Kinetics of Iodide Oxidation by Hydrogen Peroxide: Rate Law and Catalyst Effect, Slides of Law

Download Kinetics of Iodide Oxidation by Hydrogen Peroxide: Rate Law and Catalyst Effect and more Slides Law in PDF only on Docsity! - 1 - Experiment 5 Kinetics: The Oxidation of Iodide by Hydrogen Peroxide Goals To determine the differential rate law for the reaction between iodide and hydrogen peroxide in an acidic environment. To determine the activation energy and pre-exponential factor for the reaction. To determine the effect of a catalyst on the rate of reaction. Discussion When hydrogen peroxide is added to a solution of potassium iodide, the iodide ions are slowly oxidized according to the equation: 2 2 2 2 potassium iodide hydrochloric acid hydrogen peroxide iodine colorless colorless coloroless yellow - + iodide Molecular equation: 2KI(aq) + 2HCl(aq) + H O (aq) I (s) + 2H O(l) Net ionic equation: 2I (aq) + 2H (a → 2 2 2 2 hydrogen peroxide iodine q) + H O (aq) I (s) + 2H O(l)→ The rate law for this reaction should include the concentrations of iodide, hydrogen ion, and hydrogen perioxide. However, if the concentration of H+ is held constant throughout the experiment then its effect will not appear in the rate law. This results in a relatively simple rate law: rate = k[I–]n[H2O2]m[H+]p simplifies to: rate = k'[I–]n[H2O2]m where k' = k[H+]p Part 1: Determination of the Rate Law The rate law for the reaction between iodide ions and hydrogen peroxide can be determined by carrying out experiments in which the concentrations of iodide and peroxide are varied. For example, consider the following reaction and data: A + B → P Rate =k[A]n[B]m Experiment [A]0 [B]0 Initial rate       M s 1 0.10 0.10 0.45 2 0.20 0.10 0.90 3 0.10 0.20 1.80 For this reaction, doubling the initial concentration of reactant A doubles the initial rate of reaction (experiments 1 and 2). This relationship corresponds to n=1. Likewise, doubling the initial concentration of B has the effect of quadrupling the initial rate (experiments 2 and 3). This corresponds to m=2. We can now write the differential rate law for this reaction as: rate = k[A][B]2 The reaction is said to be first order in A and second order in B. Overall, the order of the reaction is 3. The value of the rate constant k can be determined by using the known values of n and m: 2 Ratek= [A][B] We can use the given initial concentrations and initial rate for each experiment and determine the value of k for each experiment. For the study we will perform, the reaction between iodide and hydrogen peroxide, there are several experimental requirements: 1. The acidity of the solution must be maintained at a constant level so that the concentration of H+ is constant. 2. The temperature of the reactants must be the same for all runs. The rate constant is dependent on the temperature of the solution. Since the heat of reaction is relatively small for this reaction the temperature should remain relatively constant throughout the process. If this were not the case then we would need to place the reaction in a constant temperature bath. 3. The reverse reaction must be suppressed. Forward reaction: 2I– + 2H+ + H2O2 → I2 + 2H2O Reverse reaction: I2 + 2H2O → 2I– + 2H+ + H2O2 (negligible) If the concentration of iodine (I2) is allowed to build up as the reaction proceeds then the rate of the reverse reaction will increase and an equilibrium mixture will result. Any rate measured for the reaction under such conditions will not represent the rate of the forward reaction but the net difference between the forward and reverse rates. In this reaction, however, we can ignore the reverse reaction because the equilibrium position is so far on the I2 side that the reaction above does not take place to any appreciable extent. time 4. We need a method to accurately measure the rate of reaction. The addition of thiosulfate ions (S2O3 2–) allows an accurate measurement of the rate at which the peroxide-iodide reaction is taking place. Suppose that you add a small and known amount of thiosulfate ion to the original mixture of peroxide and iodide. Iodine is produced slowly by the reaction between peroxide and iodide ions and the thiosulfate ions immediately react with the iodine as it is produced: 2- 2-- 2 2 3 4 6iodideiodine thiosulfate dithionatecolorlessyellow colorless colorless I + 2S O 2I + S O→ fast reaction As long as excess thiosulfate ions are present in the solution, no free iodine can accumulate because it is immediately turned into iodide ions which are colorless. The