formal lab report 3

Lab 11: Beer's Law and Crystal Violet Advanced Placement Chemistry Partners: Aruha Khan & Madeline Phillips April 7th, 2016

A. Purpose: The purpose of this experiment is to collect absorbance data using a spectrophotometer for crystal violet to generate a Beer’s law calibration curve and also to collect absorbance data as a function of time for the reaction of CV with NaOH to determine the rate law of the reaction.

B. Abstract: The problem of this lab centered around how to find the complete rate law for the reaction that took place. The reaction of crystal violet with sodium hydroxide was experimentally determined to be first order with respect to crystal violet. The pseudo rate constants of both trials of experiments were found to be k'1 = 1.373E-2s^-1 & k'2 = 1.325E-2s^-1. The rate law of the overall reaction was determined to be Rate = 0.137 [CV] [OH]. These results were found by performing an experiment that found a calibration curve for Beer's law as well as two graphs of absorbance v. time.

C. Pre-Lab: 1. a.) Based on the absorption spectrum of 25 µm crystal violet in figure 1 and taking into account the considerations that follow, what wavelength should you use for the Beer’s law calibration curve and subsequent reaction of CV with NaOH? Please explain your answer. The wavelength used should be about 590nm. This wavelength does not actually work though, as this would result in over-saturation of light. In general the solution will be most sensitive at a point right before the peak, or 565nm. b.) Simulate the instrument readings you will get in Part 1 of the experiment by doing the following: Trace Figure 1 onto your paper. Draw a vertical line at the wavelength you have chosen, intersecting the absorbance curve at that wavelength. Where your vertical line intersects the absorbance curve is the absorbance value your instrument should 1! of !7

read for the stock 25µm crystal violet solution. Keeping in mind Beer’s law from Equation 1, and being mindful that the wavelength and path length are fixed, draw X’s on your vertical line where you expect the absorbance values will be for the diluted solutions you prepare in Question 2. Use appropriate ratios of concentrations to determine where on the vertical line to make your marks. Used Beer's law equation (A = abc): 1.5 (found from graph) = a * 1 (given) * 20µm, solving for a. Use for each dilution.

2. A calibration curve requires the preparation of a set of known concentrations of CV, which are usually prepared by diluting a stock solution whose concentration is known. Describe how to prepare 10mL of a 5-, 10-, 15-, and 20-µm crystal violet using a 25µm crystal violet stock solution. The diluted solutions can be prepared by using distilled or deionized water. Specifically, use the dilution equation to get her these values. For example, the algebraic equation to find the amount of distilled water to use in the 20µm concentration should be: (25µm) (V1) = (5µm) (10mL). So the diluted solutions must use 2, 4, 6, and 8mL for 5, 10, 15, and 20µm respectively. 3. During the reaction of CV with NaOH, do you expect the spectrophotometer’s absorbance reading to change? How do you expect it to change if such a change is anticipated (increase, decrease, no change) as the reaction proceeds. Explain your reasoning. The absorbance 2! of !7

should decrease because as the reaction proceeds, there should be a lower amount of moles of crystal violet—as they are being used to drive the reaction forward. Lower moles in this case means lower absorbance. 4. Answer the following questions for a reaction of CV with NaOH in these two scenarios: a solution with a 1:1 NaOH:CV mole ration and a solution similar to what you will be using with a 1000:1 NaOH:CV ratio. a.) Using your prior knowledge of reaction stoichiometry, what is the final percentage of each reactant remaining if each reaction went to completion? Show work and reasoning to justify your answer. For the one-to-one ratio, the final percent of reactant remaining should be 0% for each substance, as there are equal amounts, so they are both limiting reactants. For the one thousand-to-one ration, about 99.9% should be remaining of the sodium hydroxide, and 0% of the crystal violet should be left, as the CV is the limiting reactant. b.) Based on this result, describe how one gets equations 3 and 4 from equation 2 in the explanation section. Equations 3 & 4 are essentially broken down and simplified versions of equation 2. These equations are approximations of equation 2, made to simplify the analysis process. 5. Describe the graphical analysis that can be done to determine the order and the value of the pseudo-rate constant k*, of a chemical reaction from concentration data collected through time. To find the rate order from a graph, divide one by the concentrations of the CV ions and/or take the natural lot of the concentrations of the CV ions. These both indicate either first or second order reactions, and when plotted, whichever one describes a straight line will be the actual rate order. Once the rate order is known, the integrated rate law can be used to find the value of k*. 6. Based on your answer to questions 3-5, design an experiment for the reaction of CV with NaOH and describe the subsequent data analysis to accomplish the Central Challenge, the determination of (i) the value of w (the order with respect to CV) and (ii) the pseudo-rate constant found in the rate law in Equation 3. For simplicity, use 10.mL for the combined

