Less than Zero: An Exciting Thermochemistry Lab
Steve Sogo, Laguna Beach High School
ssogo@lbusd.org
Overview: This lab is an exciting application of thermochemistry. Run as a contest, students
attempt to create a temperature of less than zero degrees Celsius using the endothermic reaction
of sodium bicarbonate and hydrochloric acid. In order to successfully complete this challenge,
students must make wise choices regarding:
 proper ratios of reactants to use (stoichiometric calculations)
 an appropriate vessel in which to run the reaction (glass? ceramic? metal? Styrofoam?)
 an appropriate concentration of acid (anything between 1 M and 6 M is "legal")
 an appropriate scale for the reaction
The lab starts with a scripted reaction that uses improper mole
ratios, a glass beaker, and 2-molar HCl. In this scripted reaction,
students will achieve a temperature drop of about 5 to 8C. After
calculating a theoretical temperature drop using these reactants,
students then ponder what they will change in order to achieve a
temperature drop of more than 20C.
Instructional Video:
http://www.youtube.com/watch?v=BSHTpesmhUc
Lab Set-Up:
 Prepare a large volume of 2 M HCl (at least 50 mL per lab group) See
note below regarding use of inexpensive pool-grade HCl!
 Prepare volumes of 1 M, 3 M & 6 M HCl as well (25-50 mL per lab
group)
 Set out baking soda at weighing stations. Store adequate reserves to that
each lab group can use 25-grams of baking soda.
 Provide a sampling of Styrofoam cups, metal cups, and ceramic cups for
communal use
 Computer-linked thermometers recommended, so that the results of
experiments are recorded for teacher review. The curvature of the
temperature vs. time graphs are also valuable in stimulating student
analysis/discussion of the results (e.g. temperature rising at the end of the
graph indicates heat is being transferred into the solution)
MONEY-SAVING TIP: FAIRLY CONCENTRATED SOLUTIONS OF
HCl CAN BE PURCHASED AT POOL SUPPLY STORES OR HOME
IMPROVEMENT STORES. I buy a 30% HCl solution at my local pool
store that routinely titrates to a 10 M concentration. This solution is
somewhat yellow, but the yellow color is almost undetectable when
diluted to 1 M, 2 M, or 3 M concentrations. 6 M will still be somewhat
yellow. The cost for two gallons of this concentration of acid at the
pool store is about $25, which is an immense savings over reagent
grade HCl from a chemical supplier.
Instructor Set up time: 30-90 minutes, depending on time needed to prep the HCl solutions.
Minimum Experimental time: 45 minutes (students need time to run the scripted reaction,
ponder the results, discuss alternative methods and perform the modified reactions (two
modified reactions recommended))
Recommended in class Analysis/discussion time: 30-45 minutes
Conceptual difficulty level: moderate
Procedural difficulty level: moderate
Suitable for: College-Prep Chem, Honors Chem, AP Chem, Introductory College Chem
Assignment #146: Less than Zero (LAB)
Overview: Your objective in this lab is to cool an aqueous solution to below zero Celsius. All groups
will start by running a scripted reaction as a “trial run”. Each group will then make subsequent runs
utilizing novel ideas for improvement. It is in these "improved" runs that you may be able to lower the
temperature to less than zero. There will be no write-up for this lab, but each individual will be expected
to turn in an official note page (provided on the third page of this lab) at the end of the hour.
Part I (scripted reaction):
1. Pre-weigh a 250 mL beaker. Mass of beaker = _________ grams
2. Obtain a PASCO computer-interfaced thermometer and link it to the computer. Follow your
instructor’s guidelines for using SparkVue software.
3. Click the RUN button and verify that your thermometer is working properly. Then click STOP to
terminate this experiment.
4. Weigh out between 2.80 and 3.20 grams of solid baking soda (NaHCO3) on weighing paper.
5. Use a graduated cylinder to measure out 25 mL of 2 M HCl. Pour the acid into your pre-weighed
250-mL beaker. Click RUN to measure the initial temperature of the acid.
6. While the thermometer is still running (recording data on the computer screen), carefully add the preweighed baking soda to the beaker (much fizzing will occur). Stir with the thermometer and record data
until the reaction is complete.
7. Display your graph with appropriate scaling of axes and read the STATISTICS to determine the
minimum temperature obtained in this run and the overall temperature drop induced by the reaction.
Calculation time:
The reaction that occurs in the beaker is:
NaHCO3 (s) + HCl (aq)  NaCl (aq) + H2O (l) + CO2 (g) H = +28 kJ/mol
a) Use the H value for the reaction given above to calculate the number of joules that would be
absorbed by the reaction using the mass of baking soda (NaHCO3) you weighed out in step #5.
b) Convert the joules in (a) to calories.
c) Divide your calories from (b) by the number of GRAMS OF SOLUTION you have in your beaker. This
will give you a theoretical value for how much your temperature should have dropped during the
reaction. Hint: to find grams of solution, weigh the beaker and subtract the weight of the empty beaker.
d) Compare your theoretical temperature drop from (c) to the experimental temperature drop you
recorded with the thermometer. Ponder why these numbers are not the same.
e) Use the MOLARITY concept to calculate the moles of HCl present in 25 mL of 2-molar acid.
Compare to the moles of baking soda you used in the reaction. Note: the balanced equation shows you
that the proper mole ratio is one-to-one. Is this what you actually used in this reaction?
Assignment #146: Less than Zero (continued)
Part II
Your objective now is to modify the scripted reaction so that you will achieve a colder
temperature than you got in Part I. You will be allowed to try 2 EXPERIMENTS to get the
temperature to drop as low as possible. Your grade on this lab will be based on how cold a
temperature you achieve, as shown in the following table:
Lowest temperature
Less than 20C
Less than 14C
Less than 12 C
Less than 10C
Less than 6C
Less than 1C
Best in the Class
Grade (out of 10 points)
7
7.5
8
8.5
9
9.5
10
Note: the values shown in the table are base values and may be adjusted up or down
depending on the quality of your note page (provided on the next page).
The questions shown below are designed to have you consider possible avenues for
modification. These are NOT the only things that you could change. I believe the best strategy
will be to carefully discuss the parameters you would like to change (and why) prior to
performing actual experiments. You must learn from your mistakes if you are to be successful in
this lab.
Does the container affect the results of the experiment?
Ceramic, metal and Styrofoam vessels are available.
Is the ratio of baking soda to acid that you used in the first reaction the "right" ratio?
Hint: think about the MOLES you calculated in question (e). . .
Would there be an advantage to adding some extra water to the reaction?
Is there an advantage to scaling up or scaling down the reaction?
You may want to use more moles or fewer moles in your reaction mixture
Is the volume of acid important? If so, do you want more or less volume in your
reaction?
Would there be an advantage to using a different molarity of acid?
1 M, 2 M, 3 M, and 6 M HCl are available
#146 (continued): Less Than Zero Lab note page: (turned in for points!)
Part Two:
Unscripted Experiment #1:
Type of reaction vessel used: ________________________________________
Mass of
baking soda
Moles of
baking soda
Volume and
molarity of acid
Moles of
acid
Lowest
Temperature
Theoretical T
Describe the logic behind your experimental procedures. That is, what changes did you
make to the original procedure? Why??
Unscripted Experiment #2:
Type of reaction vessel used: ________________________________________
Mass of
baking soda
Moles of
baking soda
Volume and
molarity of acid
Moles of
acid
Lowest
Temperature
Describe the reasoning behind the modifications you made in this experiment.
Theoretical T