Iodine clock experiment between potassium iodide and peroxodisulphate

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Iodine clock reaction experiment between potassium iodide and peroxodisulphate. Please give proper reference to my IB student, Winson if you use his material.

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Iodine clock experiment between potassium iodide and peroxodisulphate

  1. 1. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 2008INVESTIGATING THE RELATIONSHIP BETWEEN CONCENTRATION OFREACTANTS AND RATE OF REACTION USING IODINE CLOCK REACTIONAimTo study the effect of concentration of iodide ion solution on the rate of iodide ion I- oxidation byperoxodisulphate ion S2O82- using iodine clock reactionIntroductionIn an iodine clock reaction, two clear solutions would be added together in a common container where noapparent reaction takes place. After a short delay, at a sudden, the clear solution would turn into a blue-black solution.In this experiment, two clear solutions – potassium iodide, KI and sodium peroxodisulphate, Na2S2O8 –would be added together, with delaying additives, where a blue-black product solution would be observed.The aim of this experiment is to measure the different time taken to form the blue-black solution fordifferent concentration of potassium iodide solution used. The different rates of reaction for eachconcentration can then be determined because they are the reciprocal of the times taken.The chemical reaction that takes place is a redox reaction where iodide ion is oxidized andperoxodisulphate ion is reduced. The full ionic equation for the reaction is represented by: I- (aq) + S2O82- (aq) → I2 (aq) + 2SO42- (aq) * all potassium and sodium ions are spectator ions.However, without any delaying mechanism, the formation of the blue-black starch complex isinstantaneous. In order for the experimenter to be able to measure the time for the formation of the blue-black solution, and by extension rate of reaction, a delaying mechanism needs to be introduced.Sodium thiosulphate would be added, in additional to potassium iodide and sodium peroxodisulphate, toconvert iodine back into iodide as represented by the ionic equation: I2 (aq) + 2S2O32- (aq) → I- (aq) + S4O62- (aq)After all thiosulphate is exhausted, iodine would then be free to form a blue-black starch complex.HypothesisAccording to the Collision Theory1, for a reaction to happen, the two reacting particles involved must:i) collide with one anotherii) the collision must be energetic enough to overcome the activation energy of the reactioniii) the collision must occur with the correct geometrical alignment, i.e it must bring the reactive parts of the molecules into contact in the correct way.1 Source: Green, John & Damji, Sadru. 2001, Chemistry for International Baccalaureate 1
  2. 2. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 2008The theory also states that effective collision can be affected by the concentration of reactants andtemperature. In this experiment, the concentration factor is being studied.In terms of the iodine clock reaction, the concentration of iodide ions would, according to the CollisionTheory, affect the rate of conversion of iodide to iodine. As the concentration decreases, the iodideparticles per volume decrease.Consequently, as the iodide particles per volume decrease, with all other variables remain equal, theprobability of collision between iodide and peroxodisulphate ions, would decrease. By extension, thenumber of effective collisions would also decrease.As the number of effective collisions decrease, the rate of reaction would decrease, signaled by theincrease in time taken for formation of the blue-black solution.The presence of delayer - thiosulphate - would not have a net effect on the reaction because theconcentration of thiosulphate is fixed throughout the experiment.Variablesi) Independent Variable : Concentration of iodide solution (or concentration of KI solution used)ii) Dependent Variable : Time taken for formation of blue-black solution, rate of reactioniii) Fixed Variable : Volume of sodium peroxodisulphate Na2S2O8 solution used, volume of sodium thiosulphate Na2S2O3 solution used, volume of starch solution used, volume of iodide I- solution usedPre-Experiment Preparation / Method DesigningUsing trial and error approach, a pre-experiment exercise was performed to determine the suitable amountof solute and concentration of chemical solutions used. Most notably, the amount of sodium thiosulphateused was adjusted several times to produce a reasonable delay.A trial run