Chemistry 140

Please have the following pages ready before class on Wednesday, November 27. Write an abstract and paper-clip it to the front of your individual writeup. The abstract and the carbon-copy pages of the write-up are due in class on Wednesday, December 4.

Metapurpose: When searching for an experimental protocol, you will often find something close to, but not exactly, what you want to do. This lab is an exercise in modifying an existing protocol to fit the particular purpose and equipment you have. “Cal Poly” is the California Polytechnic University at San Luis Obispo (a place I used to teach at).

The heat of reaction of the combustion of magnesium Mg (s) + 1/2 O₂ (g) ® MgO (s) is hard to measure because, as you saw in lab 2, there was a lot of heat given off in a short period of time. The “lot of heat” and “short period of time” make it difficult to measure the amount of heat given off in Joules using typical general chemistry lab equipment.

In fact, we are going to be sneaky and use Hess’s Law to determine the heat of magnesium combustion indirectly. Go to the website:

http://chemweb.calpoly.edu/chem/124/124Experiments/Thermochem/HessExp.html

and look only at the instructions for magnesium. We are not using the Silberberg textbook; the equivalent is section 6.3 in the Zumdahl text.

Your name, your partner’s name, date of experiment

Lab 6: Determination of the heat of reaction using Hess’s Law

Part 1. Purpose

Please write a sentence or two stating the goal of this experiment, based on the information on the Cal Poly web page. Then write and balance the set of four equations found on the Cal Poly web page (#1 through #3 and the net equation).

Part 2. Materials and methods

Chemicals needed: magnesium metal strip; magnesium oxide powder; 1 M hydrochloric acid

Print out the setup shown below and label the various pieces of equipment.

Part 3. Procedure

1. Set up the apparatus as shown above. Note which type of temperature probe you have; this will determine how you will position it in the calorimeter. For the Texas Instruments CBL system probe, make sure the black plastic tip will be fully immersed in the aqueous solution. For the Amprobe (multimeter) probe, make sure the small rounded metal tip will be fully immersed in the aqueous solution. For the Electrotherm probe, make sure the conical metal tip will be fully immersed in the aqueous solution

2. Tare your calorimeter (two nested styrofoam cups) and pour approximately 25 mL of 1 M hydrochloric acid into the calorimeter. Record the mass down to the milligram. Put in a magnetic stir bar and place on a hotplate/stirrer.

3. Tear the magnesium ribbon into three roughly equal pieces (approximately seventy milligrams or more). Weigh one piece down to the milligram, and roll the ribbon into a very loose spiral. This is to keep the temperature probe away from the metal during the reaction.

4. Record the temperature of the hydrochloric acid in the cup to be sure that it has reached thermal equilibrium with the room.

5. Carefully drop the magnesium ribbon into the calorimeter and record the temperature change until it reaches thermal equilibrium.

6. Record the appearance of the solution in the cup. Note any solid residue.

7. After thermal equilibrium is reached, pour the solution including any solid residue into the appropriate waste container. Rinse out the Styrofoam cup and dry it as best as you can.

8. Repeat the procedure for the other two pieces.

9. Repeat the procedure for approximately 0.250 grams of magnesium oxide powder instead of the magnesium ribbon. Be sure to record the mass of the MgO to the milligram.

Waste disposal — Dispose of any magnesium-containing materials in the container designated for magnesium waste disposal in the hood.

Part 4. Original data

Since you should end up running six trials total (three for the Mg, three for the MgO), a series of six tables, one for each table that lists time versus temperature, would be appropriate. Don’t forget to identify what is being reacted and which trial number it is for each table. Include a spot for the description of the contents of the cup after the reaction for each.

Summarize the main results of all six trials in one final table that lists the mass of both reactants (identify them), the initial temperature (T_i), the final temperature (T_f) and the temperature difference (DT) for each trial.

Part 5. Calculated results

For one trial, show how you calculated the enthalpy of reaction of magnesium and HCl in J/g and kJ/mol.

For one trial, show how you calculated the enthalpy of reaction of magnesium oxide and HCl in J/g and kJ/mol.

For the two numbers above, show how you calculated the enthalpy of combustion of magnesium (DH_combustion) in kJ/mol Mg.

Show the calculation of DH_reaction for Mg (s) + 1/2 O₂ (g) ® MgO (s) using DH_f° from the textbook.

Calculate a mean for your three trials (no standard deviation). Remember to throw out the outliers.

Calculate a percent error between your value of DH_combustion and the reference value DH_reaction.

Part 6. Group results

Write the mean of your experimental magnesium combustion enthalpies on the board and record all groups' experimental magnesium combustion enthalpies. Calculate the mean and standard deviation, and comment about where your results fit in (for instance, are your results outliers?). Finally comment on whether the group or your partnership was closer to the calculated combustion enthalpy and why this might be so.

Part 7. Questions

1. How can you tell when the reaction (either one) has reached thermal equilibrium?

2. What assumption did you make about the Styrofoam cup and the Teflon-covered stir bar? Why is this a reasonable assumption?

3. a. How would the temperature difference be affected by not fully immersing the sensitive part of the temperature probe? Show your reasoning.

b. How would DH for that reaction be affected by the error in part a? Again, show your reasoning.

c. How would DH_combustion be affected by the error in part a? Or can you tell without further information? Again, show your reasoning.

4. How would the temperature difference be affected by the magnesium not completely reacting with the acid?

5. How would the temperature difference be affected by not completely drying out the Styrofoam cup?

Part 8. Conclusion

A sentence each on:

• Report your partnership’s three measurements of DH_combustion.

• Report the percent error between the mean of your three measurements of DH_combustion and the calculated value of DH_combustion.

• Suggest a source of either systematic or random error, as appropriate.

• Report the class mean and standard deviation.

• Comment on whether the true value was determined by the class or not.

• Comment on whether you have more confidence in your results or the class’s results.

Abstract

Again, a short (less than 100 word) summary of the major result(s) of your experiment, and the method by which you achieved this result.

Name, school affiliation, title, text as usual. Keep in mind: What was the major result (and uncertainty)? By what method did you achieve the result?