Iodine clock reaction experiment using potassium iodide and hydrogen peroxide

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How changing potassium iodide concentration affect the rate of iodine clock experiment, a reaction between hydrogen peroxide and potassium iodide. …

How changing potassium iodide concentration affect the rate of iodine clock experiment, a reaction between hydrogen peroxide and potassium iodide.
Please give proper reference to my IB student, Eileen if you use her material.

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  • 1. 2206-008 Chemistry HLCandidate Name : Eileen Cham Yee LinCandidate Number : 2206-008Date of Practical : 4th May 2009 Practical Assessment 22 – Iodine clock reactionResearch QuestionHow does the concentration of a reactant (manipulated by using different concentrations of potassiumiodide solution) affect its rate of reaction with another reactant (hydrogen peroxide solution, of a fixedconcentration and volume), which is calculated as the inverse of the time measured for the triiodide ionformation from the reaction to completely obscure the “X” mark at the bottom of a flask?IntroductionThere are a few factors that affect the rate of reaction: temperature, reactant concentration, reactantsurface area, pressure, the presence of catalyst and the physical states of reactants. This experiment willinvestigate the effect of reactant concentration on the rate of reaction in the reaction between potassiumiodide and hydrogen peroxide. The hydrogen peroxide concentration is fixed while the concentration ofpotassium iodide is manipulated; the formation of the product – triiodide ions can be detected with theblue-black starch complex. Because both of these reactants react quickly with each other, sodiumthiosulphate solution is used as a delaying mechanism to delay the formation triiodide ions by reactingwith the triiodide ions to reform more iodide ions. When the sodium thiosulphate is exhausted, remainingtriiodide ions form a blue-black starch complex with the starch present in the solution.The chemical equations involved in this reaction are: H2O2 (aq) + 3 I- (aq) + 2 H+ (aq) → I3- (aq) + 2 H2O (l) I3- (aq) + 2 S2O32- (aq) → 3 I- (aq) + S4O62- (aq)A stopwatch is used to measure the time taken for the triiodide ions to completely obscure a mark “X” atthe bottom of the flask; the times recorded are then inversed and the rate of reactions for individual runscan be found.This experiment is also known as the Iodine Clock Experiment. 1
  • 2. 2206-008 Chemistry HLHypothesisThe collision theory states effective collisions between reactant molecules must occur in order for thereaction to occur. The more reactant particles present, the more frequently they collide effectively, and themore often the reaction occurs to form products. Therefore, the increase in reactant concentration willincrease the rate of reaction. Rate of reaction  frequency of effective collision  concentration of reactants In this experiment, the concentration of one of the reactant, which is potassium iodide, is increased; thismeans that there will be more potassium iodide available to effectively collide with hydrogen peroxide asthe concentration increases.As such, the hypothesis for this experiment is that the rate of reaction will increase with the concentrationof potassium iodide.The expected graph from this experiment is as follows: Rate of reaction Concentration of potassium iodide Figure 1: Expected graph of rate of reaction against concentration of potassium iodideVariablesDependent variable : Rate of reaction – This can be determined by measuring the time taken for a mark “X” to be completely obscured by the blue-black of triiodide – starch complex. The inverse of the time taken is the rate of reaction.Independent variable : Concentration of potassium iodide – 0.1 mol dm-3 potassium iodide solution is prepared. The solution is then diluted to form potassium iodide solutions of different concentrations using dilution technique. 2
  • 3. 2206-008 Chemistry HLControlled variables : 1. Amount of hydrogen peroxide solution – To ensure that the same amount of hydrogen peroxide is used in the reaction, the same volume of the same concentration of hydrogen peroxide solution is reacted with potassium iodide solution. – However, hydrogen peroxide solution has to be in excess; therefore, a solution with high concentration (6 %) is used. 2. Amount of starch – Starch is the indicator of the presence of triiodide, the final product of the iodine clock reaction. – The same volume of homogenous starch should be used in the reaction as to ensure that the formation of black-blue colour is standard for all reactions. 3. pH or H+ concentration – Low pH or high H+ concentration causes the reactants to react more quickly. – The same volume of the same concentration of dilute hydrochloric acid should be used to acidify the hydrogen peroxide solution. 4. Temperature – Reactants react more quickly at high temperatures. – All iodine clock runs are carried out at room temperature.Apparatus and materials  6 % hydrogen peroxide, H2O2 solution  0.025 mol dm-3 potassium iodide, KI powder  Sodium thiosulphate, NaS2O3 solution  Starch solution  Dilute hydrochloric acid, HCl solution  Digital stopwatch  (10.00 ± 0.02) cm3 pipette  250 cm3 conical flask  (100.0 ± 0.1) cm3 volumetric flask.  Pipette filler 3
