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Name: Elizabeth Charlton
Status: student
Age: 17
Location: N/A
Country: N/A
Date: Around 1999


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?


Replies:
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.

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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