Kinetics - Experimental determination of a Rate Law with a modified Iodine clock Grade: 50 points towards total grade No Lab Report document - You write this one up. Include an Abstract (write this last), Procedure, Results, Pictures, Discussion, and Conclusion. Your calculations should be in your Discussion section. introduction The reaction witnessed in this lab is between iodine and starch. The hydrogen peroxide and vitamin C (ascorbic acid) have a reaction where they are protecting the iodine and starch from contacting one another. Once the acid and peroxide reaction has completed, the iodine contacts the starch and the color of the solution changes. The color change may be immediate or it may be slow. It is up to the student to decide where the color change can be reliably and consistently be timed. Materials Beakers or cups Graduated cylinders, 10mL & 50mL H2O2 (3% from a pharmacy - Unless otherwise marked, the hydrogen peroxide sold at pharmacies in the U.S. is 3%) Iodine - brown tincture or Lugol’s iodine. NOT Decolorized. Pipets, eye droppers, or syringes (5 for best results) Starch solution - liquid laundry starch OR make a corn starch solution Stop Watch 1g of Vitamin C - not chewable - spoons to crush the tablet(s) Background The reaction rate is dependent on the concentration of the reactant. If a reaction has two reactants and the rate is dependent on both of them, the reaction rate law could be: rate = k[A]x [B]y . aA + bB → cC + dD The exponents x and y are determined experimentally by inspecting the data OR by calculating. Sometimes it is easier to note the changes by inspection. [F2] [ClO2] Rate s-1 1 0.10 0.010 0.0012 2 0.10 0.040 0.0048 3 0.20 0.010 0.0024 Rate = k[F2]x[ClO2]y To determine the exponents x and y, with the experimental data, start with experiments 1 and 3. The concentration of ClO2 is constant which allows for a comparison of the rate in relation to the concentration of the other reactant. As the concentration of F2 doubles, the rate doubles as well. This is a proportional relationship that would be expressed as an exponent of 1 for [F2]. Rate = k[F2][ClO2]y Comparing experiment 1 and 2 keeps the concentration of F2 constant to inspect the change in rate when the concentration of ClO2 changed. The concentration of ClO2 multiplies by 4, so does the rate. That means y is 1 as well. Rate = k[F2][ClO2] To complete the rate law, k should be calculated. Insert the data from any of the experiments and solve for k. Rate = 1.2 M-1 s-1 [F2][ClO2] The rate is first order with respect to each of the reactants, but this is a 2nd order reaction rate because the sum of the exponents is 2. If the rate were to quadruple when the concentration doubled, that would necessitate an exponent of 2. The units of k need to adjust for each reaction rate order. Procedure Preparation: Starch Solution If you have opted to make your own starch solution rather than buy liquid laundry starch then measure 2 teaspoons of cornstarch into 30mL of clean water. Pour the starch slurry into 470mL of boiling water. Cool. This is your starch solution. The exact volume and mass are not crucial. Vitamin C Solution Crush and dissolve enough tablets to make a solution of 1g (expected) of Vitamin C in 60mL clean water. The mass and exact volume are not crucial. Procedure Students will create a procedure to test the dependence of the rate of the reaction on the concentrations of the acid, I-, and H2O2 solutions. As a baseline, students are encouraged to perform the following baseline reaction. Words of caution and wisdom: Be sure to use a dedicated pipet for each solution. Do not cross contaminate. Label the pipet with the name of the chemical if you think you’ll set it down and forget. Be consistent with the starting of the stopwatch. Be consistent with the declaration of the color / stopping of the stopwatch. You may wish to put a white piece of paper under your well with an X drawn on it. That may make it easier for you to gauge when the reaction is complete. i.e. when you cannot see the X any longer. Don’t try to fudge your data. Report what you see and explain what you can. (I’m happy to help you work through so reach out if you aren’t sure.) Baseline reaction - what to expect To gauge the reaction you are going to witness, start with the following: This is how to set up row #1: In a clean, dry beaker mix 2.5mL iodine, 2.5mL vitamin C solution, and 46.5mL water. This will be mixture A for this trial. In a clean, dry beaker, mix 1.0mL starch and 7.5mL H2O2. This will be mixture B for this trial. With a stopwatch at the ready, pour mixture B into mixture A. Start the stopwatch. Swirl the beaker to mix the solutions and wait for the color to change. You will have to decide when the reliable color change occurs. Some solutions allow for this to occur within 30seconds; some take 10minutes. Without knowing what your solutions are comprised of (and your raw materials), that is as much as I can say should be “normal.” Plan your experiment. You will need to create an experiment that tests the concentration dependence of the acid, I-, and H2O2 solutions. A minimum of 2 trials of each set of conditions need to be recorded. When you record the time each reaction takes to occur, you will have to calculate the rate. The rate is 1/time. If a reaction takes 22.4seconds then the rate is 1/22.4 = 0.0446 1/s. For example, the first 2 rows of the table below shows the suggested reaction amounts indicated above. When changing the amount of substance, use the amount of water to balance the total volume. The total volume of solution should be constant throughout the experiment. (The rate time here is illustrative. You will determine this time with your solutions.) There are 2 trials showing for the first set of conditions and 2 trials for the next set of conditions. The conditions illustrated below would be testing the concentration dependence of the rate on I-. Notice how the volume of I- are doubled from trials 1 & 2 to trials 3 & 4. The balance of drops is made up by adding drops to the water column so that the total drops of solution is kept constant. I suggest you experiment a little more than what is provided below. Iodine Vit C H2O Starch H2O2 Time (s) Rate (1/time) Average rate of the 2 trials #1 2.5mL 2.5mL 46.5mL 1.0mL 7.5mL (1): (2): #2 5.0mL 2.5mL 44.0mL 1.0mL 7.5mL (3): (4): #3 7.5mL 2.5mL 41.5mL 1.0mL 7.5mL (5): (6): #4 5.0mL 5.0mL 41.5mL 1.0mL 7.5mL (7): (8): #5 5.0mL 2.5mL 44.0mL 1.0mL 3.0mL (9): (10): Test the Rate of Reactions Write the procedure for the experiment you will perform. Create Data Tables in the Results section of the Lab Report Document. The data table should include amounts of the stock solutions used, time the reaction took to take place, and the rate of the reaction. The rate is the reciprocal of the time (rate = 1/time). Perform your experiment. Take pictures at various places to show your progress for your lab report. Data Treatment The concentrations of the stock solutions are relative ratios of volume / total volume. The concentration of iodine in row 1 is therefore calculated as 2.5mL/60.0mL * 100 = 4.17%. Other concentrations can be calculated similarly. The rate of the reaction is the reciprocal of the time in seconds. I.E. if the time of the reaction is 37 seconds then the rate = 1/37s = 0.027 s-1 The exponents of each reactant can be found by the following equation: Remember to compare two rows where the only thing that changes is the concentrations you are comparing. To solve this equation, take the log of both sides and use log rules to get: Solve for x: This exponent, when averaged with the other x values found for that reactant, will be rounded. The whole number value will be the exponent used in the rate equation. For example, if row (1) had a rate of 0.027s-1 and row (2) had a rate of 0.058s-1, then the calculation would follow as: This x value would be averaged with other x values found when Iodine is changed but the other solutions’ concentrations are held constant. If the average was 1.11, then that exponent value would be rounded to 1 for the rate equation. State the rate equation. At a minimum, you will be comparing the following conditions: #1 vs #2 : I- concentration changes #1 vs #3 : I- concentration changes #2 vs #4 : Vitamin C (acid) concentration changes #2 vs #5 : H2O2 concentration changes