Experiment 7
Enthalpy of Solution

Students will work in teams of two and the experiment will be split over two lab classes. A computer will be used for data acquisition and data graphing.

A. Calibration of Temperature Probe and Alcohol Thermometer

1. Set up a 400-mL beaker on a ring stand equipped with a wire screen. Add about 350 mL of distilled water to the beaker and heat with a Bunsen burner until the water boils.

2. Choose "Calibrate" from the "EXPERIMENT" pull-down menu, and then click "Perform Now."

3. Place the temperature probe into the boiling water. Be extremely careful to avoid allowing the wire to contact either the flame or hot ring stand. Grasp the stick portion of the probe and use it to stir the water gently. Do not hold the probe by the wire.

4. When "Input 1" of "Reading 1" stabilizes, enter the value 100 and click the "Keep" button. You have just told the computer that the particular electrical signal corresponds to 100 ºC. Adjust the Bunsen burner to a lower flame, but keep the water boiling.

5. Add 50 mL of distilled water to 250 mL of crushed ice and stir with the temperature probe until the value of "Input 1" of "Reading 2" stabilizes. Enter the value 0 (zero) and click "Keep." Then click OK to exit the calibration program. You have just specified that this electrical signal corresponds to 0 ºC and you have calibrated the probe over the temperature range encompassed by the experiment. Warning, the calibration will be lost if you close or quit the program. Consult your instructor if you have questions. Keep the ice slurry for Step 7.

A. 6	A. 7

6. Place the alcohol thermometer in the boiling water and use it to stir gently for a minimum of 60 seconds and continue stirring until the temperature reading is stable. Keep the bulb immersed completely while stirring. Read the temperature to 0.1 ºC and record it on line "a" of Pg. 6-15.

7. Place the alcohol thermometer into the ice slurry and use it to stir gently for a minimum of 60 seconds and until the temperature reading is stabilized. Read the temperature to 0.1 ºC and record it on line "b" of Pg. 6-15.

8. Set the rate of data collection by selecting "Sampling" from the "EXPERIMENT" pull-down menu. Click on "minutes" as the time unit, and then type 25 into the box for sampling rate. Also enter 10 as the experiment length and exit by clicking OK.

B. Heat Capacity of the Calorimeter

1. Assemble the calorimeter and use a 100-mL graduated cylinder to transfer exactly 50.0 mL of water into the calorimeter. Slide the Teflon-coated stir bar into the water being careful not to splash the water.

2. Place the coffee cup calorimeter on the magnetic stirrer and adjust the stirring rate so that the stir bar moves freely and continuously. Keep the speed low to prevent loss of water by splashing.

B. 3	B. 4

3. Attach a utility clamp to the ring stand and use it to clamp the temperature probe into position with its tip immersed about ½ inch into the water in the calorimeter. Ensure that the stir bar does not hit the temperature probe.

4. Rinse a clean 250-mL beaker with distilled water but do not dry it. Use a 100-mL graduated cylinder to transfer exactly 50.0 mL of water into the beaker. Place the alcohol thermometer in the water.

5. Use a Bunsen burner on a low flame to heat the water to just over 60 ºC. Remove the beaker, using beaker tongs, and place it on a wire screen. Use the alcohol thermometer to stir the water continuously.

6. Click "Collect" on the computer to begin collecting data. After 30 to 45 seconds, slide the cardboard cover off the calorimeter in preparation for adding the hot water. Read the temperature of the alcohol thermometer to 0.1 ºC and record it on line "2a" of Pg. 6-15 and then quickly, carefully, and completely pour the hot water into the calorimeter and replace the cover. The system will automatically stop at the end of the collection period. Your graph will look something like this.

B. 6

B. 7

7. Re-scale the temperature axis to improve the accuracy of reading the data. First click on the data table to make it the active window. Scroll down through the data and note the highest and lowest temperatures. Now click on the graph to make it the active window. Select "Graph Options" from the "VIEW" pull-down menu, and click on the "Axis Options" tab. Add 1 to the highest temperature and enter the result as the maximum for the y-axis. Subtract 1 from the minimum temperature and enter the result as the minimum for the y-axis. Click "Apply" to see if the adjustments are satisfactory. If the flat portion of the curve is too close to the bottom or top of the graph, make a further 1-degree change in the minimum or maximum. Make sure that the values displayed on the y-axis are whole numbers and not decimals, before clicking OK. Your graph should now look like this.

8. Select "Print" from the "FILE" pull-down menu and input the names of the partners on the first line and the title of the experiment, such as "Calorimeter Calibration," on the second line. Click OK and then enter the number of copies (one for each partner) before clicking "Print" on the next pop-up window.

9. Turn off the magnetic stirrer and remove the temperature probe and stir bar from the calorimeter. Carefully dry the probe and stir bar. Place the stir bar back on the magnetic stirrer for safekeeping. Pour the water from the calorimeter down the drain and dry the calorimeter with a paper towel.

C. Heats of Solution

C. 1	C. 2

1. Assemble the coffee cup calorimeter and calibrate the temperature probe using the procedure described in Section A above. Also set the data collection rate as described in step 8 of Section A.

2. Use a 100-mL graduated cylinder to transfer exactly 100.0 mL of water into the calorimeter and add the stir bar. Clamp the temperature probe into place, add the cover, start the stirrer, and allow the water and calorimeter to equilibrate thermally for 5 minutes.

3. Use a weighing boat to weigh about 4 grams of calcium chloride, CaCl₂, and record the exact mass to 3 decimal places. Warning, the calcium chloride is hygroscopic (absorbs water readily), so do not prepare it until it is needed.

C. 3

4. Click the "Collect" button and allow the system to collect data on the pure water for about 45 seconds before sliding the cover off the calorimeter and adding the calcium chloride. Quickly replace the cover. Check to ensure that the stir bar is operating properly, because the addition of the solid chemical might knock it off center. If necessary, quickly stop andthen restart the stirrer to get back on center. The system will stop automatically after 10 minutes.

5. Re-scale the y-axis, add names and tiles, and then print the graph using the procedures described in section

B. 6

6. Turn off the magnetic stirrer and remove the temperature probe and stir bar from the solution. Use water to rinse the probe and stir bar, and then carefully dry them. Place the stir bar back on the magnetic stirrer for safekeeping. Pour the solution from the calorimeter into the waste container on the side bench. Finally rinse the calorimeter with water and dry it with a paper towel.

7. You will need to re-scale the y-axis before proceeding with the next step. Set the maximum temperature to 25 ºC and the minimum to 0 ºC.

8. Repeat steps 1 through 6 except use 5 grams of ammonium nitrate, NH4NO3, instead of calcium chloride. Remember to record the exact mass of ammonium nitrate to three decimal places.