Grade 12 Thermochemistry Lab

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 ΔH_x of the combustion of candle wax in air:

C29H60 (s)   +   44 O2 (g)

30 H2O (g)  + 29 CO2 (g)

ΔHx  =

Use bond energies to estimate ΔH, and use that estimate as your theoretical heat of combustion.

If your experimental value of ΔH_x differs  from the theoretical ΔH, calculate the % error, and account for the missing energy.

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.

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.

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.

Let's Take A Closer Look at the Box Diagram

I am going to approach this idea several times, from different directions. Please bear with me as I go through the introduction.

My experience is that it takes students, and teachers, several attempts to "get" this idea, so be prepared for a spiral curriculum experience. It might appear to be a little repetitious, but stick with it. Your students will thank you.

The first thing to note is that the student "progresses" through five stages. It might be more appropriate to say that the student considers five kinds of actions.

One of the purposes of the "box" is to arrange aspects of these actions so that they are adjacent. In history, and in rhetoric, they would not appear to be so obviously related.

 Now pay attention to the coloured bars in the diagram. Boxes 2 and 5 are tied together by a magenta bar labelled "theory - thinking." Boxes 2 and 5 are theoretical in their intention: they deal with scientists' representations of nature. As such, the first row is theoretical.
Box 2 is concerned with concepts and theories. This is the place where the student works to make representations of her thoughts about her focus question. Box 5 is the place where the student makes representations of theoretical implications of her findings.

The yellow bar that connects boxes 3 and 4 indicate the methodological emphasis of those phases in the investigation. These boxes are about doing things. They are concerned with method, its design, its instrumentality, its effectiveness, its limits.

Now note the cyan-coloured vertical connecting boxes 2 and 3. These boxes are concerned with model-building. In box 2, the student-scientist considers possible theoretical models that might be brought to bear upon the focus question. In box 3, the student-scientist considers how to hive off a little bit of nature, so as to construct a methodological model that could test the student-scientist's theoretical model of the situation.

The magenta analysis bar connects boxes 4 and 5. In box 4, the student-scientist makes representations of the records. Note that the term records indicates a much broader collection of data than "observations." This would include video, audio, and transducer records of nearly infinite variety. Box 4 also includes all attempts to methodically transform the records. This could be as simple as constructing an ordered table of records, or as complex as computationally modifying a photograph of a supernova. In any case, the student-scientist understands that she is methodically making records of data relevant to the focus question, and transforming  that data into relevant structures. This is methodical analysis.

In box 5, the student-scientist attempts to analyze the theoretical implications of all that has preceded.

OK. Is this just more government-funded BS from OISE? Or does this actually apply to real situations?

First disclaimer: I have not received a dime of government money.
Second disclaimer: OISE will not allow me into their elevators.
Third disclaimer. This isn't BS.

In seven days, I will be back with two lines of evidence.

First line: many articles published in scientific journals apparently reflect this structure.
Second line: many students have provided me with convincing examples of this kind of work.

Our Epistemological Assumption... Don't Be Afraid!

e·pis·te·mol·o·gy

  [ih-pis-tuh-mol-uh-jee]

noun

a branch of philosophy that investigates the origin, nature, methods, and limits of human knowledge.

The form that science takes in the classroom reflects the epistemological assumptions of the teacher. For example, the "purpose - equipment - method - observations - conclusions" structure that has prevailed for the last century makes some huge assumptions. Some examples...

• Knowledge is "out there" just waiting to be "discovered."
• We can "discover" that knowledge simply by "observing nature" in a disciplined way.
• The learner has all of the other mental equipment required to "discover" knowledge.

Contrary to that model of science, may I propose another model? My colleagues and I have found this model much more plausible, and much more fruitful, both for us as teachers, and for the teenagers who are trying to learn.

"Science is not the study of nature.
Science is the study of human representations of nature." 

Let's consider Max Planck, for example. Planck had worked out a representation of light as a quantum with energy related to frequency. As a result, scientists all over the world were studying this representation: is this a good representation? can it be true in all cases? does it have a more complex structure? does it need adjusting? Note that the scientists are twiddling the representation, and then testing the representation against experimental evidence.

Consider Niels Bohr: the act of actually observing the hydrogen spectrum was trivial. But now, Bohr was able to represent the hydrogen spectrum as a quantum structure. Perhaps he could represent the atom as a quantum structure, since they were related. Bohr re-represented the planetary model of the atom with quantum concepts, and modified the representation of the hydrogen atom. Note that he did not "discover" the Bohr atom. He created it.

We can involve students in the very heart of science by having them make representations of nature, test their representations of nature, and refine their representations. As they become familiar with this method, they become less afraid, more confident. As learners, all they have to do is refine their representations. It's not that different from refining their drawings of a horse or a car.

How Can We Teach High School Students to Do Real Science?

First Light.

Of three centuries' experience of science education, one of the science teacher's enduring difficulties is the problem of teaching students "what science is."
Francis Bacon's description of a "method of science" became a prescription in science texts. Every teacher has since found that it violates the very essence of science at some point.
When education leaders emphasize one key aspect of science, such the importance of "controls," we find that all of the experiments look like industrial quality control measurements.
Yet if we simply leave the students to "discover" science, we find that most students cannot articulate anything more than the most rudimentary patterns.
So... year after year, we see our weakest students trying to "discover" how to please the teacher, the middle students trying to "game the system", and our strongest students frustrated by the artificiality of the educational enterprise.
The Ross Box Diagram is a simple heuristic with great depth. In the pages to follow, I will describe each part of the Ross Box Diagram, and work with any questions that interested respondents might raise.

So.. Post away, and form whatever circles and associations you can to keep the questions alive.