Blue food coloring can be oxidized by household bleach (which contains OCT-) to

Question

Blue food coloring can be oxidized by household bleach (which contains OCT-) to form colorless products, as represented by the equation above. A student used a spectrophotometer set at a wavelength of 635 nm to study the absorbance of the food coloring over time during the bleaching process. In the study, bleach is present in large excess so that the concentration of OCI- is essentially constant throughout the reaction. The student used data from the study to generate the graphs below. (a) Based on the graphs above, what is the order of the reaction with respect to the blue food coloring? (b) The reaction is known to be first order with respect to bleach. In a second experiment, the student prepares solutions of food coloring and bleach with concentrations that differ from those used in the first experiment. When the solutions are combined, the student observes that the reaction mixture reaches an absorbance near zero too rapidly. In order to correct the problem, the student proposes the following three possible modifications to the experiment. Increasing the temperature Increasing the concentration of the food colorin Increasing the concentration of the bleach Circle the one proposed modification above that could correct the problem, and explain how that modification increases the time for the reaction mixture to reach an absorbance near zero. (c) In another experiment, a student wishes to study the oxidation of red food coloring with bleach. How would the student need to modify the original experimental procedure to determine the order of the reaction with respect to the red food coloring?

Explanation / Answer

a) The first order rate law is written as

First, write the differential form of the rate law.

Rate=d[A/]dt=k[A]

Which can be further rearranged to

ln[A]=kt+ln[A]o

This is in the form of

y=mx +b

where y = ln[A] and mx = kt

so plotting ln[A] versus t will give a straight line the slope of which is -k

SO from the plots above it is clear that the reaction is first order with respect to blue food colouring.

b) The rate of the reaction is first order with respect to bleach and first order with respect to blue coloring.

Now if the absorbance reaches zero very fast it means the blue color is bleached very fast. Since we have seen that in the presence of excess of bleach we can still get absorbance based kinetics. It means in this case increasing bleach will not increase the time for blue coloring disappearance.

Increasing temperature will only make the reaction faster.

The only alternative is to incraese the concentration of the blue coloring so that the starting point of the kinetics being higher the reaction will have longer life time.

c) If the student needs to study the oxidation of red food coloring with bleach in the said experiment which he conducted he can replace the blue food coloring with the red food coloring and study the same kinetics in the presence of excess bleach to check the kinetics with respect to red food coloring.

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