Activation Energy of Quenching of Pyrene by Iodide
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Activation Energy of Quenching of Pyrene by Iodide Joel Salem, Roman Bargo, Kayla Shellhammer, Casey Dubrawka Department of Chemistry, The Penn State University, University Park, PA 16802 Abstract Methods Results References Acknowledgments Future Recommendations: The immediate task ahead is to study this reaction at different temperature intervals, to investigate the reproducibility at different temperatures. As pyrene is used in many facets of identification, it is also useful to study quenching by other ions and molecules. Conclusions and Recommendation Introduction Figure 1 and Figure 2 are useful together in finding the quenching rate constant at individual temperatures. As temperature increases, we observed that the slopes of the fits become more negative, indicating an increase in decay rate. This is supported by the proportional relationship between temperature and rate. The slopes of Figure 1 and Figure 2 are k obs and k 0 , respectively. Using Eq (3), relating the observed rate, natural decay rate, and concentration, we found the quenching rate at each individual temperatures. The primary objective of this experiment was to measure the activation energy of the quenching of pyrene by Iodide. This was performed using time-resolved laser photolysis, at multiple temperatures; the Arrhenius model was utilized to obtain the Activation Energy of 28.7±.2 kJ/mol. Pyrene is excited easily and is useful in finding quenching agents in known environments. Figure 1: Natural log of intensity vs. time for quenched sample Figure 2: Natural log of intensity vs. time for non- quenched sample An Arrhenius plot is important in chemical reactions because it provides a way to calculate the activation energy of the reaction. Observing Eq (5), by plotting the natural log of the quenching rate constant vs. inverse temperature leads to a linear model between the two, with activation energy divided by the ideal gas constant as the slope. From this plot we calculated the Activation Energy, E a =28.7±.2 kJ/mol We see in Figure 3 a concave down shape, not ideally linear. To find out the reproducibility of this effect, a viscosity test was performed at the same temperature interval. The viscosity test, if of the same shape, will confirm the shape of our Arrhenius plot. Figure 3: Arrhenius plot for the quenching of pyrene by Iodide Fluorescence: A 40 mM solution of potassium iodide in 40% ethanol in water was prepared. Two milliliters of pyrene were added and the sample was purged with nitrogen. The sample was placed in a water bath and laser photolysis was run at various temperatures ranging from 6⁰ C to 45⁰C. Ice and a heating coil were used to control temperature. A second solution of 40% ethanol in water was prepared without potassium iodide. Two milliliters of pyrene were added, the sample was purged, and the same procedure using laser photolysis was repeated. Viscometer: A viscometer and heat bath were used to measure the viscosity of a solution of 40% ethanol in water in a temperature range of 20⁰C to 45⁰C. Using a balance, 8.3 g of the solution was weighed and time was recorded at each temperature. All compounds were purchased from Sigma-Aldrich and used without further purification. All instruments and materials used were supplied by the Penn State Chemistry Department. Figure 4: Fluorescence setup Figure 5: Viscometer setup Pyrene was used because of its ability to achieve an excited state under laser photolysis. Pyrene is a polycyclic aromatic hydrocarbon (PAH) and contains four fused benzene rings. These aromatic benzene rings allow the molecule to become excited. Fluorescence spectroscopy is a powerful tool used to gain information regarding the electronically excited states of various molecules. Excited pyrene will undergo redox chemistry in the presence of another chemical species with a sufficiently low or high reduction potential. A photon of sufficient energy is absorbed by pyrene to yield an excited state pyrene molecule. An electron is promoted from the ground state energy level into an excited state. This excited state can then relax back to the ground state either by fluorescence of a photon or by radiationless decay as the molecule loses energy in the form of heat. Excited state pyrene is a good electron acceptor. In the presence of I - , excited pyrene undergoes a reductive transition back to a lower energy state through the formation of the Py - anion and the I . radical. The lifetime of the excited state can be treated in the same manner as one would treat reactants in basic reaction kinetics. Using this kinetics model, The fluorescence intensity as a function of time, to describe the decay, can be seen in Eq(1), were I is the intensity, I0 is the initial intensity, k is the rate