Chemical Kinetics: Finding the Rate Law (Kathryn Smith)

Research Question and Hypothesis

• Research Question: What is the rate law, the concentration change over time, of the reaction between crystal violet and sodium hydroxide measured under constant pressure and temperature?

• Hypothesis: If the data is linear, the rate law is zero order; if the data has a logarithmic relationship, the rate law is first order; If the data has an inverse relationship, the data is second order.

Standards

• HS-PS1-5 Students who demonstrate understanding can apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.

• DCI PS1:B Chemical Reactions Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy.

• CCC Patterns Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.

Before teaching this lab, students need to be familiar with the graphs and formulas associated with rate laws and integrated rate laws.

Experimental Design

• In this experiment, students use a colorimeter, a spec 20, or a UV/VIS spectrometer to measure the absorbance of light of a sample of crystal violet. When sodium hydroxide is added to the sample, the crystal violet looses it's color. Beer's law can be applied to find the concentration of the sample over time. Students will plot their absorbance (concentration) versus time data, ln(abs) vs time, and 1/Abs versus time. The graph that is linear is the one that matches a zero, first, or second order rate law.

  • Phenolphthalein or Malachite Green could also be used for this type of reaction instead of or in addition to crystal violet.

Independent variable

Time is the independent variable in this experiment. Some spectrometers come with software that can measure the absorbance over time for the student. Personally, I have the kids measure the time with a handheld timer or a cell phone app.

Dependent variables

Absorbance of light at 592 nm. The absorbance values need to be between 0.2 and 1.0 on most spectrometers to be considered accurate.

Series

Study the concentration of colored compounds reacting with sodium hydroxide (dependent variable) as a function of time (independent variable) for crystal violet, phenolphthalein, and Malachite Green(series).

Constants and Controls

The concentration of sodium hydroxide should be held constant at about 0.05 M. Most lab manuals describing this lab suggest a 0.02 M solution, but in my lab I found that these reactions were two slow to measure in one class period. Increasing the concentration to 0.05 M is more practical for those of use who teach in 55 minute periods.

Materials

• Lab Manual:

  • This lab is in several lab manuals I've encountered, but I think that the AP Chemistry Lab manual from Flinn (Vonderbrink) is really spectacular at explaining why the crystal voilet changes color.

• Spectrometer

  • There are several options for equipment that measures the absorbance of light. If you are low on funds, Vernier has a visible light range spectrometer called the SpectroVis Plus that runs around $470. However, with all Vernier equipment, you need to have an interface, the most recent of which is the LabQuest 2. The LabQuest 2 is around $330.

  • Flinn and several other suppliers also sell stand alone spectrometers, the Flinn Scientific Spectrophotometer ($950) or the Spectronic 200 ($1500).

• Chemicals

  • Crystal Violet Solution

  • Sodium Hydroxide

Procedures

• Set a spectrometer to read absorbance at 592 nm, the maximum absorbance of crystal violet (it would be a different wavelength for phenolphthalein or Malachite Green).

• Place 1.5 mL of 25 mirco M crystal violet solution in a cuvette.

• Place the cuvette in the spectrometer.

• Simultaneously add 1.5 mL of 0.05 M NaOH to the cuvette and start the timer.

• Record the absorbance value every 20 seconds until the absorbance is the same for one minute (takes about 640 seconds).

Sample data and graphs

Include raw data and graphs. Make certain to include units in all measurements, titles on all graphs, and labels on all axes.

Raw Data:

Analysis & Conclusions

According to Beer's Law, the absorbance of light is directly proportional to the concentration of a solution. In this lab, the absorbance of the crystal violet was measured every 20 seconds for about 12-15 minutes. The absorbance versus time was plotted and the result was not a linear relationship, therefore the reaction is not zero order. The natural log of the absorbance was plotted versus time and the relationship was linear, which implies that this reaction is first order. Just to be sure, the inverse of absorbance was plotted versus time, which also does not imply a second order reaction.

To find the rate law of the reaction between crystal violet and sodium hydroxide, the rate constant needs to be determined from the linear graph. In a first order reaction, the slope of the ln(abs) vs time graph should the equal to the negative slope, k = -m. By adding a linear trendline to the graph, the best fit equation provides the slope.

The slope in this case is -0.0019, therefore the rate constant is 0.0019.Conclusion: The rate law for the reaction between crystal violet is:Rate = 0.0019[CV⁺]

Photos & Movies

Include photos and movies of your longitudinal research. Post your photos in the class photo album and your movies in your Youtube account.

References

Laboratory Experiments for Advanced Placement Chemistry (Sally Ann Vonderbrink). "Kinetics of a Reaction." Publisher: Flinn Scientific.