Chemical Kinetics Lab. Prepared for: Mrs. Freeman. February 6, 2014

Transcription

1 Running Head: CHEMICAL KINETICS LAB 1 Chemical Kinetics Lab Prepared for: Mrs. Freeman February 6, 2014 Introduction

2 CHEMICAL KINETICS LAB 2 The science of chemistry is, in the simplest context, the formation of substances from other substances; in other words, the concept of the chemical reaction is at the very core of all chemistry. In turn, chemical kinetics, the study of the rates these chemical reactions, is also of paramount importance (Kinetics, n.d.). Chemical kinetics involves the concepts of reaction rates and rate laws. Reaction rates can be the rate at which a product/reactant is formed/used in a reaction and rate laws are the definitions that govern the aforementioned rates of appearance/disappearance. Many factors affect the rate of a reaction. First, the concentration of the reactants plays an important role, as increased concentrations usually correlate to a faster reaction. Increased temperature similarly affects rate, as well as increased surface area of the reactants. Finally, the presence of a catalyst decreases the activation energy of reaction, in turn speeding up the reaction (Intro to Kinetics, n.d.) Furthermore, these concepts are not confined to theoretical use. One example of a real world application of rate law is found in the use of pesticides. When pesticides are used in an environment to keep insects away, rainfall and other sources of water, such as sprinklers, will wash off the pesticides from the plants, and the runoff will contain traces of these pesticides. To prevent the contamination of freshwater sources, such as rivers, scientists must find and create pesticides that dissolve into non harmful chemicals when reacted with water, and this rate of reaction is of utmost importance (Mathematical Prediction, n.d.) Another use of rate law can be found in enzymes. Enzymes are biological catalysts that cause biochemical reactions. The rate of these biochemical reactions is very important in many processes, such as food digestion. Digestive enzymes located in the stomach help break down

3 CHEMICAL KINETICS LAB 3 food that is ingested, and the determination of the average rate of this digestion can be used to develop digestive supplements for those with digestion problems (Chymotrypsin, n.d.). Finally, the concept of reaction rates are used in nuclear decay and areas that deal with nuclear decay. For example, the dating of ancient organisms using Carbon-14 is a manipulation of rate law. All living creatures contain carbon, and this technique of dating is widely accepted Carbon-14 slowly decays, and using the concepts of decay kinetics and the rate of decay of Carbon-14, scientists can accurately determine the age of fossils and other decomposed organic material (Decay Kinetics, n.d.) Purpose: In this lab, the pseudo rate law governing the reaction between Crystal Violet (CV) and Sodium Hydroxide (NaOH) will be derived from the gathered data. Methods: 1. All the materials needed for the lab were obtained. 2. The colorimeter was calibrated. To calibrate the colorimeter, use a pipette and fill one cuvette 75% full with distilled water and place the cuvette in the colorimeter. The absorbance should be (Note: The colorimeter was set at 565nm because of the CV s purple color). 3. In 11 different test tubes, dilutions of the 24.5 μm solution of CV were created with distilled water, as shown in Table With 11 DIFFERENT PIPETTES for each test tube, 11 different cuvettes were filled about 75% full of each of the 11 dilutions of CV solution. The absorbance of each was collected and recorded.

4 CHEMICAL KINETICS LAB 4 5. Then, the concentration of each of the dilutions was found using the formula M 1 V 1 = M 2 V 2. (Note: The first dilution was essentially only 24.5 μm solution of CV; this information was used to calculate the concentrations of the other dilutions as shown in Table 1.) 6. A chart showing the concentration of CV dilutions vs. Absorbance of CV dilutions was created, as well as a graph of this collected data (Table 1 and Figure 1). The graph was created with a trend line. 7. The equation of the trend line and the coefficient of determination (R 2 ) of this line was then found, as shown in Figure 1. The R 2 value should be high (>.90). This high value ensures that the trend line accurately represents the data plotted. Both the equation and R 2 value was recorded on the graph created in the previous step (Figure 1) 8. A separate cuvette was filled about 50% full of the 24.5 μm solution of CV using a different pipette. 9. Next, with a new pipette, a small amount of NaOH solution was obtained. This solution was then put in the cuvette filled in the previous step, which started to react with the CV. The cuvette was placed into the colorimeter. 10. The absorbance of the solution of CV and NaOH was then recorded immediately and every 25 second interval thereafter until the absorbance shown on the colorimeter is 0.00 (or close to it), (Table 2). 11. The equation found in step 7 is defined as absorbance = m(concentration of CV) (where m is a definite number). Using this equation, the concentration of the solution of CV at each of the 25 second intervals was determined. This was done by substituting the

