The Chemistry of Fire and Other Oxidation Reactions

Barbara Pawela                 Retired
                          

Objectives:

The students will :
1.  Gain an understanding of the process of combustion.
2.  Realize that fuel and oxygen are necessary in order to have combustion.
3.  Discover that air contains about 20% oxygen (actually 21%).
4.  Gain knowledge of some types of oxidation reactions.
5.  Discover that oxidation is an exothermic reaction.

Materials:

    Activity I                Activity II                Activity III
   wooden matches            2 small glasses            steel wool pad
   small candle              2 large glasses              without soap
   lighter                   2 glass chimneys           1/4 cup vinegar
   paper                     6 birthday candles          cooking or outdoor
   4 saucers                 9 corks                      thermometer
                             test tube                  jar with lid (The
                             dry yeast                    thermometer must fit
                             wooden splint                inside the jar.)

Strategies:

Activity I:  Teacher strikes the match and lights the candle.  Ask the students 
what they observed.  After their comments, discuss the history of man's use of 
fire.  Strike another match.  Hold the yellow part of the flame under the bottom 
of one of the saucers.  Blow out the match and examine the underside of the 
saucer.  Follow the same procedure, using candle, lighter, and burning paper. 
Discuss what was observed and conclude that whenever the yellow part of a flame 
came in contact with a cool dish, a black substance was deposited.  Moisture 
also was present.  Introduce and define vocabulary:  fire, flame, combustion, 
fuel, and ignition.  Explain to the students that anything that will burn can be 
called a fuel.  Most fuels contain carbon, hydrogen, or both.  When carbon burns 
incompletely, which is usually the case, it glows with a yellow color.  A flame 
is made of tiny particles of very hot carbon.  When the particles cool quickly, 
as they did on striking the cool saucer, they were deposited as black carbon.  
When they cool more slowly, as above an open flame, they join with the atoms of 
oxygen from the air and become carbon dioxide (CO2), a colorless gas.  The 
moisture formed on the saucers, because every flame gives of water vapor.  This 
is because the fuel contains hydrogen, which reacts with oxygen in the air to 
form water vapor. 

Activity II:  On a fireproof tray, arrange the candles so that they are about 
6-8 inches apart.  Fasten them to the tray by placing the candles on some clay.  
Arrange the glassware as follows: a small glass behind each of the first two 
candles, a large glass behind each of the next two candles, and a glass chimney 
behind each of the last two candles.  Put three corks near the second, fourth, 
and sixth candles.  Light candle No. 1.  Cover it with the inverted small glass. 
Observe what happens.  Light candle No. 2.  Arrange the corks in a triangle 
around its base and place the glass on the corks.  Observe what happens.  Repeat  
the procedure with the remaining candles and the glassware behind them.  
Students observe the behavior of the flames.  Several students can be 
timekeepers and data recorders.  Teacher and students compare observations.  
Discuss and conclude that both fuel and air were necessary to maintain the 
flame.  The teacher pours 6-8ml of hydrogen peroxide into a test tube.  Add 1/2 
teaspoon dry yeast into the test tube.  The yeast will react with the hydrogen 
peroxide releasing oxygen.  Immediately light a wooden splint.  Blow out the 
flame gently, so that the splint is still glowing.  Stick the splint into the 
test tube.  It will reignite, and if enough oxygen is present, a popping sound 
may occur.  Discuss and conclude that although oxygen itself does not burn, it 
must be present to have fire, which is an oxidation reaction. 

Activity III:  Place the thermometer inside a jar and close the lid.  Have 
student data recorders note the temperature after five minutes.  Soak one-half 
of the steel wool in vinegar for two minutes.  Squeeze excess liquid from steel 
wool, and wrap it around the bulb of the thermometer.  Place the thermometer 
with the steel wool inside the jar and close the lid.  Record the temperature 
after five minutes.  The temperature rises.  The vinegar removed the protective 
coating from the steel wool, allowing the iron in the steel wool to rust.  
Rusting is a slow combination of iron with oxygen, which is an oxidation 
reaction.  Oxidation is an exothermic reaction, and heat is always released. 
   
References:

     Mullin, Virginia L. Chemistry Experiments for Children, Copyright, 1968, 
   Dover Publications, Inc.