Lab: Determining the Rate Law for the Catalyzed Decomposition of H2O2
Hydrogen peroxide can be purchased as a dilute solution in a pharmacy. It is used as a topical antiseptic for
minor cuts. In this investigation, you will use the I- ion, in the form of aqueous sodium iodide, to catalyze
the decomposition of H2O2. The purpose of this lab is to derive the rate law equation for the decomposition
of hydrogen peroxide: rate = k[H2O2]m[I-]n
Materials
6.0 % (m/v) H2O2 (0.060 g/mL)
0.3000 mol/L NaI (Actual [ ] is _____ mol/L)
distilled water
three 10 mL graduated cylinders (H2O2, NaI, H2O)
two medicine droppers (for H2O2, NaI…water in wash bottle)
retort stand
1 L beaker
large test tube (plastic tubing,stopper)
50 mL gas collecting tube
burette clamp
utility clamp
Procedure:
1. Set up the apparatus as directed by your teacher.
2. Add 10.00 mL of hydrogen peroxide solution to the large test tube.
3. Add 10.00 mL of sodium iodide solution to the large test tube containing the hydrogen peroxide.
Stopper the test tube right away and start the timer. Swirl the reaction mixture gently periodically in
order to help the bubbles of O2 gas leave the solution. (the total volume in the test tube should be 20.00
mL assuming volumes are additive).
4. Measure the volume of gas collected after 60 seconds and record it in the data table for TRIAL 1.
5. Repeat this process for Trials 2 and 3 but prepare the solutions as outlined in the data table.
Example For Trial 2:
H2O2
Dilute 5.00 mL of 6% H2O2 up to 10.00 mL by adding water (5.00 mL assuming additive volumes)
NaI
Just 10.00 mL of NaI (the concentration of NaI will be the same as in Trial 1)
Record the volume of gas collected in the data table.
6. Clean up as directed by your teacher.
7. Complete the final two columns in the data table.
Trial
Volume
of 6.0 %
H2O2(aq)
(mL)
1
2
3
10.00
5.00
10.00
Volume
of
distilled
water
(mL)
0
5.00
0
Volume
of
_____ M
NaI(aq)
10.00
10.00
5.00
Volume
of
distilled
water
(mL)
0
0
5.00
Volume Molarity Molarity
of Gas
H2O2
NaI
produced
Upon
Upon
in 60 s
Mixing
Mixing
(mL)
Reactants Reactants
Lab Report
1. Write a balanced chemical equation for the catalyzed decomposition of H2O2.
2. What is the general rate law equation?
3. Express the rate of hydrogen peroxide consumption in terms of the rate of oxygen production.
4. For each trial, determine the initial rate of hydrogen peroxide consumption:
First, determine the initial rate of oxygen production.
Second, use the answer in question 3 above to determine the rate of hydrogen peroxide consumption.
To determine the initial rate of oxygen production in mol/L/s:
- use the Ideal Gas Law to convert the volume of oxygen produced during the 60 s to the number of
moles of oxygen produced during the 60 s (if the gas was collected over water, the P value used in
the Ideal Gas Law calculation should be corrected by subtracting the partial pressure of water
from the atmospheric pressure…Dalton’s Law of Partial Pressures!...we will ignore this for
simplicity…thus assume all the gas collected is pure oxygen)
- divide the number of moles of oxygen by 0.02000 L (the volume it was produced in)
- divide the molarity of oxygen by 60 s to express the rate in mol/L/s
Show an example of your calculations for trial 1.
Trial
Rate of O2 Production
(mol/L/s)
Rate of H2O2 Consumption
(mol/L/s)
1
2
3
5. For the trials in which the concentration of NaI was kept constant (trials 1 and 2), determine the order
of reaction for H2O2 by drawing and analyzing an appropriate rate vs. initial concentration graph
(these initial concentrations are those upon mixing the H2O2 and the NaI). DONE BY HAND!
6. For the trials in which the concentration of H2O2 was kept constant (trials 1 and 3), determine the
order of reaction for the I- ion catalyst by drawing and analyzing an appropriate rate vs. initial
concentration graph (these initial concentrations are those upon mixing the H2O2 and the NaI). DONE
BY HAND!
7. Determine the value with units of the rate constant, k, for the reaction (obtain a k value for each trial
and then calculate an average from these). Show your work for trial 1.
Trial
Rate Constant
Average Rate Constant
1
2
3
8. Write the rate law equation for this reaction including the average k value with units.
9. Use the rate law equation to determine the rate of hydrogen peroxide consumption in mol/L/s if the
concentration of hydrogen peroxide is 30.0 % (m/v) and the concentration of NaI and thus I- is 0.50
mol/L.
10. The reaction mechanism for the iodide ion catalyzed decomposition of hydrogen peroxide is believed
to be:
Step 1: H2O2 + I-  H2O + IO-
(slow)
Step 2: H2O2 + IO-  H2O + O2 + I-
(fast)
a)
b)
c)
d)
Show that this mechanism supports the overall stoichiometry of the reaction.
How does this mechanism account for the iodide ion, I-, being a catalyst?
What is IO- an example of?
Sketch and label a potential energy vs. reaction progress graph that is consistent with the
mechanism.