3 pages Chemistry LAB

V.S2016 LD-1

THERMODYNAMICS: DETERMINATION OF THE ENTHALPY AND ENTROPY OF FUSION OF TRIPHENYLPHOSPHATE

OBJECTIVE The objective of this experiment is to become familiar with experimental thermodynamics; this will be accomplished by using graphical methods to determine thermodynamic quantities (enthalpy and entropy of fusion) from experimentally acquired data. INTRODUCTION

Triphenylphosphate (TPP, MW = 326.27 g/mol) is an organic molecule with molecular formula (C6H5O)3PO and its structure is shown in Figure 1. Like most substances, at TPP’s freezing point equilibrium exists between its solid phase and liquid phase represented by RXN 1. TPP(s) ⇌ TPP(l) (RXN 1) The equilibrium constant, Keq, is given by eq. 1, where only the concentration in molarity of TPP in the liquid phase is needed, as

solids are not represented in equilibrium expressions. (eq. 1) It is more convenient to use an alternative form of eq. 1, where Keq can be expressed as eq. 2 with respect to mole fraction of liquid TPP, χTPP (l). In eq. 2 nTPP (l) is the number of moles of TPP in the liquid phase, nTOT is the total number of moles of a solution containing TPP as the solute, and Kχ is the equilibrium constant with respect to mole fraction of TPP. (eq. 2) In this experiment solutions of TPP will be made with another organic molecule called camphor (C10H16O, MW = 152.24 g/mol) so that the quantity χTPP (l) may be varied to investigate the thermodynamic quantities of interest. For example, if the same number of moles of TPP is used where the number of moles of camphor is varied systematically, than χTPP (l) is always less than 1 and differs from trial to trial because nTOT = nTPP (l) + ncamphor. In this experiment the thermodynamic quantities that will be investigated are the change in enthalpy (ΔHo) and entropy (ΔSo) of fusion, which are related to the change in Gibbs free energy (ΔGo) by eq. 3, with all reactants and products being in their standard states. (eq. 3) The Gibbs free energy at constant temperature can be related to the equilibrium constant, in this experiment Kχ, by eq. 4, where R is the ideal gas constant (8.314 J/mol·K) and T is the ambient temperature in Kelvin. (eq. 4)

V.S2016 LD-2

Recall at equilibrium the change in Gibbs free energy, ΔGo, is zero; therefore, eq. 3 and eq. 4 may be set equal to each other once equilibrium is established and this is shown as eq. 5(a,b) below. (eq. 5a) (eq. 5b) Equation 5b is a manipulation of eq. 5a and shows the variance of the equilibrium constant, Kχ, with the thermodynamic quantities of interest. This manipulation was made because now eq. 5b represents the equation of a line (y = mx + b), where the slope, m, is the quantity –ΔHo/R, the y- intercept, b, is the quantity ΔSo/R, the y-axis is the natural log (ln) of Kχ, and the x-axis is 1/T. Thus, repeating the experiment at several different values of χTPP (l), recording the freezing points of each solution, and finally plotting the data as ln (Kχ) vs. 1/T will yield the thermodynamic quantities of interest. From the slope of this line, ΔHo can easily be determined. The value of ΔSo could be obtained graphically by extending the line to the origin (because it is the y-intercept); however, this is 1/T = 0, which is ill-defined. A better approach in this experiment is to obtain ΔSo analytically (i.e. using equation 3). In this calculation, use the value of T from Trial 1 and the value of ΔHo obtained from the slope of the graph. Remember that for a system in equilibrium, ΔG = 0. It should be noted the freezing point of interest in this experiment is the temperature when freezing just begins. As freezing occurs, the number of moles of TPP in the liquid phase decreases and thus so does the quantit χTPP (l); correspondingly the freezing point will then change as well. Thus, to a good approximation the temperature when freezing just begins will have a known value of χTPP (l) (and of course Kχ) for that given temperature, which is the initial value of the TPP mole fraction that was made before freezing began. PROCEDURE Work in pairs. Each pair obtains from the stockroom a stirring ring and split cork. Assemble a freezing point apparatus by clamping an 18 x 50 mm clean, dry test tube vertically about 15–30 cm above the lab bench. Insert the stirring ring and a thermometer in the test tube and use the split cork to ensure the thermometer does not touch the walls of the test tube. Make sure the graduations on the thermometer can be read all the way along its length (through the split cork as well). Preparing the TPP/Camphor solutions: For TPP use the triple beam balance and for camphor use the analytical balance; record your sample mass according to the instrumental precision. You will need one 15.00-gram portion of TPP and three 2.0000-gram portions of camphor. You should weigh these out on weigh paper. You will have time to weigh out the camphor portions as you proceed through the experiment. Table 1 shows the solution compositions you will make.

