Heat of Solution and Voltmeter Circuits - Experiment 4 | CHEM 461, Lab Reports of Physical Chemistry

Download Heat of Solution and Voltmeter Circuits - Experiment 4 | CHEM 461 and more Lab Reports Physical Chemistry in PDF only on Docsity! Revised 6/06 Chemistry 461 EXPERIMENT 4—HEAT OF SOLUTION AND VOLTMETER CIRCUITS 1, WASTE DISPOSAL 1. Ail salt solutions (including BaSOq precipitate)—Waste bottle labeled “Experiment 4 Salt Solutions,” located in the hood. Clean up spills in the hood promptly. DO NOT FILL THE WASTE JUGS COMPLETELY TO THE TOP! 2. Reaction products from the standardization of the Parr Solution Calorimeter and any remaining HCl—Adjust pH to between 5.5 and 12 using NaHCO; (check final pH with pH paper), then wash down the drain. 3. Any remaining solid salts (including TRIS)—Waste jar labeled “Experiment 4 Waste Salts,” located on the shelf under the windows on the north wall. Clean up spills in the balances promptly. Ii. INTRODUCTION In this experiment, you wiil make experimental measurements of the heats of solutions for the following reactions: Dissolution Reaction No. of Runs NaNO; in H20 3 Ba(NO3)2 in H2O 2 Na,SOx in H20 2 Ba(NO3)2 + NagSO, in H,O 2 Ba(NO3)o in 0.2 M NaSOx 1 (Extra Credit) The number of experimental runs required is also listed above. The first three measurements directly give the heats for the following three reactions: I Ba(NO3), (s) + HyO ——> Ba** (aq) + 2 NO} (aq) AH, IL Na,SO, (s} + H,O ——> 2Na” (aq) + so, {aq) AHy Il. NaNO, (s) + H,O ——> Na* (aq) + NO; (aq) AB The reaction for the solvation of BaSO4 may be written: IV. BaSO,(s) + H,O ——> Ba” (aq) + SO; (aa) AByy The actual fourth reaction you run in the laboratory is 4, Na,8O,(s) + Ba(NO3), (s) + H,O —> BaSO, (8) + 2.Na* (ag) +2. NO (aq) AH,, which involves formation of a precipitate, and is the sum of reaction I plus reaction II minus reaction IV. Hence we have that AHy = AH, + AH, - AH. Finally, note the possibility of the solid phase reaction: V. Ba(NO,), (s) + Na,SO,(s) —> BaSO, (s) + 2 NaNO; (s) AHy, which can be written approximately as 1+0-20 -IV so AH, = AH, + AHy - 2AHg - AHy. Note that AHy may be calculated from standard heats of formation, easily found in the Handbook of Chemistry and Physics. This serves as a literature check on the heats determined above. Note also that literature values for some of the heats of solution are posted on the bulletin board in the lab. The final reaction [Ba(NO3)2 in 0.2 M Na2SO,] can be run for extra credit. It is simply a linear combination of two of the other measured reactions. It thus serves as an internal check on the measured heats. These runs can be made on either a “homemade” calorimeter, or on the Parr 1455 Solution Calorimeter. You are, however, required to make no less than 2 and no more than 4 of these runs on the Parr instrument. The rest must be made on a “homemade” calorimeter. Since the “homemade” calorimeter has poorer insulation and other problems, higher concentrations are always used when using it. IV. GENERAL USE OF “HOMEMADE” APPARATUS The apparatus is shown schematically in Figure 1. It consists of a thermally insulated Dewar flask for aqueous solutions. A thermistor (temperature-sensitive resistor) is placed into the water in this flask to measure its temperature. This is accomplished by measuring the thermistor resistance (R,) via a Wheatstone bridge circuit, which includes a galvanometer, G. Such measurements are well described in the “electricity” section of your undergraduate physics textbook. It is also necessary to read in such a book about the temperature coefficient of resistivity to learn how a thermistor works. Also placed into the flask of water is an electrical heater. The heats of solution will be counterbalanced or mimicked exactly by the energy dissipated by this electrical heater. Your physics textbook will also tell you how to calculate this electrical energy by knowing the time (t) over which a fixed current (i) flows through this heater (resistor, Ry) and the voltage drop (V) that is maintained across this heater while this current flows. What is this necessary mathematical relationship? Voltages are measured using a potentiometer (digital voltmeter), current is determined by measuring the voltage drop across a 0.100 ohm calibrated resistor in series with the heater. Special Funnel Thermistor Leads (Red) Heater Leads Thennistor Black/White) [—— Dewar Flask Heating