LAB 11B:
HEAT OF SOLUTION OF CaCl2 USING CALORIMETRY
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This lab does NOT require a lab report.
You will work with a partner for this lab and will be assigned either :ab 10A OR Lab 10B.
I Introduction
This is both a simple lab AND a tutorial on how to solve common calorimetry problems. Our
specific goal is to determine the heat transferred when a sample of calcium chloride is
dissolved in water. This heat is called the enthalpy of solution (∆Hsoln). The process is:
CaCl2(s) → CaCl2(aq)
ΔH = ?
We can calculate the heat transferred in this process using the equation:
q = m∙c∙ΔT + ccup∙ΔT
The first term is our usual calorimetry equation, where m is the mass of the
substance, c is the specific heat of this same substance, and ΔT is the temperature
change of this substance (ΔT = Tfinal – Tinitial).
For m, use the combined mass of the water AND the CaCl2 dissolved in it.
 The specific heat value we will use is that of pure water: cH2O = 4.18 J/g∙oC.
Although this is not quite accurate because the solution is not pure water, it’s
close enough for our work.
 Once you know the change in temperature, you can calculate the heat, q,
transferred to or from the solution. Because this process occurred under
constant pressure (1 atm), the heat transferred is equal to the change in
enthalpy, ΔH, of this process (q = ΔH)
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The solution isn’t the only thing absorbing (or releasing) heat, however: the
calorimeter cup itself also absorbs (or releases) some heat. For maximum accuracy,
our calculations need to take this into account. As a result, we include the second
term in the above equation to represent the amount of heat absorbed by the
styrofoam cup calorimeter.
For this lab, we will assume a heat capacity for our cups of ccup=30 J/oC.
Thus, for every one-degree increase in the temperature of the cup, the cup
absorbed 30 joules of energy. Note that we don’t need a mass of the
calorimeter.
 Assume the calorimeter has the same initial and final temperatures as the
water.
 By multiplying ccup by the change in temperature, you can calculate the
amount of heat transferred to or from your calorimeter.
By adding together the heats gained or lost by the solution and calorimeter, you can
determine the total amount of heat gained or released in the process. (Although we
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could include additional terms to the above equation to calculate the heat gained or
released by the air, desktop, you, etc. we will assume that the universe, as the
solution knows it, ends at the cup. A discouraging assumption from the point of view
of the solution, but a most necessary simplification for us.)
Equipment and Reagents
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Styrofoam® cup
Beaker
Graduated cylinder (?)
Magnetic stirrer and stir bar
Thermometer OR LabPro with
thermometer probe
Ringstand and thermometer
clamp
Analytical balance
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Distilled water
CaCl2, calcium chloride
! Warnings!
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No warnings other than the usual “Do not eat the Chemicals” warning.
Procedure
Prepare the Materials
1) Prepare your lab notebook.
2) First we need a calorimeter containing a known mass of water. Add about 75 g
of room temperature distilled water to a CLEAN AND DRY Styrofoam cup (you
decide how to do this). Record the exact mass of water used.
3) Drop in a magnetic stirbar, place the cup in a beaker for stability, then place
the beaker on a stirrer. Start stirring. Stirring will ensure the compound
completely dissolves and will prevent “hot spots” from forming in the solution.
4) Weigh out about _____ g of CaCl2(s). Record the exact mass used. Do not add it
to the water yet.
5) Your calorimeter is good to go. Be sure you have a lid handy.
Preparing your Equipment (If you’re using a LabPro, follow all steps; if you’re using a
thermometer, jump to Step 9)
6) If you’re using one, make sure the temperature probe is plugged into Channel 1
of the LabPro. Also make sure your calculator is linked to the LabPro.
7) Start the DATAMATE or EASYDATA program on your calculator (under the blue
APPS key if you have a TI-83+).
8) The LabPro should automatically detect the temperature probe and should
display the temperature in the upper right hand corner of the calculator’s
display.
9) Place the temperature probe through the hole in the lid and place the lid on
the cup of water. Be sure the stirrer doesn’t hit the probe. Clamp the
thermometer in place.
Start the Experiment
10) Record the initial temperature displayed in the upper right-hand corner of the
calculator or on the thermometer.
11) Add the CaCl2(s). Monitor the (hopefully) changing temperature in the upper
right-hand corner of your calculator or on your thermometer.
12) Once the temperature has stabilized, record the highest temperature, flush the
solution down the drain, and clean up.
Data and Calculations (to be recorded on your lab Data Sheet)
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Procedure and observations
Masses of water and CaCl2 used
Initial and final temperatures
Enthalpy change of process with supporting calculations
Enthalpy change of dissolution of CaCl2 (in units of kJ/mol) with supporting
calculations
? Questions to Answer
1) Is the dissolving of calcium chloride an endothermic or exothermic process?
How do you know?
2) Draw an energy diagram for CaCl2(s) and CaCl2(aq).
3) Compare your calculated ΔH to the value calculated by your classmates who
used a different amount of CaCl2. Do you expect to have the same values? Why
or why not?
4) Compare your results of ΔH/mol with your classmates who used a different
amount of CaCl2. Do you expect to have the same values? Why or why not?
5) Are there any errors that could affect your results? If so, how would they
affect the calculated enthalpy of hydration? (If there are no errors, I’ll expect
your ∆H is perfect)
6) Suppose that a significant amount of heat were lost to the air during the
experiment. What effect would this have on the calculated value of ∆H? Justify
your answer.