thiosulfate ions are the limiting reagent. So once all the thiosulfate ions are consumed, iodine starts to form in the solution. Iodine is a pale yellow. If starch is added to the solution then a more dramatic blue solution is formed by the complex of starch–iodine. The color change is sharp, and the time elapsed to this point is determined simply by use of a timer. The time from the addition of the peroxide solution to the appearance of the blue color is ∆t for the reaction. Since the stoichiometry of the thiosulfate–iodine and the peroxide–iodide reactions is known, it is not difficult to calculate how many moles of peroxide were reduced in the known interval of time. Consequently, the average rate (moles of hydrogen peroxide consumed per liter per second) of the reaction during this period can be calculated. [ ]2 2∆ H O Rate = - ∆t For the peroxide–iodide reaction the average rate of the reaction over the period taken for the measurement is a good approximation of the initial reaction rate. change in time, measured with a stopwatch change in peroxide concentration, calculated from known amounts of reactants - 5 - 9. Rinse the beaker with tap water and deionized water. Prepare and measure the rate of reaction for the next mixture in a similar manner. Since you will be using the same graduated cylinders for the same reagents each time, there is no reason to rinse them between experiments. Part 2: The Effect of Temperature on the Reaction Rate Data will be shared between groups for this part of the experiment. The mixture described in this section should be prepared and placed into the appropriate bath at the beginning of the laboratory. To study the effect of temperature on reaction rate we need to perform the experiment at temperatures above room temperature and at temperatures below room temperature. Since we are measuring the effect of temperature and not concentration we need to keep the concentrations of all reactants constant for this part. To do this we will choose the concentrations used in experiment #3 from part 1. Expt # Deionized Water (mL) HCl (mL) KI (mL) Starch (mL) Na2S2O3 (mL) H2O2 (mL) 3 405 10 30.0 5 25.0 25.0 or Expt # Deionized Water (mL) HCl (mL) KI (mL) Starch (mL) Na2S2O3 (mL) H2O2 (mL) 3 162 4 12.0 2 10.0 10.0 1. Each group should prepare one #3 solution, without peroxide, in a beaker. 2. Place 25.0 mL (or 10.0 mL) of peroxide should be in a 100 mL graduated cylinder. 3. Place the mixture into the cold bath or hot bath, as directed by your instructor. 4. Once the temperature of the mixture has equilibrated with the bath, measure the rate of reaction as you did in part 1. The temperature and time data should be recorded in the data sheet. Part 3: The Effect of a Catalyst on the Reaction Rate 1. Prepare a #3 solution, without peroxide, in a 600 mL beaker. 2. Place 25.0 mL of peroxide should be in a 100 mL graduated cylinder. 3. Add the peroxide to the mixture at the same time as you add 2 mL of iron (II) ions to the mixture. Measure and record the time of reaction. Data Sheet Experiment 5. Kinetics: The Oxidation of Iodide by Hydrogen Peroxide Name Name(s) of Partner(s) Stock solution Concentrations* concentration of KI __________ concentration of Na2S2O3 __________ concentration of H2O2 __________ *From the reagent bottles. Part 1: Effect of Concentration Expt # Time (s) 1 2 3 4 5 Part 2: Effect of Temperature* Temperature (°C) Time (s) cold bath cold bath hot bath hot bath *concentrations are the same as experiment 3 in Part 1. Part 3: Effect of a Catalyst Time (s)* * concentrations are the same as experiment 3 in Part 1. Calculations Sheet Experiment 5. Kinetics: The Oxidation of Iodide by Hydrogen Peroxide Name Part 1 Complete the calculations in Tables 1–3. Table 1—Initial Moles* The number of moles of each reactant is found by multiplying the volume of reactant solution added by the molarity of the reactant. For example: ( ) mol0.01L KI 1.0 KI =0.01molKI L       Expt # [KI] mol/L volume KI (L) moles KI [Na2S2O3] mol/L volume Na2S2O3 (L) moles Na2S2O3 [H2O2] mol/L volume H2O2 (L) moles H2O2 1 1.0 0.01 0.01 2 3 4 5 *The molarities used in this table are those of the stock solutions. We wish to find the orders of the reactants, m and n: Initial rate = k'[I–]0 n[H2O2]0 nm By taking the log of both sides of the rate law we can do this. log (Initial rate) = log (k') + mlog[H2O2]0 + nlog[I–]0 Now, suppose that we make multiple measurements of the rate in which we vary the initial concentration of H2O2 but keep the initial concentration of I– constant. The