THE PROBLEM: Determine if iron(III) chloride, FeCl₃, or potassium iodide, KI, is a better catalyst for the decomposition of a 3% solution of hydrogen peroxide, H₂O₂. Describe the method you developed to solve this problem.

This problem is designed to assess student skill in designing an experiment to test the effectiveness of a catalyst. A student must be able to demonstrate control of variables, and to know the meaning of the term catalyst.

1. The student develops a strategy for deciding how to determine which is a better catalyst. A suitable strategy would be to measure rate of decomposition of H₂O₂ for a fixed mass of catalyst, judging the reaction to be complete when the bubbling stops.

2. The student adds a known, fixed quantity of each catalyst to a known, fixed volume of H₂O₂ solution and records the time for complete decomposition. Note: An alternate approach would be to prepare solutions with the same concentration for each catalyst, as they both are soluble. Then a drop of each catalyst solution could be added to a known, fixed volume of H₂O₂ solution and the time recorded as before.

3. The student concludes that a faster bubbling rate, or a shorter time for reaction completion, indicates the better catalyst.

1. Development of experimental strategy 2 pt

a) The student plans to combine each catalyst with H₂O₂ solution, but fails to control either the mass of the catalyst or the volume of solution. 0 pt

b) The student plans to combine each catalyst with H₂O₂ solution, but fails to control one of the variables. 1 pt

c) The student plans to use the likely approach procedure. 2 pt

2. Carrying out the reactions 2 pt

a) The student runs the reactions but never decides how to judge the endpoint. 0 pt

b) The student runs the reactions, recording time, but does not record how the endpoint has been judged. 1 pt

c) The student runs the reactions, using the likely approach procedure. 2 pt

3. Conclusions drawn from experimental data 1 pt

a) The student does not draw any conclusions 0 pt

b) The student draws a conclusion, but no reasonable evidence is cited. 0.5 pt

c) The student draws conclusions based on the likely approach analysis. 1 pt

Extra credit could be awarded if the student

a) writes a chemical equation for the reaction.

b) discusses the uncertainties and sources of error in this determination.

Chemicals Equipment Possible Distracters 3 % hydrogen peroxide, H2O2 , solution potassium iodide, KI(s) iron(III) chloride, FeCl3 (s) water Beral-type pipets, micro-tip reaction plate, 24-well balance that weighs to 1 mg timer, clock, or watch with second hand

1. The hydrogen peroxide solution should be fresh and tested for reactivity before it is used.

2. The problem can be made more challenging if yeast is added as a third choice of catalyst.

3. The problem is made considerably more challenging if the students are asked to prepare the
catalysts by reacting soluble copper, nickel and cobalt salts with sodium hydroxide. The salt
solutions are not good catalysts, but the insoluble hydroxides are. Working out the details of this
project might well be a group project, not an individual assessment.

1. Do not use manganese(IV) oxide, MnO₂,for a catalyst in this reaction as it is a suspected carcinogen.

DATE CLASS NOTES