CONSTANT PRESSURE CALORIMETRY: A STUDY OF GLYCINE PROTON-TRANSFER ENTHALPIES 1

Transcription

1 CONSTANT PRESSURE CALORIMETRY: A STUDY OF GLYCINE PROTON-TRANSFER ENTHALPIES 1 OBJECTIVES 1. To determine the reaction enthalpies for the proton transfer reactions of glycine. 2. To use a high-precision calorimeter. 3. To determine heat capacities experimentally using the electrical heating method. INTRODUCTION Chemical Overview Glycine, H2N CH2 COOH, is the simplest of the 20 amino acids which form the building blocks of proteins. Although its formula implies that the glycine molecule contains a carboxyl group ( COOH) and an amino group ( NH2), glycine's physical and chemical properties are incompatible with this structure, and indicate that in its crystalline form and in neutral aqueous solution it exists as a self-ionized species called a dipolar ion or zwitterion ("hybrid ion" auf Deutsch): + H3N-CH2-COO. A zwitterion is electrically neutral but has two poles of opposite electric charge. Amino acids can act as acids or bases in aqueous solution, and their acid-base properties determine many of the properties of proteins. In this experiment you will determine the molar enthalpy changes for the two stages of glycine proton transfer: H2Gly + (aq) = H + (aq) + HGly(aq) (1) HGly(aq) = H + (aq) + Gly (aq) (2) where H2Gly is + H3N CH2 COOH, HGly is + H3NCH2COO (the zwitterion), and Gly is H2N CH2 COO. The zwitterion is a base in reaction 1 and an acid in reaction 2. The calculated enthalpy changes won't be the standard values because the experiments are carried out at electrolyte concentrations of about 0.3 M. However, the ionic strength doesn't have a large effect on the H values. Thermodynamic Overview The calorimetric determination of H for a reaction in solution involves a simple application of the thermodynamic cycle shown in Figure 1. Reaction enthalpies are usually reported at a particular temperature, so we are interested in H for the isothermal process Reactants (Ti) Products (Ti), shown as the lower path in Fig. 1. Because H is a state function, the desired enthalpy change, H(Ti), must equal the enthalpy change for the upper, two-step path Reactants (Ti) Products (Tf) Products (Ti). The first step in the two-step path is the reaction that occurs in the calorimeter: Reactants (Ti) Products (Tf), where Ti and Tf are the initial and final temperatures of the solution, respectively. Since the reaction occurs in a thermally insulated container at constant pressure, we must have both q = 0 and q = H for the calorimeter process, i.e. the enthalpy change for the calorimeter process is equal to zero. Consequently, the only contribution to H for the upper (two-step) path comes from constant pressure temperature change of the products, which to a good approximation is equal to Cp(TI Tf), where Cp is the heat capacity of the solution. Therefore, 1

2 calorimeter process H 1 = q p = 0 Products (T f ) H 2 = C p (T i -T f ) Reactants (T i ) H(T i ) Products (T i ) Figure 1. Thermodynamic Cycle for the Calorimeter Process Hreaction(Ti) = Cp(Ti Tf). (3) The temperatures Ti and Tf are measured when the reaction is carried out in the calorimeter. The heat capacity of the solution is found separately by measuring the temperature change Theat (not necessarily equal to Tf - Ti) produced when a known amount of heat is added to the solution via an electrical resistance heater, since Q = Cp Theat. Experimental Overview Each reaction of interest is carried out under constant (atmospheric) pressure in a calorimeter comprising a Dewar flask, a sensitive thermistor, an electrical heater, and a stirrer. The stirrer also serves as a container which holds one reactant (the Dewar holds the other) permitting the reactants to reach the same temperature before the reaction starts (Fig. 2). To initiate the reaction you simply force the stirrer cover open. The thermistor is a resistor whose resistance varies linearly with temperature. The resistance is measured by placing the thermistor in a "bridge" circuit similar to the Wheatstone bridge you may have encountered in physics lab. The output of the bridge is sent to a recording device, either a strip-chart recorder or a computer. APPARATUS AND CHEMICALS Parr model 1451 solution calorimeter, thermistor and heater probes, MacLab computer interface, Apple Macintosh computer with MacLab software, BK model 1601 regulated DC power supply, digital voltmeters, digital ammeter, timer, mortar & pestle, 100 ml pipets. solid glycine Stock solutions: M HCl; M NaOH; M NaCl 2

