Although bromocresol green is often used as an acid-base indicator, in this problem it is being used — Kinetics Chemistry Question
Problem Context
Although bromocresol green is often used as an acid-base indicator, in this problem it is being used as a reactant. When mixed with a dilute solution of household bleach, bromocresol green is gradually oxidized and changes to a different color, a color that happens to match that of bromocresol green at a pH of 4. Design and carry out an experiment to determine the kinetic order of this redox reaction with respect to bleach. You will be asked to describe the method you developed to solve this problem.
Give a brief description of your experimental plan. List the equipment and materials you plan to use and the steps you plan to take to solve this problem.
Model Answer
Plan: State:
a. Add a small amount of bromocresol green to a well of pH 4 buffer and to a well of pH 7 buffer to determine color at each pH.
b. Measure known amounts of bromocresol green to well plates (by counting drops). Add various amounts of bleach (by counting drops) and record time to color change.
c. Ideally, the reactions will be conducted at constant volume by adding either H2O or pH 7 buffer so that concentration ratios are directly related to volumes of bleach used for each reaction.
Replications should be made for each trial.
Record your data and other observations.
Model Answer
Data: Record:
a. color change of bromocresol green from blue at pH = 7 to yellow at pH = 4
b. drops of bromocresol green added to each well
c. drops of buffer or water added to each well, if used in plan
d. drops of bleach added to each well
e. time required for color change
Replication: multiple trials
Determine the order of the reaction with respect to bleach. Show your reasoning clearly.
Model Answer
To determine the order of reaction, use one of these equations.
Sample Data:
The observed color change for bromocresol green is from blue at pH = 7 to yellow at pH = 4.
Data gathered if reactions are buffered at pH = 7
Trial Drops of Bromocresol Green Drops of Buffer Drops of Bleach Time for color change to occur Average time for three trials
1 20 5 20 14
2 20 5 20 16 15.7
3 20 5 20 17
4 20 1.5 10 28
5 20 1.5 10 27 27.7
6 20 1.5 10 28
Data gathered if reactions are not buffered
Trial Drops of Bromocresol Green Drops of Buffer Drops of Bleach Time for color change to occur Average time for three trials
1 20 5 20 50
2 20 5 20 52 51.7
3 20 5 20 53
4 20 1.5 10 50
5 20 1.5 10 51 51.7
6 20 1.5 10 54
Note: Students do not need to do both buffered and unbuffered reactions.
Sample Calculation:
Using data from the observations from the pH 7 buffered reactions:
rate = k^1 if the reaction is carried out in the buffer.
Using data from the observations from the unbuffered reactions:
rate = k^0 if the reaction is not carried out in the pH = 7 buffer.
Conclusion:
If buffered at pH = 7, then the reaction is first order with respect to bleach.
If unbuffered, then the reaction is zero order with respect to bleach.