Exp 9: HEAT EFFECTS AND CALORIMETRY

Introduction:

In your introduction briefly describe/explain the following: heat, direction of flow of heat, cofee cup and bomb calorimeters, specific heat capacity, Law of Dulong and Petit, enthalpy, Delta H, q, heat of formation, heat of solution, and heat of reaction.
Be sure to give the mathematical formulas and/or relationships when necessary.

Procedure:

There are 3 parts to this lab--specific heat of a metal, heat of solution, and heat of a reaction. You may do them in any order you wish.

Set up computer, calibrating your thermister as you did previously.

Review of ULI setup and thermister calibration.

1. Specific Heat of a Metal

Begin boiling water in a 1000 mL beaker about 2/3 full. Mass approximately 50 grams of a metal in a large Pyrex test tube. Place the test tube containing the metal in the boiling water for ten minutes. Measure the temperature of the boiling water using a thermometer (remember to note the correct number of significant digits).

Mass 60 mL of water and place in the coffee cup calorimeter. Measure the temperature of the water using the temperature probe. Pour the metal directly from the water bath into the water in the coffee cup. Immediately cover the coffee cup calorimeter and measure the temperature using the temperature probe (i.e. begin graphing). Swirl/stir the mixture as the data is collected. When the temperature levels off, stop data collection and record the highest temperature measured.

Dry the metal before repeating the procedure for a second trial. After completing 2 trials, dry the metal and set aside as directed by your instructor.

2. Heat of Solution

Mass approximately 5 grams of NH₄Cl or NaOH. Use a 50 mL graduated cylinder to measure 40 mL of water. Mass the water and pour it into the coffee cup calorimeter. Measure the initial temperature of the water using thr ULI temperature probe. Add the solid reactant to the calorimeter, cover and begin graphing. Remember to stir contents as the reaction proceeds. When the temperature levels off, stop data collection and record the highest temperature measured. Repeat at least once using the same amounts of solute and solvent.

3. Heat of a Reaction

Mass a sample of magnesium ribbon (as provided by your instructor) on the analytical balance. Mass 20 mL of 6 M HCl using a 50 mL graduated cylinder to measure volume. Pour the HCl into the coffee cup calorimeter and record the initial temperature using a temperature probe. Add the sample of magnesium ribbon, cover and begin graphing. Remember to stir contents as the reaction proceeds. When the temperature levels off, stop data collection and record the highest temperature measured. Rinse out the calorimeter and repeat the above procedure 2 additional times, for a total of 3 trials. Dispose of any undissolved magnesium in a trash bin, not in the sink.

Note: Be sure to record and label all masses and temperatures. You need not print off the graphs obtained with each trial.

Analysis:

Specific Heat of a Metal: Report the value of C_p for each trial and the average for the metal you used. Compare to the accepted value and report % error.

Heats of Reaction and Solution: Report the value of Delta H for each trial and the average, for both heat of solution and heat of reaction. Delta H should be given in both J/g and kJ/mol.

Calculate the theoretical value for Delta H for each, using the formula

Delta H = Sum of Delta H_f(products) - Sum of Delta H_f(reactants)

Then calculate the % error for your average heat of solution and average heat of reaction.

Send questions, comments or suggestions to Gwen Sibert
Roanoke Valley Governor's School
gsibert@rvgs.k12.va.us