  • 4. 2206-008 Chemistry HL  Dropper  White tile  Beakers  Micropipette  Wash bottle  Measuring cylinder  Electronic balance, correct to 0.001 gProcedure 1. 100 cm3 of 0.1 mol dm-3 potassium iodide solution is prepared by dissolving 1.66 g of potassium iodide powder into 100 cm3 of distilled water in a volumetric flask. 2. 30 cm3 stocks of potassium iodide solutions of different concentration are prepared using the dilution technique in measuring cylinder. A micropipette is used when necessary. Volume of 0.1 mol dm-3 Volume of distilled water Concentration of potassium potassium iodide solution added / cm3 iodide solution, Mpotassium iodide measured / cm3 (± 0.4 cm3) / cm3 (± 0.4 cm3) 30 0 0.10 15.75 14.25 0.075 10.5 19.5 0.050 5.25 14.75 0.025 2.1 27.9 0.010 Table 1: Volumes required for the preparation of potassium iodide solutions of different concentrations 3. 10 ml of 6 % hydrogen peroxide, H2O2 solution is pipette into one conical flask. 6 drops of dilute hydrochloric acid is added to acidify it. The conical flask is labeled “A”. 4. In another conical flask, 7 ml of 0.10 mol dm-3 of potassium iodide solution is measured with a measuring cylinder, and poured into it. 10 ml of starch, and 1 ml of 0.025 mol dm-3 of sodium thiosulphate solution are also poured into the conical flask. The conical flask is labeled “B” and placed on a white tile with a mark “x”. 5. The contents of conical flask “A” are emptied into conical flask “B”, and the stopwatch is immediately started. The “x” is viewed through the top of the conical flask “B”. The stopwatch is immediately stopped when the “x” is obscured by the dark-blue formation in the solution in conical flask. The time taken is recorded. 6. Steps 3 – 5 are repeated 2 times to obtain 2 more replicates. 7. Steps 3 – 6 are repeated with different concentrations: 0.075 mol dm-3, 0.05 mol dm-3, 0.0025 mol dm-3 and 0.001 mol dm-3. 4
  • 5. 2206-008 Chemistry HLData collection – quantitative data Concentration of Time taken for “x” to be obscured, t’ potassium iodide /s solution, Mpotassium iodide / (± 0.01) cm3 Run 1 Run 2 Run 3 0.10 5.28 4.75 4.47 0.075 6.25 6.62 8.75 0.050 17.75 15.13 14.94 0.025 36.00 28.12 24.75 0.010 45.63 42.81 52.00 Table 2: Collected dataData collection – qualitative dataThe colourless solution in conical flask “B” turns dark blue at the end of each run.Data processingThe average time taken for each potassium iodide solution concentration is calculated: Concentration of potassium Average time taken, t 1 Rate of reaction, iodide solution, Mpotassium iodide /s / cm3 (± 0.01) / s-1 5.28+4.75+4.47 1 0.10 = 4.83 4.83 = 0.207 3 5.28+4.75+4.47 1 0.075 = 7.21 7.21 = 0.139 3 6.25+6.62+8.75 1 0.050 = 15.94 15.94 = 0.063 3 36.00+28.12+24.75 1 0.025 = 29.61 = 0.034 3 29.61 45.63+42.81+52.00 1 0.010 = 46.81 46.81 = 0.021 3 Table 3: Calculation of the average time taken and rate of reaction 5
  • 6. 2206-008 Chemistry HLData presentation Average time taken, t / s against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 0.120 Concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 0.100 4.83, 0.1 0.080 7.21, 0.075 0.060 15.94, 0.05 0.040 29.61, 0.025 0.020 46.81, 0.01 0.000 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 Average time taken, t / s Figure 2: Average time taken, t / s against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 6
  • 7. 2206-008 Chemistry HL Rate of reaction, 1/t / s-1 against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 0.12concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 0.10 0.207, 0.1 0.08 0.139, 0.075 0.06 0.063, 0.05 0.04 0.034, 0.025 0.02 0.021, 0.01 0.00 0.000 0.050 0.100 0.150 0.200 0.250 Rate of reaction, 1/t / s-1 Figure 3: Rate of reaction, 1/t / s-1 against concentration of potassium iodide solution, Mpotassium iodide / mol dm-3 7
  • 8. 2206-008 Chemistry HLUncertaintiesUncertainty in time taken due to stopwatch = 0.01 sUncertainty in time taken due to human reaction time = 0.09 sTotal absolute uncertainty in time taken = 0.1 sPercentage uncertainty in 0.10 mol dm-3 potassium iodide solution prepared in Step 1 of the Procedures= percentage uncertainty due to electronic balance + percentage uncertainty due to volumetric flask 0.001 0.1=( 1.66 × 100% ) + ( 100 × 100%)= 0.16 %Uncertainty in the different concentrations of potassium iodide solution, Mpotassium iodide: Concentration of Percentage potassium iodide Percentage uncertainty in Percentage uncertainty in uncertainty in 0.1 solution, volume of potassium iodide volume of distilled water mol dm-3 Mpotassium iodide solution measured / % measured / % potassium iodide / mol dm-3 solution / % 0.4 0.4 0.10 30 × 100 % = 1.333 0 × 100 % = 0.000 0.16 0.4 0.4 0.075 × 100 % = 2.540 × 100 % = 2.807 0.16 15.75 14.25 0.4 0.4 0.050 × 100 % = 3.810 × 100 % = 2.05 0.16 10.5 19.5 0.4 0.4 0.025 × 100 % = 7.619 × 100 % = 2.712 0.16 5.25 14.75 0.4 0.4 0.010 2.1 × 100 % = 19.048 27.9 × 100 % = 14.434 0.16 Table 4: Calculation of the percentage uncertainties in the preparation of solutions of different concentrations 8
