Experiment 8

Students will work in teams of two. A computer will be used for data acquisition and data graphing.

A. Calibration of Temperature Probe

1. Set up a 400-mL beaker on a ring stand equipped with a wire screen. Add about 250 mL of distilled water to the beaker and heat with a Bunsen burner until the water boils.

2. On the computer, 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.

6. 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. Enthalpy of Combustion of Magnesium


B. 1	B. 2

1. Assemble the calorimeter and use a 100-mL graduated cylinder to transfer exactly 100.0 mL of 1.0 M HCl into the calorimeter. Slide the Teflon-coated stir bar into the solution, being careful not to splash the solution. Wash immediately if the solution comes into contact with your skin.

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 by splashing.

3. Attach a utility clamp to the ring stand and use it to clamp the temperature probe into position with its tip immersed about 1/2 inch into the solution in the calorimeter. Ensure that the stir bar does not hit the temperature probe. Slide the cardboard cover onto the calorimeter.

4. Weigh a plastic weighing boat and enter its mass on your data sheet. Add a little more than 1 g of magnesium oxide, MgO, to the plastic weighing boat. Record the exact mass of the boat plus MgO to 3 decimal places.

B. 4

5. Click the "Collect" button on the computer screen and allow the computer to collect data for 1 to 2 minutes to establish a baseline. Then remove the cardboard cover and carefully add all of the MgO to the HCl. Replace the lid and continue to collect data until the software automatically stops at 10 minutes. Your chart should resemble the chart shown in B.5 below.

B. 5

B. 6

6. 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 look like the chart (B.6) on the right.

7. 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 "Magnesium Oxide Reaction," 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. You may find that you will need to print in landscape mode and adjust the size to 200% to provide a large, easy-to-read graph.

8. Turn off the magnetic stirrer, and remove the temperature probe and stir bar from the calorimeter. Carefully rinse and dry the probe and stir bar. Immediately wash off any HCl that gets on your skin.

9. Pour the solution from the calorimeter into the waste container at the side bench. Rinse and dry the calorimeter.

B. 10

10. Repeat Steps 1 - 9 except use about 0.5 g of magnesium ribbon instead of MgO. Remember to weigh the magnesium to 3 decimal places. Be careful to check the stir bar after adding the magnesium ball to ensure that it is turning properly. If it is not turning properly, turn off the stirrer and then immediately re-start it. Lack of proper stirring will produce poor data and will necessitate a re-run of this portion of the experiment.

11. During clean-up, dispose of the waste in the bucket at the side bench. Place the stir bar on the magnetic stirrer for safekeeping.

C. Data Anaylsis

1. Using a pencil and ruler, draw a vertical line parallel with the y-axis through the point of chemical addition. Draw the best straight line through the points after the chemical addition. This line should intersect the vertical line which identifies the time of chemical addition.

2. Extrapolate the point of intersection back to the y-axis.

3. Read the initial and final temperatures from the graph, as shown in the figure, to the nearest 0.1 ºC.

4. Use the data, formulae, and Hess' Law to finish calculations