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2.8: Kinetics- Iodine Clock using Vitamin C and the Calculation of Initial Rate-Home

  • Page ID
    387716
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    Learning Objectives
    • To observe and record the time needed for a reaction to happen
    • To develop skills to set-up a redox reaction
    • To complete the calculations to find out the initial rates
    • To graphically analyze the data to find the rate law for the reaction

     Theory

               Vitamin C (Ascorbic acid C6H8O6) is oxidized by free iodine molecule as follows. This reaction is often used in volumetry to quantify the amount of Vitamin C in a sample. This we will see in the titration lab experiment. Since iodine is involved, and  these are redox reactions not neutralization reactions, these titrations are called iodometric titrations. However, in kinetics lab experiments, this reaction is often used as an example where the speed of a reaction is measured or rather calculated. The following equation shows the 1:1 reaction between Vitamin C (Ascorbic acid) and Iodine.

    C6H8O6 + I2 à C6H6O6 + 2I- + 2H+

                  Iodine (I2) solution is a brown solution and dissolved Iodide (I-) ions are colorless. Free iodine in the presence of starch makes an I2-Starch complex that is dark blue in color. If there is vitamin C, it will react with I2. So, there will not be any free iodine to complex with starch to give the blue color. Once the vitamin C is oxidized completely, the free iodine will be available, and it will complex with starch to give the blue color. This is an indication of the completion of vitamin C oxidation. If you have more Vitamin C, it will take more time for the completion, and the appearance of blue colored is delayed. Therefore, the rate of the Vitamin C oxidation can be followed by the time taken for the appearance of the blue free iodine-starch complex. That is why this reaction is known as the iodine-clock reaction.

                  A set of four to five reactions will be set up with increasing amount of Vitamin C, keeping the amount all other reagents the same. Use a stopwatch (cellphone) to measure the time taken for the appearance of blue color in each case and are recorded. The mols of Vitamin C taken in each sample, and the corresponding time are used to calculate the initial rate of the reaction. Use a graph sheet or excel spread sheet to manipulate the data to find the rate law (zero, first, second, or?).

                 Watch the following video to see the set-up on iodine clock reaction at home using household items. The concentration calculations in molarity is more of an estimation as we are not using standard pure chemicals. We will be using a different amounts of reagents for our reactions. This video is meant only for a demonstration purpose.

     

    Procedure

    Materials Required

    Vitamin C tablets (Nature Best brand) 500 or 1000 mg supplements, 2-3 #, Tincture iodine antiseptic (2%) 30ml, Hydrogen peroxide, 3% solution, 100 ml, Starch (corn or potato), 1-2 spoons, Beakers or clear bowls for mixing, graduated cylinder, electronic balance, spoons. No filtration is required.

    Note

    Wear gloves. Iodine can cause stains.

    Part A: Calculating the Rate of Vitamin C Oxidation using Initial Rate Method

    • Even though we are doing five samples with different amounts of Vitamin C, we will do one at a time. That means prepare one set of A and B solution as shown in the video, mix and measure the time time taken for the appearance of the blue color using a stopwatch.
    • Crush 2-3 Vitamin C tablets.
    • Sample 1
      Solution A: Dissolve 200 mg (0.2g) of Vitamin C powder in 25 ml of water in a beaker or a clear cup, and then add 6 ml 2% tincture iodine solution:            Solution B: Dissolve a pinch of starch in 15 ml 3% Hydrogen peroxide, add another 16 ml water to this. Mix solutions A and B and start the stop-watch immediately to measure the time taken for the appearance of the signature blue color. The total volume of the solution is 62 ml.
    • Sample 2
      Repeat the above procedure, but with 400 mg (0.4g) Vitamin C in Solution A. Keep the measurement of all other reagents the same as in sample 1. Prepare B with no change. Record the time take to see the blue color after mixing A and B.
    • Sample 3, 4, and 5. Repeat the above procedure with various amounts of Vitamin C in making solution A, 600 mg (0.6g) in the third sample, 800 mg (0.8 g) in the fourth, and 1000 mg (1.0 g) in the 5th. Remember to keep the amount of all other ingredients the same in all the repetitions for preparing solutions A and B.. Record the time taken to see the blue color in each experiment.

