A New Twist on the Iodine Clock Reaction: Determining the Order of a Reaction
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In the Classroom 530 Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu A New Twist on the Iodine Clock Reaction: Determining the Order of a Reaction Xavier Creary and Karen M. Morris* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556 Over the years chemical educators have investigated, discussed, and used clock reactions in their classrooms to illustrate chemical kinetics (1–3). Other chemical educators have experimented with the Landolt iodine clock reaction, and their results have often been published in this Journal (4, 5). The Landolt iodine clock reaction is a reliable and well-used chemistry demonstration owing to a variety of features: ease of solution preparation, striking color change indicating reaction completion, convenience in changing reaction “clock time”, and effective, dramatic presentation. The iodine clock reaction can also be used to illustrate the kinetic order of a reaction, and an overhead projector dem- onstration was developed three years ago for General Chem- istry classes at the University of Notre Dame showing this concept. This demonstration has been used successfully with consistent results since that time. Demonstration Preparation The solution preparation for this demonstration is slightly different from other preparations (refer to 1, 3, 4 ) because sodium metabisulfite (Na 2 S 2 O 5 ) is used instead of sodium bisulfite (NaHSO 3 ). Metabisulfite serves as a source of HSO 3 { according to the equation S 2 O 5 2{ (aq) + H 2 O(,) → HSO 3 { (aq) Since the iodine clock reaction depends on the reducing properties of the bisulfite ion, using sodium metabisulfite is equivalent to the use of sodium bisulfite. If possible, the prepared solutions should be used promptly, since solutions containing bisulfite ions are subject to air oxidation. Addi- tionally, although the bisulfite solution is dilute, gaseous sulfur dioxide can also be evolved, since sulfuric acid is added to this solution (1). Keeping the bisulfite solution storage flask tightly capped will reduce exposure to atmospheric oxygen and reduce loss of gaseous SO 2 . To prepare solution A, dissolve 4.30 g of KIO 3 in 200 mL of water in a 1-L volumetric flask. Add enough water to the flask to make 1 L of solution. Stopper and label the flask. To prepare solution B, heat 300 mL of water to boiling in a 1-L beaker (a hot-plate–stirrer works well). Dissolve 4.0 g of soluble starch in the water; the resulting solution should be translucent. Dilute the starch solution with water to a volume of 900 mL and allow it to cool. Dissolve 0.20 g of sodium metabisulfite (Na 2 S 2 O 5 ) in the cooled starch solution. Pour the solution into a 1-L volumetric flask. Add 0.49 g of concentrated sulfuric acid (H 2 SO 4 ) to the solution in the flask, stopper the flask, and mix the contents. Add enough water to the flask to make 1 L of solution, tightly stopper the flask to reduce exposure to atmospheric oxygen (use stopcock grease if necessary), and mix the contents. Label flask. Demonstration Setup Obtain four glass, 90 × 50-mm crystallizing dishes (250- mL beakers may also be used; however the crystallizing dishes work best on the overhead projector). Label the dishes on their sides, consecutively, from one to four. Using a 25-mL graduated cylinder, measure the amount of solution A listed in Table 1 for dish 1 and add enough water to make the total solution volume 25 mL. Repeat for the other dishes using the volumes of solution A and water in Table 1. Just before the demonstration, fill four 25-mL graduated cylinders with solution B to the 25-mL mark. Place the four dishes containing the different concentrations of solution A and the graduated cylinders containing solution B near an overhead projector. Place a glass stirring rod nearby as well. A timer or stopwatch capable of timing to tenths of seconds will also be necessary to do this demonstration. Doing the Demonstration Place the first dish containing solution A on the over- head projector and turn on the projector so that the image is displayed. Very quickly add solution B to solution A on the overhead and stir briefly to make a homogenous solution. Start timing the reaction as soon as solution B is added. Record the time it takes for the solution to turn completely blue. Repeat this process for the remaining dishes. After the reaction times have been collected, the data must be converted to reaction rates because a plot of ln rate vs ln [IO 3 { ] is needed to determine the reaction order (see Discussion section). The method of initial rates, where rate = ∆[HSO 3 { ]/∆t, is used because the [HSO 3 { ] is very small and constant (10 {4 M). HSO 3 { is also assumed to react completely (6 ). The rate values, therefore, are determined for each reaction in which rate = 10 {4 mol L {1 /reaction time. Overhead Projector Demonstrations edited by Doris K. Kolb Bradley University Peoria, IL 61625 *Email: [email protected].
