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N-11
EXPERIMENTAL PROCEDURE
Please put on your safety glasses. Clean a 250 mL Erlenmeyer flask and pour approximately 180
mL of 1.0 M HCl into it. Label the flask "1.0 M HCl" and stopper it. Dispense from this solution
when 1.0 M HCl is required in the experimental procedure.
Part I.
Standard Heat of Formation, ?H
f
, of H2O
(l)
Assemble the calorimeter as described in the Experimental Method section just after Figure 1. Remove
the thermometer and carefully put it aside. Take off the lid and separate the two cups. Take one of the
cups to a balance. Using the correct weighing technique, accurately weigh between 0.55-0.60 g of
magnesium into the Styrofoam cup. Excess magnesium should be placed in the solid waste container. If
you have difficulty getting the magnesium metal in the cup because of static electricity, add a little water
to the calorimeter and swirl, wetting the entire inside surface. Pour out the water and then proceed as
before.
Bring the cup containing the magnesium back to your bench, and put it inside the second cup. Measure
60 mL of 1.0 M HCl into your graduated cylinder. Record the temperature of the HCl solution (to 0.1
C) on your Observations Sheet. At this point, remove the thermometer and accurately determine the
volume of HCl to 0.1 mL and record this on your observation sheet. Rinse the thermometer with
distilled water, wipe dry and carefully reinsert it through the lid of the calorimeter.
NOTE:
The temperature of the HCl solution will be t
i
(at time = 0) for the reaction. Since the
reaction occurs so quickly, it is not possible to measure the initial temperature once the
reagents have been combined.
Smoothly add the HCl solution all at once (avoid splashing) to the calorimeter, replace the top, and
immediately begin swirling gently. The instant that the acid is added is taken as time zero. Continue to
swirl gently throughout the run. Read and record the temperature and time every twenty
seconds until a
maximum temperature is reached. Once the maximum is reached, record the
temperature every 30 seconds for at least four more minutes. You should keep recording data until the
decrease in temperature shown in Figure 2 is well defined (i.e. at least 4 minutes after reaching a
maximum temperature, t
max
). The accuracy of your results depends on this. On your graph, you will
obtain t
e
to 0.1 °C by extrapolation. You will use this result to calculate ?H
6
(see equation (6)).