3 EXPERIMENTAL PROCEDURE An outline of the procedure is given in Reference 1. Follow the published procedure, but use 25 mmol of glycine rather than 20 mmol. The details of operating the calorimeter are given below. A. Set up 1. Set up the electrical circuits shown in Fig. 3, then plug in all the power cords and set the calorimeter selector switch to ZERO. Allow the calorimeter to warm up for 10 min before calibrating it (in part B). 2. Turn on the DC power supply, and with the DC VOLTAGE OUTPUT switch set to DC ON adjust the output to about 15 V. Switch the power supply to STANDBY mode, but don't turn the power off. 3. Data Collection set up: If you have not done so already, connect the + and output terminals of the calorimeter to the Channel 1 input on the MacLab interface. Connect the output cable of the MacLab to the SCSI port on the computer. Turn on the MacLab interface and the computer in that order. Start up the Chart software. Turn off all input channels except Channel 1, and open the Window for the DVM (digital voltmeter) for channel Calorimeter calibration Note that there are five positions of the selector switch on the calorimeter: OFF, ZERO, NULL, CAL, and READ. Between runs the switch should be set to ZERO. (The switch should be placed in the OFF position at the end of the day.) To calibrate the output of the bridge, carry out the following steps: a. You should already have set the selector switch to ZERO and allowed a 10 minute warmup period. Click the Start button at the lower right corner of the Chart window to begin reading the voltage across the + and output terminals of the calorimeter bridge. Adjust the ZERO control knob on the calorimeter to give a reading of zero volts on the DVM window. b. Set the selector switch to NULL, then adjust the NULL control knob to give a reading of zero volts on the DVM window. c. Set the calorimeter selector switch to CAL. This sets the bridge output to 1000 mv. Adjust the CAL control knob until the DVM window displays V. d. Stop the data collection and return the calorimeter selector switch to ZERO. The calibration is now complete, and you shouldn't touch the ZERO, NULL, and CAL knobs on the calorimeter for the remainder of the experiment. 5. Determination of room temperature: (Room temperature may drift slightly during the several hours it takes to perform the experiment. Use the following procedure to check the temperature occasionally.) With Chart s y-axis Range set to about 50 mv, turn the calorimeter selector switch to the READ position. Start the Chart recorder. The pen will move to a position on the chart which corresponds to the difference between the temperature sensed by the thermistor and 3