Transcript of A New Twist on the Iodine Clock Reaction: Determining the Order of a Reaction
In the Classroom
530 Journal of Chemical Education • Vol. 76 No. 4 April 1999 • JChemEd.chem.wisc.edu
A New Twist on the Iodine Clock Reaction:Determining the Order of a Reaction
Xavier Creary and Karen M. Morris*Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556
Over the years chemical educators have investigated,discussed, and used clock reactions in their classrooms toillustrate chemical kinetics (1–3). Other chemical educatorshave experimented with the Landolt iodine clock reaction,and their results have often been published in this Journal(4, 5). The Landolt iodine clock reaction is a reliable andwell-used chemistry demonstration owing to a variety offeatures: ease of solution preparation, striking color changeindicating reaction completion, convenience in changingreaction “clock time”, and effective, dramatic presentation.The iodine clock reaction can also be used to illustrate thekinetic order of a reaction, and an overhead projector dem-onstration was developed three years ago for General Chem-istry classes at the University of Notre Dame showing thisconcept. This demonstration has been used successfully withconsistent results since that time.
Demonstration Preparation
The solution preparation for this demonstration isslightly different from other preparations (refer to 1, 3, 4 )because sodium metabisulfite (Na2S2O5) is used instead ofsodium bisulfite (NaHSO3). Metabisulfite serves as a sourceof HSO3
{ according to the equation
S2O52{(aq) + H2O(,) → HSO3
{(aq)
Since the iodine clock reaction depends on the reducingproperties of the bisulfite ion, using sodium metabisulfiteis equivalent to the use of sodium bisulfite. If possible, theprepared solutions should be used promptly, since solutionscontaining bisulfite ions are subject to air oxidation. Addi-tionally, although the bisulfite solution is dilute, gaseous sulfurdioxide can also be evolved, since sulfuric acid is added tothis solution (1). Keeping the bisulfite solution storage flasktightly capped will reduce exposure to atmospheric oxygenand reduce loss of gaseous SO2.
To prepare solution A, dissolve 4.30 g of KIO3 in 200mL of water in a 1-L volumetric flask. Add enough water tothe flask to make 1 L of solution. Stopper and label the flask.
To prepare solution B, heat 300 mL of water to boiling ina 1-L beaker (a hot-plate–stirrer works well). Dissolve 4.0 gof soluble starch in the water; the resulting solution shouldbe translucent. Dilute the starch solution with water to a
volume of 900 mL and allow it to cool. Dissolve 0.20 g ofsodium metabisulfite (Na2S2O5) in the cooled starch solution.Pour the solution into a 1-L volumetric flask. Add 0.49 g ofconcentrated sulfuric acid (H2SO4) to the solution in the flask,stopper the flask, and mix the contents. Add enough waterto the flask to make 1 L of solution, tightly stopper the flaskto reduce exposure to atmospheric oxygen (use stopcockgrease if necessary), and mix the contents. Label flask.
Demonstration Setup
Obtain four glass, 90 × 50-mm crystallizing dishes (250-mL beakers may also be used; however the crystallizing disheswork best on the overhead projector). Label the dishes ontheir sides, consecutively, from one to four. Using a 25-mLgraduated cylinder, measure the amount of solution A listedin Table 1 for dish 1 and add enough water to make the totalsolution volume 25 mL. Repeat for the other dishes usingthe volumes of solution A and water in Table 1.
Just before the demonstration, fill four 25-mL graduatedcylinders with solution B to the 25-mL mark. Place the fourdishes containing the different concentrations of solution Aand the graduated cylinders containing solution B near anoverhead projector. Place a glass stirring rod nearby as well.A timer or stopwatch capable of timing to tenths of secondswill also be necessary to do this demonstration.