  • 9. 2206-008 Chemistry HL Concentration of potassium iodide solution, Total percentage uncertainty / % Mpotassium iodide / mol dm-3 0.10 1.333 + 0.16 = 1.493 0.075 2.540 + 2.807 + 0.16 = 5.507 0.050 3.810 + 2.05 + 0.16 = 6.02 0.025 7.619 + 2.712 + 0.16 = 10.491 0.010 19.648 + 14.434 + 0.16 = 33.684Table 5: Calculation of the total percentage uncertainties in the preparation of solutions of different concentrations Concentration of potassium iodide Total percentage uncertainty Absolute uncertaintysolution, Mpotassium /% / mol dm-3 iodide / mol dm-3 0.10 1.493 0.10 × 1.493 % = 0.001 0.075 5.507 0.075 × 5.507 % =0.004 0.050 6.02 0.050 × 6.02 % =0.003 0.025 10.491 0.025 × 10.491 % =0.003 0.010 33.684 0.010 × 33.684 % =0.003 Table 6: Calculation of the absolute uncertainties in the preparation of solutions of different concentrations 9
  • 10. 2206-008 Chemistry HL 1Uncertainty in the different rates of reaction, : Concentration of potassium iodide Percentage uncertainty Absolute uncertainty in rate of solution, Mpotassium time taken / % reaction / mol dm-3 iodide / mol dm-3 0.1 0.10 4.83 × 100 % = 2.070 0.207 × 2.07 % = 0.004 0.1 0.075 7.21 × 100 % = 1.387 0.139 × 1.387 % = 0.002 0.1 0.050 15.94 × 100 % = 0.627 0.063 × 0.627 % = 0.0004 0.1 0.025 29.61 × 100 % = 0.338 0.034 × 0.338 % = 0.0001 0.1 0.010 46.81 × 100 % = 0.214 0.021 × 0.214 % = 0.00004 Table 7: Calculation of the percentage and absolute uncertainties in rate of reactions for different potassium iodide solution concentrationsConclusionThe rate of reaction increases as the concentration of potassium iodide increases; the graph of therelationship between the rate of reaction and the concentration of potassium iodide shows a positive linearregression curve. This result is in agreement with the hypothesis that the rate of reaction will increasewith the increase in concentration of potassium iodide solution.EvaluationTwo reactions occur when contents in conical flask “A” is poured into conical flask “B”: 1. Between hydrogen peroxide solution and the iodide ions from the potassium iodide solution to form triiodide ions: H2O2 (aq) + 3 I- (aq) + 2 H+ (aq) → I3- (aq) + 2 H2O (l) 2. Between triiodide ions and thiosulphate ions to re-form iodide ions. I3- (aq) + 2 S2O32- (aq) → 3 I- (aq) + S4O62- (aq)Triiodide ions and starch molecules combine to form triiodide-starch complexes which give the solutionits dark blue colour. 10
  • 11. 2206-008 Chemistry HLWhen the thiosulphate ions in the 2nd reaction is fully consumed, and thus hinder the reformation ofiodide ions, the triiodide ions will form complexes with starch and present the black-blue colour in thesolution. The stopwatch is stopped when the dark blue triiodide-starch complexes completely obscures the“X” mark at the bottom of the flask.If the time taken for the triiodide-starch complexes to obscure the “X” is to be prolonged, theconcentration of the sodium thiosulphate solution should be increased; and vice versa. This is because asthe more the amount of sodium thiosulphate solution present, the later the sodium thiosulphate solutionwill be used up and thus delaying the formation of blue black triiodide-starch complexes.The experiment can be carried out with potassium iodide concentrations of more than 0.200 mol dm-3 toextend the linear regression curve in Figure 3. However the rate of reaction will stop increasing with theincrease of potassium iodide concentration at a point, where the hydrogen peroxide concentrationbecomes the limiting factor. The rate of reaction remains constant; unless the concentration of hydrogenperoxide solution is increased.If the conical flasks are left on their own for some time, the dark-blue colour becomes more intense asmore triiodide – starch complexes are formed. The colour will stop darkening when the hydrogenperoxide solution or potassium iodide solution is completely used up; the reactant that is exhausted is thelimiting factor of the reaction.There is a “human reaction time” that is included as part of the uncertainty due to stopwatch. This isbecause there may be a delay between the first detection of an obscured “X” with when the thumb pressesthe stop button.Ways to improve experimentThe starch solution added into the beaker must be clear and does not cloud up the solution and decreasethe visibility of the “X” mark before the potassium iodide solution is added.The preparation process of starch solution used in this experiment included boiling the starch solution toensure that it became homogenous; hence the starch was warm when added into the conical flask withhydrogen peroxide solution. The starch should be cooled down to room temperature first, as hightemperature causes rate of reactions to be faster. 11