Major Chemical Concept Expected Student Background Time Safety Materials Advance Preparation Pre-Laboratory Discussion Teacher-Student Interaction Anticipated Student Results

Answers to Data Analysis and Concept Development Answers to Implications and Applications Post-Laboratory Activities Extensions Assessing Laboratory Learning Relevant National Science Education Standards

Major Chemical Concept

This exercise allows students to determine experimentally how various factors—notably concentration and temperature—affect the time for a reaction to occur, and thus how they affect the reaction rate. Emphasis is placed on quantitative results.

Expected Student Background

This activity is appropriate for all general chemistry students. Students should have a general idea of the concept of a reaction rate. It would be helpful if they have seen examples of varying rates, at least on a qualitative basis.

Time

The activity can be done in one or two class periods. The length of time available will determine how many different trials of concentration and temperature variations you will assign each student group.

Safety

Students should be cautioned about the hazards of an open burner flame at their bench.

General Safety Guidelines

Materials

(For 24 students working in pairs)

Apparatus

24 test tubes, 18 × 150 mm
12 beaker, 250 ml
12 graduated cylinders, 10 ml
12-24 thermometers, -10 to 110ºC
12 clocks with sweep second hand, or digital watches with seconds
12 burners
12 ring stands with rings
12 ceramic/wire gauze

72-96 ice cubes for cooling water bath
300 ml distilled water at room temperature
1 L solution A (4.3 g/L KIO₃; see Advance Preparation Note 4 for further dilution to be prepared.)
1 L solution B (0.2 g Na₂S₂O₅, 4 g soluble starch, 5 ml 1 M H₂SO₄/L) Note that Na₂S₂O₅, is sodium META-bisulfite, not sodium bisulfite (see Advance Preparation Notes 2 and 3).

Advance Preparation

1. To prepare each liter of Solution A, add 4.3 g KIO₃ to a 1 L volumetric flask about half-full of distilled water. Stopper the flask and swirl until the solid dissolves. Fill to the 1 L mark with distilled water. Pour into suitable smaller containers for student use. Large burets work well for group dispensing, as do squeeze bottles such as laboratory wash bottles. If you choose to latter method, be certain the bottles are well-labeled to distinguish them from other bottles normally found in the laboratory.

2. For each liter of Solution B, make a paste of 4 g soluble starch in a small volume of water. Add this slowly to about 900 ml boiling water, and boil it for a few minutes. Allow the system to cool; then add the 0.2 g Na₂S₂O₅ and stir. This solution can be stored for a week or more prior to using it in this laboratory activity. (Second Option: Some teachers purchase commercial spray starch and use this to make the starch solutions very quickly by simply spraying the starch into a large volume of water. Try this method beforehand to determine how much spray starch is needed to give the expected deep blue color. After the starch solution has been prepared in this way, add the 0.2 g Na₂S₂O₅, sodium metabisulfite, and stir.)

3. On the day of the activity, add the sulfuric acid to Solution B just before use. Stir the solution well and do a trial run with 10 ml A and 10 ml B. You may need to add more acid until the time is within the 10-15 second range. If the blue color appears faded, or if it forms gradually through the tube, you may need to add more Na₂S₂O₅. If the reaction occurs too slowly, at lower dilutions the reaction time will be too long for reasonable results. If the blue color appears too quickly, you can dilute Solution A. If the reaction occurs too rapidly (less than 10 sec) at room temperature, reactions at higher temperatures will be too rapid to measure accurately, or they may not happen at all.

4. To prepare for Part II, dilute Solution A to about half its original concentration and test the room-temperature reaction time. It should be 20-25 sec in order to prevent higher-temperature reactions from occurring too rapidly.

5. Be sure to pour back and forth three times in order to establish a standard mixing technique, ensuring more precise results. The technique should take only 5 or 6 seconds.

Pre-Laboratory Discussion

1. Avoid giving students any preconceived ideas about the effect of concentration on reaction rate (time). Previous student exposure has probably focused on qualitative observations rather than quantitative data.

2. A discussion of the steps in the reaction can be mentioned now, or can be dealt with as part of the post-laboratory activities. It may be sufficient to mention here that formation of blue color indicates presence of iodine molecules. Students may recall the iodine test for starch in previous biology classes.

3. You may find it helpful to demonstrate the reaction to students so that they will know what to expect when they do the activity themselves. This may prevent the loss of first-round results as students become amazed and miss the time of the color change when it finally appears (the “ooh and aah!” factor). If you want them to experience the surprise while carrying it out with their own hands, you may tell them to do a practice run before their timed runs.

Teacher-Student Interaction

Move around the laboratory to monitor the uniformity of the solution mixing process. If you have each team do the 5 ml A + 5 ml H₂O reaction first and record the result on the board, you will be able to detect procedural flaws quickly. For Part II ascertain the water bath temperature to help prevent overheating. Monitor the central supply of chemical substances to prevent contamination.

Ask students what heating is doing to the molecules. Inquire what molarity the diluted A is as they use it. You can anticipate difficulty with the dilution calculation. Inquire what dilution does to the probability of reactant molecules colliding.

Anticipated Student Results

Part I

Post-Laboratory Activities

1. Discuss the results with students. Summarize the laboratory investigation (or have students do it verbally) in terms of relationships between concentration and time (inverse) and concentration and rate (direct), and between temperature and time (inverse) and temperature and rate (direct).

2. Ask students if it is valid to compile results taken over several days for graphing purposes. Discuss problems associated with such a compilation.

3. A computer program such as Vernier’s Graphical Analysis III can be very helpful to students in their search for a linear relationship involving the variables temperature and time, as discussed in answers to Implications and Applications. Functions of the variables can be easily changed within the program with little effort. Consider investigating this as part of class discussion. Results can be displayed on a screen via a computer overhead projection device, or on a large-screen television monitor for the entire class to view simultaneously.

4. Depending on the level of the students, you might want to show them the following equation and discuss the reason for delay in appearance of the iodine-starch complex (blue color):
   
   

Extensions

Consider challenging students to find a linear relationship between various functions of the two variables via computer program or "by hand." This can provide a challenging exercise for more able students.

Assessing Laboratory Learning

1. Ask students to summarize their findings in a short in-class paper.

2. Provide students a sheet detailing the various steps in this reaction (listed in the student laboratory write-up). Ask students to explain why the blue color takes so long to appear. This provides a way to express their understanding of the mechanism of the reaction.

Additional Teacher Resources

Reaction Simulator — a set of computer models simulating the kinetics of various types of reactions, from the University of Southern California. In these models, variables such as reactant concentration may be manipulated in order to observe their effect on the reaction rate. The simulator features animations of reactant molecules and graphical data during simulated reactions.

Relevant National Science Education Standards

Unifying Concepts and Processes — The activity involves studying how parts of a system interact to contribute to its behavior, the measurement of parameters that change when variables are changed, and the drawing of conclusions based on evidence the students themselves have observed.

Physical Science — The activity studies a chemical reaction, and how the interaction of matter and energy (heat) affects the rate of the reaction.

Science in Personal and Social Perspectives — Understanding reaction rates is important to studying the reactions that destroy ozone. Studying the rates of reactions involving potential CFC replacements helps us determine whether they are ozone friendly.

This activity has been adapted from Orna, Mary Virginia, Schreck, James O., and Heikkinen, Henry, editors. SourceBook Version 2.1. New Rochelle, NY: ChemSource, 1998.