# Kinetics Activity

-Introduction

-Background Step 1

-Background Step 2

-Step 1: Making a kinetic trace

-Step 2a: Getting information from a kinetic trace

-Step 2b: Determining the pseudo rate constant k'

-Step 3: Determine the effect of bleach on the reaction

-Step 4: Converting absorbance to concentration

Kinetics Studies of the Bleaching of Food Dyes >> Step 1: Making a kinetic trace

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# Kinetics Studies of the Bleaching of Food Dyes

## Step 1. Follow the disappearance of the dye by measuring the absorbance of the dye during the reaction

We are now ready to move to the first step of our experiment: mixing the dye with bleach and observing how the absorbance of the dye decreases with time: we will be creating a kinetic trace.

In the simulation below, you will be working with a 10.00 mL solution, which you will make up of the dye and the bleach. You will be using a 10 mL serological pipet to deliver the dye and a 1 mL syringe to deliver the bleach to a vial, which you will place into the spectrophotometer. You will have to ensure that the solution is mixed well throughout the experiment by means of a magnetic stirrer. You are given a dye, Yellow 6, whose concentration is 3.40 x 10-5M and two choices of bleach solution: 0.090M and 0.180M.

From the movie you saw at the beginning of this tutorial it is clear that the concentrations of dye and bleach in solution determine the length of time it takes for the colour of dye to disappear. The spectrophotometer takes a reading every second, so your reaction must be long enough for you to be able to observe a clear kinetic trace. We recommend to make your reaction last at least 20 seconds. Try out the simulation first to obtain a long enough reaction and then leave its kinetic trace on the screen and move on to the next step of the tutorial.

The sequence of steps in the simulation is as follows:

1. Use the serological pipet to deliver between 9.00 mL and 9.90 mL of dye into the vial
2. Move the vial into the spectrophotometer
3. Drop the magnetic bar into the vial
4. Use the 1 mL syringe to deliver between 1.00 mL and 0.10 mL of bleach into the vial (to make up the volume to 10.00 mL)
5. Immediately cover the vial with the black lid to start the measurement.
6. When the simulation has completed, select a plot (abs versus time, ln(abs) versus time, 1/abs versus time) that is most useful for determining the order of the reaction and the pseudo rate constant k'. When you are satisfied with your kinetic trace, print it out. 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).
7. Complete the section of the homework document corresponding to step 1.

Note: Simulation may take a few minutes to completely load, depending on your internet connection speed, please be patient.

The following tutor will lead you through the calculations required to determine the initial concentrations of dye and bleach in solution you have just prepared in your simulation. Choose the volumes and concentrations you have actually used to create your kinetic trace.

## Step 2a. Getting information from a kinetic trace

Beer's law tells us that absorbance is directly proportional to concentration of the colour species, and so the observed decay of absorbance versus time has the same shape as the decay of concentration versus time. We can collect our data using the kinetic traces for absorbance vs time and use them to determine reaction orders.

Use the tabs in the upper left hand corner in the simulation above to change the manner in which the data is plotted. The choices are absorbance, ln(absorbance), or 1/absorbance versus time. Remember that the type of plot that leads to a straight line gives you the information you need to determine the order of the reaction with respect to dye.

Now that you have determined the order of reaction with respect to dye, predict what will happen to the rate of your reaction if the concentration of the dye is doubled?

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