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Factors Affecting Reaction Rates

Description of Activity

Title: Factors Affecting Reaction Rates

Overview: Working in pairs, students will investigate the effects of temperature and concentration on the rate of a chemical reaction between the iodate ion (I03-) and hydrogen sulfite ion (HSO3-). This is a clock reaction with a certain amount of time elapsing before the reaction reaches completion. The end of the reaction is signaled by a sudden color change as I2 combines with starch to give a blue-black iodine-starch complex. Students will graph their results and use the graphs to predict other concentration or temperature rates.

This lab should take two laboratory periods to complete.

Materials:

Solution A--.02 M solution KIO3 (Dissolve 4.3 g of potassium iodate per liter solution)
Solution B--Dissolve 2 g sodium bisulfite (NaHSO3), 5 mL 1.0 M H2SO4, and 4 g soluble starch per liter of solution
Distilled water
Ice Cubes
Hot water
Tap water
Hot plate or Bunsen burner
Thermometer
Timer

Micro-scale:
96 well micro-plate
micro-tip pipets

Macro-scale:
Beakers
Graduated cylinders
Test Tubes

Background

The rate of a chemical reaction is the speed at which reactants are converted to products. Some reactions are very fast and some are very slow. In order for a chemical reaction to occur, particles of the reactants involved must collide with one another at the correct angle and with the correct amount of energy. The factors which affect the rate of a reaction are the surface area, nature of the reactants, concentration, temperature, and catalysts.

In this experiment, two solutions will be mixed, and the completion of the reaction will be marked by a color change. One solution contains the iodate ion (IO3-). The other contains the hydrogen sulfite ion (HSO3-) and soluble starch. The entire reaction takes place in two stages. The ionic equations for these stages are:

IO3- + 3 HSO3- --> I- + 3 SO42- + 3 H+

5 I- + 6 H+ + IO3- --> 3 I2 + 3 H2O

In the presence of starch molecules, molecular iodine produces a characteristic blue color. The rate of the entire reaction can be determined by timing the interval between the time the two solutions are mixed and the appearance of the blue color. By varying the concentration of one of the reactants (at constant temperature) and then varying the temperature alone, you can observe and record the effects of these two factors on reaction rate.

Teaching and Learning Strategies

Check the solutions before the students use them. If the reaction is occurring too fast, dilute the KIO3 solution. If it is too slow, add another drop of acid or additional NaHSO3. The skill development procedure suggested should take about 15 seconds.

When adjusting the temperature of the solutions the reaction at 0C is very slow and difficult to observe. At temperatures above 40C the HSO3- ion concentration is decreased so much that misleading results are obtained. Above 50C the iodine starch complex is unstable. You may wish to warn students of these problems or you may want the students to deal with them.

The most common problem occurs when students don't thoroughly mix the reactants.

If using the microscale methods, please note that the microplates float on water.

Prerequisite instruction:

A working definition of reaction rate as a change in concentration divided by time or as a function of time is necessary.

Students must be familiar with the dilution of solutions.

Safe Operating Procedures:

Safety goggles and a lab apron must be worn for this experiment. Dispose of all chemicals appropriately. See Flinn Chemical Catalog.

Skills Development Lab

Measuring Reaction Rate--Micro-scale Procedure

1. Fill one micro-tip pipet with the KIO3 solution. Fill the second pipet with the HSO3-/starch solution. Another pipet can be used for mixing the solutions.
2. Add 10 drops of KIO3 solution to well A1.
3. Add 10 drops of the HSO3-/starch solution to well B1.
4. Draw up the solution from well A1 into the mixing pipet. Be ready to start timing.
5. Add the A1 solution to well B1. Start timing the instant the solutions mix. Keep timing but immediately draw up the combined mixture in well B1 into the pipet and then return it to well B1.
6. Observe the mixture in well B1 until a change takes place. Record the elapsed time.
7. Rinse the mixing pipet with distilled water.
8. Repeat the procedure until you achieve a consistent time +/- 2 seconds. You must complete at least 3 trials.
9. Discard the solutions as directed by your teacher.

Measuring Reaction Rate--Macro-scale Procedure

1. Using a clean, dry, 10-mL graduated cylinder, measure exactly 10.0 mL of the IO3- solution (A) and pour it into a 100 mL beaker.
2. Using a second 10 mL graduate, measure exactly 10.0 mL of the HSO3- solution (B) and pour it into a second 100 mL beaker.
3. Prepare to time the reaction. While one lab partner pours Solution A into Solution B, the second partner should immediately start timing the reaction. Pour the solutions back and forth several times from one beaker to the other to ensure thorough mixing. Then allow the mixture to stand. At the instant a color change occurs, the partner timing the reaction should note the elapsed time. Record this in your data list. Rinse and dry the beakers and graduated cylinders.
4. Repeat the procedure until you achieve a consistent time +/- 2 seconds. You must complete at least 3 trials.
5. Discard the solutions as directed by your teacher.

Invitation to Learn

Now that you know how to measure reaction rate how does temperature and concentration affect that rate? Design and carry out an experiment which will answer these questions. Test at least 5 different temperatures and 5 different dilutions.

Assessment of Learning:

The student lab report should include data tables, graphs, and appropriate conclusions.

Suggested Evaluation Tools

1. Graph each of the relationships you tested.
2. Describe each of the graphs. Do they show linear, hyperbolic, or parabolic relationships? Do they increase or decrease regularly?
3. From each graph, give a one sentence conclusion that explains how temperature and concentration affect reaction rate. Make a generalization.
4. Based on the information you collected, why must bakers carefully control the amount of levening agent added to breads and pasteries?
5. Based on the information that you collected in the laboratory, why is a car harder to start in the winter than in the summer?

Alternative Evaluation Tools

1. Based on the graph you constructed, the time for the reaction to be completed at 18¡C should be approximately
A. 5 seconds.
B. 11 seconds.
C. 14 seconds.
D. 19 seconds.

2. What conclusions can be drawn about the relationship between concentration of the reactants and the rate of the reaction?
A. There is an inverse relationship between the concentration of the reactants and the rate of the reaction.
B. There is a direct relationship between the concentration of the reactants and the rate of the reaction.
C. The rate of the reaction varies exponentially with the concentration of the reactants.
D. It is not possible to determine the relationship between concentration and rate from the data given.

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This document was submitted for posting to the Internet by the State Science Specialist. Any questions concerning content should be directed to that individual.

Updated September 26 1997 by Michelle Dumas