  # 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 >> Background Step 2

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# Kinetics Studies of the Bleaching of Food Dyes

## Finding the order of the reaction with respect to dye.

We will consider three possible orders for the reaction: which may be simplified to: Integrating these differential equations gives the dye concentration as a function of time: The following plots show how [dye] changes with time for each of these cases. For zeroth-order reaction, the rate of the reaction is independent of dye concentration, so we get a straight line with slope k'.

For a first-order reaction, the rate is proportional to [dye], so the rate decreases as the reaction proceeds and [dye] decreases.

For a second-order reaction, the rate is proportional to [dye]2, and the decrease in rate as the reaction proceeds is more rapid than for the first order reaction.

A useful technique for identifying first order reactions is to plot ln([dye]) versus time: Note that only the first order reaction gives a straight line.

If we plot 1/[dye] versus time, only the second order reaction gives a straight line: This leads to a useful means to identify the order of reaction. Given a kinetic trace, we compare the following three plots: For this data, the plot of 1/[dye] versus time gives a straight line. This indicates that the reaction is a second order reaction and the rate law is: Suppose the kinetic trace you measured in the lab led to the following three plots: Which of the following is the rate law for the reaction? The description above explains how to use this plotting technique to determine the order of a reaction. To understand why it works, please see the explanation of "integrated rate laws" in the textbook and your class notes.

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