Clocking a Reaction
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        Introduction
        Background
        Purpose
        Safety
        Materials and Apparatus
        Procedure
        Data Analysis and Concept Development
        Implications and Applications

      Introduction

      Every one of the scientists we have met in our exploration of the chemistry of ozone depletion has been very concerned with the chemical reactions that destroy ozone. A big part of studying these chemical reactions is learning how fast they take place, that is, their reaction rates.

      This matters because we need to know how fast a particular compound might destroy the ozone that protects us from the sun's ultraviolet radiation. Ozone is regenerated in the atmosphere at a certain rate, and in order to keep ozone levels up it is important that the compounds we release into the atmosphere don't destroy ozone faster than it can be regenerated. Scientists like Susan Solomon have investigated how atmospheric conditions over Antarctica accelerate the rate of ozone depletion. Others like Joseph Francisco have studied the rates at which potential CFC replacements break down in the lower atmosphere, because if they break down quickly, they are less likely to linger in the air, rise to the stratosphere, and damage ozone.

      Chemical reactions can be fast, or they can be slow. When paper burns, a chemical reaction is taking place at a very fast rate. Meanwhile, when iron reacts with oxygen to form rust, the chemical reaction normally takes place very slowly. The study of how fast something happens, and just how and why it happens as fast or slow as it does, is called kinetics.

      Background

      Various factors affect reaction rates. Concentration and temperature are two such factors that will be explored in this laboratory activity. You will investigate a chemical reaction called a clock reaction to determine the effect of each of these two factors on reaction rate. Part I deals with concentration changes of one reactant. Part II involves temperature changes. You will carefully control other important variables as the activity is performed.

      This clock reaction involves mixing of the two solutions. Solution A is a dilute solution of potassium iodate, KIO3. This solution is the source of iodate ions, IO3-. Solution B contains soluble starch and the second reacting species, hydrogen sulfite ions, HSO3-. The initial step of the reaction is represented by the equation

      reaction of iodate ion with bisulfite ion

      When hydrogen sulfite ions, HSO3-, are used up, iodide ions, I-, react with remaining iodate ions, IO3-, to produce iodine, I2:

      reaction of iodate ion with iodide ion

      Molecular iodine (I2) forms a blue substance÷an iodine-starch complex÷in the presence of starch from Solution B. The formation of blue color thus indicates that the reaction has proceeded to this point:

      complexation of iodine with starch

      The iodine starch complex involves a b-amylase-starch cage containing I2, I-, and H2O in a three-dimensional structure inside the cage.

      Purpose

      To investigate concentration and temperature as two factors affecting the rate of chemical reactions.

      Safety

      1. Wear protective goggles throughout the laboratory activity.

      2. Heating of beakers of water over an open flame is always potentially hazardous; burners should be handled with care.

      3. Dispose of the used materials as your teacher directs.

        General Safety Guidelines

      Materials and Apparatus

      • Solution A (prepared by your teacher)
      • Solution B (prepared by your teacher)
      • Test tubes (18 × 150 mm)
      • 10-ml Graduated cylinder
      • 250-ml beaker
      • Rubber stoppers

      Procedure

      Part I

      To investigate the effect of changing concentration of one reacting species on reaction time, you will prepare dilutions of Solution A, in which iodate ion, IO3ö, concentration will vary. The concentration of the other reacting species, hydrogen sulfate ion, HSO3ö, contained in Solution B, will be held constant. Since temperature change is not a consideration in this part of the laboratory activity, all solutions should be kept at room temperature throughout. Your teacher will assign each laboratory group several concentrations to investigate. By exchanging data with other groups at the end of the activity, all laboratory groups will receive all the data. From these data you should be able to draw conclusions concerning the effect of concentration on reaction time.

      1. Use a clean graduated cylinder to measure 10.0 ml Solution A; pour it into a clean test tube (18 × 150 mm). Rinse the graduated cylinder. In a similar manner place 10.0 ml Solution B into another test-tube. If the solutions have been in the laboratory for some time you may assume that they are at room temperature. Otherwise, place the two test-tubes containing solutions into a 250-ml beaker about two-thirds full of water at room temperature and let them stand for several minutes.

      2. Record the time to the nearest second as you pour Solution A into Solution B and then pour the mixture back and forth quickly three times to obtain uniform mixing. Alternately, stopper the test-tube and invert to mix. Time should be recorded from the first instant the two solutions are in contact.

      3. Observe the solution in the tube carefully and record the time (to the nearest second) corresponding to the first sign of a reaction.

      4. Repeat this procedure to check your results if so directed by your teacher.

      5. Prepare different concentrations of the KIO3 solution by diluting Solution A as specified below. Do as many dilutions as specified by your teacher.

        ml Solution A
        ml distilled water
        9.0
        1.0
        8.0
        2.0
        7.0
        3.0
        etc.
        etc.

        Note that the total volume is always 10.0 ml. Thoroughly mix each diluted solution (Solution A and distilled water) prior to allowing it to react with Solution B.

      6. Repeat Steps 1-4 by adding one of the diluted solutions of KIO3 to 10.0 ml Solution B, both at room temperature.

      Part II

      To investigate the effect of temperature on this reaction's rate you will determine the time for reaction at room temperature and at other temperatures within a range of 20°C of room temperature. Your teacher will assign your laboratory group particular temperatures to investigate. By exchanging data with other groups, you will be able to draw conclusions regarding the effect of temperature on reaction time.

      1. Pour 10.0 ml Solution A (labeled for Part II) into one test tube (18 x 100 mm) and 10.0 ml Solution B into another. These solutions must be brought to the desired temperature before they are mixed. Put both tubes into a 250 ml beaker about two-thirds full of water at the temperature you were assigned to investigate. Let them stand for about 10 min so the solutions come to the temperature of the water bath.

      2. Record the time to the nearest second as you pour Solution A into Solution B and then pour the mixture back and forth quickly three times to obtain uniform mixing. Time should be recorded from the instant both solutions are first in contact.

      3. Place the test-tube back in the water bath. Observe carefully. Record the time (to the nearest second) corresponding to the first sign of a reaction.

      4. Repeat the procedure at the same temperature to check your results if so directed by your teacher.

      Data Analysis and Concept Development

      Part I

      1. The concentration of potassium iodate in prepared Solution A is 0.020 M. Calculate the number of moles of KIO3 contained in each milliliter of Solution A.

      2. Calculate the initial molar concentration of KIO3 in each mixture of A and B prepared in Part I.

      3. Prepare a graph of your concentration-time data, with time on the vertical axis (ordinate) and concentration of KIO3 on the horizontal axis (abscissa). Include the data of all other laboratory groups in your plot.

      Part II

      1. Prepare a graph of your temperature-time data, with temperature on the horizontal axis (abscissa) and time on the vertical axis (ordinate).

      Implications and Applications

      Part I

      1. Why is it important to maintain the total volume at 10.0 ml during the dilutions of Solution A?

      2. What generalizations can you make concerning the effect of varying the concentration on the time of the reaction?

      3. How is the time of a reaction related to the rate of the reaction?

      Part II

      1. What general relationships can you derive from your temperature-time graph?

      2. Predict the time of reaction at 0°C and at 50°C, assuming other variables in the laboratory procedure are kept constant. If possible, test your prediction experimentally.

        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.
      Copyright ©2001 The Chemical Heritage Foundation