Coil Magnetic Stir Bar Stirring Motor Figure ta. “Homemade” Calorimeier for Heat of Solution  Digital Voltmeter C L isv T Dry Cell Ammeter 1 Coil I Wheatstone Bridge Figure 1b. Circuit Diagram of “Homemade Calorimeter” General Measurement Instructions: The following preliminary operations should not be done at this time. They are merely operations that you will need to repeat several times in the Experimental Procedure section. Voltage Measurements: Plug the leads from the calorimeter console labeled “POT” into the digital voltmeter (DVM), the black lead into the jack labeled “COM” and the red lead into the lead labeled “V-Q”. To conserve the battery, leave the DVM power off except when actually making a reading. Whenever turning “on” a DVM to read voltages or currents, always set it to the least sensitive (maximum range) before turning it “on”, then adjust for maximum precision. Unless otherwise noted, set the toggle switch on the calorimeter console so that actual voltages are read (i.e., in the “Vx1” position). When in the “V/100” position, the DVM sees 1/100th of the true voltage. (Prove in your lab report why this is true.) Adjustment of Heater Current to 1.000 Ampere: Caution: Never turn on heater current unless the heater coil is immersed in water. a, Place the knife switch on the front of the plexiglass case in the CURRENT position. This puts the DVM leads across the 0.100 Q resistor. (See Figure 1b.) b. Switch “on” the power to the DVM and the function and range dial to “DCV, 200m.” On this scale, the maximum voltage you can read is 200 mV. c. Bring the voltage reading to exactly 100.0 mV using the current-adjust control (the multi-turn dial on the front of the plexiglass case). Note that the analog (needle-type) ammeter may not read 1 ampere after the current has been adjusted. Do not be concerned about this. The value that is determined by using the DVM is much more accurate; in this case the analog ammeter should be used only as an on-off indicator rather than a measuring device. d. Turn off the power to the DVM. What you have done is to adjust the voltage across a calibrated 0.100 ohm resistor {in series with the heater coil) to be exactly 0.1000 volts (100.0 mv). Thus, from Ohm’s Law, the current flowing through the resistor (and the heater coil) is 1.000 ampere. Determination of the Voltage Drop Across the Heater Coil: a. Place the knife switch in the VOLTAGE position. As shown in Figure 1b, this puts the DVM leads across the heater resistance, Ry. b, Measure the voltage drop across Ry (i.e., the heater coil) using the DVM. c. Switch the toggle switch temporarily to the “V/100” position, and repeat the measurement of step (b) above. Sometimes, the actual voltage drop across some element in a circuit is too great to be measured directly using a normal DVM. (Nowadays this usually happens only above a few kV.) What you have done is to measure the potential drop across the smaller resistance of a potential divider in parallel with Ry. The potential divider is set in a ratio of 100:1; this means that the actual voltage drop across the heater should be 100 times that value read on the DVM. Note: When determining this voltage, you may observe some fluctuations in the reading; at other times it may be quite stable. If these fluctuations, which are the result of fluctuations in the AC line voltage, are present, use the best average value for the voltage that you are able to obtain. d. Switch the toggle switch back to the “Vx1” position and turn off the DVM. 8. 10. 1. As the water temperature nears 25°C, turn off the heater; note that the temperature continues to rise for several seconds. Try to determine the approximate magnitude of this temperature lag so that you will not overshoot the required temperature. (This may be easier to determine by monitoring the change in the thermistor’s resistance; see below.) Approach the 25° mark using the “momentary” switch. When the temperature has been adjusted to exactly 25°, determine the resistance of the thermistor which corresponds to this temperature. This exact resistance value should serve as the reference point for all experimental measurements, since it is a more accurate measurement to reproduce this reference temperature than to measure the temperature with a thermometer. To estimate the magnitude of the temperature lag of the heater by monitoring the thermistor resistance, adjust the