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IB Chemistry on rate of reaction, iodine clock reaction using visible spectrophotometer


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IB Chemistry on rate of reaction, iodine clock reaction using peroxide with potassium iodide measured using visible spectrophotometer. Please cite and give proper reference to Seo Young if you use her work.

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IB Chemistry on rate of reaction, iodine clock reaction using visible spectrophotometer

  1. 1. Seo Young MyaengCandidate Number: 002213-064 Internal Assessment – Investigating the relationship between KI concentration and its rate of reaction with H2O2, measured using a spectrophotometerResearch Question: How will changing the concentration of KI (potassium iodide) affect itsrate of reaction with hydrogen peroxide (H2O2), calculated as inverse of time taken for theblue-black coloration of tri-iodide ion and starch solution, measured using spectrophotometer?Introduction: One of the several factors that affect the rate of reaction is the concentration of one ofthe reactant. According to the collision theory, as the reactant’s concentration increases,successful collisions will occur more frequently with the other reactant, which exists inexcess. Thus, the overall rate of reaction increases. In this internal assessment, the iodideclock reaction experiment was carried, and the concentration of a reactant, potassium iodide,was changed to investigate its relationship with the rate of reaction. In the iodide clockreaction, there are two steps of chemical reactions. The first reaction’s chemical equation isas following:Excess hydrogen peroxide was added to assure KI concentration is the only factor influencingthe reaction rate. Hydrogen chloride, or HCl, was added to provide the hydrogen ion neededto start the chemical reaction. The second chemical reaction involves addition of starch solution. One of theproducts, tri-iodide ( ), will react with the starch and lead to a color change from clear toblue-black. This acts as an evidence of iodide dissociation into tri-iodide ion. Then,thiosulfate, or was added in order to slow down the reaction time; the tri-iodide willreact first with the thiosulfate and then react with starch to form the blue-black color. Thisreaction can be expressed as: The overall reaction time can be measured using a spectrophotometer. A spectrumchange can be observed when the solution turns blue-black. Thus, the time taken for acomplete reaction to occur can be measured, and the rate of reaction can be calculated by theinverse of the time. Pg 1 of 10
  2. 2. Seo Young MyaengCandidate Number: 002213-064Hypothesis: The collision theory states that in a chemical reaction, the concentration of thereactant is directly proportional to the rate of reaction. This relationship can be represented asfollowing:The positive correlation is shown because as the concentration increases, the frequency ofcollision between the reactants increases. The increase in collision will increase occurrence ofsuccessful reactions, thus increasing the reaction rate. Therefore, my hypothesis is that theconcentration of potassium iodide will be directly proportional to the rate of reaction. Rate of Reaction Concentration of Potassium Iodide Figure 1: Expected trend of rate of reaction against concentration of potassium iodide Pg 2 of 10
  3. 3. Seo Young MyaengCandidate Number: 002213-064Variables Variable Measured Method of Measure or ControlIndependent Concentration of Performing a 2 fold serial dilution Variable potassium iodide (0.1M, 0.05M, 0.025M, 0.0125M, 0.00625M) Dependent Time it takes for color Measured using the spectrophotometer; Variable change to occur When color change occurs, change in absorbance will also be observed Rate of reaction Calculated as inverse of the time it takes for color change to occur Constant Temperature Will be kept constant to room temperature (25 ) Amount of solutions A micropipette is used to add the solutions; added volume of each solution is kept constant as: - KI: 500µl - H2O2: 500µl - HCl: 500µl - Starch: 500µl - S2O3: 1500µl (1.5ml) Concentration of Concentration of solutions are kept as: solutions - H2O2: 3% - HCl: 0.1M - Starch: 1% solution - S2O3: 0.0001M Pg 3 of 10
  4. 4. Seo Young MyaengCandidate Number: 002213-064Apparatus and Materials - Potassium iodide - Test tubes - H2O2 - Micropipette - HCl - Spectrophotometer - Starch - Cuvette - S2O3Procedures A. Preparation of Solution 1. Using test tubes and a micropipette, perform a 2 fold serial dilution to prepare different concentrations of KI solutions 10cm3 distilled water 10cm3 0.1M 0.05M 0.025M 0.0125M 0.00625M (20ml of 0.1M KI solution) Figure 2: 2 fold serial dilution for KI B. Measuring the Time of Color Change 1. Using a micropipette, add 500µl of H2O2, HCl, and starch solution, and 1.5ml of S2O3 to a cuvette 2. Set up the spectrophotometer and put the cuvette into the machine 3. Data collection is started and 5 seconds are waited 4. Using a micropipette, add 0.1M KI to the same cuvette 5. Data is collected for 300 seconds 6. Repeat steps 1-5 three times using 0.1M KI to compute the average for the specific concentration 7. Repeat steps 1-6 using different KI concentrations to compute the average for different concentrations Pg 4 of 10
