Ask A Scientist© Chemistry Archive

 > >    name         Elizabeth Charlton
 > >    status       student
 > >    age          17

 > >    Question -   In theory, a candle is made of a solid hydrocarbon called
 > > paraffin.  When the solid burns, the hydrocarbon reacts with oxygen to
 > > form carbon dioxide and water.  The reaction is exothermic, meaning it
 > > releases heat to the atmospherre along with the other products.  Does
 > > oxygen really affect the heat of the actual flame?  If so, why does it
 > > vary throughout different parts of the flame?

Elizabeth,

     The rate of heat release depends on the rate of the chemical reaction
between the (vaporized) hydrocarbon and the oxygen in the air.  The rate of the
chemical reaction depends on the concentration of the two species (oxygen and
hydrocarbon), among other things.  The concentration of oxygen is greatest on
the outer edge of the flame since the source of the oxygen is the atmosphere.
The concentration of the hydrocarbon is greatest near the wick but at that
point there is almost no oxygen (because it is used up reacting with the
hydrocarbon near the outer edges of the flame).  As a result, the hydrocarbon
doesn't burn well in the center of the flame and is hottest near the edges.

Greg Bradburn

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Combustion chemistry is quite complicated.  Different reactions are
occurring in the different regions of a flame.  A flame will burn hotter and
faster in pure oxygen than in air, because the flame's heat is not carried
away by unreactive nitrogen molecules.

If you look at a candle flame, you will see that near the wick, the flame is
nearly invisible, and that a yellow luminous zone surrounds this.  Near the
wick, the paraffin vapors are breaking down, releasing hydrogen and creating
long, unsaturated carbon chains.  This pyrolysis process is actually
endothermic (heat-consuming), and can only occur because of the heat
supplied by the reactions occurring in the luminous region.  Here, the
carbon chains have gotten so large that they are actually tiny particles of
soot.  These soot particles burn just like charcoal, and the yellow light
released by the candle flame is from these hot, burning soot particles, just
as the light from a light bulb is from the hot filament.  If there is enough
oxygen (or not too much paraffin vapor being created at the wick), the soot
particles all burn up in the flame, and the candle releases only heat,
light, water, and carbon dioxide.  If the flame is too "rich," unburned soot
particles can escape from the flame.  (You can see that soot is present in
the yellow region of the flame by placing a metal knife blade or wire into
the flame:  the part in the yellow zone, and only in the yellow zone, will
become covered with soot.)

The hottest part of the candle flame is the top, because that's where all
the hot product gases go.

The faint blue glow at the bottom of the flame is from a short-lived
molecule formed in the flame:  diatomic carbon, C2.

Bradburn
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