AP Chemistry Laboratory #15: Reaction Rate of Crystal Violet and Sodium Hydroxide. Lab days: Thursday and Friday, February 1-2, 2018

Transcription

1 AP Chemistry Laboratory #15: Reaction Rate of Crystal Violet and Sodium Hydroxide Lab days: Thursday and Friday, February 1-2, 2018 Lab due: TBD Goal (list in your lab book): The goal of this lab is to determine the rate law for the reaction of crystal violet (CV) and sodium hydroxide (NaOH). Introduction/Background (DON T write in your lab book): If you re making something, you might think making it to last would always be a good thing. But what if you re making a pesticide with known detrimental impacts on human health? Then you may only want it to stay intact for a few days after it has been applied to crops before it decomposes into what often are less harmful products. If its molecules stay intact for too long, the pesticide can persist in the environment and build up in drinking water. In the year 2000, over 20 million kilograms of the pesticide 1,3-dichloropropene (known as 1,3- D) were applied to crops in the United States. Scientists investigated the rate of decomposition of 1,3-D in acidic, basic, and neutral solutions as well as in soil. For each case, they generated plots of the amount of intact 1,3-D persisting versus time and found that the reaction could be characterized as pseudo first-order. Knowing the order of the reaction allowed them to determine the half-life of intact 1,3-D. In acidic media, they found that the half-life of the decomposition of 1,3-D was about eight days, but in the presence of excess NaOH, the half-life was reduced to about four days. Experimentally-determined data like these are vital to the ability of society to use chemicals wisely in improving food production while not endangering the end-consumers or the people who work with the chemicals during the growing process. Beer s Law (also called the Beer-Lambert Law), which we utilized in labs 10 and 11, uses a spectrophotometer (or colorimeter) to obtain a calibration curve that is used to convert raw absorption data to concentration of a chemical in solution. In this lab, students will first use a colorimeter to generate a calibration curve for crystal violet (CV) and then use the colorimeter again to follow the change in the concentration of CV as it reacts with NaOH. By recording these changes through time and analyzing them graphically, students will be able to obtain the rate law of the reaction, which then may be used to predict the behavior of the system under different experimental conditions (concentrations) without doing the actual experiments. In the reaction of CV and sodium hydroxide (see Figure 2 below), the dye s color will fade as it reacts with sodium hydroxide. A spectrophotometer will be used to follow the disappearance through time of CV by measuring the absorbance of a solution of CV during its reaction with NaOH. The raw absorbance measurements from the spectrophotometer can be transformed to molar concentration of CV via the use of a Beer s Law calibration curve.

2 The net ionic equation for the reaction can be written as CV + (aq) + OH - (aq) > CVOH (aq) The rate law for this reaction is then rate = k [CV + ] w [OH - ] z [Equation 1] where k is the rate constant and w and z are the order of the reaction with respect to CV + and OH -, respectively. Under certain experimental conditions (which we will be using see research question 1c), the rate law above can be simplified to the following equation: rate = k* [CV + ] w [Equation 2] where k* = k [OH - ] z [Equation 3] and k* is the pseudo-rate constant. Equation 2 is referred to as the pseudo-rate law, since it is an approximation of Equation 1, the actual rate late, and significantly simplifies our analysis. Research questions (answer in your lab book in complete sentences; don t write the questions): NOTE: Research questions 1 through 4 must be completed before the lab begins; research questions 5 through 8 may be completed after Day 1 of the lab. 1) In this lab, we will use 25 ìm crystal violet solution and M sodium hydroxide solution. (a) What does ì stand for? (b) Which of the two solutions is more concentrated? How much more concentrated is it (ratio)? (c) Based on the ratio and the reaction equation, which reactant do you predict will be limiting? Which will be in excess? 2) Look at the graph of absorbance vs. wavelength for the 25 ìm crystal violet solution in a spectrophotometer (below). A spectrophotometer is typically not sensitive enough to reliably measure absorbance values much above 1.0, yet the absorbance at the chosen wavelength should be high enough that it can vary over a wide range of values during the reaction. Our colorimeters have the wavelength choices of 430 nm, 470 nm, 565 nm, and 635 nm. Based on the graph and information above, what wavelength should you use for this lab? Please explain your answer.

3 3) A calibration curve requires the preparation of a set of solutions with known concentrations of CV which are usually prepared by diluting a stock solution. Describe how to prepare 10. ml each of 5.0 ìm, 10 ìm, 15 ìm, and 20 ìm CV solutions using 25 ìm CV stock solution. (Both show your calculations and indicate how you would use the results of the calculations to make the actual solutions.) 4) Simulate the instrument readings you will get during the calibration curve part of this lab by doing the following: (a) Trace the absorbance graph from research question 2 into your lab book. (b) Draw a vertical line at the wavelength you chose in research question 2, intersecting the absorbance curve. Where your vertical line intersects the absorbance curve is the absorbance value your colorimeter should read for the stock 25 ìm CV solution. (c) Draw X s on your vertical line where you expect the absorbance values will be for the diluted solutions you described preparing in research question 3. (Hint: Think Beer s law!) 5) During the reaction of CV and NaOH, do you expect the spectrophotometer s absorbance readings to change? If so, how do you expect the readings to change (increase or decrease) as the reaction proceeds? If not, why is no change in the absorbance readings expected during the reaction? For either answer, explain your reasoning. 6) (a) How is a differential rate law different from an integrated rate law? (b) What is the differential rate law equation for this reaction? (Use variables for the parts we will solve during this lab.) (c) What does the integrated rate law look like for this reaction in terms of crystal violet? (d) Why will we focus on the changes in the crystal violet molarity instead of the sodium hydroxide molarity during this experiment? Be specific. (There are two good answers, but you only need to include one.) 7) (a) What is meant when a reaction is called 0th, 1st, or 2nd order? (Be specific!) (b) Describe the graphical analysis that can be done to determine the order (considering only 0th, 1st, and 2nd order) and the value of the pseudo-rate constant k* of a chemical reaction from concentration data collected through time. (Hint: See Chapter 12.4 or use an Internet resource such as (Question 8 is on the next page.)

