Teaching Combustion in a First-Year High School Chemistry Class Tim McLinden - Catholic Central High School - Springfield, Ohio

Students’ first exposure to combustion is in learning to properly adjust a Bunsen burner. On the left, a student shows a flame that is

much too rich. Without enough oxygen, the flame is not completely burning the fuel, resulting in the production of soot and a very un-

predictable flame. On the right, the student has adjusted the mixture of fuel and oxygen to produce a proper flame, one that is hotter,

cleaner, and more controlled. By holding a piece of copper wire at various points in the flame and observing the heating of the wire,

students discover that the hottest part of the flame is at the top of the inner blue cone of flame, where the temperature is high enough to

actually melt the wire.

Poster 2P01

What is a combustion reaction?

A combustion reaction usually (though not always*) involves

a substance combining with oxygen, releasing a large

amount of energy in the form of heat and light. *Titanium, once ignited in the air, will continue to burn in pure nitrogen. Some sources also mention potassium, magnesium, sodium, and lithium burning in pure nitrogen.

Challenges in teaching combustion to beginning

chemistry students:

If the chemistry teacher is not an expert on combustion, having access to a

knowledgeable “mentor” is very helpful.

Concepts must be learned at a time when the students have enough back-

ground knowledge to understand what’s going on, so it becomes a year-long

process, and combustion is woven into many topics throughout the year.

Many concepts can be understood using simple lab equipment, but to pique

their interest and enhance their understanding , students should be exposed

to some modern technology, which can be rather expensive.

The student on the left is observing a “typical” combustion reaction in which a hydrocarbon fuel (ethanol) combines with

oxygen to produce carbon dioxide and water vapor. The flame originates above the surface of the liquid, and the students

realize that it is actually the evaporated ethanol that is burning. This is an example of a diffusion flame, which is not sharply

defined and which moves around due to convection currents in the heated air. The student on the right is demonstrating the

reaction between magnesium metal and oxygen to form magnesium oxide. This reaction, which can be classified as both a

synthesis reaction and a combustion reaction, shows that the rate at which the reaction proceeds can determine whether it is

categorized as combustion (producing heat and light) or just oxidation, as would be the case for iron rusting.

Combustion Science Topics To Be Addressed

1. The basic idea of combustion reactions – that they generally involve the reaction of a sub-

stance with oxygen in a fast reaction that produces heat and light.

2. The idea that you need a fuel, an oxidant (air) and an ignition source (spark, heat, tempera-

ture, whatever) to make something burn. If you’re missing one of these, you won’t have com-

bustion.

3. The combustion of natural gas in a Bunsen burner – how fuel and air are adjusted to achieve a

clean and controlled flame.

4. Pre-mixed vs. diffusion flame

5. Laminar vs. turbulent flame

6. The speed with which flame from various fuels moves

7. Combustion stoichiometry – both theoretical and real-world applications, including:

Fuel/air ratio based on stoichiometry

While the combustion is determined by the chemical interaction of the constituents, in the real

world one sets a flow rate of fuel and a flow rate of air

Introduce the idea of fuel/air ratio actual, like kg/hr, lbm/sec, etc.

Equivalence ratio is used to compare the F/A-act to F/A-st, ɸ is the symbol and you divide ac-

tual by stoichiometric. Rich mixture is ɸ > 1, lean mixture is ɸ < 1

8. Average chemical formulas, as used for gasoline, jet fuel, etc.

9. Current combustion science research

Using Modern Technology to Monitor a Combustion Reaction

Students use home-made combustion chambers to burn a known amount of ethanol in a watch glass. The ethanol is ignited using an in-

duction coil, and the pre– and post-combustion oxygen and carbon dioxide levels and temperature are monitored using Vernier® equip-

ment. The picture on the right shows the pre-combustion levels. Note that the readings show a reduced oxygen level and an increased

carbon dioxide level in the room, compared with “normal” values, caused by earlier trials and students continuing to breathe. Holding

their breath for the full 50-minute class period proved to be impossible.

How were grant funds used?

Funds from a Combustion Institute grant were used to purchase the following:

2 Omega® digital thermometers and a variety of thermocouple probes and connectors

2 Vernier® CO2 sensors*

6 Vernier® O2 sensors*

2 Vernier® LabQuest interface devices*

60 feet of copper pipe, end caps, and solder for making combustion troughs

Miscellaneous combustion fuels and containers for use in the troughs

Materials for constructing combustion chambers

* The school already owned some of the needed equipment.

Participation by Dr. Vince Belovich

Dr. Vince Belovich is a combustion research scientist at Wright Patterson Air Force Base. He has served as a

much-appreciated mentor in helping me understand the basics of combustion in the real world. He has been

involved in helping design some of the labs, recommending needed equipment, and most importantly, serving as

a guest presenter in the chemistry classes. Dr. Belovich presented a series of demonstrations and discussed such

concepts as average chemical formulas, fuel/air ratios, equivalence ratio, diffusion vs. pre-mixed flame, jet

engine and combustion chamber design, and pressure waves during combustion. He shared a section of an

actual jet engine combustion chamber and a turbine section with the class, as well as amazing them with a dem-

onstration of the combustion of alcohol vapor in a pipe that was closed at one end.

Chemistry knowledge involved in doing the Combustion Chamber Lab

Students needed to use stoichiometry with the balanced equation C2H5OH + 3O2 → 2CO2 + 3H2O to find

the theoretical mass of oxygen consumed and the mass of carbon dioxide produced in the combustion of a

known amount of ethanol. Because of the time lag between when the ethanol was placed in the watch glass

and when it was ignited, they also had to determine the rate of evaporation of the ethanol and account for

that in their calculations. The ideal gas law was used to calculate the number of moles of gas contained in

the combustion chamber. The students also learned how to use percent composition and ppm readings to

determine the amounts of oxygen and carbon dioxide present in the gas within the chamber. As with most

labs, error analysis was an important component of the lab.

Dr. Belovich explains a real-life combustion reaction, focusing

on the pollutants and greenhouse gases that are formed.

An introduction to the organic chemistry of hydrocarbon fuels.

Students learn about how a flame maintains itself by traveling

upstream against the flow of fuel and air. “It’s like walking up

a downward-moving escalator and staying in the same place.” Students learn about the basic operation of a jet engine.

Dr. Belovich followed up on this theoretical discussion of a com-

bustion chamber by showing an actual segment from a jet engine. This video showed a 120 Hz pressure wave oscillation.

Due to its high volatility, alcohol readily ignites when a burning

match is dropped in it; jet fuel will actually extinguish the

match! Jet fuel is designed to have low volatility.

This demonstration shows that a flame is hollow, and filled with unburned fuel.

It was first performed by Michael Faraday, who used a candle flame.

This video showed a comparison of synthetic jet fuel, con-

taining of only straight-line hydrocarbons, and fuel contain-

ing aromatics. The synthetic fuel produced much less soot. Dr. Belovich pours a small amount of alcohol into a closed tube and

allows it to evaporate before igniting the vapors through a small hole

at the closed end of the tube A loud “whoosh!”, a flame shooting out the open end of the tube, and a

group of startled students means the demonstration was a success!