1. Determination of the Rate Law for Food Coloring Bleaching

2. Pseudo-Order Kinetics
dye + OCl-  products
Rate = k[dye]a[OCl-]b
Analyzing rate law is difficult when 2 concentrations are varying. So…
Force one to be constant

3. Pseudo-Order Kinetics
dye + OCl-  products
Rate = k[dye]a[OCl-]b
If [OCl-] is large and [dye] is small then the [OCl-] is small
k’ = kx[OCl-]b
Rate = k’x[dye]a

4. Pseudo-Order Kinetics
dye + OCl-  products
Rate = k’x[dye]a
Now the problem is easier. Run the reaction and graphically analyze the data as 0th, 1st, or 2nd order.
Now you have the reaction order in dye. The apparent rate constant can be calculated too.

5. Pseudo-Order Kinetics
dye + OCl-  products
Rate = k’x[dye]a

6. Pseudo-Order Kinetics
dye + OCl-  products
Rate = k’x[dye]a
Linearize and
calculate k’

7. Pseudo-Order Kinetics
dye + OCl-  products ?
Rate = k’x[dye]a
But how do you get the order in OCl- and the true rate constant?
Change the [OCl-] and get a new k’, of course

8. Pseudo-Order Kinetics
dye + OCl-  products
Rate = k’x[dye]a
Linearize and
calculate the
new k’

9. Pseudo-Order Kinetics
dye + OCl-  products
Rate = k’x[dye]a
Remember:
k’ = kx[OCl-]b
From the change in the [OCl-] and k’, deduce ‘b’ and k

10. Procedure notes 25 mL volumetric flasks
Kinetic runs should last ~ minutes
Recommend graphing before you leave lab
READ the lab carefully

11. Absorption Spectra of FD&C Food Dyes
630 nm

12. Procedure Design Considerations
How will you determine the volumes of FD&C blue #1 and bleach to use for the reaction to be complete in approximately minutes.
A good set of starting concentrations are
[dye]o = ~2.5 x 10-6 M [OCl-]o = ~0.02 M
(Stocks: [dye] = ~5 x 10-6 M; [OCl-] = 0.67 M )
What kind of data will you obtain from the Spectronic-20 spectrophotometer and how will that help you with this determination?
How will you set up the Spectronic-20 spectrophotometer?
4. What should you use as your blank?

13. Procedure Design Considerations
How will you manipulate the data to determine the rate law?
How will you deal with the 2 variables in the general rate law? Remember: [dye]o must stay constant.
How will you determine k?
How will you determine the coefficients a, b?
Hint: Do you absolutely need to convert absorbance to concentration?
Note: Method of initial rates will not work well here