass bottles.
• 2 rubber corks - one for each of the bottles.
• A 2 ml plastic syringe.
• Two 23 gauge syringe needles.
• 3 short latex tubes.
• 3% H 2 O 2 solution.
• Crystalgraph MnO 2 .
• 2 pressure sensors
• A MultiLog
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• 3 Way Valve
Equipment Setup Procedure
1. Connect the MultiLog to the serial port of the computer and to the power supply.
2. Connect the pressure sensors to the I/O 1 port and I/O 2 port of the MultiLog
3. Assemble the equipment as illustrated in figure 1 below.
4. Turn the MultiLog on. Set the MultiLog up according to the setup specified below.
You can set up the MultiLog in two ways:
- use the keyboard of the MultiLog, or
- select the Control Panel from the Logger menu.
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Fig. - 1
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A syringe needle (no 23) is inserted through the cork, till its tip projects somewhat
out of the cork (figure 2)
Fig. - 2
To the other end of the syringe, projecting out of the upper side of the cork, a three
way valve (the one used in infusions) is attached via a very short latex tube (its
length must be just enough to hold together the valve and the syringe). A pressure
sensor is connected to the valve, through another short latex tube.
Turn the valve till its opening is directed vertically. In this position, air can flow
through the valve. In order to stop air flow, turn the valve till its opening reaches an
horizontal position.
To one of the corks, insert an additional needle. A syringe filled with 6% H 2 O 2
solution will be later attached to this needle.
MultiLog Set Up
• Input 1: Pressure
• Input 2: Pressure
• Rate: 1/sec.
• Samples: 500.
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Experimental Procedure
1. Mark the bottles with labels 1 and 2.
2. Fill the plastic syringe with 2 ml of 3% H 2 O 2 solution.
3. Add to bottle 1 - 8 ml water and 2 ml 3% H 2 O 2 solution.
4. Add to bottle 2 - 8 ml water and a few crystals of MnO 2. Mix the solution gently
5. Close tightly the bottles with the rubber corks.
6. Attach to bottle 2, the syringe filled with H 2 O 2 solution, through the additional
needle inserted through the cork.
7. Start the MultiLog by pressing the Run/Stop key in the MultiLog’s keypad. You
can also start the MultiLog using the DB-Lab software: Press the Run button from
the Control Panel, or the (Run) button from the left toolbar.
8. Follow the pressure level registered on the computer monitor.
9. Turn the valves attached to the corks of the two bottles, till an atmospheric
pressure is obtained in both of them.(0 level is an atmospheric pressure: about
1000 mBars).
10.Inject the H 2 O 2 solution into bottle 2 and immediately turn the valves of the two
bottles, to stop air flow through them.
11.Follow changes in pressure registered on the computer monitor during the
experiment.
Data Analysis
1. Calculate the change in pressure in each of the bottles: What was the initial
pressure value? The final value? The difference between the two values?
Use the Markers or the Grid to find the appropriate values:
Markers - double click with the left mouse button on the first required value. A marker will pop-up on
the graph. Drag the marker with the left mouse button to the point on the graph you want to
measure. Double click again on the second value. You get a second marker. Repeat the “drag”
process with the second marker.
You get both readings at the bottom of the display as well as the difference in the
values - ∆t and ∆y.
Grid - choose View\Display from the menu. Mark the option Grid in the Option Display Window.
2. Calculate the reaction rate of H 2 O 2 decomposition:
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i) To receive a net reaction rate, prepare a “difference” graph: Reduce the
graph obtained in the control system from that of the experimental system.
From the Process menu select the More... option, and then select “-“
(Minus).
ii) Prepare a linear regression of this graph, by selecting Linear Regression
from the Process menu. The graph and the formula of the linear regression
will appear in the same window. The slope of the regression graph is the
reaction rate measured in this experiment.
An example of the graph obtained in this experiment is shown below:
Questions
1. How is the pressure developed in the experiment related to decomposition of
H 2 O 2 ?
2. Compare the changes in pressure in the two bottles: Did you observe a change in
bottle 1? in bottle 2? Explain the differences.
3. Which of the bottles serves as a control? Explain.
4. Why is a control system needed in the experiment?
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5. What is the effect of adding MnO 2 crystals to the experimental bottles?
6. What can be the effect of adding increasing amounts of MnO 2 on the reaction
rate?
7. What can be the effect of temperature rise in the bottles during the experiment,
on the measurement of H 2 O 2 decomposition rate?
Further Suggestions
1. Add increasing amounts of MnO 2 to the reaction mixture and follow the reaction
developed in each case.
2. Calculate the reaction rate obtained in each experiment.
3. Compare the effect of different types of chemical catalysts: HBr, HI, Fe +2 ions,
platinum metal.
4. Change the concentration of H 2 O 2 added to the reaction mixture. Compare the
effect of reactant concentrations on the reaction rate, with that of the catalyst.
5. Follow temperature changes occurring during the reaction. Evaluate the effect of
temperature on the decomposition rate of H 2 O 2
Advantages of Using the MultiLog in
Studying Chemical Catalysis
1. Real-time measurements of a catalysis reaction - the students watch bubbles of
oxygen released during the experiment, and receive immediately the quantitative
data of the process.
2. Use of an easy to operate system, enabling the students to design and perform
experiments in order to answer problems of interest for them.
3. Learning and comparing effects of chemical catalysts by performing short and
simple experiments and analyzing the results.
4. Use of a sensitive pressure sensor enabling detection of small changes.
5. Simple and convenient means to calculate graph slopes and net reaction rate.
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