of decay, and t is the time of decay, in seconds. (1) Linearizing this equation yields a plot in which the slope of the line is equal to –k obs , as seen in Eq (2). (2) To determine the quenching rate constant, k q , of the reaction of *Py with I - , at different temperatures, Eq (3) is used, with k obs as the observed rate constant for the fluorescence system, and k 0 the normal decay of pyrene. (3) The fluorescence rate constant is already known from working with the single component system, but the quenching rate constant still has yet to be determined. The quenching rate constant can be related to temperature via the Arrhenius equation. The utility of the Arrhenius equation is to determine the activation energy of the decay of pyrene by Iodide. In Eq (4), Ea is the activation energy. (4) Linearizing this by taking the natural log of both sides yields Eq (5), from which the activation energy of quenching can be determined. 1 Ethanol (absolute, ≥99.8%). Sigma-Aldrich Co. LLC. Web. 22 November 2013. 2 Milosavljevic, B. Chem 457 Experimental Physical Chemistry: Lab Packet. The Pennsylvania State University. Fall 2013. Revised July 2013. 3 Wen, S. et al. Photocatalytic reactions of pyrene at TiO2/water interfaces. Chemosphere. 50. 1. Jan 2003. Pp 111- 119. 4 Sengupta, S., Karmakar, S., Dasgupta, C., & Sastry, S. (2013). Breakdown of the Stokes-Einstein relation in two, three, and four dimensions. The Journal of chemical physics, 138, 12A548. We would like to thank Dr. Milosavljevic, Chris Grieco, and Jovan Livada for their guidance and assistance in the experiment. We would also like to thank the Chemistry department, and the University. Conclusions: The activation energy for the quenching reaction of pyrene by iodide was found via time resolved fluoresce to be 28.7±.2 kJ/mol. The quenching reaction was faster at higher temperatures, as predicted by kinetic theory. The arrhenius plot had a downward curvature, which was investigated with the viscosity of the solvent. This feature of the plot was a negative slope, indicating diffusion related quenching. Figure 4: Natural log of viscosity versus the inverse of temperature for ethanol water mixture
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This was the final project for my physical chemistry lab. It was a group project that allowed us to use what we learned during the lab and apply it to a special project that we designed. This poster was presented at the poster symposium at the end of the semester to our peers and professors.
Transcript of Activation Energy of Quenching of Pyrene by Iodide
Slide 1
Activation Energy of Quenching of Pyrene by Iodide
Joel Salem, Roman Bargo, Kayla Shellhammer, Casey DubrawkaDepartment of Chemistry, The Penn State University, University Park, PA 16802
Abstract Methods ResultsReferencesAcknowledgmentsFuture Recommendations:The immediate task ahead is to study this reaction at different temperature intervals, to investigate the reproducibility at different temperatures. As pyrene is used in many facets of identification, it is also useful to study quenching by other ions and molecules.Conclusions and RecommendationsIntroductionFigure 1 and Figure 2 are useful together in finding the quenching rate constant at individual temperatures. As temperature increases, we observed that the slopes of the fits become more negative, indicating an increase in decay rate. This is supported by the proportional relationship between temperature and rate. The slopes of Figure 1 and Figure 2 are kobs and k0, respectively. Using Eq (3), relating the observed rate, natural decay rate, and concentration, we found the quenching rate at each individual temperatures. The primary objective of this experiment was to measure the activation energy of the quenching of pyrene by Iodide. This was performed using time-resolved laser photolysis, at multiple temperatures; the Arrhenius model was utilized to obtain the Activation Energy of 28.7.2 kJ/mol. Pyrene is excited easily and is useful in finding quenching agents in known environments. Figure 1: Natural log of intensity vs. time for quenched sampleFigure 2: Natural log of intensity vs. time for non-quenched sample
An Arrhenius plot is important in chemical reactions because it provides a way to calculate the activation energy of the reaction. Observing Eq (5), by plotting the natural log of the quenching rate constant vs. inverse temperature leads to a linear model between the two, with activation energy divided by the ideal gas constant as the slope. From this plot we calculated the Activation Energy, Ea=28.7.2 kJ/mol
We see in Figure 3 a concave down shape, not ideally linear. To find out the reproducibility of this effect, a viscosity test was performed at the same temperature interval.
The viscosity test, if of the same shape, will confirm the shape of our Arrhenius plot.