Doing the Demonstration
Place the first dish containing solution A on the over-head projector and turn on the projector so that the image isdisplayed. Very quickly add solution B to solution A on theoverhead and stir briefly to make a homogenous solution.Start timing the reaction as soon as solution B is added. Recordthe time it takes for the solution to turn completely blue.Repeat this process for the remaining dishes.
After the reaction times have been collected, the datamust be converted to reaction rates because a plot of ln ratevs ln [IO3
{] is needed to determine the reaction order (seeDiscussion section). The method of initial rates, where rate =∆[HSO3
{]/∆t, is used because the [HSO3{] is very small and
constant (10{4 M). HSO3{ is also assumed to react completely
(6 ). The rate values, therefore, are determined for each reactionin which rate = 10{4 mol L{1/reaction time.
Overhead Projector Demonstrations edited byDoris K. Kolb
Bradley UniversityPeoria, IL 61625
*Email: [email protected].
In the Classroom
JChemEd.chem.wisc.edu • Vol. 76 No. 4 April 1999 • Journal of Chemical Education 531
Disposal and Safety
Since the bisulfite solution is acidic, some gaseous sulfurdioxide can be released, which can irritate the respiratorysystem. CAUTION: People hypersensitive to sulfites should avoiddirect contact with the solutions in this demonstration (1).
To dispose of the products of this demonstration, combinethe deep blue solutions in a large beaker. While stirring themixture, slowly add solid sodium thiosulfate (Na2S2O3?5H2O)until the mixture is no longer blue. Flush the mixture downthe drain with large quantities of water (1).
Discussion
The concentration of the bisulfite solution is very smallwhen compared to the iodate solution ([bisulfite] ~10{4 M;[IO3
{] = 0.004–0.016 M). Since early studies of the Landoltreaction indicated that the reaction is first order in [IO3
{] andfirst order in [HSO3
{] (1) and since the [IO3{] doesn’t change
appreciably during the course of the reaction, the rate expres-sion is pseudo-first-order and can be written as rate = kobs[IO3
{]1
(6 ). Since students do not know the reaction order, the rateexpression for the reaction is written as rate = kobs[IO3
{]n. Takingthe natural logarithm of both sides gives ln rate = ln(kobs[IO3
{]n),which is converted to ln rate = n ln[IO3
{] + ln kobs, a straight-line equation with slope = n (order of reaction). Using class-room data, therefore, the order of reaction can be determinedby plotting ln rate vs ln[IO3
{]. Refer to the graphing examplebelow (Table 2, Fig. 1).
Literature Cited
1. Shakhashiri, B. Chemical Demonstrations; The University of Wis-consin Press: Madison, WI, 1992; Vol. 4, pp 3–25.
2. Lambert, J. L.; Fina, G. T. J. Chem. Educ. 1984, 61, 1037–1038.3. Tested Demonstrations in Chemistry; Alyea, H. N.; Dutton, F. B.,
Eds.; Journal of Chemical Education: Easton, PA, 1965; p 19.4. Autuoir, M. A.; Brolo, A. G.; Mateus, Al. L. J. Chem. Educ. 1989,
66, 852–853.5. Brice, L. K. J. Chem. Educ. 1980, 57, 152.6. Bromberg, J. P. Physical Chemistry, 2nd ed.; Allyn and Bacon:
Newton, MA, 1984; pp 890–892.
Figure 1. Order of reaction for the iodine clock. Slope = n =1.00;r = .998.
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.oNhsiD AnoituloSLm/loV
retaWLm/loV OI[ 3
{]
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7991gnirpS,yrtsimehClareneGmorfataD.2elbaTOI[ 3
{ /]Llom {1
OSH[ 3{ /]
Llom {1/emiT
s/etaR
Llom( {1 s {1) nL OI[ 3{ ] etaRnL
400.0 01 {4 65 01 {4 65/ { 125.5 { 632.31
800.0 01 {4 62 01 {4 62/ { 828.4 { 864.21
210.0 01 {4 81 01 {4 81/ { 324.4 { 101.21
610.0 01 {4 41 01 {4 41/ { 531.4 { 948.11