decade resistors to bring the galvanometer on scale (to the left of zero) at some temperature below 25°. (The heater should be on.) Turn the heater off and observe how long the galvanometer continues to rise, and measure the magnitude of this rise. When the water temperature has been adjusted to exactly 25° and the thermistor resistance measured, REMOVE THE LONG-STEM DIGITAL THERMOMETER and insert the “special” sample addition funnel. Set the electric timer to zero time; make sure the stirrer is operating. Note: In all runs, the total stirring time must be determined; this is the elapsed time between the addition of the salt and the final temperature adjustment of the calorimeter contents. This will be required to calculate the “stirring correction” (step 16 below). You are now ready to make your first practice run measurement of the heat of solution of NaNO3. Add the first sample of NaNO3 to the water, and start the digital stopwatch. Close the telegraph key; the galvanometer in the thermistor circuit should deflect to the left indicating a drop in temperature (thermistor resistance), which occurs due to the endothermic reaction: NaNO,(s) + H0(é) ——> Na” (aq) +NO3(aq) Tum on the heater to counteract the cooling effect of the salt and check the approach to 25° by monitoring the thermistor resistance. As the endpoint of 25° is approached, change to the “momentary” heater switch so as to insure that the endpoint is just reached, but not exceeded. The apparatus is not equipped with a calibrated cooling device so no correction can be made for overheating; any run in which the endpoint is overshot must be discarded and repeated. When the endpoint is reached, stop the digital stopwatch and record the elapsed time (the total stirring time). Record the heating time required (from the analog timer). The energy (heat) of solution can be determined by multiplying this heating time 10 12. 13. 14, 15. 16. by the heater power. Do an energy balance to prove this is the case before the lab starts. Check the heater current and voltage drop across the heater after each run. If there has been any change in either value, the average must be used in your calculation of the heater power (P=I-V). Remember that the heater must be ON before measuring these quantities. Repeat the above procedure for the two real samples of NaNO; and for the 2 samples of Ba(NO3)2. For the (slightly) exothermic salt Na»SO, a different procedure must be used. The water temperature is adjusted to the same reference point (thermistor value) as for the other salts, and the Na,SO, is added through the funnel. However, in this case, the heater is not turned on. Instead, the change in thermistor resistance (temperature) is monitored. The galvanometer needle should deflect to the right of zero upon salt addition until it has reached a stable value indicating that all of the Na2SOy has been dissolved. When this occurs, the new thermistor resistance (temperature) is measured by adjusting the decade resistors to bring the galvanometer back to zero; this value is recorded. Reset the decade resistors back to that value corresponding to 25° and cool the solution below this point by immersing a cold glass rod. Turn on the heater and bring the temperature of the solution to exactly the 25° thermistor reading. Reset the decade resistors to that “temperature” value which was obtained after the dissolution of the Na)SOq, and reset the timer to zero. Turn on the heater and record the time required to heat the solution to this final “temperature;” this time is treated in the calculations exactly as the times recorded for the endothermic salts. Prove by energy balance that this is the case. After both runs have been made on each of the individual salts, the sample consisting of a mixture of Ba(NO3)2 and NazSO, is run. The overall process of solution plus reaction will still be endothermic in this case. After the last heat of solution run is made, the energy contribution to the system by stirring plus that energy lost due to non-perfect insulation (the so-called “stirring correction”) is determined. Adjust the initial water temperature to 25° as in the previous runs, Run the stirrer for about 30 minutes (recording the actual length of time the stirrer operates); at the end of this time, determine whether the temperature has increased or decreased. If the temperature has increased, the same procedure as for an exothermic salt is followed. Measure the new thermistor resistance, cool the water to below 25° with a cold glass rod, adjust the water temperature to 25° with the heater, and determine the length of time the heater must be on in order to raise the water temperature to that which was obtained after 30 minutes of stirring. From this time and the heater current and voltage, the energy input per second of stirring is calculated. If the water temperature has decreased after 30 minutes (indicating the system has lost 1 17. 