Figure 3: Arrhenius plot for the quenching of pyrene by Iodide
Fluorescence:A 40 mM solution of potassium iodide in 40% ethanol in water was prepared. Two milliliters of pyrene were added and the sample was purged with nitrogen. The sample was placed in a water bath and laser photolysis was run at various temperatures ranging from 6 C to 45C. Ice and a heating coil were used to control temperature. A second solution of 40% ethanol in water was prepared without potassium iodide. Two milliliters of pyrene were added, the sample was purged, and the same procedure using laser photolysis was repeated.
Viscometer:A viscometer and heat bath were used to measure the viscosity of a solution of 40% ethanol in water in a temperature range of 20C to 45C. Using a balance, 8.3 g of the solution was weighed and time was recorded at each temperature. All compounds were purchased from Sigma-Aldrich and used without further purification. All instruments and materials used were supplied by the Penn State Chemistry Department.
Figure 4: Fluorescence setupFigure 5: Viscometer setupPyrene was used because of its ability to achieve an excited state under laser photolysis. Pyrene is a polycyclic aromatic hydrocarbon (PAH) and contains four fused benzene rings. These aromatic benzene rings allow the molecule to become excited.
Fluorescence spectroscopy is a powerful tool used to gain information regarding the electronically excited states of various molecules. Excited pyrene will undergo redox chemistry in the presence of another chemical species with a sufficiently low or high reduction potential. A photon of sufficient energy is absorbed by pyrene to yield an excited state pyrene molecule. An electron is promoted from the ground state energy level into an excited state. This excited state can then relax back to the ground state either by fluorescence of a photon or by radiationless decay as the molecule loses energy in the form of heat. Excited state pyrene is a good electron acceptor. In the presence of I-, excited pyrene undergoes a reductive transition back to a lower energy state through the formation of the Py- anion and the I. radical. The lifetime of the excited state can be treated in the same manner as one would treat reactants in basic reaction kinetics. Using this kinetics model, The fluorescence intensity as a function of time, to describe the decay, can be seen in Eq(1), were I is the intensity, I0 is the initial intensity, k is the rate of decay, and t is the time of decay, in seconds.(1)Linearizing this equation yields a plot in which the slope of the line is equal to kobs, as seen in Eq (2). (2)To determine the quenching rate constant, kq, of the reaction of *Py with I-, at different temperatures, Eq (3) is used, with kobs as the observed rate constant for the fluorescence system, and k0 the normal decay of pyrene.
(3)The fluorescence rate constant is already known from working with the single component system, but the quenching rate constant still has yet to be determined. The quenching rate constant can be related to temperature via the Arrhenius equation. The utility of the Arrhenius equation is to determine the activation energy of the decay of pyrene by Iodide. In Eq (4), Ea is the activation energy.
(4)Linearizing this by taking the natural log of both sides yields Eq (5), from which the activation energy of quenching can be determined.
(5)Viscosity, a measure of inertial flow, can prove that a reaction is diffusion-limited. In such a process, a reaction depends on the time taken for individual molecules to interact, or diffuse towards each other. In this experiment, we apply this theory to investigate the quenching reaction as diffusion-limited.
1Ethanol (absolute, 99.8%). Sigma-Aldrich Co. LLC. Web. 22 November 2013. 2Milosavljevic, B. Chem 457 Experimental Physical Chemistry: Lab Packet. The Pennsylvania State University. Fall 2013. Revised July 2013.3Wen, S. et al. Photocatalytic reactions of pyrene at TiO2/water interfaces. Chemosphere. 50. 1. Jan 2003. Pp 111-119.4Sengupta, S., Karmakar, S., Dasgupta, C., & Sastry, S. (2013). Breakdown of the Stokes-Einstein relation in two, three, and four dimensions. The Journal of chemical physics, 138, 12A548.
We would like to thank Dr. Milosavljevic, Chris Grieco, and Jovan Livada for their guidance and assistance in the experiment. We would also like to thank the Chemistry department, and the University.
Conclusions:The activation energy for the quenching reaction of pyrene by iodide was found via time resolved fluoresce to be 28.7.2 kJ/mol. The quenching reaction was faster at higher temperatures, as predicted by kinetic theory. The arrhenius plot had a downward curvature, which was investigated with the viscosity of the solvent. This feature of the plot was a negative slope, indicating diffusion related quenching.
Figure 4: Natural log of viscosity versus the inverse of temperature for ethanol water mixture
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