Experiment 11 - Kinetics of Food Dyes >> Step 2: Getting information from a kinetic trace

Experiment 11 - Kinetics of Food Dyes

Step 2. Getting information from a kinetic trace

Determining the pseudo rate constant k'

For a first order reaction the integrated rate law is:

However, we measure the absorbance, which is proportional to the concentration of dye. So we instead write the integrated rate law as:

[Note: this takes advantage of the properties of logarithms. The absorbance is proportional to dye concentration (absorbance = const [dye]), and so ln(absorbance) = ln(dye) + ln(const). The ln(const) appears on both the left and right hand side of the integrated rate law, and so cancels: ln(dye) + ln(const) = - k' t + ln(const) + ln(dye)₀)]

We plotted ln([absorbance]) on our y axis and time, t, on our x axis. The result is a straight line whose slope is m = - k', and whose intercept is b = ln(absorbance)₀. So the slope we measure from our plot gives us the quantity we want, the pseudo rate constant k'.

Record the k' value on your feedback form.

At this point, you have determined the order of the reaction with respect to dye, and calculated the pseudo first-order rate constant, k'. What we next need to do is determine the effects of the bleach concentration on the rate of the reaction.