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volume of CV and NaOH because it is a bit more than enough to fill cuvettes appropriately. Experiment/procedure is listed in the procedure section of this lab report. 7. Answer the following questions after examining figure 3 to address the issue of when to stop collecting data. a.) For early parts of the three different reactions in Figure 3, all three curves seem relatively linear with different slopes. But as the reactions progress through time, at roughly what concentration level would you say some graphs start to look non-linear? The lowest concentration level where the nonlinear behavior of the graph is revealed is at about 3M. This shows that both the second and first order reactions are curved, meaning the reaction must be zeroth order. b.) Given that you don’t yet know the order of the reaction of CV with NaOH, how might Figure 3 help you to decide when to stop collecting data? Think in terms of percent completion instead of concentration. I would not stop collecting data until after 80% of the crystal violet has been used in the reaction. This would ensure that the curves in the slopes for whatever orders the reaction isn't are revealed.

D. Procedure: Part I 1.) Use a cuvette of distilled water to calibrate your spectrophotometer according to its instructions. 2.) Configure the spectrophotometer’s wavelength to 565nm. 3.) Prepare 10-mL of the following crystal violet solutions in a 10-mL graduated cylinder: 25µm solution, 20µm solution, 15µm solution, 10µm solution, & 5µm solution, separately. 4.) Pour and appropriate amount of each aforementioned solution into a cuvet (clean between uses with distilled water & handle with ridged sides). Place each cuvet of solution into spectrophotometer and record absorbance. Part II 1.) Pour 5-mL of CV and NaOH into two separate 10-mL graduated cylinders. 2.) Start a timer as soon as you combine the crystal violet and sodium hydroxide into a single graduated cylinder. 4! of !7

3.) Pour the combination solution prepared in step two into a cuvette and insert the cuvette into the spectrophotometer. 4.) Record the absorbance of the solution into the spreadsheet every twenty seconds until eight minutes gave passed, then stop timing and record exact time elapsed. 5.) Repeat steps one through four with a a combination of 6-mL of crystal violet and 4-mL of sodium hydroxide, as illustrated in steps one and two. 6.) Clean all laboratory equipment and dispose of solutions as instructed.

E. Materials Used: • Berol-type pipet • 10-mL graduated cylinder • Sodium hydroxide, NaOH, 9-mL, 0.200M • Crystal violet, CV, 11-mL, 25µm • Water, distilled or deionized • Timer • Spectrophotometer • Blank cuvets • MacAir (w/ LoggerPro software)

F. Data:

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G. Calculations/Results: Statement of Results: The complete rate law for the crystal violet - hydroxide reaction was found to be Rate = 0.137 [CV] [OH]. This was done by using Beer's law, the dilution equation and an equation which related k', hydroxide concentration, and k. Firstly, find the slopes of the natural log graphs of T1 and T2. These values will be equal to k'. k'1 = 1.373E-2s^-1 & k'2 = 1.325E-2s^-1 of T1 & T2 Secondly, use the dilution equation to find the molarities of the hydroxide in T1 and then in T2. (0.200M) (5mL) = (M2) (10mL); M2 = 0.10M (T1) (0.200M) (6mL) = (M2) (10mL); 0.12M (T2) Thirdly, plug the aforementioned values into this equation: ([OH-]1/[OH-]2)^m = (k')1/(k')2. Solve for m. (0.10M)/(0.12M)^m = (1.373E-2s^-1)/(1.325E-2s^-1) 0.83^m = 1.036 m = 1 (approximately) Fourthly, use the k' and the OH concentration values from T1 in this equation: k = k'/[OH-]^m. Solve for k. k = 1.373E-2s^-1/0.10M k = 0.137M^-1•s^-1 6! of !7

Lastly, write the full rate law. Rate = 0.137 [CV] [OH-]

H. Analysis: The point of this lab was essentially to determine the full rate law of the crystal violet - sodium hydroxide reaction. In doing this, the lab taught me how to use a calibration curve to find the concentration value of crystal violet (with Beer's Law). It also taught me how to use graphs of the concentrations, or natural log or reciprocal of the concentrations, to find which rate order the crystal violet was. Then I learned how to find a k' value for the hydroxide, and from that calculate the actual k value and then the full rate law. In doing this, the lab succeeded, as I was able to calculate a full rate law for the reaction. Possible sources of error in the lab include the use of Berol-type pipets and the remnants of distilled water in the cuvets used. Using Berol-type pipets to measure out milliliters of hydroxide and crystal violet decreased the precision with which my group was able to attain our amounts of said chemicals. This means that we quite possibly had more or less moles of either crystal violet or hydroxide. The possibilities caused by this are quite numerous, and our rate constant value—even the order attained for the hydroxide—could be inaccurate/incorrect as a result of this. Also, the leftover distilled water in the cuvets would've diluted both the crystal violet - distilled water solutions more than they should've been, it also (likely) slightly diluted the crystal violet - hydroxide solutions. This would've decreased the absorbance values that we attained more than they should've been, meaning our k', and thus our k, could possibly be lower than it should be.

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