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Discussion The reaction takes place in two stages. Stage 1: IO3–(aq) + 3HSO3–(aq) → I–(aq) +3SO42–(aq) + 3H+(aq) Stage 1 is the rate-determining step of the reaction mechanism. Decreasing the concentration of the IO3– ions by dilution will cause a decrease in the rate of the overall reaction (Part A). Stage 2: 5 I–(aq) + 6H+(aq) + IO3–(aq) → 3I2 (aq) + 3 H2O(l) Stage 2 is a very fast reaction. The molecular iodine, I2, produced in Step 2 reacts with the starch (not shown in the equations), to produce the deep blue-black solution. The color change indicates that the first reaction is complete and the second one has begun to take place. Increasing temperature (Part B) will increase the reaction rate. Sample Results
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| Solution Preparation These solutions should be prepared fresh, a day or so before the experiment. Solution A
Solution B
To prepare 1.0 M H2SO4 solution:
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GENERAL COMMENTSThis lab requires more preparation time than other labs in this unit. It is recommended that the teacher practice the lab before assigning it to the students.
There are several variations on the clock reaction, using different sets of reactants. The general results, however, will be the same – increasing the concentration of the reactants will increase reaction rate, and increasing temperature increases reaction rate.
The objectives of this lab are similar to the objective of the Rates of Reaction Lab. The clock reaction, however, allows the student to collect quantitative data on reaction rates, which may then be graphed.
The uncertainty can be very high in this experiment unless the students are careful in recording reaction times. Class averages should be used to make the results more meaningful.
If the reactions occur too rapidly, the solutions can be further diluted.