This lab is a classic: determine, experimentally, the heat
of combustion of candle wax. It has been used in secondary schools for decades.
All chemistry teachers are aware that this lab provides a very crude
measurement of the heat of reaction. Simply "doing the experiment" is
hardly an experience in experimentation. To improve the learning experience,
I've modified the lab in two significant ways.
First, I left the details of the method up to the students.
I have provided a diagram as a starting point. Given the general harmlessness of the materials, the
students are free to modify the design. They must be able to provide reasons
for their design, however it was modified.
Second, I shifted the analytical focus of this lab to error
analysis. Students must scrutinize their experimental design to identify, and
quantify, possible sources of experimental error. Does their analysis account
for the discrepancy between the prediction (based upon a bond energy
calculation) and their measurement?
Here are the instructions that I provide the students.
Lab 1.3: Heat of Combustion of
Candle Wax
What=s The Question? Measure the experimental ΔHx of
the combustion of candle wax in air:
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C29H60 (s) +
44 O2 (g)
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30 H2O (g) + 29 CO2 (g)
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ΔHx =
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Use
bond energies to estimate ΔH, and use that estimate as your theoretical heat of
combustion.
If
your experimental value of ΔHx differs from the theoretical ΔH, calculate the %
error, and account for the missing energy.
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What
Are We Doing?
1. Make a simple calorimeter out of a pop
can.
2. Measure any masses and temperatures you
think you need, both before burning, and again after burning.
3. Record the temperature each minute until
the T has risen to about 40EC
4. Carry your data through any necessary
calculations to find the heat released by your candle, and the molar heat of
reaction of candle wax.
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What
Are We Thinking About?
Human
error (incompetence?) is unacceptable as a source of experimental error. It=s
an excuse, to be sure, but a miserable excuse. Instead, do this...
1. Carry your experimental data through the
calculations, just as you always do. .
2. Estimate the size of any
error. Don=t
just say Awe
mighta spilt some wax.@ Estimate the maximum mass of the spilled
wax.
3. ACorrect@ your experimental data by applying
your best estimate of the size of the error.
4. Carry your Acorrected@ data through the same calculations
as (1). Compare the answers. Did the error cause an increase or a
decrease in the measurement? Find the
% eror.
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Questions
For Later...
Calculate the effect of each of
the following uncertainties. Do they
make your measurement too big, too small?
Express these uncertainties as a % of the Acorrected@ value.
1) 0.1
g of wax dripped onto the desk top during combustion.
2) your
thermometer always reads 2.0EC too low.
3) you
spilled about 1 g of water from the calorimeter after weighing it.
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