Florida Tech CHM 3011 Experiment 10 Fall 2016
Heats of Solution The p-chem lab manual is written by Clayton Baum at Florida Tech.
References: Engel, T.; Reid, P. Thermodynamics, Statistical Thermodynamics & Kinetics, Pearson, San Francisco, 2013
(2010, 2006); Chapter 4. Selected Values of Chemical Thermodynamic Properties, National Bureau of Standards Technical Note 270-3, pp. 13, 25-27 (1968); NBS Technical Note 270-8, pp. 23-26, 64-65 (1981). (photocopy, on reserve) Purpose: Determine the molar heat of an exothermic ionic reaction involving HCl and NaOH and the endothermic solution process involving solid KNO3 when dissolved in water. Method: The heats will be measured using a constant-pressure solution calorimeter instead of the constant- volume bomb calorimeter employed in experiment 2, but in many respects the procedure will be the same. Refer to the Background in Experiment 2. You will be using a Parr Solution Calorimeter (Figure 1) with the microprocessor-based thermometer. Recall that the thermistor probe and vacuum flask (dewar) are fragile and expensive ($300) so please handle them with extreme care. Clean up any spills in the calorimeter immediately. The calorimeter automatically corrects the temperature by assuming the heat leak between the bucket and the jacket is proportional to their temperature difference. The details were given on the last page of the Experiment 2 handout.
Figure 1. Parr 1455 Calorimeter Figure 2. Parr Sample Cell This experiment is based on the relationship between the heat at constant pressure (qP) and the temperature rise (ΔT), which is analogous to equation (6) in experiment 2: qP = -Cs ΔT, where Cs is the heat capacity of the system consisting of everything inside the Dewar (1) The heat capacity of the calorimeter is obtained by measuring the temperature rise produced by the standard reaction of TRIS, tris(hydroxymethyl)aminomethane, dissolved in 0.1 M HCl. The known heat at 25oC is -58.738
cal/g of TRIS. Then, measurements of the temperature change associated with the reactions of interest are made to obtain the enthalpies of these reactions. Experimental Procedure: Note: Calculate the volumes of concentrated HCl (Fisher Scientific, Reagent ACS Grade) and the mass of NaOH (Fisher Scientific, Pellets) required to make the solutions before coming to the laboratory. A. Standardization with TRIS (two trials) 1. Prepare exactly 100 mL of 0.100 M HCl. (Concentrated HCl is 12.4 M.) CAUTION: Remember to add the concentrated acid to the flask partially filled with water, swirl the uncapped flask to mix the solution, and then add water to the mark.. Carefully pour this solution into the Dewar flask. 2. Weigh approximately 0.50 g of granulated (i.e., no lumps) TRIS (Sigma-Aldrich, Trisma base, 99.9%) into the teflon dish to a precision of ±0.0001 g. See Figure 2. Be careful not to drop any of the sample into the push rod socket. 3. Assemble the rotating cell; set the dish on a flat surface, grasp the glass bell, and carefully press the bell firmly onto the dish. Do not grasp or press the thin-walled glass stem during this operation since it is fragile and will break easily. Attach the cell to the stirring shaft by sliding the plastic coupling onto the shaft as far as it will go and turning the thumb screw finger tight. Do not over tighten the screw or the Teflon will crack. Place the cell in the calorimeter and attach the drive belt. 4. Turn on the Parr 6772 Calorimetric Thermometer which is attached to the 6725 Semimicro Calorimeter and the network. After 10-15 s, the touch screen Main menu will appear. The Par 6772 will determine the initial temperature (Tinit) and the corrected temperature change (T) for your samples, and send this information automatically to its web site. The Parr 6772 also will provide a real-time temperature vs. time plot. 5. Turn on the computer (user: pchem, password: chemistry) and monitor. Open Explorer and go to the Calorimetric Thermometer home page at http://22.214.171.124. Check the last few digits of the IP address by looking at the inet addr on the Parr 6772 (Main → Communication Control → Network Interface → Network Status). Sample Data on the home page contains the calorimeter reports, and LCD Image displays the Parr 6772 screen.
6. Test the stirring assembly before each trial by starting and then stopping the motor (Main→Calorimeter Operation → Stirrer). Do not start the motor unless there is liquid in the dewar. 7. Set up the Data Logger (Main → Diagnostics → Data Logger) before each trial. The interval should be 12 s, the destination should be Logfile Only in Data Format. Delete Data Log File to clear the Data Logger and make certain the Data Logger is on. 8. In Main → Calorimeter Operation, press the Start button to begin the preperiod. This activates the stirring motor and prompts for your sample ID number. Enter a unique name that includes your group initials and sample number (e.g., run #1 for group C on Tuesday afternoon could be TUPMC1). The calorimeter will allow 7- 8 minutes for the system to equilibrate before signaling the operator to initiate the reaction. The run may be aborted at any time by pressing Abort button. Press the Temperature Graph button to see a real time plot of temperature vs. time. Press the Escape button to return to the Calorimeter Operation menu. 9. When the “Mix Reactants” dialog box appears, combine the reactants by pressing the push rod downward to drop the sample out of the rotating cell. This should be done without undue friction from the finger. Push the rod down as far as it will go. Then immediately press the <Continue> button to begin the postperiod phase. It is important that this phase is synchronized with the initial mixing to obtain the correct temperature rise. The temperature of the calorimeter should change within 30 seconds after mixing.
