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Determination of a Rate Law by the Method of Initial Rates
Determination of a Rate Law by the Method of Initial Rates
PROCEDURE
Table 1 summarizes the volumes required for trials 1-12. All trials will be carried out in 150-mm test tubes.
- Preparation of solution A: Pipette the listed volumes for the various solutions into a clean, dry test tube. Add the starch from a dropper. Prepare all of solution A for trials 1-6 at the same time.
- Preparation of solution B: Pipette the indicated volume of 0.10 M H 2 O 2 into a small container (10-mL beaker). Prepare just before use in each trial.
- Prepare to time the reaction. Place a piece of white paper behind the test tube rack so that you can easily see the color change.
- Add solution B rapidly to the test tube with solution A for the trial. START TIME immediately. Stopper the tube and invert twice (only 2 times!). Remove the stopper and place into the test tube rack.
- The change to a deep purple color will be sudden. Be prepared! STOP TIME when the blue color appears.
- Record the time in seconds. Measure and record the temperature of the reaction mixture.
- Repeat steps 2-6 for the additional kinetic trials.
- Repeat steps 1-7 above for trials 7-
- Run a Trial “Zero” that is just like trial #1, but excludes the thiosulfate solution. This should help to illustrate dramatically the effect of the thiosulfate on the reaction mixture.
DATA ANALYSIS
- Prepare a results table to summarize the data below.
- Calculate the moles of S 2 O 32 −^ ions consumed in each trial. From the stoichiometry of the reaction, calculate the moles of I 2 consumed by the reaction with the thiosulfate. We will designate this ∆(mole I 2 ). These values are the same for each trial and serve as the “clock” - when the moles of I 2 produced exceeds the stoichiometric ratio to the thiosulfate, it will complex with the starch to give the deep blue color of the starch-iodine complex.
- Calculate the initial rate of the reaction for each trial with the equation: where ∆t is the elapsed time.
- Calculate the log (rate) for each trial.
- For each trial, calculate the initial concentrations of hydrogen peroxide [H 2 O 2 ] 0 and iodide [I−] 0 as well as log [H 2 O 2 ] 0 and log [I−] 0. Note: Consider the volume of the drops in the calculation of the initial volumes of the reactants. 1 drop ≈ 0.05 mL
∆t
∆mol rate
( I 2 )
Prepare a data/analysis table that summarizes, for each trial, all of the measured and calculated values describe above.
Preliminary order determinations:
- Select two trials in which the [I−] 0 is constant between trials. Analyze the difference in the [H 2 O 2 ] 0 and the rate of reaction to make a preliminary determination of the order of the reaction for [H 2 O 2 ].
- Select two trials in which the [H 2 O 2 ] 0 is constant between trials. Analyze the difference in the [I−] 0 and the rate of reaction to make a preliminary determination of the order of the reaction for [I−].
Graphical order determinations:
- Graph #1: Plot log [I−] 0 (x-axis) vs. log (rate) (y-axis) for trials 1-6. Determine the slope of the line. The slope should be a good approximation of the order of the reaction for I−.
- Graph #2: Plot log [H 2 O 2 ] 0 vs. log (rate) for trials 7-12. Determine the slope of the line. The slope should be a good approximation of the order of the reaction for H 2 O 2.
RESULTS
- State the differential rate law expression for the reaction of the iodide ion and hydrogen peroxide.
- Calculate k´ for each of the seven trials and determine the average value. Calculate the standard deviation.
Table 1. Composition of Reaction Mixtures
Solution A Solution B
Trial
Buffer (0.5 M CH 3 CO 2 H / 0.5 M NaCH 3 CO 2 )
0.020 M
Na 2 S 2 O 3 Starch DI Water 0.30 M KI 0.10 M H 2 O 2
1 1.0 mL 1.0 mL 5 drops 6.0 mL 1.0 mL 3.0 mL 2 1.0 mL 1.0 mL 5 drops 5.0 mL 2.0 mL 3.0 mL 3 1.0 mL 1.0 mL 5 drops 4.0 mL 3.0 mL 3.0 mL 4 1.0 mL 1.0 mL 5 drops 3.0 mL 4.0 mL 3.0 mL 5 1.0 mL 1.0 mL 5 drops 2.0 mL 5.0 mL 3.0 mL 6 1.0 mL 1.0 mL 5 drops 1.0 mL 6.0 mL 3.0 mL 7 1.0 mL 1.0 mL 5 drops 7.0 mL 2.0 mL 1.0 mL 8 1.0 mL 1.0 mL 5 drops 5.5 mL 2.0 mL 2.5 mL 9 1.0 mL 1.0 mL 5 drops 4.0 mL 2.0 mL 4.0 mL 10 1.0 mL 1.0 mL 5 drops 3.0 mL 2.0 mL 5.0 mL 11 1.0 mL 1.0 mL 5 drops 1.5 mL 2.0 mL 6.5 mL 12 1.0 mL 1.0 mL 5 drops none 2.0 mL 8.0 mL 0 1.0 mL 1.0 mL none 6.0 mL 1.0 mL 3.0 mL
- State the purpose of each of the following solutions in this experiment.
A) Buffer solution
B) Starch solution
C) Sodium thiosulfate solution
D) Deionized water
- Consider the reaction studied in this experiment (Equation 24.2). It is an oxidation-reduction reaction. Write the ½-reactions for this overall reaction.
- Write the equation for the reaction that serves as the “clock” for this reaction.