V.S2016 LD-3

TABLE 1: MASS OF TPP AND CAMPHOR TO BE USED IN TRIALS

Trial Mass of TPP (g) Mass of Camphor (g)

1 15.00 0.0000 2 15.00 2.0000 3 15.00 4.0000 4 15.00 6.0000

Trial 1 Place the amount of TPP in the freezing point apparatus as specified by Table 1 (you will have to remove the stirrer and thermometer first) and then arrange the thermometer so all graduations are visible and there is no contact of the thermometer with the test tube walls. Gently heat the system until the TPP has melted completely. DO NOT allow the temperature of the system to exceed 55oC. This can be accomplished with your Bunsen burner by wafting the flame around the bottom of the test tube until about half of the TPP is melted and then remove the flame. Wait until the remainder of the TPP has melted and when you have a homogenous liquid make sure the thermometer is submerged in the liquid. Stir the contents of the test tube slowly with an up-and-down motion and when the temperature is at 55oC begin recording the temperature to the nearest 0.1oC at 30-second intervals while continuing to stir. Continue recording until the temperature has remained constant for at least 6–8 readings (see Figure 2 below) after some solid has appeared in the tube and record this data on a separate piece of paper in an organized table. Trial 2 Using the same test tube and TPP as in trial 1 and add the amount of camphor specified by Table 1, heat the sample test tube gently, and follow the same general procedure as you did in Trial 1. Start the measurements at 50oC (you may have to heat the system after adding the camphor) and record to the nearest 0.1oC at 30-second intervals while continuing to stir. Continue recording until the temperature has remained constant for at least 6–8 readings after some solid has appeared in the tube and record this data on a separate piece of paper in an organized table. Trial 3 Using the same test tube and TPP as in trial 2, follow the same general procedure as you did for the first two trials. Add the amount of camphor specified by Table 1; note: you are only adding another 2.0000-g portion to the 1st 2.0-g portion you used in trial 2, giving you a total of 4.0-grams in the apparatus. Be sure to start your measurements at 45oC.

V.S2016 LD-4

Trial 4 Using the same test tube and TPP as in trial 3, follow the same general procedure as you did for the first two trials. Add the amount of camphor specified by Table 1; note: you are only adding another 2.0000-g portion to the 2 preious 2.0000-g portion you used in trial 2, giving you a total of 6.0000-grams in the apparatus. Be sure to start your measurements at 40oC.

WASTE DISPOSAL/CLEAN UP After Trial 4 heat the test tube gently until the contents are liquid. Pour the liquid TPP and liquid camphor into the waste container provided. DO NOT DISCARD THE CONTENTS IN THE SINK. Allow the tube to cool and then use a brush, detergent, and hot water to clean thoroughly. Discard the washing liquid in the sink.

Freezing Curves Plot all four freezing curve trials on the same graph using a software package such as Microsoft Excel (see Appendix X, Using Excel to Make Plots). Each freezing curve should look somewhat similar to Figure 2 above. Temperature should be plotted on the y-axis and time should be plotted on the x-axis. Adjust your scales appropriately so all four curves are clearly visible. Make sure to label both axes appropriately and include a title on the graph. Use different shaped markers (include a line through the makers) for each trial and include a legend that clearly labels each trial (you may also pick different colors for the lines if you are going to print in color). You must print 1 graph that contains all four trials and attach it to the end of this report for full credit. On your printed freezing curves, you may hand draw a line similar to the dotted line shown in Figure 2. The line should run from the plateau region to the liquid cooling region. Record the temperature at which these lines intersect in the DATA AND RESULTS Table on the next page.

V.S2016 LD-5

DETERMINATION OF THE ENTHALPY Name: ____________________________ AND ENTROPY OF FUSION OF TRIPHENLYPHOSPHATE Partner’s Name:____________________________ Section: Day ________ Hours: ________ DATA

Data Trial 1 Trial 2 Trial 3 Trial 4 “initial” equilibrium freezing point (oC)

“initial” equilibrium freezing point (K)

1/T (K–1)

mass of TPP (g)

moles of TPP

mass of camphor (g)

moles of camphor

Mole fraction of TPP(l) ( χTPP (l) )

Kχ

ln (Kχ)

RESULTS Using a software package such as Microsoft Excel, plot your data with ln (Kχ) on the y-axis and 1/T (K–1) on the x-axis (See Appendix 7, Using Excel to Make Plots). Fit the data with a best-fit line and report your results below (see eq. 5b). Make sure to label both axes appropriately and include a title. You must print your graph with the best-fit line results and attach it to the end of this report for full credit. NOTE: You will prepare 5 graphs in total for this lab, 4 freezing curves plotted on the same graph and 1 best-fit line plotted on a different graph. You do not need to conduct a best-fit line analysis on the freezing curves. Write the best-fit line equation from your results:

V.S2016 LD-6

Calculate the Enthalpy of Fusion, ΔHo, from your best-fit line and show your work below. Record your answer in the space provided. Enthalpy of Fusion, ΔHo, in units of kJ/mol = _____________________ Calculate the Entropy of Fusion, ΔSo, using your Trial 1 data and the ΔHo you obtained from the best-fit line. Show your work below and record your answer in the space provided. Entropy of Fusion, ΔSo, in units of J/K mol = _____________________

V.S2016 LD-7

QUESTIONS

1. Why was ΔSo obtained analytically rather than graphically?

2. Do you expect melting of TPP to be endothermic or exothermic? Why? Justify your answer using no more than 3 complete sentences.

3. Does your data and results confirm or deny your expectations from question 2? Why? Justify your answer using no more than 3 complete sentences.

4. What can you conclude about the relative “order” or “disorder” in the solid and liquid phases of TPP from the sign of ΔSo? Explain using no more than 3 complete sentences.

5. Calculate the value of ΔGo for the melting of TPP 10oC above its normal melting point and 100C below its normal melting point. Discuss the results of these calculations in terms of spontaneity of the melting process at these temperatures. You may assume ΔHo and ΔSo are independent of temperature.