10. At the end of the postperiod, the Parr 6772 will turn off the stirring motor and prepare a calorimeter report indicating the initial temperature and the temperature rise, fully corrected for all systematic heat leaks. 11. Immediately return to the Temperature Graph screen on the Par 6772, enter Setup to expand the temperature scale (Bucket Min Value and Bucket Max Value), and use LCD Image on the computer to print out the graph. (After several minutes the temperature change will be lost.) 12. With the report on the Par 6772 screen (Report → Select from List → Display), go to LCD Image on the computer web page and print out the screen. If the Report does not contain the initial temperature and the temperature rise, copy the data from the Data Logger file on the computer under Data Log to Notepad. This data consists of the date, time, Bucket temperature, and Jacket temperature in comma delimited format. This can be imported into Excel and the Bucket temperature versus time can be plotted.
13. Rinse out the cell and dewar when the run is finished. B. Exothermic heat of neutralization 1. Prepare 100 mL of 0.25 M HCl using concentrated (12.4 M) HCl. See the CAUTION in step 1 of Part A. Carefully pour exactly 90 mL of the 0.25 M HCl solution into the Dewar flask. 2. Prepare 50 mL of 2.5 M NaOH from solid NaOH. NaOH is corrosive so immediately clean up any NaOH powder spilled on the balance or the bench. Dissolve the solid in a 50-mL beaker with 30-40 mL of water and pour this solution into the 50-mL volumetric flask, adding water to the mark. Using a volumetric pipet, introduce 10 mL of the 2.5 M NaOH solution through the glass stem into the assembled cell. Carefully attach the cell to the stirring shaft as in step A.3.
3. Carry out the determination runs by following steps A.6-A.13 above.
C. Endothermic heat of solution 1. Pour exactly 100 mL of distilled water into the Dewar flask. 2. Weigh approximately 0.70 g of granulated solid potassium nitrate (Fisher Scientific, Certified ACS, > 99.0%) into the teflon dish to an accuracy of 0.1 mg.
3. Carry out the determination runs by following steps A.6-A.13 above. Adjust the scale, if necessary to accommodate a temperature decrease. Cautiously neutralize the remaining acid and base solutions with one another before pouring them down the drain.
Calculations and Discussion: 1. Verify the temperature rise for each trial by comparing it with the T estimated from the corresponding plot.
2. Standardization. Calculate the heat capacity of the calorimeter using the TRIS standard. a. The value of qp (in cal) used for the reaction involving m grams of TRIS is given by
qP = -m[58.738 + 0.3433(25 – T.63)] where T.63 = 0.63T + Tinit. The term 0.3433(25 – T.63) adjusts the heat of reaction to any temperature above or below the 25oC reference temperature. b. Use equation (1) along with the value of qP in the previous step and the temperature rise to obtain the value of Cs for the calorimeter. Calculate the average Cs for the two trials. Note that the value of Cs is for 100 mL of aqueous solution in the calorimeter so that this volume must be maintained for the samples. 3. Determination of H for the neutralization reaction. Use equation (1) along with the value of Cs from above and the temperature rise to calculate qp in cal. Determine n, the number of moles of limiting reactant, from the balanced stoichiometric equation and the volumes and concentrations of each reactant. Finally, calculate qp (kcal/mol) for the neutralization reaction. 4. Determination of H for the KNO3 solution. Use equation (1) along with the value of Cs from above and the temperature rise to calculate qp in cal. Calculate qp (kcal/mol) from the mass of the salt used in this experiment. 5. Compare your heats of reaction (kcal/mol) to the values obtained using the literature data for the heats of formation. Include the uncertainties for your values. (The uncertainty in your ΔH value can be estimated by comparing the ΔH value calculated using your average Cs with the ΔH value calculated using the Cs value for one of your trials.) The NBS Technical Notes concentrations are expressed in # mols solvent/1 mol solute. Choose the values closest to your concentrations expressed in molarity. For example, a 1.0 M solution could be expressed as 55.6 mols H2O/1 mol solute, assuming 1 liter of solvent. The state “c” represents crystalline solid. 6. For the neutralization reaction, calculate the differences between the ΔH values for the concentrations used in this experiment and the values that would apply at infinite dilution. Are these differences within the qualitative estimate of the experimental uncertainty in measuring the ΔH values?