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Determination of the Rate Law for Food Coloring Bleaching

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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

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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 = k x [OCl - ] b Rate = k x [dye] a

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

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

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

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

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

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

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

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Absorption Spectra of FD&C Food Dyes 630 nm

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Procedure Design Considerations 1.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 ) 2.What kind of data will you obtain from the Spectronic-20 spectrophotometer and how will that help you with this determination? 3.How will you set up the Spectronic-20 spectrophotometer? 4. What should you use as your blank?

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Procedure Design Considerations 5.How will you manipulate the data to determine the rate law? 6.How will you deal with the 2 variables in the general rate law? Remember: [dye] o must stay constant. 7.How will you determine k? 8.How will you determine the coefficients a, b? 8.Hint: Do you absolutely need to convert absorbance to concentration? Note: Method of initial rates will not work well here

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