5 CHEMICAL KINETICS LAB 5 absorbance at a specific time into the equation and solving algebraically for the concentration (Table 3). 12. Next, the natural log of the concentration of CV was determined for each concentration found in the previous step, as well as the reciprocal of the concentration of CV. (Table 3). 13. A graph was then created containing 3 different graphs: time vs. concentration of CV, time vs. ln(concentration of CV), and time vs. 1/(Concentration of CV) (Figure 2). These 3 were also graphed individually with linear trend lines for clarity (Figures 3, 4, and 5). 14. The R 2 value of each of the individual graphs (Figures 3, 4, and 5) was found in order to determine the order of CV in the reaction. 15. With the order of CV determined, the pseudo rate law was found.

6 CHEMICAL KINETICS LAB 6 Data Table 1: Concentration of CV Dilutions vs. Absorbance Dilutions (ml of CV/mL of water) Concentration (μm) Absorbance 10ml/0ml ml/1ml ml/2ml ml/3ml ml/4ml ml/5ml ml/6ml ml/7ml ml/8ml ml/9ml ml/10ml 0 0 Table 2: Time vs. Absorbance during reaction between CV and NaOH Time (s) Absorbance

7 CHEMICAL KINETICS LAB 7 Table 3: Time vs. Concentration, ln (Concentration), and 1/Concentration of CV Time Concentration Ln[Concentration] 1/Concentration

8 CONCENTRATION (MICROMOLAR) ABSORBANCE CHEMICAL KINETICS LAB 8 Figure 1: Concentration vs Absorbance of Dilutions of CV y = x R² = CONCENTRATION VS. ABSORBANCE OF DILUTIONS OF CV CONCENTRATION (MICROMOLAR) Figure 2: Time vs. Concentration, ln (Concentration), and 1/Concentration of CV 20 T I M E V S. C O N C E N T R A T I O N, L N ( C O N C ENTRATION), 1 / C O N C ENTRATION Concentration ln(concentration 1/Concentration TIME (S)

9 LN[CONCENTRATION] (MICROMILAR) CONCENTRATION [MICROMOLAR] CHEMICAL KINETICS LAB 9 Figure 3: Time vs. Concentration of CV (0 th Order) T IME(S) VS. CONCENTRATION OF CV 20 R² = TIME (S) Figure 4: Time vs ln (Concentration of CV) (1 st Order) T IME (S) VS. LN[CONCENTRAT ION OF CV] R² = TIME (S)

10 1/CONCENTRATION (MICROMILAR) CHEMICAL KINETICS LAB 10 Figure 5: Time vs 1/Concentration of CV (2 nd Order) 2.5 T IME (S) VS 1/[CONCENTRAT ION OF CV] 2 R² = TIME (S)

11 CHEMICAL KINETICS LAB 11 Calculations Concentrations of Dilutions: (Known information: Concentration of 10 ml/ 0 ml = 24.5 μm) Using M 1 V 1 = M 2 V 2 : Ex) 9mL/1mL: (24.5 μm)(9 ml) = (? M)(10 ml); (24.5 μm)(9 ml) 10 ml =? M = μm Process repeated for each dilution Equation/ Coefficient of Determination (R 2 ) of Trend Line of Figure 1 (Found on Excel) y = x R² = Concentration of CV when reacted with NaOH (Table 3) Using above trend line Ex) y = absorbance of CV = 1.01 x = μm of CV =? x = 1.01 x = x =? = μm of CV Repeated for all absorbances in Table 2 R 2 value of Figures 3, 4, and 5 Figure 3 (0 th order) Figure 4 (1 st order) Figure 5 (2 nd order)