18. energy), the procedure for an endothermic salt is followed. Determine the length of time the heater musi be on in order to heat the water back to 25°. From this time, the energy loss per second is calculated. This value should be used as a correction in your energy balance equations. At the end of each day be sure to do the following: a) Turn HEATER AND TIMER and MAIN POWER switches (on the calorimeter console) to OFF. b) Turn stirring motor off. c) Turn Fluke Electronic Galvanometer and DVM function switches to OFF. d) Unplug the leads to the DVM and return it to the stockroom. When you have finished the experiment, don't forget to remove any unused samples from the drying oven. Correctly dispose of any salts you have stored in a desiccator, and return the desiccator to the shelf. 12 Vii. EXPERIMENTAL PROCEDURE FOR THE PARR CALORIMETER CAUTION! The glass sample cell must only be handled by either the large diameter glass base or the plastic collar. The small diameter glass stem is thin-walled and fragile (and the cell is expensive) and will break very easily if any stress is placed on it. A. Calibration: 1. Tum on the calorimeter power (switch on rear of instrument on upper left) and the chart recorder; allow them to warm up for at least 10 minutes before making a run. Check that the recorder CM/MIN-OFF-CM/HR switch is set to OFF, the STANDBY/RECORD switch is set to RECORD, and that the INPUT RANGE SELECTOR is set to 1 volt. (See Figure 3 for the location of recorder controls.) Raise the pen from the paper using the PEN LIFT LEVER and remove the cap from the pen; place the cap in a location where it may be easily found at the end of the lab period. Rotate the ATTENUATOR control to its counter—clockwise limit. In order to make good comparisons with the numbers obtained with the “homemade” calorimeter, all runs should be made at 25°C. Before starting the runs, place an adequate supply of the solvents used, either 0.1 M HC! or deionized water, in one of the 25°C water baths to equilibrate; ten or fifteen minutes should suffice. The first operation is the calibration of the calorimeter, i.e. determination of the calorimeter's heat capacity. The calorimeter should be calibrated each day, or whenever operating conditions change (e.g., if a new Dewar flask or sample cell is used). The calibration should be repeated at least once, more if the results do not closely agree, It is essential that the calorimeter be accurately calibrated because all subsequent results will depend upon the calibration. The calibration consists of reacting a known quantity of tris(hydroxymethyl)aminomethane, commonly call TRIS, with dilute hydrochloric acid. TRIS is an organic base for which the heat of reaction with hydrochloric acid is accurately known; its formula is (HOCH2)3;CNHz. The TRIS sample should be ground before the calibration runs, but need not be dried. In each of the runs it will be necessary to set the calorimeter span (number of degrees Celsius represented by a full-scale deflection of the recorder) and offset (the temperature corresponding to a recorder reading of zero). These are most easily estimated by first performing a trial run. 2. Remove the drive belt from the pulleys and remove the cover from the calorimeter. Lift the Dewar flask out of the metal can; dry the Dewar thoroughly if necessary. Tare the Dewar on the large triple beam balance and weigh about 100g of (approximately) 0.1 M HCl to the nearest 0.1g. Place the black plastic spacer ring around the Dewar and place the filled Dewar in the calorimeter. 