4 8) Given that you don t yet know the order of the reaction of CV with NaOH, how might Figure 3 (below) help you decide when to stop collecting data? (Hint: Think in terms of percent completion instead of concentration.) NOTE: Reactions A, B, and C are different reactions with the same value for k and same initial reaction concentrations. Materials (DON T list in your lab book): 1 cuvet 1 plastic test tube rack 1 small test tube brush 1 LabQuest with colorimeter probe 3 10 ml pipets 1 pipet pump 3 30 ml beakers 1 test tube brush 1 permanent marker lint-free wipes 25 ml 25 ìm crystal violet solution 10 ml M sodium hydroxide solution 6 16 x 125 mm test tubes Hazards (list in your lab book): (Include the safety contract and the hazards of crystal violet and sodium hydroxide.) NOTE: You must wear gloves when handling the crystal violet (because it will stain everything) and the sodium hydroxide (because of its hazards). Procedure - Day 1 (DON T list in your lab book): 1. Physically and chemically clean the the pipets, test tubes, and beakers 2. Obtain a sample of crystal violet stock solution in a labeled 30 ml beaker 3. Obtain a sample of distilled water in the second labeled 30 ml beaker 4. Make 10 ml samples of the following dilutions in the test tubes using the pipets: 5 ìm crystal violet solution 15 ìm crystal violet solution 10 ìm crystal violet solution 20 ìm crystal violet solution (see research question 3 for specific details) 5. Prepare a blank cuvet using distilled water. (a) Fill the cuvet 3/4 full with distilled water (b) Wipe the outside of each cuvet with a lint-free tissue. (c) Handle cuvets only by the top edge of the ribbed sides. (d) Dislodge any bubbles by gently tapping the cuvet on a hard surface. (e) Always position the cuvette so the light passes through the clear sides. 6. Connect the colorimeter to LabQuest.

5 7. Calibrate the colorimeter. (a) Place the blank in the cuvet slot of the colorimeter and close the lid. (b) Press the < or > buttons on the colorimeter to set the wavelength to the wavelength you chose in Research Question 2. (c) Calibrate by pressing the CAL button on the colorimeter. When the LED stops flashing, the calibration is complete. 8. Collect the absorbance values of the diluted solutions and stock solution using the colorimeter. 9. Prepare a blank solution for the reaction in a 16 x 125 mm test tube (a) Add 6.00 ml distilled water to the new, labeled test tube using a pipet. (b) Add 4.00 ml of sodium hydroxide solution to the test tube using a second pipet. 10. Add the mixture to a cuvet to blank the colorimeter. 11. Re-calibrate the colorimeter. (a) Place the new blank in the cuvet slot of the colorimeter and close the lid. (b) Use the same wavelength as before. (c) Calibrate by pressing the CAL button on the colorimeter. When the LED stops flashing, the calibration is complete. 12. Choose New from the File menu. 13. Set up the data-collection mode. (a) On the Meter screen, tap Rate. (b) Change the data-collection rate to 1 sample/second. (c) Change the data-collection duration to 200 seconds. Select OK. 14. Prepare a sample of crystal violet and sodium hydroxide. (a) Add 6.00 ml stock crystal violet solution to a new, labeled test tube (b) Add 4.00 ml sodium hydroxide solution to the test tube 15. Quickly add the crystal violet and sodium hydroxide mixture to a clean cuvet 16. Measure the absorbance of the crystal violet and sodium hydroxide mixture in the colorimeter until the reaction is complete. 17. Save the file with partner names and hour. 18. Clean up. Post-lab calculations (Answer in lab book and show work; some may be completed on a computer or graphing calculator): 1) Make a graph of absorbance vs. concentration for the crystal violet solutions. 2) Determine the equation of the best-fit line of that data using a graphing calculator or computer spreadsheet program. List the equation of the line [y = mx +b; Absorbance = (slope)(concentration) + intercept] and the R or R 2 value of the linear regression. 3) Convert the absorbances of the crystal violet and sodium hydroxide mixture over time to concentrations over time using your equation from post-lab question 3. 4) Make a graph of concentration vs. time for the crystal violet and sodium hydroxide mixture. 5) Use your knowledge of the graphical analysis of integrated rate laws (see research question 7b) to create two other graphs to determine the order of the reaction. 6) Determine the order w of the pseudo-rate law for this reaction in terms of the crystal violet. Describe why you chose that order. (NOTE: Choose only 0th, 1st, or 2nd order.) 7) Determine the pseudo-rate constant k* for this reaction. Describe how you determined it. Lab handout based on the experiment Investigation 11: What is the Rate Law of the Fading of Crystal Violet Using Beer s Law? in AP Chemistry Guided-Inquiry Experiments: Applying the Science Practices (College Board, 2013)