Record the equation of the straight line that appears on your LabQuest screen in your lab notebook, then tap on OK. To determine T f , set x equal to 4.0 mins, and solve for y. The value of y is equal to T f , the final temperature at the 4 min mark. Do this calculation in your lab notebook and record the value of T f to the nearest 0.1 o C. For example, using the equation of the straight line in the figure above: y = – 0.1153x + 35.303; when x = 4.0, solving for y = T f = – 0.1153(4.0) + 35.303 = 34.8 o C. 12. Repeat steps 3 – 11 using 6.00 – 7.00 g of ammonium chloride, NH 4 Cl. To start a new run (Run 2), tap on the file cabinet icon next Run 1 and you should see: Be sure to record, in your lab notebook, the mass of NH 4 Cl used, T i , the equation of the straight line for data collected after the 6 min mark, and calculate the value of T f . Part B: Heat of Reaction for Magnesium and Hydrochloric Acid 13. Your instructor will prepare 1 L of 2.0 M NaOH and 2.0 M HCl solutions. These solutions can be found under the hood in a dispensing bottle. 14. Re-assemble the calorimeter. Transfer 50.0 mL of the HCl (aq) solution to the calorimeter using a graduated cylinder. Turn on the switch that controls the stirring mechanism and stir the solution at a moderate speed. Carefully lower the clamped temperature probe through the hole in the lid. Adjust the height so that the probe is in the HCl solution but does not touch the magnetic stirrer bar.
8 15. Obtain a piece of magnesium ribbon between 25–26 cm long. Fold the ribbon back and forth to form a zigzag chain and weigh on an analytical balance. Record its mass in your lab notebook. 16. Using a piece of copper wire, form a cage around the magnesium ribbon. Your instructor will demonstrate how to make the cage. This will allow the magnesium to remain on the bottom of the calorimeter as the reaction occurs. 17. Tap on the file cabinet icon to start Run 3. Tap the green arrow, , in the lower left of the screen to start data collection. 18. Keep your eye on the time (lower right of screen). At the 3.5 min mark get ready to add the encaged magnesium ribbon into the HCl solution. Lift the lid up carefully but do not move the temperature probe. 19. At the 4 min mark add the encaged magnesium ribbon as quickly as possible, and then replace the lid carefully, making sure that the solution does not splash out of the calorimeter. You may have to turn down the stirrer speed. You should see the temperature rise rapidly. You do not need to stop the data collection. LabQuest will stop automatically at the 10 min mark. 20. Follow steps 9 – 11 as outlined above, and be sure to record all relevant data in your lab notebook. 21. It is vital that you wipe off all traces of hydrochloric acid from the temperature probe. Use a wet paper towel to do this several times over, and then wipe dry. Any traces of acid will corrode the temperature probe.
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