rate will change because [H2O2]0 changes. But any change in the rate depends only on the change in initial H2O2 concentration. The rate depends on H2O2 only. We can generate a graph that shows this to be the case. If we make a graph of log (Rate) versus log [H2O2]0 then we get the following result: log (Initial rate) = log (k') + mlog[H2O2]0 + nlog[I–] where the constant is equal to log (k') + nlog[I–] log (Initial rate) = mlog[H2O2]0 + Constant y = mx + b This result is the familiar linear relationship. In this case, the order of peroxide (m) is the slope of the graph. The y-intercept is unimportant to us since we merely want the order n. Note that this graph only works for the case where the concentration of iodide is constant. If the iodide and peroxide concentrations change simultaneously then we can't use this analysis. If we want to find the order of iodide (n) we can do the same analysis. But we need to pick data in which the initial iodide concentration changes and the initial peroxide concentration remains constant. If we review the initial concentration data from Table 2 we can find data in which this is the case. We can then generate a second graph log(Initial rate) = log (k') + mlog[H2O2]0 + nlog[I–] log(Initial rate) = nlog[I–] + constant where the constant is log (k') + mlog[H2O2]0 Again, the slope of the graph gives us the order of the reactant, in this case, iodide. Table 5– Rate Constants Using the values of m and n to 2 significant figures (from the graphs), the initial rate, and the initial concentrations of the peroxide and iodide, calculate the rate constant k for each of the five runs. The values should be the same because k is only a function of temperature for a given reaction. However, due to experimental error the five values of k that you calculate will probably be somewhat different. The equation to calculate k is: m - n 2 2 initail ratek = [H O ] [I ] Experiment 5. Kinetics: The Oxidation of Iodide by Hydrogen Peroxide Name The values of m and n should be entered to two significant figures. The average value of k is found by adding the five values and dividing by five. Expt [H2O2]0 * [H2O2]0 m [I–]0 * [I–]0 n initial rate (M/s) k 1 2 3 4 5 Value of m (2 sig fig) = Value of n (2 sig fig) = average value of k = standard deviation of k = *From Table 2. Calculate the standard deviation of your values of k. The standard deviation s, is a measure of how close the measured values are to one another and is defined as: 2 2 2 1 2 n(k -k) +(k -k) +...+(k -k)s = n-1 where k1, k2, k3, k4, k5 are the five rate constants and k is the average value of the rate constant. n = the number of measurements, in this case n = 5. The standard deviation is a measure of the variability of your measurements. If the only errors made in the experiment are random errors, then the smaller the standard deviation, the less the random error in your measurements. Part II: The Temperature Dependence of the Rate Constant and Determination of the Activation Energy and Pre-exponential Factor In the first part of this experiment, we took data which allows us to calculate k, the rate constant for the reaction, and m and n, the partial orders of the reactants. But k, while not sensitive to concentration, is temperature dependant, i.e., it will change if you change the temperature of the reaction. We can take advantage of this temperature dependence of k. Consider the Arrhenius equation: aE- RTk = Ae where A is known as the frequency factor or pre-exponential factor, e is the base of the natural logarithm system, Ea is the activation energy in joules/mole, R = 8.31 J Kmol is the ideal gas law constant, and T is the temperature in kelvin. Exponential equations like the Arrhenius equation produce curved lines when plotted. It is desirable to convert the equation into that of a straight line, because then the activation energy and the pre-exponential factor can be found. The way to convert an exponential equation into a straight line equation is to take the natural logarithm (ln) of both sides of the equation: aE 1ln (k)=- +ln (A) R T       y = m x + b Notice that taking the natural log of both sides results in a linear equation (just as in Part 1). If the natural logarithm of k (ln (k)) is plotted versus the reciprocal of the temperature (1/T) a straight line is produced. The slope, m of the line, is then the negative of the activation energy divided by the ideal gas constant, and the y- intercept, b, is the natural logarithm of the pre-exponential factor: m = – Ea/R b = ln (A) Ea = –mR A = eb