Practice Problem 10
Use the experimental data found in the table at the beginning of this lesson to determine whether the reaction between phenolphthalein (PHTH) and the OH- ion is a first-order or a second-order reaction.
The first step in solving this problem involves calculating the natural log of the phenolphthalein concentration, ln (PHTH), and the reciprocal of the concentration, 1/(PHTH), for each point at which a measurement was taken:
|(PHTH) (mol/L)||ln (PHTH)||1/(PHTH)||Time (s)|
We then construct graphs of ln (PHTH) versus time (Figure 1)and 1/(PHTH) versus time (Figure 2). Only one of these graphs, Figure 1, gives a straight line. We therefore conclude that these data fit a first-order kinetic equation, as noted in the beginning of this lesson.
Rate = k(phenolphthalein)
Figure 1. A plot of the natural log of the concentration of phenolphthalein versus time for the reaction between phenolphthalein and excess OH- ion is a straight line, which shows that this reaction is first order in phenolphthalein.
Figure 2. A plot of the reciprocal of the concentration of phenolphthalein versus time for the reaction between phenolphthalein and excess OH- ion is not a straight line, which shows that this reaction is not second order in phenolphthalein.