COUNT ON IT!

The Iodine Clock Challenge

Purpose:Can you predict the time it will take for the iodine clock to ring?

Background:Kinetics is the study of the rates of chemical reactions. As reactants are transformed into products in a chemical reaction, the amount of reactants will decrease and the amount of products will increase. The rate of the reaction describes how fast the reaction occurs. The greater the rate of the reaction, the less time is needed for a specific amount of reactants to be converted to products. Some of the factors that may affect the rate of a chemical reaction include temperature, the nature of the reactants, their concentrations, and the presence of a catalyst. The demonstration of an “iodine clock” involves a chemical reaction that suddenly turns blue due to the formation of the familiar iodine-starch complex. The color change occurs abruptly, like an alarm clock ringing.

Materials:0.1M KIO30.2M Na2S2O5starch solutionsmall cups

10mL and 100mL graduated cylindersDI waterstirring rod

Safety:Potassium iodate solution is moderately toxic by ingestions and a body tissue irritant. Sodium meta-bisulfite is also irritating to skin, eyes, and other body tissues. Avoid contact of all chemicals with eyes and skin. Wear goggles.

Procedure:DEMONSTRATION

  1. Pay close attention to the procedure used by the teacher in the iodine clock demonstration. Start timing as soon as Solutions A and B are mixed and stop when the blue color appears. Observe the reactions and record the reaction times.

EXPERIMENTATION

  1. Answer questions 1-4 on the other sheet. Then brainstorm the following questions and fill out the graphs as instructed.
  2. Why was the total volume of Solution A kept constant in the iodine-clock reaction? Be able to explain why.
  3. In order to investigate the effect of KIO3 concentration on the rate of the iodine clock reaction, the composition and amount of Solution B was not changed. Be able to explain why.
  4. Prepare graphs of a) reaction time versus volume of KIO3 solution in Solution A and b) 1/time versus volume of KIO3 solution in Solution A. Explain the shape of each graph. See graphs on back of this sheet. When doing the graphs, be sure to label X and Y axes, label units and numbers. Connect points in a best fit line/curve in PENCIL.
  5. Which graph would probably give a more accurate prediction of the amount of KIO3 solution needed to make the iodine clock “ring” at 25 seconds?
  6. Use your graphs to determine the column of KIO3 needed for a 25 second “ring”.
  7. Once you have answered questions 1-4 and completed your graphs, get your teacher to initial your paper.
  8. Read the list of materials and safety precautions.
  9. Get enough reactants from your teacher to run three trials of the iodine clock reaction. – two trials to gather additional data for the graphs and a final “challenge” run which must ring in 25 seconds. Your total volume of Solution A should not exceed 100.0mL.Your total volume for Solution B is 40 mL. This final CHALLENGE run will be done in front of your teacher who will time your reaction!
  10. Carry out the trial runs, record the data on the other sheet, and graph the results on the same graph as the three demonstration points. Redraw the curve or line for each.
  11. Solution B is made by adding 15 mL of 0.2M Na2S2O5 to 45 mL starch and then adding 60mL of water. The total volume is divided into thirds (40mL each) for the three trials.

ALL DATA and CALCULATIONS

Experiment/Trials / Solution A
0.1MKIO3 mL / Reaction time, s / Reaction time, s-1
Demo 1 / 25.0mL
Demo 2 / 50.0mL
Demo 3 / 12.5mL
Trial 1
Trial 2
Challenge

GRAPH 1:Reaction time in seconds versus Volume of KIO3 in mL

GRAPH 2:Inverse Reaction time in seconds versus Volume of KIO3 in mL

Name ______Period _____ Date ______

COUNT ON IT!

The Iodine Clock Challenge

DEMONSTRATION

Experiment / Solution A
0.1M KIO3 / Solution A
Water / Reaction time, s
1 / 25.0mL / 75.0mL
2 / 50.0mL / 50.0mL
3 / 12.5mL / 87.5mL
  1. Calculate the volume percent (concentration) of the potassium iodate (KIO3) for solution A for each experiment. Show your work below.
  1. How did the reaction time change as the KIO3 concentration was changed?
  1. How is the rate of the reaction related to the reaction time?
  1. Write a general statement describing the effect of the reactant concentration on the rate of a reaction.

EXPERIMENTATION

Trials / Solution A
0.1M KIO3, mL / Solution A
Water, mL / Solution A
Total Vol., mL / Reaction Time, s
1
2
3
  1. Graph your three trials on the same graph as 2c but in a different color. Connect points in a best fit line/curve in INK.
  2. For the final test run, how close did the actual reaction time come to the predicted time of 25 seconds?
  1. Based on the results of the demonstration and your trial runs, predict the reaction time in seconds if 63mL of KIO3 solution were used for Solution A. Prove it.
  1. Explain the effect of concentration on reaction rate in terms of collisions between molecules: When the concentration of reactants increases, the reaction time ______, because increasing the ______of molecules or ions in solution increases the rate of ______between them.