12 CHEMICAL KINETICS LAB 12 Analysis The purpose of this lab was to find the pseudo rate law of the reaction of Crystal Violet (CV) and Sodium Hydroxide (NaOH). The reaction of CV and NaOH can be written in net ionic form as shown below: CV + + OH yields CVOH This, in turn, means that the rate law of CV and OH is defined as Rate Law = k [CV] x [OH] y Where x is the order of CV in the reaction, y is the order of OH in the reaction, and k is the rate constant that governs the reaction. However, the concentration of OH - ions in the reaction was so high compared to the CV ions that the value of [OH] y almost remains constant, allowing the rate shown above to be substituted with a pseudo rate law defined as Rate Law = k [CV] x k = [OH] y Now, with the pseudo rate law properly defined as an equation, the missing component that made the pseudo rate law complete was the value of x, the order of CV. To find the order of a reactant in any reaction, two aspects of the reaction are needed: the time within which the reaction takes place, and the changing concentration of the reactants at during this time. To find these aspects, the properties of Beer s law that connects absorbance and concentration were utilized. First, as shown in Table 1, dilutions of CV and distilled water were made, and the absorbance of each was found using a colorimeter. Then, using the known molarity of the 10 ml/ 0mL dilution (24.5 μm) and M 1 V 1 = M 2 V 2, the concentrations of all the dilutions were calculated (Table 1). This information was then rearranged and graphed as Concentration vs.

13 CHEMICAL KINETICS LAB 13 Absorbance of CV dilutions in Figure 1, and the equation of the trend line was found. This information was crucial later in the lab. Next, a small amount of CV and NaOH were reacted in a cuvette, and the absorbance of this solution was measured consistently every 25 seconds. The absorbance fell dramatically, due to the fact that when the hydroxide ions reacted with the vibrantly purple CV, the solution slowly became transparent. This data is shown in Table 2. Now, with the equation of the trend line found in Figure 1, the concentration that correlated with each absorbance in Table 2 was found, as shown in the calculations section of this report. This means that, after some manipulation of charts and data, time and concentration of CV, the two components needed to find the order of the CV, have been found (Table 3). The final step to finding the order of CV is to determine which manipulation of the concentration graphed against the time yields the straightest line. These graphs are defined as time vs concentration of CV (Figure 3), which represents the 0 th order, time vs. ln (concentration of CV) (Figure 4), which represents the 1 st order, and time vs. 1/ (concentration of CV) (Figure 5), which represents the 2 nd order. To determine which yielded the straightest line, the R 2 value of a linear trend line was found for each graph. Statistically speaking, the higher the R 2 value, the more of the data can be explained by the trend line. The first order graph had the highest R 2 value (.9962), which means that 99.62% of the data could be explained by the linear trend line. In other words, 99.62% of the data was completely in line with the trend line, making it the straightest data regression of the 3 orders. Therefore, the order of CV was found to be 1.

14 CHEMICAL KINETICS LAB 14 Error Analysis The results of the experiment and the subsequent analysis of the data seem to be very sound. While there was a rush for time and the reaction could not completely go to completion (absorbance of CV and NaOH solution is 0), this does not seem to have had a large effect on the data at all. Conclusion The pseudo rate law that governs the reaction between Crystal Violet and Sodium Hydroxide was found to be: Rate Law = k [CV] 1 k = [OH] y

15 CHEMICAL KINETICS LAB 15 References Chymotrypsin. (n.d.). - Chemwiki. Retrieved February 5, 2014, from II Decay Kinetics - variation of decay rate with time. (n.d.). Decay Kinetics - variation of decay rate with time. Retrieved February 5, 2014, from Introduction to Chemical Kinetics, First Order Precesses, Half-Life. (n.d.). Introduction to Chemical Kinetics, First Order Precesses, Half-Life. Retrieved February 5, 2014, from Kinetics - The Rate Law. (n.d.). Kinetics - The Rate Law. Retrieved February 5, 2014, from Mathematical Prediction of Cumulative Levels of Pesticides in Soil. (n.d.). - Organic Pesticides in the Environment. Retrieved February 5, 2014, from