Is Hess’ Law True?
(LAB GUIDELINES:1,2,3,4,5&6[handout],7,9,10,11,12)
Purpose:
To confirm Hess’ Law experimentally.
Pre-lab Info:
A refresher course on concentration: How many grams of solid sodium hydroxide are
necessary to prepare 100.0 mL of a 0.50M solution.
Materials/equipment:
solid NaOH, water, styrofoam cup, 1.00M HCl, 1.00M NaOH, 0.50M HCl,
weighing paper, 50 mL and 100 mL graduated cylinders, temperature probe
CAUTION: • TOUCH NO CHEMICALS----the NaOH is VERY
CAUSTIC. WEAR GOGGLES at ALL TIMES
• USE CLEAN GLASSWARE!!!
Procedure: (Note: Empty and Rinse the cup before each Part.)
Part ONE
1. Place 100.0 mL of water in a calorimeter.
2. Record the temperature of the 100.0 mL of water.
3. Put enough solid NaOH in the 100.0 mL of water to make a 0.50 M solution.
4. Stir and record the final temperature after all the NaOH has dissolved.
NOTE: DO NOT TOUCH THE NaOH or
LET IT TOUCH THE METAL OF THE SCALE--USE WEIGHING PAPER
Part TWO
1. Place 50.0 mL of 1.00M HCl in a calorimeter.
2. Record the temperature of the 50.0 ml of 1.00M HCl.
3. Mix 50.0 ml of 1.00M NaOH with the 50.0 ml of 1.00M HCl.
4. Stir and record the final temperature of the solution.
Part THREE
1. Place 100.0 mL of 0.50M HCl in a calorimeter.
2. Record the temperature of the 100.0 ml of 0.50 M HCl.
3. Add 2.00 grams of solid NaOH to the calorimeter.
4. Stir and record the final temperature after all the NaOH has dissolved and reacted.
Data:
Record all intial and final temperatures!
Calculations & Questions: (Show Your Work For Calculations!)
Key assumptions: specific heat of all solutions = specific heat of water
density of all solutions = density of water
Part ONE:
1) Calculate the heat change in part one as performed (note: the mass of your solution is
102.00 grams not 100.00 grams – do you know why?).
2) Write the equation for the “reaction” which took place in part one.
3) Now calculate the heat change for the reaction as written in the balanced equation. (i.e.
what is the heat change for the dissolving of an entire mole of NaOH?)
Part TWO
4) Calculate the heat change in part two as performed (note: we are assuming that the
final mass of solution is 100.00 grams).
5) Write the net ionic equation for the reaction which took place in part two.
6) Now calculate the heat change for the reaction as written in the balanced equation.
7) Show how the two reactions above (#2 and #5) add together to give the following
reaction.
NaOH (s) + H+(aq) -----> Na+(aq) + H2O (l)
8) Now, using your answers above (#3 and #6), determine the heat change for this
reaction (as written).
Part THREE
9) Calculate the heat change in part three as performed (what is the mass of solution?).
10) Write the net ionic equation for the reaction which took place in part three.
11) Now calculate the heat change for the reaction as written in the balanced equation.
12) What should be true about your answers to #8 and #11?
Error Analysis:
1) Use data from the appendix of your book to determine the actual heat chage for the
reaction written in #7 (which should be the same as the reaction written in #10). This
will be considered your accepted value for the heat change for this reaction.
2) Calculate 2 percent error values: one for your answer to #8, and one for your answer to
#11, and comment on possible sources of error.
Conclusion:
Write a thorough conclusion which discusses how you accomplished the purpose and the
usefulness of Hess’ Law. Don’t forget to discuss the math involved in the process as well
as necessary equations and assumptions.
Remember, here is a great guide for writing your conclusion:
1) Restate the purpose
2) Summarize the procedure and data collection
3) Discuss any important principles / equations used
4) Explain the graphs and/or math
5) Report your results
Also, please include any other interesting information you would like to add.