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Iodine Clock and an Introduction to Kinetics
Introduction:^ Edward Conley, Noah Felvey, Jill Hung, and Bradley Harris This introduction to kinetics lab makes use of the iodine clock reaction to instruct students at an early undergraduate chemistry level with a set of mini-experimentscarried out in groups of 4-5 students. At standard conditions, the iodine clock reaction takes ~40 seconds to complete and ends with a stark transition from a colorless solutionto dark blue/purple. Through modifying the temperature and reactant ratios, students can qualitatively observe the Arrhenius Equation and reaction orders of the reactants.The lab is designed for 2 lab sessions of (2-3) hours and contains several exercises designed for entire class collaboration.
Materials (per Group): 2x 50mL beaker2x 25mL graduated cylinder 1x 100 mL beaker1x Red+blue coloring dye 1x Thermometer2x 500 mL beaker 2x 1L beaker1x Hot plate 1x Stir bar
PART ONE: Observable Kinetics**** _The instructor may want to perform the activities of part one in front of the class. The endpoint of the coloring dye experiments is subjective, and the instructor may want todefine reaction completion. Secondly, this expedites the first two activities for more advanced lab groups.**_
Activity 1: Procedure Fill a 50 mL beaker with deionized water. Prepare stopwatch, or watch a clock. Add 1 drop of coloring dye. Record time when the dye has dispersed into the solution. (The lab should agree upon an endpoint. A tutorial is advisable.) Time: _________________________
Fill the 100 mL beaker with deionized water. Prepare stopwatch, or watch a clock. Add 1 drop of coloring dye. Record time when the dye has dispersed into the solution. Time: _________________________
Activity 1: Questions For all questions on the WS, please work together and do not hesitate to ask for help.
What was the difference in timescale between the two experiments?
Why wasn’t/was there a difference in the timescale?
If I mixed red and blue coloring dye I could make a purple solution. Is this a chemical reaction?
Why does the solution go from clear to spontaneously purple after ~40 seconds? Whatdoes this tell you about the chemical reaction. Hint- look at Appendix A at the back
PART TWO: Temperature Effects Use the thermometer to check the temperature.
Activity 1: Procedure Create an ice bath with the other 1L beaker. Fill about ½ way with ice.Fill with water to create a slurry. If available add salt. Fill one of the 50 mL beakers with deionized water. Place this beaker into the ice bath. Fill another 50 mL beaker with deionized water. Place this beaker on the hot plate and heat to about 30-40 °C Remove both beakers from ice bath/hot plate and recording with stopwatch/clock add 1 drop of red/blue dye and record the time for it to diffuse. Time Cold: _________________________
Time Hot: _________________________ Time (from Part One): _________________________
Activity 1: Questions What did you observe between the different rates of diffusion at different temperatures?
What do you expect to happen if you continue to increase/decrease the temperature? Are there limits?
What would happen if the beaker was heating or cooling during the diffusion process? Explain.
Activity 2: Procedures For this reaction we will be using 25 mL of A and a 25 mL 2:3 dilution of B to deionized water. Explain to a grad student how you will make the diluted B solution before you startheating or cooling.
Select and write on the board 4 temperatures between 51 °C and 0 °C that you want to examine for the Iodine Clock reaction.
Arrhenius gives the following equation for temperature dependance of rate:
k = Ae −^ RTEa ,
where k is the rate constant, A is an exponential prefactor (constant), e is the mathematical value 2.718…,constant, and T is the temperature. Ea is the reaction energy barrier, R is the ideal gas
From this equation, how do you expect temperature to effect the rate constant? Does this behavior match the graph?
Examine the other terms of the Arrhenius Equation, explain them in detail.
Arrhenius Equation aside, can you describe why temperature influences rate at a particle level?
What happens at above 50 °C? Why do you think this happens? Hint- See Appendix A for more information.
Some chemical processes are exothermic. What does exothermic mean? How mightthis influence the rate of reaction and why might this be a concern?
PART THREE: Concentration Effect on Kinetics Procedure: In this experiment, you will measure the time of reaction until color change. Procedure will be repeated four times, and the reaction mixture will be diluted with a different amount of watereach time.
Choose 4 dilutions to perform, and write them below in the format A : B : H Volumes of solutions A and B will stay the same, but you will vary the amount of water. Choose 2 O. volumes of water between 0 mL and 50 mL. Measure 25 mL of A into the 50 mL beaker using a graduated cylinder. Measure 25 mL of B into the other 50 mL beaker using a graduated cylinder. Measure desired amount of water into the 100 mL beaker. Record this volume below in the dilution ratio format. Prepare to measure the time of reaction. Simultaneously pour the contents of both beakers into the 100 mL beaker containingwater. Start the timer/stopwatch/smartphone.
Record time when the solution becomes purple-blue. Dilution One: _________________________ Time: _________________________ Dilution Two: _________________________Time: _________________________
Dilution Three: _________________________ Time: _________________________ Dilution Four: _________________________Time: _________________________
Record your dilution factors and reaction times on the graph.
A + B → AB
How would the rate change if:(a) I increased the amount of A and B uniformly? (b) I decreased the amount of A and B uniformly?(c) I increase the amount of A? (d) I increase the amount A and decrease the amount of B uniformly?(e) I increase the amount of AB? Explain each response.
PART FOUR: Temperature and Concentration Together Procedure: In this part of the lab you will design your own experiment. Choose to hold either the temperature or the dilution of B constant hold at 30 °C and do 5 different dilutions of B. and then vary the other parameter. For example,
Create 5 data points and construct a graph on the whiteboard. Choose which parameter you will hold constant:
Temperature / Dilution Factor Data Point One: _________________________ Time: _________________________ Data Point Two: _________________________Time: _________________________
Data Point Three: _________________________ Time: _________________________ Data Point Four: _________________________Time: _________________________
Data Point Five: _________________________ Time: _________________________
Questions: What were you attempting to learn from your data set? What did you learn?
How would you optimize the reaction to go as fast as possible? What are your limits?
Can you think of some real-life applications of these sorts of kinetics experiments? Why might scientist do these?
There is unavoidably some error in your results. Explain possible sources of error, and how (given unlimited funding) to remove them.control? Is there some error that you cannot
-ACKNOWLEDGEMENT-
This project would not have been possible without funding from the United States Department of Education We would also like to thank the Chemistry Department at American River College andthe Departments of Chemical Engineering and Materials Science and Engineering at University of California Davis for their support. -Thank You-
