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EXPERIMENT 2: KINETICS Introduction The concentration of a colored solution is proportional to how dark the solution appears. If a light were passed through a series of solutions, less light would pass through the darker, more concentrated solutions. A spectrometer can be used to measure the amount of light a solution allows to pass through it, transmittance, or conversely the amount of light that does not pass through, absorbance. Using Beerโ€™s Law, the concentration of a solution can be found from the measured absorbance. In Experiment 1_Solutions, this technique was used to find the concentration of an unknown solution and a personal solution. In Experiment 2_Kinetics, similar techniques will be used to monitor a chemical reaction between bleach (sodium hypochlorite) and a common blue dye. Using the data recorded in the lab and data provided by this handout, the specific rate law and the activation energy, Ea, for the reaction can be determined. This lab involves the decomposition of a blue dye, Brilliant Blue, found in many blue colored food products. Brilliant Blue dye has a molar extinction coefficient, or molar absorptivity, of ฮต = 1.38 x 105 M-1cm-1 at 630 nm. Reacting the dye with household bleach (a solution of sodium hypochlorite) forms colorless products in solution via the decomposition of the dye. Scheme 1. Reaction of Brilliant Blue dye and Bleach. Brilliant Blue (aq) + NaClO (aq) products blue colorless colorless For this reaction, measuring the change in concentration of the sodium hypochlorite (NaClO) via spectroscopy is not possible due to it being a colorless solution. The reaction progress can be measure, however, by tracking the change in the absorbance of the Brilliant Blue. For this lab, the general rate law is shown in Equation 1, where m and n are the orders of the reaction with respect to brilliant blue and bleach, respectively. Equation 1. Rate law of the reaction between Brilliant Blue dye and Bleach. rate = k [Brilliant Blue] m [NaClO]n A spectrometer will be used to monitor the absorbance of the Brilliant Blue at various time intervals. The absorbance and time data, as well as the initial rate vs concentration data in Table 1, can be used to determine the specific rate law for the reaction, including the rate constant k, at two temperatures. Using the results at the two different temperatures and the two-point form of the Arrhenius equation (Equation 2), the reactionโ€™s activation energy can be determined. Equation 2. Two-point Arrhenius equation. ๐‘™๐‘› ๐‘˜2 ๐‘˜1 = ๐ธ๐‘Ž ๐‘… ( 1 ๐‘‡1 โˆ’ 1 ๐‘‡2 ) Safety Bleach is an oxidizer that can cause severe skin and eye damage as well as damage clothing. The solution used in the lab is diluted. Exercise caution and immediately rinse any skin that comes into contact with the chemical. Wear lab coat and goggles when performing this lab. Gloves will be provided if desired. Procedural Guidelines Two experiments will be performed using the same initial concentrations of approximately 1.1 x 10-5 M Brilliant Blue and 0.0244 M NaClO: at room temperature and at approximately 45 ยฐC (check actual concentrations on bottles prior to starting the experiment). Confirm the spectrometer is set to 630 nm and โ€œzeroโ€ the spectrometer with DI water. To begin the room temperature experiment, mix 5.0 mL of the Brilliant Blue solution with 5.0 mL of the 0.0244 M sodium hypochlorite in a small beaker and immediately start a timer. Fill a cuvette with the mixture. At intervals of about 4 minutes, measure and record the absorbance and the exact time. Continue until the absorbance reading drops below 0.05 or until 10 data points have been recorded. Next, perform the experiment at 45 ยฐC. Using the same setup as the room temperature experiment, mix the same volumes of solutions and fill a cuvette. Place the cuvette in a 45 ยฐC water bath. At intervals of about 1 minute, measure and record the absorbance and the exact time, then return the cuvette to the water bath. Post-Lab Calculations Using the absorbance and time data from both experiments in the lab, determine the order with respect to Brilliant Blue. Using the data in Table 1, determine the order with respect to sodium hypochlorite. Determine the rate constant k at the two temperatures, then determine the activation energy using the Arrhenius equation. Table 1. Initial reaction rates measured at different concentrations of sodium hypochlorite bleach. [brilliant blue] (M) [NaClO] (M) Initial Rate (M/s) 1.12 x 10-5 0.0122 3.945 x 10-9 1.12 x 10-5 0.0244 7.899 x 10-9

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The obtained value of activation energy Ea was 89.3 kJ/mol, while the actual value given in the lab handout is 26 kJ/mol. Considering the experimental and theoretical data, the percent error of the experimental activation energy was 243%, meaning that the measurement was inaccurate...

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