was performed to identify possible weaknesses and shortcomings, and thus in the process,design actual experimental procedures.Control of Variables Concentration of iodide solution The concentration of iodide solution is equivalent to the concentration of potassium iodide KI solution since potassium ions are spectator ions. The concentration of iodide solution would be manipulated, acting as independent variable. Using a standard KI 1.0 mol dm-3 solution as stock solution, a dilution will be performed to produce solutions of varying concentrations (0.8, 0.4, 0.2, and 0.1 mol dm-3). Time taken for blue-black solution to form The time taken for the formation of blue-black solution will be measured using a digital stop watch to reduce uncertainty. The time taken is therefore the dependent variable. To reduce random errors, three repetitions will be performed for each set of concentration. 2
  3. 3. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 2008 Rate of reaction The rate of reaction is the reciprocal of time taken for formation of blue-black solution. Since there are three repetitions for each set of concentration, the rate of reaction is the reciprocal of the average time taken. Primarily, this is the dependent variable being studied. Volume of sodium peroxosulphate Na2S2O8 solution used The volume of sodium peroxodisulphate will be fixed at 5 cm3 throughout the experiment. Volume of sodium thiosulphate Na2S2O3 solution used The volume of sodium thiosulphate used will be fixed at 2 cm3 throughout the experiment. Volume of starch solution used The volume of starch solution used will be fixed at 5 drops throughout the experiment. Volume of iodide I- solution used The volume of iodide solution will be fixed at 10 cm3 throughout the experiment. The volume is related to concentration through the equation where M is concentration, V is volume and n is amount of solute. Since the manipulated variable is the concentration of iodide solution, it is imperative that the volume of iodide solution be made constant.Apparatus / MaterialsPipette (25.00 ± 0.03) cm3, pipette (10.00 ± 0.02) cm3, pipette (5.00 ± 0.01 ) cm3, burette (50.00 ± 0.05)cm3, micropipette (1.000 ± 0.003) cm3, digital stop watch (± 0.01s) , electronic balance (± 0.001g),sodium peroxodisulphate Na2S2O8 0.04 mol dm-3 solution, standard potassium iodide KI 1.0 mol dm-3solution, sodium thiosulphate Na2S2O3 0.05 mol dm-3solution, starch solution, test tubes, test tube rack,beakers, volumetric flasks, conical flask, distilled water, retort stand with clamp, pipette filler, spatula,dropperExperimental Proceduresi) Preparation of chemical solutions1. To prepare 1000 cm3 of standard potassium iodide KI 1.0 mol dm-3 solution, 166.000g of KI is needed. However, to prevent wastage, only 83.000g (to 3 d.p) was instead used to produce 500 cm3 of standard potassium iodide KI 1.0 mol dm-3 solution. After weighing, the weighted mass of KI solute was diluted with distilled water to produce 500 cm3 of standard KI 1.0 mol dm-3 solution.2. A dilution was then performed using the standard KI solution as stock solution. For concentration 0.8 mol dm-3, 80 cm3 of stock solution was transferred into a 100 cm3 container using a burette. Using a second burette, 20 cm3 of distilled water was added. For concentration 0.4, 0.2 and 0.1 mol dm-3, the stock to distilled water ratios are 4:6, 2:8, and 1:9 respectively. 3
  4. 4. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 2008*Shading is for visual aid only. Actual solutions are colourless. 1.0 mol dm-3 0.8 mol dm-3 0.4 mol dm-3 0.2 mol dm-3 0.1mol dm-3 Stock solution 8:2 4:6 2:8 1:9 Figure 1.1 Stock to distilled water ratios for different concentrations3. To prepare 250 cm3 of sodium peroxodisulphate Na2S2O8 0.04 mol dm-3 solution, 2.381g of Na2S2O8 was weighed on the balance. After weighing, all the solute was diluted and transferred into a volumetric flask. Distilled water was added drop-wise till the calibration mark.4. To prepare 250 cm3 of sodium thiosulphate Na2S2O30.05 mol dm-3solution, 1.977g of Na2S2O3was weighed on the balance. After weighing, all the solute was diluted and transferred into a volumetric flask. Distilled water was added drop-wise till the calibration mark.ii) The Iodine Clock Reaction1. Three test tubes were labeled A, B, and C respectively.2. Using a pipette (10.00 ± 0.02) cm3, 10 cm3 of standard potassium iodide KI 1.0 mol dm-3 solution was transferred into test tube A.3. Using a pipette (5.00 ± 0.01) cm3, 5cm3 of sodium peroxodisulphate Na2S2O8 0.04 mol dm-3 was transferred into test tube B.4. Using a micropipette (1.000 ± 0.003) cm3, two batches of 1cm3 of sodium thiosulphate Na2S2O3 0.05 mol dm-3 were transferred into test tube C. Sodium thiosulphate acts as a delaying mechanism.5. Solutions from test tube A and C were added together in a conical flask. Then, 5 drops of starch solution were added to allow the formation of blue-black complex in the presence of iodine.6. The mixture in the flask was swirled to ensure a homogenous mixture. As the solution from test tube B was added into the flask, the digital stop watch was started instantaneously.7. The flask was swirled gently as the formation of blue-black complex was observed. Once the blue- black solution was formed, the stop watch was stopped.8. The time taken was recorded in a table. The experiment was then repeated to obtain two more readings.9. Step 1 till 8 were repeated using different concentrations of iodide solution in the order of 0.8, 0.4, 0.2, and 0.1 mol dm-3.CALCULATION10. The average time TAVE was calculated using the formula: AVE .11. The rate of reaction for each concentration was calculated using the formula: 4
  5. 5. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 200812. A graph of rate of reaction against concentration was plotted after an error analysis was conducted. A best fit line was drawn to connect the plots.Data Collection OBSERVATION All solutions in test tube A (KI 1.0 mol dm-3), B (Na2S2O8 0.04 mol dm-3), and C (Na2S2O3 0.05 mol dm-3) was colourless. As solutions of both test tube A and C were added into the conical flask, the mixture remained colourless. After 5 drops of starch solution were added, the mixture resembled rice water. After solution in test tube B was added into the flask, there was no initial reaction observed. For some time, no changes were detected. Suddenly, the mixture in the flask turned into a blue-black solution. The blue-black intensified as time passed by. Concentration of KI, Time for blue-black solution to form, t / s ( ± 0.01s) MKI / mol dm-3 t1 t2 t3 1.0 29.84 28.31 30.21 0.8 38.84 38.32 38.54 0.4 117.44 110.06 116.97 0.2 329.50 338.09 351.97 0.1 1464.38 1585.35 1558.94 Table 1.1 Concentration of iodide solution and time for blue-black solution to formData Processing and Analysis Concentration of KI, tAVE / s / s-1 MKI / mol dm-3 AVE 1.0 29.45 339.6 x 10-4 0.8 38.57 259.3 x 10-4 0.4 114.82 87.09 x 10-4 0.2 339.85 29.42 x 10-4 0.1 1536.22 6.509 x 10-4 Table 1.2 Rates of reaction for different concentrationsInference on observationsWhen potassium iodide (test tube A) was added together with sodium thiosulphate (test tube C) in a flask,no reaction was occurring. Rather, all ions were present in aqueous form, preparing to react withperoxodisulphate ion to be added later. Thus, the mixture remained colourless.The mixture resembled rice water after the addition of starch solution because the starch solution washomogenous with the mixture. Starch solution by itself resembles rice water. Physical diffusion occurredbut no chemical reaction was occurring. 5
  6. 6. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 2008After peroxodisulphate ion (test tube C) was added into the solution, no initial reaction was observed dueto the delaying mechanism in action. As iodide ion is oxidized by peroxodisulphate ion to form iodine,the formed iodine is reduced by thiosulphate ion back into iodide. 2I- + S2O82- → I2 + 2SO42- + starch 2S2O32- + I2 → 2I- + S4O62- Delaying mechanism (reduction of I2)As a result, no iodine is free to react with starch to form blue-black starch complex which forms thedistinct blue-black solution.However, once the thiosulphate is exhausted, iodine is free to react with starch. Thus, there seemed to bea sudden formation of blue-black solution. The blue-black intensified as time passed by due to theincreasing concentration of blue-black complex formed. As time passed by, more free iodine reacts withstarch, thus the blue-black seemed to be more intense.Error AnalysisUncertainty on x -barUncertainty due to dilution of KI Concentration of KI, Uncertainties M / mol dm-3 Volume of 1.000 molar Volume of distilled Total Error for aqueous KI solution / water added / Concentration of KI / mol dm-3 mol dm-3 mol dm-3 1.0000 - - - 0.8000 0.8000 ± 0.0001 = 0.2000 ± 0.0001 = ± 0.0625 % 0.8000 ± 0.0125% 0.2000 ± 0.0500% 0.4000 0.4000 ± 0.0001 = 0.6000 ± 0.0001 = ± 0.0417 % 0.4000 ± 0.0250% 0.6000 ± 0.0167% 0.2000 0.2000 ± 0.0001 = 0.8000 ± 0.0001 = ± 0.0625 % 0.2000 ± 0.0500% 0.8000 ± 0.0125% 0.1000 0.1000 ± 0.0001 = 0.9000 ± 0.0001 = ± 0.1111 % 0.1000 ± 0.1000% 0.9000 ± 0.0111% Table 1.3 Uncertainties due to dilution of KI solution*The uncertainties due to preparation of solutions are minimal and assumed to be negligible.Uncertainty due to pipette = ± x 100 = ± 0.12%Uncertainty due to pipette = ± x 100 = ± 0.20%Uncertainty due to micropipette = ± x 100 = ± 0.30% 6
