Experiment 21 Data and Calculations: Rates of Chemical Reactions, II. A Clock Reaction (2) A. Dependence of Reaction Rate on Concentration Reaction: 61) + Bro,aq) + 6H(aq) 31. (aq) + Br(+ 3H.O() In all the reaction mixtures used in this experiment. rate - 111MBO, TH- number of moles of Bro, had been used up in the reaction. The color clock" allows you to measure the time required for the color to change; since in Equation 2 the change in concentration of Bro, ion, Bro,). nguined for this filmber of roles of Bro," pracy. The rule of each reaction is determined by the time is the same in each mixture, the relative rate of each reaction is inversely proportional to the time 1. Since we being equal to 1000V. Fill in the following table, first calculating the relative reaction rate for each mixture. are mainly concerned with relative rather than absolute rates, we will for convenience take all relative rates as ABO, color change occurred when constant predetermined Temp. in (C) Reactant Concentrations in Reacting Mixture (M) [Bro, (H) 0.0080 0.02e 21 20 0.0080 0.020 0.02 Timer (sec) Relative Rate Reaction for Color of Reaction Mixture to Change 1000/ 11") 1 96 10.4 0.0020 2 41 24.39 0.004 21 3 45 22.22 0.0020 0.016 20 4 27 37.04 0.0020 0.0080 5 14 8.771 0.0016 0.004 The reactant concentrations in the reaction mixture are nor those of the stock solutions, since the reagents were diluted by the other solutions. The final volume of the reaction mixture is 50 mL in all cases. Since the number of moles of reactant does not change on dilution we can say, for example, for I ion, that moles of r = [1]stock X Vstock = [1°misture X Vanixture For Reaction Mixture I. 0.04 0.03 20 [1stock = 0.010 M. Vstock = 10 mL Vimixture 50 mL Therefore, 11-mare 0.010 Mx10 mL = 0.0020 M 50 mL Calculate the rest of the concentrations in the table using the same approach. Determination of the Orders of the Reaction Given the data in the table, the problem is to find the order for each reactant and the rate constant for the reac- tion. Since we are dealing with relative rates, we can modify Equation 2 to read as follows: relative rate = k1151"[Br0,1"(H (5) (continued on following page) 168 Experiment 2+ Rates of Chemical Reactions, I. A Clock Reaction We need to determine the relative rate constant k' and the orders a, #, and p in such a way as to be consistent with the data in the table. The solution to this problem is quite simple, once you make a few observations on the reaction mixtures. Each mixture (2 to 4) differs from Reaction Mixture in the concentration of only one species (see table), This means that for any pair of mixtures that includes Reaction Mixture 1. there is only one concentration that changes. From the ratio of the relative rates for such a pair of mixtures we can find the order for the reactant whose concentration was changed. Proceed as follows: Write Equations below for Reaction Mixtures 1 and 2, substituting the relative rates and the concentra- tions of r. Bros. and Hions from the table you have just completed. Relative Rate 10 - K'I 0.0020 | 0.0080 II 0.020 V Relative Rate 2 = 24 lo.colo l' 0.020 P Divide the first equation by the second, noting that nearly all the terms cancel out. The result is simply Relative Rate 1 Relative Rate 2 = kl 0.004 If you have done this properly, you will have an equation involving only m as an unknown. Solve this equation for m. the order of the reaction with respect to lion. m = 7.7 (nearest integer) Applying the same approach to Reaction Mixtures 1 and 3, find the value of n, the order of the reaction with respect to Bro, ion. Relative Rate 1 = 10 = klo 11.000 1'1 .Ozo Relative Rate 3 = =k'l o.orzo 10.016 1" 0.002 Dividing one equation by the other: n = 1.2=1 Now that you have the idea, apply the method once again, this time to Reaction Mixtures 1 and 4, and find p, the order with respect to H ion. Relative Rate 4 = = kl .000 0.00301" 0.04 IP Dividing the equation for Relative Rate I by that for Relative Rate 4, we get p= 0.40 Having found m. n, and p (nearest integers), the relative rate constant, k'. can be calculated by substitution of m. n, p, and the known rates and reactant concentrations into Equation 5. Evaluate k' for Reaction Mixtures 1 to 4. Mixture 1 2 3 4 k' man Standard deviation in k' (See Appendix VIII) Why should k' have nearly the same value for each of the above reactions? Using kucu in Equation 5, predict the relative rate and time, I prad. for Reaction Mixture 5. Use the concen- trations in the table. Relative ratepeed "peed lots Experiment 2+ Rates of Chemical reactions. Alex Resin 159 B. Effect of Temperature on Reaction Rate: The Activation Energy To find the activation energy for the reaction it will be helpful to complete the following table. The dependence of the rate constant. k', for a reaction is given by Equation 4: In k' =- E RT + constant Since the reactions at the different temperatures all involve the same reactant concentrations, the rate constants, K. for two different mixtures will have the same ratio as the reaction rates themselves for the two mixtures. This means that in the calculation of Eg. we can use the observed relative rates instead of rate con- stants. Proceeding as before, calculate the relative rates of reaction in each of the mixtures and enter these values in (c). Take the natural logarithm of the relative rate for each mixture and enter these values in (d). To set up the terms in 1/T, fill in (b), (e), and (1) in the table. Approximate Temperature in °C 20 40 10 0 (a) Time 1 in seconds for color to appear 96 یا llo (b) Temperature of the reaction mixture in °C 21 41 10 (c) Relative rate = 1000/t (d) In of relative rate (e) Temperature T in K (1) 1/T, K- To evaluate Eg, make a graph of In relative rate vs. 1/T, using Excel or the graph paper provided. (See Appendix V.) Find the slope of the line obtained by drawing the best straight line through the experimental points. Slope к The slope of the line equals -EJR, where R culate the activation energy, Ey, for the reaction. (8.31 joules/mole K) if E, is to be in joules per mole. Cal- Ex = joules/mo Optional C. Effect of a Catalyst on Reaction Rate Reaction 1 Catalyzed Reaction 1 Section Name Experiment 21 Data and Calculationst Rates of Chemical Reactions, II. A Clock Reaction
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