Transcript 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.

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
0.25
Absorbance (630 nm)
0.20
Linearize and
calculate k’
0.15
0.10
0.05
0.00
0
100
200
300
400
Time (s)
500
600
700
800
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
0.25
Absorbance (630 nm)
0.20
Linearize and
calculate the
new k’
0.15
0.10
0.05
0.00
0
100
200
300
400
Time (s)
500
600
700
800
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
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?
Procedure Design
Considerations
5.
6.
7.
8.
8.
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