Lab 4 Report Draft 2

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Chemistry

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Apr 3, 2024

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Joshua Dawkins CHM1046L Professor Hawthorne 3/15/24 How Fast Does Crystal Violet Decolorize? Chemical kinetics, a subfield of physical chemistry, concerns itself with understanding the speeds of chemical reactions. A rate law elucidates how the pace of a reaction correlates with the concentration of its reactants. The objective of this experiment is to ascertain the rate at which crystal violet fades in color. Through the examination of three distinct plots and their correlation values, we aim to infer the reaction rate. A correlation value nearing 1 in [CV] vs. Time signifies zero order, ln[CV] vs. Time denotes first order, and 1/[CV] vs. Time represents second order. The reaction, depicted by the equation CV+ + OH- → CVOH, will be evaluated using absorbance spectroscopy—a technique that exploits the wavelength-dependent absorption properties of materials to construct an absorbance-concentration plot, following Beer's law. Beer's law posits that absorbance is directly proportional to both the path length and the concentration of the absorbing species, expressed as A = mx + b, where A represents absorbance, m denotes the slope of the line, x signifies concentration, and b indicates the length of the light path. This methodology facilitates the collection of absorbance, slope, and path length data from a single mixture, thereby enabling the calculation of the unknown concentration. To commence our experiment, we will employ a technique called serial dilution to prepare our test samples. Serial dilution involves gradually diluting a sample through a sequence of standardized volumes of sterile diluent, which may include distilled water. In our experiment, crystal violet serves as our sample, and sodium hydroxide (NaOH) serves as our diluent. Dilution of our crystal violet sample is necessary to prevent absorbance values from exceeding 1.0, as this could lead to molecular proximity causing deviations in absorptivity. This method allows us to establish the relationship between the concentration of a compound and its absorbance.

Subsequently, a measured volume of each diluent is added to create a range of diluted concentrations. The mixture is then placed into a cuvette, which is inserted into the colorimeter to measure absorbance. The colorimeter, an instrument used to gauge color intensity, compares the amount of light passing through the solution to that passing through a sample of pure solvent. As previously mentioned, we can utilize the formula A = mx+b to determine the unknown concentration. For example, in this experiment, we acquired the values A = 0.287, m = 1.5624 x 10^-4, and b = 0.156, resulting in the equation 0.287 = (1.5624 x 10^-4)(x) + 0.156. Solving for "x" using algebraic techniques, we find x = (0.287 – 0.156) / (1.5624 x 10^-4), yielding x = 835.45 when inputted into the calculator. After concluding the experiment and obtaining absorbance values, as shown in Table 1, we proceed to compute our [CV], ln [CV], and 1/[CV] values. Initially, we employ the equation [CV] = (A – b)/slope to determine the concentration of crystal violet. For instance, utilizing the values b = 0.156, slope = 1.5624 x 10^-4, and A = 0.900, we calculate [CV] to be 4761.9. Subsequently, we use this crystal violet concentration ([CV] = 4761.9) to compute the natural logarithm, resulting in ln [4761.9] = 8.4684. Lastly, we determine 1 / 4761.9 to yield the value 2.1 x 10^-4. These individual values are then compared to our time, such as [CV] vs. Time, to ascertain slope and regression values. To discern the order of our reaction as zero, first, or second order, we scrutinize the regression value, which indicates the linearity of our plot and its proximity to 1. In our case, the closest value obtained is -0.9945 with ln [CV] vs. Time, leading us to conclude that the order of our experiment is first order. This implies a chemical reaction in which the rate of reaction is directly proportional to the concentration of the reacting substance. Upon comparison with another group's regression value of -0.998 in ln [CV] vs. Time and (1/[CV]) vs. Time, we observe their values to be closer to 1 than ours. Furthermore, they exhibit identical values in two plots, rendering it impossible to determine the reaction order definitively.

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