Determination of the Rate Law for Food Coloring Bleaching

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

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

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.

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

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

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

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

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

Procedure notes 25 mL volumetric flasks Kinetic runs should last ~10 - 15 minutes Recommend graphing before you leave lab READ the lab carefully

Absorption Spectra of FD&C Food Dyes 630 nm

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 10-15 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?

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