Relative rate of change problems

Experiment 21 Data and Calculations: Rates of Chemical Reactions, Il. A Clock Reaction A. Dependence of Reaction Rate on Concentration Reaction: 61(ag + BrO,ag)+ 6H(aq)3aqp+ Brap+HO0 rate irBeO, Tr-- In all the reaction mistures used in this experiment, the color change occurred when a constant predetermined aumber of moles of BrO," had been used n in the reaction The color "clock" allows you to measue the ime mgaired for this fixed number of moles of BrO," to mact. The rate of each reaction is determined by the time I required for the color to change: since in Fouation 2 the change in concentration of BrO, ion, ABeO, 1 is the same in each mixture, the relative rate of cach reaction is inversely proportional to the time 1. Since we are mainly concerned with relative rather than sbuolute Mes we will for convenience take all redative rates as being equal to 1000h. Fill in the follow ing table, first calculating the relative reaction rate for each mixture Reactant Concentrations in Reacting Misture (M) H' Relative Rate Time t (sec) Temp in (C) of Reaction Reaction for Color BrO," Mixture to Change 1000 Q.0020 00ZO 4.854 ZO6 0.0020 O.0040 00020 Q0z0 O.0020 0.0010O 0020 0-0020 00020 0040 Q00l6 0.0010 O.030 9.615 9.524 104 2 105 3 49 20.41 4 4.545 220 5 The reactant concentrations in the reaction mixture are not those of the stock solutions, since the reagents were diluted by the other solutions. The final volume of the reaction mixture is 50 mlL in all cases. Since the number of moles of reactant does not change on dilution we can say, for example, for r ion, that moles of I For Reaction Mixture 1, V 50 ml V 10 mL 0010 M. Therefore 0010 Mx10 ml.00020 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 (5) (continued on following page)
Raes of Chemical Rescon ILA Coc Retion 168 Experiment 21 We neod to determine the relative rate constant& and the orders am, n, 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 mixtur Fach mixre (2 to 41 differs from Reaction Mixture I in the concentration of only one species (see tables This means dthat for any pair of mixtures that includes Reaction Mixture 1, there is only one concentration thut chanors, From the ratio of the relative rates for such a pair of mixtures we can find the ceder for the reactane whose concentration was changed. Proceed as follows Write Fomation 5 below for Reaction Mixtures I and 2, substituting the relative rates and the concenta- tions of I, BrO,", and H" ions from the table you have just completed. Relative Rate 1 Relative Rate 2 Divide the first equation by the second, noting that nearly all the terms cancel out. The result is simply Relative Rate I Relave Rate 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 r ion. (nearest integer) Applying the same approach to Reaction Mixtures I and 3, find the value of n, the order of the reaction with respect to BrO, ion. Relative Rate 1 Relative Rate 3 Dividing one equation by the other: 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 = Dividing the cquation for Relative Rate 1 by that for Relative Rate 4, we get Having found m,n. and p (nearest integers), the relative rate constant, A, can be calculated by substitution of m. n. p. and the known rates and reactant concentrations into Equation 5. Evaluate ' for Reaction Mixtures I to 4. Mixture 2 4 Sdand e Appendit TH Why should k' have nearly the same value for each of the above reactions? Using k in Equation 5, predict the relative rate and time, L trations in the table for Reaction Mixture 5., Use the coacen- Relative rate