    Part B-Graphical Analysis to Find the Rate Law (Graph Paper or Excel Spreadsheet)

    • Create a table with Concentration of Vitamin C (millimols) in each sample versus the time recorded
    • Create two other manipulated tables. Table 2,  take the natural logarithm of the concentrations of Vitamin C vs the time. Table 3, take the inverse of all the Vitamin C concentrations versus the time.
    • Check which of the graphs give a straight line. If it is the first one, the rate law is zero order. If it is the second, it is a first order rate law. The thirs one would be second-order if it gives a straight line.

    Report Sheet Part A: Calculating the Rate of Vitamin C Oxidation using Initial Rate Method

    • The vitamin C tables are not standardized are pure ascorbic acid available in a real laboratory. In order to find the concentration or mols of vitamin C in a particular amount of the tablet, it is titrated with a standardized base like NaOH. Since we do not have NaOH. We are going to calculate the millimols/mg of the tablet based on the titration values given in the video. This is why we need to use the same brand of Vitamin C tables as used in this reaction shown in the video. This can be used to find the mols of Vitamin C used in the following 5 samples.
    • M1V1=mols of NaOH. Since it is a 1:1 reaction between Vitamin C and NaOH, the number of mols of Vitamin C should be equal to the mols of NaOH used. Divide the mols of Vitamin C with the weight in milli grams of the tablet used we will get the mols per every mg of Vitamin C tablet. This calculations in the video is used to find  the amount of Vitamin C is milli mol/mg of the tablet. We are going to borrow this to calculate the mols of Vitamin C in the report table for each sample.

      It comes to 0.001milli mol Vitamin C per every milli gram of the Vitamin C tablet. For example, for sample 1

    • Molarity of Vitamin C would be
      200mg Vitamin C tablet X 0.001mmol Vitamin C/mg tablet = 0.2mmol Vitamin C in sample 1

    • To calculate molarity M, divide the mols by volume of the solution (62 ml is the total volume of solutions in all the samples).
      For sample 1, it would be Molarity, M=0.2mmol/63 ml = 0.003M. Calculate the molarity of vitamin C for all the other sample using the same method and enter in the table below.

    • Initial Rate calculation

             
    Sample    Vitamin C in grams Mols of Vitamin C

    Molarity, M of Vitamin C

    milli mol/milliliter

     

    Time recorded

    Δt

    Rate calculated =

    Rate=

    (-) Δ [Vitamin C]/Δt

     

     

    Sample 1-200mg (0.2 g)  Vitamin C

    200mg tablet X0.001mmol Vitamin C /mg tablet

    =0.2mmol Vitamin C

           0.2 mmol/62ml=0.003M              

     

    (-)(0-0.003) M/(Δt) s

    =0.003M/(Δt)s =
    Sample 1-400mg (0.4 g) Vitamin C        
    Sample 1-600mg (0.6 g)Vitamin C        
    Sample 1-800mg(0.8 g) Vitamin C        
    Sample 1-1000mg (1.0 g) Vitamin C        

    Report Sheet-Graphical: Vitamin C Analysis to Find the Rate Law

    Complete the following tables and plot the graphs of dependent variable vs. independent variable (t) in each case and check for the rate law. Watch the following video to review the concept.

    Case 1

    Concentration = Molarity of Vitamin C Time, s
    Sample 1 =0.003M  
    Sample 2  
    Sample 3  
    Sample 4  
    Sample 5  

    Case 2

    Ln (concentration) Time, s
    Sample 1, Ln (0.003) = (-5.809)  
       
       
       
       

    Case 3

    1/(Concentration of Vitamin C)    1/M Time, s
    Sample 1= 1/(0.003)= 333.3  
       
       
       
       

    Data Analysis

    • Are the initial rates for all the samples the same?
    • Is the initial rate increasing or decreasing with increasing concentration of Vitamin C?
    • How would the temperature impact the rate? Will it increase or decrease the speed of the reaction?
    • The change in concentration is (final concentration - initial concentration) for each sample. What is the final concentration of Vitamin C for all the samples?
    • In PART B, which of the three cases resulted in a straight line plot? Include all the three graphs with the lab report.
    • What is the rate law for the reaction, zero, first, or second? Write down the rate law equation.
    • Calculate the rate constant, k from the slope of the straight-line plot. k=

    Contributors and Attributions

    Manjusha Saraswathiamma, Minnesota State University, Moorhead has developed this experiment to be performed at home with less hazardous and cost-effective lab supplies. Manjusha would like to acknowledge all the contributors of the YouTube videos embedded on this page.


    2.8: Kinetics- Iodine Clock using Vitamin C and the Calculation of Initial Rate-Home is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Manjusha T Saraswathiamma.