CH3511 EXPERIMENT: Determination of Thermal Properties Using a Differential Scanning Calorimeter

Transcription

1 CH3511 EXPERIMENT: Determination of Thermal Properties Using a Differential Scanning Calorimeter INTRODUCTION A Mettler Toledo 823E Differential Scanning Calorimeter (DSC) will be used to determine the thermal properties of naphthalene and 2-naphthol. Results will be compared with that of the National Institute of Standards and Technology (NIST).[1, 2] BACKGROUND Chemical and physical changes are almost always accompanied by energy changes. Much has been learned by studying the laws that explain these changes. Physical chemistry places importance on the energy possessed by matter which is determined by it s structure. [3] A DSC allows us to study thermal energy changes and determine thermodynamic properties. [4] It is used in a wide variety of applications from food analysis to polymer analysis. [5] Phase and other structural transitions are studied in order to obtain measurements of heat capacities and enthalpies of decompositions, phase transitions, and reactions. A DSC (c.f. Figure 1), is comprised of a heating chamber (called a furnace) that will hold two small pans (called crucibles), precision controlled heating and cooling units, a computer control, and software to analyze the results. Figure 1. A basic DSC [6] Two crucibles are placed in the DSC furnace chamber. One crucible contains a small amount of the sample to be tested, while the other is an empty reference crucible. The sample and reference crucibles are placed in specific locations designated for each, because the chamber will heat the

2 two crucibles separately in order to make absolutely sure that the heating rate for both is the same. The difference in the amount of heat, i.e., heat flow, that is required to increase the temperature of the sample vs. the reference is regulated and monitored very carefully by the instrument. This heat flow will be shown in units of heat, q supplied per unit time, t (Equation 1): (1) The heating rates is the temperature increase, T per unit time (Equation 2) (2) The heat capacity of the sample at constant pressure, Cp, is calculated by dividing the heat flow, q/t by the heating rate, T/t: (3) We can obtain more than the heat capacity of a sample from a DSC. A compound going through a phase change will have a large change in it s heat capacity. For example, heating a solid to its melting point requires a great deal of heat with little to no temperature change. The enthalpy of the transition, Hfusion, can be determined by integrating the curve depicting the heat flow as a function of temperature: (4) (5) Thus, the area above the curve (assuming endothermic processes are plotted downward) will give us the enthalpy of fusion of our sample. The same is true for enthalpies of vaporization, sublimation, and even more subtle transitions such as glass transitions. According to the 2 nd Law of Thermodynamics, ds = qrev/t. Therefore, the entropy of fusion, Sfusion, is calculated from Equation 6 [7] : (6)

3 Figure 2. A DSC thermogram depicting the melting of tetrabutylammonium fluoride PRELIMINARY QUESTIONS 1. Complete Table I with values and include in your preliminary report: AMT Converts to: Converts to: 3.57 Watts Joules/s V. C/s 43 amps coulomb/s 9.88 Watt. s Joules Table I. Example table to use for answering Preliminary Question #1. 2. Draw a DSC thermogram that you would expect to see for a sample with a Tm of 45 C and a Tvapor of 205 C. Indicate which baseline change direction is endothermic and/or exothermic. 3. What is the melting point temperature of tetrabutylammonium fluoride (c.f. Fig. 2)? What is this compound s enthalpy of fusion? 4. A 35.4mg sample of cyclohexane was heated at a rate of 2 o C/minute. The melting point occurs at approx o C. The enthalpy of fusion, HFusion of cyclohexane, obtained via integration of

4 the heating curve, was found to be watts. s. Calculate the entropy of fusion in units of kjmole -1 K Report the melting point and enthalpy of fusion for naphthalene and 2-naphthol in a table in your preliminary report. Please use the National Institute of Standards and Technology (NIST) [2] web site. Table II is a good example of the table you should include. COMPOUND TM (K) TM ( o C) Hfusion (UNITS?) Table II. Example Table for you to complete and include for Pre-lab Question #5 for the Experiment, Determination of Thermal Properties Using a DSC EXPERIMENT Use the procedure link following this experiment to operate the DSC. (It isn t necessary to repeat the entire procedure in the Procedure portion of the report. Instead, briefly write a procedure and include a reference for the procedure link.) First you will create method files (starting temperature, final temperature, and temperature change rate) to find the melting point of the two compounds (naphthalene and 2-naphthol). You will use data from other students enrolled in CH3511 to determine reproducibility and precision. Once the data is obtained, use the DSC software to determine each compound s enthalpy of fusion. Calculate the entropy of fusion, then compare your values with those on the NIST web site in the final report. Also, discuss structural differences in the two compounds that may cause these values to differ from one another in your final report. Finally, include the literature, class, and your group s data in a table similar to Table III.

5 TM HFusion SFusion COMPOUND ( o C) (J/mol) (J/mol. K) VALUE % (RSD) VALUE % (RSD) VALUE % (RSD) (literature) (class) (group/individual) (literature) (class) (group/individual) Table III. Example table for the Results and Discussion Section for the Determination of Thermal Properties Using a Differential Scanning Calorimeter Final Report. ACKNOWLEDGEMENT The author wishes to thank Dr. Patricia A. Heiden, Katlyn Mehne, Lindsay Williams and Michigan Technological University s Department of Chemistry for assistance in the development of this experiment. REFERENCES 1. D Amelia, Ronald, Franks, Thomas, Nirode, William, J. of Chem. Ed., 84, March 2007, pp National Institute of Standards and Technology Home Page. Dec. 2006). 3. Glasstone, Samuel, Textbook of Physical Chemistry, 2 nd Edn, D. Van Nostrand Company, Inc., New York, Vyazovkin, S. Anal. Chem. 2004, 76, pp. 3290R-3312R. 5. Folmer, J.C.W; Franzen, S., J. Chem. Educ., 80, 2003 p Polymer Science Learning Center, Department of Polymer Science, University of Southern Mississippi, Differential Scanning Calorimetry, Polymer Science Learning Center, 8/ Folmer, J.C.W., Franzen, Stefan, J. of Chem. Ed., 80, July 2003, pp