Experiment 21 Data and Calculations: Rates of Chemical Reactions, II, A Clock Re

Question

Experiment 21 Data and Calculations: Rates of Chemical Reactions, II, A Clock Reaction A. Dependence of Reaction Rate on Concentration 3H200 AIB In all the reaction mixtures used in this experiment, the color change occurred when a constant predetermined number of moles of Bro, had been used up in the reaction. The color "clock" allows you to measure the time required for this fixed number of moles of Bro, to react. The rate of each reaction is determined by the time required for the color to change; since in Equation 2 the change in concentration of Bros ion. AlBroy is the same in each mixture, the relative rate of each reaction is inversely proportional to the time r. Since we are mainly concerned with relative rather than absolute rates, we will for convenience take all relative rates as being equal to 1000lt. Fill in the following table, first calculating the relative reaction rate for each mixture Reactant Concentrations Time (sec) Relative Rate in Reacting Mixture (M) Temp. eaction for Color of Reaction in (C) Bro, 1 Mixture to Change 1000/l 21 20.00 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 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 X tture Wmixture For Reaction Mixture 1 50 mL 0 mL, 0.010 M Therefore, 0.010 MX10 mL. 0.0020 M 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 k'ITT"[Bros THY (continued on following page)

Explanation / Answer

Rate = k [I-]^m[BrO3-] ^n[H+] ^o

In reaction mixture 1 and 2 , the concentration id BrO3- and H+ is kept constant while concentration of I- is varied.

Relative rate 1/relative rate 2 = k [0.002] ^m [0.008]^n[0.02]^o/k[0.004]^m [0.008]^n[0.02]^o

Silimar terms on the right side cancels out . So, the eqn becomes

or, 10.21/16.95 = (0.002/0.004) ^m

or, 0.60 = (0.5) ^m

or, m =0.7 ~1

In reaction mixture 1 and 3, concentration of I- and H+ is kept constant while BrO3- is changed.

Relative rate 1/relative rate 3 = k [0.002] ^m [0.008]^n[0.02]^o/k[0.002]^m [0.016]^n[0.02]^o

or, 10.21/19.90 = (1/2) ^n

or, (1/2) = (1/2) ^n

or, n = 1

Consider relative rate 1 and 4. Here, concentration of I- and BrO3- is changed while H+ is changed.

Relative rate 1/relative rate 4 = k [0.002] ^m [0.008]^n[0.02]^o/k[0.004]^m [0.008]^n[0.04]^o

10.21/29.94 = (1/2)^o

or, 0.34 = (0.5) ^o

or, o = 1.5 ~2

m = 1, n = 1 and o = 2

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