4 the temperature displayed by the temperature switch and rheostat. Adjust the temperature switch and temperature rheostat on the calorimeter until the average DVM reading is zero volts and/or the Chart trace is centered about the 0 mv baseline. The bridge is now nulled, and the temperature selector displays the room temperature. Return the calorimeter selector to ZERO. B. General procedure for running a reaction and recording a thermogram Because concentrations are important, all parts of the calorimeter must be dry before you begin. 1. Carefully remove the thermistor and heater probes from the Dewar and lay them in a safe place. 2. Pipet the HCl, NaCl, or NaOH solution into the Dewar flask. Weigh about 25 mmol of glycine into a clean, dry sample dish on an analytical balance (be sure to record the precise mass). Assemble the stirrer cell by placing the Teflon dish on a flat surface and carefully pressing the glass bell onto the dish. Do not put pressure on the glass stem during this operation; it is extremely fragile. 3. Insert the metal stirring shaft through the hole in the calorimeter cover, then attach the stirrer cell to the metal shaft and turn the thumb screw finger tight followed by 1/2 turn with a screwdriver. With the cell resting on a firm, flat surface insert the push rod through the metal shaft and press the end of the rod into the socket in the Teflon dish. 4. Carefully insert the thermistor and heater probes, then assemble the calorimeter as shown in Fig. 2. Install the drive belt and start the stirrer motor. Let this run for about 10 minutes to let the reactants reach thermal equilibrium. 5. Open a New Chart window. Set the sampling speed to 40/min. Set the y-axis Range: for the HCl and NaCl reactions set the y-axis Range to 100 mv. For the NaOH reaction use 50 mv 6. Turn the calorimeter selector switch to READ. Begin recording a thermogram (temperature vs. time graph) by clicking on the Start button on the Chart window. (With a sampling speed of 40/min, the trace will move from left to right at about 2 cm/min. Feel free to change these settings after recording the first thermogram.) 7. As shown in reference 1, each thermogram has 5 parts. These five parts, and the amount of time you should allow for them before starting the next part, are listed below. a. pre-reaction baseline: 3 minutes At the end of this time start the reaction by carrying out the following steps in rapid succession: stop the motor push the rod down to open the cell start the motor b. temperature change due to reaction: time varies After perhaps 2 or 3 minutes the curve will straighten out into the post reaction baseline. c. post reaction baseline: 3 minutes At the end of this 3 minute period turn on the heater power and start the timer 4

5 simultaneously. d. temperature rise due to heating: time varies Be sure to make measurements that will enable you to calculate the amount of electrical energy dissipated in the heater. Heat the solution at constant voltage until the pen is again near the top of the paper. At the end of this time turn off the heater and timer. e. post reaction baseline: 3 minutes 8. Stop the scan. Set the calorimeter selector switch to ZERO. 9. Record all the experimental parameters (ambient temperature, heater circuit data) in your notebook. Save the voltage vs. time file in the CHM 340 folder. This can also be saved as a text file that is readable by Excel. 10. At the end of the experiment be sure to rinse all wetted parts. DATA ANALYSIS 1. For each thermogram determine the temperature changes due to the reaction and the heating by finding the distance in millivolts between the extrapolated baselines, and converting these distances to the T values. THE CALORIMETER BRIDGE OUTPUT CHANGES BY mv FOR EVERY C CHANGE IN TEMPERATURE. Measure Treaction at the time at which the temperature change due to the reaction is 63% complete, as shown in the diagram in Reference Prepare a table of the following data and calculated values: glycine masses, Ti, T, qheating, Cp, and H (in kj/mol). 3. Calculate the enthalpy changes for reactions 1 and 2. DISCUSSION In your discussion you should include a) Comparisons of your experimental enthalpy changes with those obtained from King's equilibrium constant data. 2 Be sure to compare values at the same temperature! [See the discussion of equations 3, 4, and 5 in reference 1. Note that ln K = (2.3026)log10 K. Equation 5 in reference 1 should read d ln K/dT = H/RT 2. b) Values of H 298 for reactions 1 and 2 obtained from tables of thermochemical data (e.g. enthalpies of formation), and comparisons of these values with your experimental values of H1 and H2. c) A discussion of the algebraic signs of the enthalpy changes of reactions 1 and 2. Why, in molecular terms, are reactions 1 and 2 endothermic (or exothermic)? d) A discussion of the "problem of incomplete protonation of glycine" (see Reference 1). REFERENCES 1. Ramette, R. W. J. Chem. Educ. 1984, 61, King, E. J. J. Amer. Chem. Soc. 1951, 73,

6 Figure 2. Cross-Section of the Parr 1451 Solution Calorimeter 6

7 Figure 3. Electrical Connections HEATER CIRCUIT heater ( 100 Ω) V + A power supply RECORDER CIRCUIT SCSI port MacLab interface ch. 1 input coaxial cable SCSI port Macintosh computer + Parr 1451 calorimeter 7