*Chemical Kinetics Lab

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Grand Canyon University *

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115

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Chemistry

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Feb 20, 2024

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Abstract An abstract is a brief, one paragraph, statement of the results obtained and the method(s) used. Make sure to report actual values for one or two of your most important results. 2

Introduction Chemical reactions occur constantly in the world with many factors effecting the reactions such as polarity, concentration, absorbance, and molarity. The speed in which these reactions occur is called the rate of the reaction. In the lab, Chemical Kinetics , students measured the rate at which the reaction of crystal violet (CV+) and Sodium Hydroxide (NaOH) occurred. Crystal violet is primarily used as a stain in the Gram Staining method identifying bacteria. There are two types of bacteria, Gram Negative and Gram Positive. Gram Negative have a lower amount of peptidoglycan in the walls of the cell membrane. A lower amount of this proteoglycan means staining can occur. The substances in the reaction of the chemical reaction between CV+ and NaOH depend on charges and affinity for electrons. The reaction results in a loss of the brightly pigmented crystal violet because of the dispersion of charges and pH of the solution. The rate of this reaction is measured in the lab, Chemical Kinetics . The effectiveness of a reaction is directly related to the rate at which it operates. Rate is the change in the concentration of products and the concentration of reactants over a period of time. The rate law helps determine this change in concentration. The law is as follows: Rate= k[A] m [B] n In this lab, CV+ was placed in an environment stimulating a reaction with NaOH. In the rate equation, the variables are as follows: [A] and [B] refer to the concentration of the reactants, m and n refer to the order of the reaction, and K refers to the specific rate constant of the reaction. There are three orders of a reaction discussed in this lab: zero order, first order, and second order. Zero order rate reactions are those in which the rate of the reaction is independent of the concentration of the reacting substances. [A] t =[A] 0 -kt 3

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First order are reactions in which the rate is directly proportional to the concentration of the reacting substance. ln[A] t = ln[A] 0 -kt Second order reactions are reactions in which the rate is indirectly proportional to the concentration of the reactants. (1)/([A] t )=[A] 0 -kt When finding the overall rate of the reaction, the R 2 value closest to 1 will be the most accurate. In the lab, Chemical Kinetics , students used the absorbance value of CV+ vs. Concentration (s) previously attained in the Data Acquisition lab. Using this data, the concentration values can be found using the rate law order equations. Procedures in this lab involved data acquisition module and spectrophotometer. Students added 0.100 M of NaOH solution to a cuvette of 1mL of CV+. The reacting solution was then placed quickly into the spectrophotometer and according absorbance values in the reaction were recorded over a span of 150 seconds. The values of these absorbance rates are logged in the data section of the lap report. Using the rate law equations, students were tasked to find concentration and time relationships for zero-order, first-order, and second-order rate reactions. Data and Calculations Table 1: Stock Stock Solution Concentration Molecular Formula Lot # Crystal Violet 0.025M C 25 N 3 H 30 Cl 76-59-5 Sodium Hydroxide 0.100M NaOH CAS 131073-2 Table 2: Glassware Name Manufacturer Volume Serial # Model # Tolerance Graduated Cylinder Elab 10mL E1272 N/A ± 0.10mL Table 3: Data Acquisition Equipment Component Manufacturer Model # Serial # 4

Data Acquisition Module LabQuest LQZLE 16007291 Spectrophotometer Vernier CDVSV15 PL0010655 Chart 1: Time before each sample was placed in the spectrophotometer: Trial 1 12 seconds Trial 2 17 seconds Trial 3 16 seconds All Trial 1 graphs: 5

Trial 2 Graphs: Trial 3 Graph: 6

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Results Final results should be reported with a literature value for comparison whenever possible. If possible the results and literature values should be placed in a single table. Always cite the literature references from which values were obtained. Discussion In this lab, the data acquisition module and spectrophotometry were used in order to evaluate the absorbance of crystal violet (CV+) in a sodium hydroxide (NaOH) solution. In reactions, the rate (speed) of reaction may directly or indirectly relate to the concentrations of the products. In some reactions, concentration may have no effect at all. The intermediate time values in the runs were accounted for in adding the amount of time to the data graph. For example, in trial one, it took 12 seconds to transfer the solution into the spectrophotometer in order to collect data. Absorbance values at a specific time value were recorded. After the data was collected, the Absorbance vs. Concentration graph from the Data Acquisition lab and the equation. The goal of this lab was to determine which reaction order rate the chemical reaction of CV+ and NaOH fall into. As discussed in the introduction, the first order equation is [A] t =[A] 0 - kt, the second order reaction is ln[A] t = ln[A] 0 -kt, the third order reaction is (1)/([A] t )=[A] 0 -kt. [A] 0 is the concentration of the reactants, t is the time, and k the equilibrium constant in the specific reaction. In order to determine the order of the reaction, the K constant of equilibrium must be found. In order to determine the K value for the 3 run trials the equation k = -m/[b] 0 should be used. The m represents the slope of the trend line from the zero-order reaction ( graph 1) and the 7

y-intercept b in the equation. Using those values from the trend line the K value can be determined per trial based on its zero-order graph. This K value is then used to create the first and second order graphs. In order to create graphs for the experiment a concentration vs absorbance graph is used based on a spectrophotometer analysis of the absorption rates of crystal violet. The equation of the trend line for the crystal violet concentration vs absorbance is y = 55676x - .1161. Using this the concentration values for the trails runs can be gathered based on the absorbance values by using the equation (Absorbance – b)/m. The time values, found on the x-axis of the graphs, were adjusted based on the amount of time that it took to get the sample into the spectrophotometer once the chemicals were combined. The Adjusted time values were then graphed in conjunction with the concentration values. This gives the adjusted time values vs concentration. This calculation and graph deriving was done for each of the trials. This is a zero order of the reaction graph. The next step is getting the first order of reaction graphs. To do this the equation ln[A] t = ln[A] 0 -kt is used. Using the k values which were previously discussed. The Concentration in this case will be [A] 0 in the equation. After all of the [A] t values are gathered they are graphed against the adjusted time. This gives the first order of reaction graph. In order to obtain the second order of reaction graph a similar process is used. This time however the equation (1)/([A] t )=[A] 0 -kt is used. The concentration values from the zero-order reaction are used for the [A] 0 in the equation. These values for all times that data points were collected are then graphed against the adjusted times to create a second order of reaction graph. In the data analysis graphs, it was found that the most accurate R 2 value lied within the first order graph (ln[A] t = ln[A] 0 -kt). These values were the closest to one meaning it is the most accurate representation of the concentration. Because the R 2 value is closest to 1 in this graph, 8

the rate of the reaction is therefore first order. The definition of a first order reaction is one in which the rate is directly proportional to the concentration of the reacting substance. Sources of error may include faulty equipment readings regarding absorption. Faulty equipment or left-over residue from a previous experiment could have affected the rate of the reaction as well. Another source of error could have been the time interval of mixing the reactants and the movement to the spectrophotometer. The important thing to note is the R 2 value in the zero order, first order, and second order never equals 1. The R 2 value shows the true accuracy of the graph to the experiment itself. The accurate rate of the experiment cannot be known due to some of these factors discussed. Data Sheets Attach copies of the notebook pages that were used in this laboratory. 9

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References 10

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