LAB EXERCISE 8: ENTHALPY CHANGE

CHEMISTRY I - LABORATORY EXERCISE MANUAL

LABORATORY EXERCISE 8: Enthalpy Change (Hess' Law)

PART I: Memo to Lab Groups
Energy Research Foundation, Inc.
Theoretical Research Section
Washington, D.C.

TO: GenChemCo - Colorado Mountain College
Glenwood Springs, CO

Re: Determining the enthalpy change of a reaction

As our standard quality assurance program requires, we would like your organization to duplicate the experimental determination of the enthalpy change of the following reaction:

NaOH + NH₄Cl ---> NaCl + NH₃ + H₂0;

In our laboratory, we were successful in determining the enthalpy changes for the following reactions:

1. NaOH + HCl --> NaCl + H₂0

2. NH3 + HCl --> NH₄Cl

3. NaOH + NH₄Cl --> NaCl + NH₃ + H₂0

We used Hess' Law and reactions 1 and 2 to predict the results of reaction 3. Since the primary purpose of this project is the replication of results, we require that you follow our methodology (attached) as closely as possible.

We are looking forward to the results of your study.

Sincerely yours,

D.H. Calorimas
Vice President of Research

PART II: InterLab Memo

GenChemCo Industries
COLORADO MOUNTAIN COLLEGE

InterLab Memo:

Re: Determining the enthalpy change of chemical reactions

As you can see from the attached letter, our group was asked to determine enthalpy changes for several chemical reactions. Since we are supposed to replicate a series of previously performed reactions, the instructions you are supposed to follow are included.

As usually, the class as a whole should brain-storm what we are supposed to do, whether the instructions are clear, and some of the details of the techniques involved. I recommend that each group perform one reaction, calculate the enthalpy change of the reaction, and share information with the other groups. Ultimately, each group will have to check how closely the three reactions approximate Hess' Law.

Each work-group will submit a report to me (due two weeks from today) which should include the following sections:

1. Initial Working Hypothesis.

2. Initial Working Plan.

3. Your data.
---Please show ALL measurements and calculations of your groups share of the project.

4. Analysis of your data and of the results of the other groups.
---Does the data support your hypothesis? Why? Why not?

5. Further Steps Taken.
---Did you do anything beyond your initial working plan? Why? What did you find?

6. A Letter of Response.
---Each group should submit a response directed to the client outlining what you did, what you found, and how this addresses the questions proposed by the client.

Part III: PROCEDURE FOR DETERMINING ENTHALPY CHANGES
1. Obtain exactly 50 mL of 2.0 M HCl in a clean, dry graduated cylinder

2. Obtain exactly 50 mL of 2.0 M NaOH or NH3 solution in another clean, dry graduated cylinder.

3. Measure the temperature of each of the solution, using the same thermometer. Rinse and dry the thermometer after each measurement. If the temperatures of the various solutions are not within +/- 0.2 ^oC, warm or cool the solution until the temperatures are nearly equal. Record the mean temperature (initial temperature).

4. Add the acid to the calorimeter. In order to account for incomplete draining, record the volume of the solution that remains in the graduated cylinder.

5. Add the base to the calorimeter. Again, read the volume left in the graduated cylinder.

6. Immediately place the top on the calorimeter and begin stirring.

7. Record the temperature to the nearest 0.1 C after 30 seconds and every 30 seconds thereafter for 4 minutes.

8. Plot the temperature against time, using a spreadsheet or graph paper. Extrapolate your results to the time of mixing (time = 0 seconds). Record this extrapolated temperature.

9. Calculate q(system) using 4.184 J/(g.^oC) and 1.0 g/mL for the specific heat and density of the solution, respectively, and 10 J/ C for the specific heat capacity of the calorimeter.

10. Calculate the enthalpy change, delta H, from q(system) and the number of moles of the acid and base. If you used unequal volumes of the acid and base solutions, use the number of moles of the limiting reactant.

11. Repeat steps 1 through 10 for the NH₄Cl - NaOH acid-base pair.

Theoretical considerations:

The heat released by the reaction is absorbed by the calorimeter and its contents. The quantity of heat absorbed by the calorimeter is C_cal x deltaT. This represents the same magnitude of heat absorbed by the reaction, but has opposite sign, i.e.

q_rxn = - C_cal x deltaT

q(system) = - (C_soln x deltaT x mass_soln) - (C_cal x deltaT)

mol NaOH or NH₃ or HCl = vol (L) x 2.0 mol/L

deltaH = q(system) divided by moles of reactant

All contents copyrighted (c) 1998
Peter Jeschofnig, Ph.D., Professor of Science, Colorado Mountain College
All Rights reserved

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This page was created by Peter Jeschofnig and was last updated: 9/7/2001