Kinetics Studies of the Bleaching of Food Dyes >> Step 3: Determine the effect of bleach on the reaction

Kinetics Studies of the Bleaching of Food Dyes

Step 3. Determine the order with respect to bleach.

Let's summarize what we discovered until now.

In Step 1 we discovered that the more concentrated the bleach, the faster the colour of dye disappeared.

In Step 2 we established that:

• Rate = k' [dye] and
• k' = k [bleach]ⁿ

As the bleach is colourless, you will not be able to determine directly what happens to it in the reaction, however, you will be able to deduce the effect of bleach by analyzing the equation k' = k [bleach]ⁿ : how the concentration of excess bleach influences the rate of reaction.

This rate of the reaction can again be determined from a kinetic trace of the absorbance of the solution, since this tells us how rapidly the dye disappears. From your first experiment you know the order of the reaction in dye, and the pseudo-rate constant, k'. Pseudo indicates that k' is a product of the actual rate constant, k, and the concentration of bleach, k'= k [bleach]ⁿ. Think about what happens to the pseudo-constant, k', when you change the concentration of bleach.

Consider the hypotheses corresponding to the reaction being zeroth, first or second order in bleach. For each of these three hypotheses, what would be the effect on the kinetic trace of changing the concentration of bleach? Design an experiment that you can use to decide which hypothesis is supported.

When you are satisfied with your kinetic trace, print it out. As in step 1, make sure to include your name, Andrew ID, and a descriptive title, which will identify your experiment (i.e. 9.80 mL dye with 0.20mL 0.090M bleach)

    Complete the section of the homework document corresponding to step 3.