  7. 7. Candidate Name: Winson Lee Weng Hoe IB Chemistry HL Candidate Number: 002206 007 Internal Assessment Date: March 28th, 2008 Concentration, MKI / Uncert. in MKI, Uncert. in MKI, mol dm-3 ∆MKI (mol dm-3) ∆MKI (%) 1.0 0.0000 0.0000 0.8 0.0005 0.0625 0.4 0.0002 0.0417 0.2 0.0001 0.0625 0.1 0.0001 0.1111 Table 1.4 X-bar uncertainties Uncertainty in y - barConcentration, Uncert. in TAVE Uncert. in TAVE MKI / TAVE / s Uncert. in T (%) Uncert. in T (x 10-4 s -1) ∆T (s) ∆T (%) AVE AVE mol dm-3 1.0 29.45 0.01 0.03396 0.03396 0.11533 0.8 38.57 0.01 0.02593 0.02593 0.06724 0.4 114.82 0.01 0.00871 0.00871 0.00759 0.2 339.85 0.01 0.00294 0.00294 0.00086 0.1 1536.22 0.01 0.00065 0.00065 0.00004 Table 1.5 Y-bar uncertainties 7
  8. 8. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 2008The graph of rate of reaction against concentration is plotted to determine the relation between the two variables. A best fit line is then constructed torelate the x-bar and the y-bar. The proposed model and R-squared value of the model, which is a measure of correlation between the two variables, arealso attached. Both x and y-error bars are also set according to the uncertainties we have processed, though on the graph they look slightly small. Rate of reaction, against concentration of KI 400 350 y = 379.0x - 45.15 R² = 0.993 300 Rate of reaction / x 10-4 s-1 250 200 150 100 50 0 0 0.2 0.4 0.6 0.8 1 1.2 -50 Concentration of KI, MKI / mol dm-3 8
  9. 9. Candidate Name: Winson Lee Weng Hoe IB Chemistry HLCandidate Number: 002206 007 Internal AssessmentDate: March 28th, 2008From the graph, it seems there exists a linear relationship between the concentration factor and the rate ofreaction. The R-squared value of the model is at R2 = 0.993, a relatively high value that signals that only asmall residual variability remains unexplained. In order words, the linear relationship model between thetwo variables is statistically very strong.Conclusion and evaluationBased on the experimental findings, it can thus be concluded that it is very likely that the concentrationfactor is affecting the rate of reaction of the iodide oxidation linearly. Based on the graph, as theconcentration of the iodide solution increases, the rate of reaction of iodide oxidation increases too. Thislinear relationship only applies to the named reaction in this experiment; other named reaction may ormay not behave the same.These findings support our hypothesis that the concentration factor does affect the number of effectivecollisions and by extension, rate of reaction.As the concentration of the iodide solution increases, the number of particles per volume increases whichincreases the probability of collision. Given the higher probability in collision, the number of effectivecollisions – those with sufficient activation energy and minimal steric stress – increases. In effect, the rateof reaction also increases proportionally.Possible sources of errorThroughout the experiment, very precise instruments were used. With greater apparatus accuracy, andhigher precision, the total uncertainty for the experiment is very low. Nevertheless, there may be someerror introduced during:i) the prepared iodide solutions was kept overnight in 100cm3 air-tight containers since the experiment cannot be completed in a single day. On the next day where the experiment commenced, some solutions in their containers was slightly yellowish suggesting the presence of iodine. That indicated the iodide solution may have been oxidized by oxygen since there was a small volume of air stored in each container.ii) all the solutions involved were colourless; the solutions may have been contaminated during the transfer and addition of solutions. If contaminants were introduced into the solutions, it is very difficult for the experimenter to take note since all solutions are colourless.Suggestions to improve future experimentsi) the experiment should be conducted in batches. In view that the experiment may not be completed in one single go, dilution of KI solutions should be done prior to the iodine clock reaction. Overnight storage of iodide solution should be avoided.ReferenceGreen, John & Damji, Sadru. 2001, Chemistry for International Baccalaureate, 2nd Edition, IBID Press, Victoria, Australia.IB Data Booklet. Table 5. 9

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