  5. 5. Seo Young MyaengCandidate Number: 002213-064Data Collection and ProcessingQualitative Data- Before any KI is added, the mixed solution is colorless and transparent.- When KI is first added, the solution turns light yellow but is still transparent.- After a certain amount of time passes, a cloud of blue forms in the solution- The blue cloud gradually spreads and turns into blue-black; this continues until the solution turns completely blue-blackQuantitative Data:Raw DataConcentration of KI Time Taken Before Absorbance Change (M) (Seconds) ±0.5 Trial 01 Trial 02 Trial 03 0.1 18 16 18 0.05 34 33 28 0.025 70 73 74 0.0125 134 131 124 0.00625 225 217 234 Table 1: Raw data of number of seconds before absorbance change occurred Pg 5 of 10
  6. 6. Seo Young MyaengCandidate Number: 002213-064 0.1M 0.025M 0.0125M 0.00625M 0.05M Figure 3: Data graph of spectrophotometer for different concentrations of KI Pg 6 of 10
  7. 7. Seo Young MyaengCandidate Number: 002213-064Processed DataFive seconds were waited before the KI was added. Therefore, the number of seconds beforeabsorbance change will be: E.g.) 0.1M Trial 01: secondsThe average number of seconds was calculated as:The rate of reaction is calculated as: Concentration Time Taken Before Absorbance Change of KI (Seconds) (M) ±0.5 Trial 01 Trial 02 Trial 03 Average Rate of Reaction ( AU sec-1) 0.1 13 11 13 12.3 0.05 29 28 23 26.7 0.025 65 68 69 67.3 0.0125 129 126 119 125 0.00625 220 212 229 220 Table 2: Processed data on the average time and on rate of reaction Pg 7 of 10
  8. 8. Seo Young MyaengCandidate Number: 002213-064UncertaintiesUncertainties for Serial Dilution of KI SolutionUncertainty of a 10cm3 pipette: 0.02cm3Concentration Volume of Volume of Total Absolute of KI (M) Ethanol Distilled Water Percentage Uncertainty Solution Added/cm3 Error/% Added/cm3 0.02 0.02 0.00625 10 0.02cm3 10 0.02cm3 0.25 0.0125 10 0.02cm3 10 0.02cm3 0.5 0.025 10 0.02cm3 10 0.02cm3 1.0 0.05 10 0.02cm3 10 0.02cm3 2.0 0.1 20 0.02cm3 - 1.0 Table 3: Calculations of uncertainty for the concentrations of KI solution Pg 8 of 10
  9. 9. Seo Young MyaengCandidate Number: 002213-064Data Presentation Rate of Reaction Against KI Concentration 90 80 Rate of Reaction (AU sec-1) X10-3 70 60 y = 830.33x - 2.9633 50 R² = 0.996 40 30 20 10 0 0 0.02 0.04 0.06 0.08 0.1 0.12 Concentration of KI (M) Figure 4: Graph on rate of reaction against concentrationNote: the error bars for “Rate of Reaction” and “Concentration of KI” are unseen because the values are too small KI Concentration (M) Rate of Reaction ( AU sec-1) + 0.00625 +0.25 0.0125 +0.5 0.025 +1.0 0.05 +2.0 0.1 +1.0 Table 4: Presented data on rate of reaction for different KI concentrations Pg 9 of 10
  10. 10. Seo Young MyaengCandidate Number: 002213-064ConclusionIn figure 4, the graph reflects a positive, linear relationship between the concentration of KIand the rate of reaction. The R value, which reflects the accuracy of the equation shown inthe figure, is close to 1. This means the linear equation provided in the figure is almostperfectly relative to the data values. Considering the minor errors and uncertainties that couldhave occurred during serial dilution and during the experiments, the data corresponds to thelinear equation very well. Thus, it can be concluded that the KI concentration and the rate ofreaction has a good positive correlation.Evaluation Limitations ImprovementsError of KI concentration from serial This limitation may be improved by creatingdilution: The different KI concentrations the concentrations one by one instead ofwere made using a 2-fold serial dilution. going through a serial dilution.However, if error occurred during thedilution of one concentration, this errorwould have been carried onto the nextdilution. Thus, the overall concentrations ofthe solution would have been influenced.Timing of adding KI: 5 seconds were It may be possible to improve this limitationwaited before addition of KI solution. by carrying out the three trials at once. Then,However, there may be error behind this it would be possible to add the KI in morebecause the solutions were added by hand similar timings. Or, it may be possible toand thus differences in timing would have change the method to start the data collectionoccurred. This would affect the data of the as soon as the KI is added. Then, it would nottime taken for absorbance change. be necessary to wait for 5 seconds and increase the chances of error.Time of change in absorbance: The change It would be possible to improve thisin color caused by the reaction between KI limitation by using a different method toand starch are represented by the change in measure time. For example, a stop watch,absorbance in the graphs. The specific time which can measure to the 100th decimal placeof change in absorbance was obtained from of a second, may be used. It would only bethe spectrophotometer’s table. However, the that a standard for when to start and stop theabsorbance was only measured by the timing should be set before the actualinterval of 1 second due to the limitations of measurement to increase accuracy.the spectrophotometer program. Therefore,the time measurement on occurrence ofabsorbance change is not as exact.Not having a negative control: The overall Improvements can be made by performinginvestigation did not have a negative control. the experiment with a negative control. ItThus, it cannot be assured whether the would be that everything would done exactlychanges in absorbance were due to addition the same as other trials, but instead of KIof KI or whether even without KI, the solution, distilled water will be added. Thus,solutions will go through a change in this will assure whether certain changes inabsorbance. the absorbance are only occurring by the reaction with KI. Pg 10 of 10