15  PEN COVER: rtf) J ATIENTUATOR PEN LIFI/PAPER RELEASE LEVER STAND BY/RECORD SWITCH | INPUT RANGE * SELECTOR ZERO CONTROL PER SPROCKETS CHART SPEED PA oe — +: SELECTOR * < ec UW ® iil fl e £ PAPER HOLD DOWN POWER INDICATOR CM/MIN-OFF-CM/HR CORE SUPPORT KNOB CORE SUPPORT DISCS CHART PAPER CONNECTOR col oo ON-OFF POWER ENTRY MODULE CHASSIS LOCAL/REMOTE SWITCH Wit VOLTAGE SELECT GND SWITCH AND FUSE SIGNAL- SIGNAL+ Figure 3. Top and Rear Panels of Chart Recorder 16 . Check that the glass sample cell, the glass push rod, and the Teflon sample dish are all clean and dry. Remember to handle the sample cell only by the plastic collar or the large diameter glass base. Weigh ~0.5g of TRIS to the nearest 0.1 mg directly into the tared Teflon sample dish; be careful not to drop any of the sample into the push rod socket in the center of the dish or on the lip around the outside of the dish. Given that the molecular weight of TRIS is 121.14, verify that the amount of 0.1 M HCl in the Dewar is sufficient to completely neutralize your TRIS sample. Set the dish on a flat surface and carefully press the glass cell over the dish to seal the sample inside. Be careful to grasp only the sides of the thick-walled giass cell during this operation, not the stem or collar; press the glass cell down over the Teflon dish as far as it will go to insure a good seal. Attach the cell to the calorimeter cover stirring shaft by sliding the plastic collar of the cell onto the shaft as far as it will go and turning the thumb screw finger tight. Hold the cover in a horizontal position and lower it carefully until the bottom of the cell rests on the bench top; then, while firmly grasping the large diameter glass cell base, insert the glass push rod through the pulley hub and press the end of the rod into the socket of the Teflon sample dish. . Lower the cover assembly with the cell into the Dewar and set the cover in place on the calorimeter can. Rotate the cover so that the thermistor probe opening is pointed to the rear of the calorimeter; install the thermistor probe in the cover opening and press the bushing firmly into place to anchor the probe in its proper position. Check that the two plugs in the cover are completely seated; they can occasionally be knocked loose if the cover rests on a flat surface. Note: Handle the thermistor probe carefully. It can be damaged if dropped. . Drop the geared drive belt over the motor and stirrer pulleys. The belt may appear to be unusually loose, but it is intended to operate under very light tension to minimize friction in the stirrer bearing. Turn on the stirring motor by pressing (FI). Allow the system to equilibrate for several minutes before setting any calorimeter parameters. , Set the recorder CHART SPEED SELECTOR to a setting of 3. (Runs will be recorded at a chart speed of 3 cm/min.) Start the chart paper by setting the CM/MIN— OFF-—CM/HR switch to CM/MIN, and lower the pen using the pen-lifting lever on the left side of the recorder. Note: If the sample is slow to react or dissolve, it may be desirable to use a chart speed of 2 cm/min or even 1 cm/min. This can also be determined during the trial run. . Observe the liquid crystal display above the calorimeter keyboard. The top line gives prompts when entering parameters such as the span and offset. The bottom line gives various output data, the majority of which is used only for other applications. Observe the bottom line until you see “CHNL 1” followed by a temperature reading; 17 12. 13. 14. 15, 16. 17. 18. When a linear preperiod drift line has been established, combine the reactants by pressing the glass push rod downward to drop the sample out of the rotating cell. Use the palm of the hand to push straight down on the rod; do not grasp the rod, as this will interrupt the rotation of the cell and the stirring of the solution. Very soon after combining the reactants, there should be a pen deflection corresponding to the temperature change within the calorimeter. Allow the stirrer to operate during the postperiod until the drift line becomes linear and remains linear for at least 3 minutes indicating that the reaction has reached completion. Usually this condition will be reached within 10 minutes or less after combining the reactants. After a satisfactory postperiod drift line has been established, set the CM/MIN-OFF— CM/HR switch to OFF and raise the pen from the chart. Note that raising the pen disengages the chart drive so that the thermogram may be easily pulled out and torn off against the paper hold down bar. Do not try to manually advance the chart unless the pen is raised! Label the thermogram, identifying the reactants; record all pertinent parameters used during the run. Stop the calorimeter stirring motor: Press Press Note: When using the (SHIFT) key, it need not be held down as on a keyboard; simply press it once and then press the second key. Remove the drive belt. Remove the thermistor probe and wipe dry. Carefully raise the calorimeter cover and lift the cell assembly out of the Dewar flask. Remove the sample dish from the end of the push rod, then remove the rod; carefully loosen the thumbscrew holding the glass sample cell to the stirring shaft and gently remove the sample cell. Remember: Handle the glass sample cell only by the large diameter base or the plastic collar, not the small diameter stem. Lift the Dewar flask out of the calorimeter can and empty it. Wash and dry all wetted parts carefully. At the end of the day: Carefully wash and dry all wetted parts of the cell assembly and the Dewar flask. Place the Dewar in the calorimeter can; replace the cover and drive belt. Check that the recorder pen is lifted from the chart, replace the pen cover, and turn off the calorimeter and recorder. 20 Calculation of the Parr calorimeter constant: In order to determine the net temperature change produced by the reaction, it is necessary to locate a point on the thermogram at which the temperature reached 63% of its total rise (or fall). This can be easily done using the graphic method described below and shown in Figure 4. 1. Place a straight edge over the preperiod drift line and extend this line well past the point at which the cell was opened to start the reaction. 2. Move the straight edge to the postperiod drift line and extrapolate this line backward to the time when the cell was opened. If there are fluctuations in the drift lines due to noise or other variations in the signal, use the best average when drawing the extrapolations. 3. Using a centimeter scale, measure the vertical distance, R, between the two extrapolated lines at a point near the middle of the reaction period, and convert it to temperature units using the chart recorder’s scale factor from above. This corresponds to the temperature change due to the process, AT,. As shown in Figure 4, this is most accurately measured at a point where this temperature change is ~63% completed, since this minimizes errors associated with differing slopes in the preperiod versus postperiod. Note that while Figure 4 illustrates an exothermic reaction, the same procedure is used for an endothermic reaction. 4. The energy change, Q, is calculated by Q=AT, «© where e is the energy equivalent of the calorimeter and its contents: © = Soy + (mo sow) where Ccqj = the calorimeter constant (or heat capacity of the empty calorimeter), Msoly = the mass of the solvent, and Csoly = the specific heat of the solvent. The calorimeter constant is then easily calculated given that 58.738 calories per gram of TRIS is evolved when TRIS is dissolved in 0.1 M HCI, and that the specific heat of 0.1 M HCl is 0.99894 cal/g - deg at 25°C. B. Real reaction measurements with the Parr calorimeter: 21 Follow steps 2-17 in the procedure above for each salt, using 100g of water for the solvent; make a trial run for each salt to estimate the calorimeter span. Weigh samples of each salt to give final concentrations as shown in the table below: Final Suggested Span Setting Concentration For Trial Run Ba(NO3)2 0.02 M Lc NaySOg 01M 0.25°C NaNO; 0.04 M re For runs involving a mixture of Ba(NO3). + Na,SO,, weigh samples to give the same final concentration of Na” and NO, ions as above; use 1°C as the span setting for the trial run. Note: Ideally, you would want the same final concentration of NazSO4 as Ba(NO3)2 when determining the heats of solution of the pure salts so that the solutions have the same ionic strength. However, the small amount of heat evolved when NazSQq is dissolved makes this impractical; any error introduced here should be very small. Remember, at the end of the day: Carefully wash and dry all wetted parts of the cell assembly and the Dewar flask. Place the Dewar in the calorimeter can; replace the cover and drive belt. Check that the recorder pen is lifted from the chart, replace the pen cover, and turn off the calorimeter and recorder. Determine AT, as outlined in steps 1-4 of the calculation section; the energy change is calculated as above: Q= AT, - Coat + OMg1y (Ceotv » and the molar enthalpy change at the reaction temperature is calculated by an =22 n where n is the number of moles. 22