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Recently I was introduced to organic chemistry for the first time.

In the section of complete and incomplete combustion of carbon compounds, they provide example of two different objects, namely a cooking gas burner and a candle, and said that "complete combution can be seen in burner but incomplete combustion can be seen in candle"

The explanation was

Apart from unsaturated carbon compound present in candle while saturated carbon compound present in burner's gas help in complete combution as less energy is required to break single bond in gas than double and triple bond in candle wax"

But also that

the holes in burner help in mixing of oxygen more to perform complete combution but candle's thread dont allow much easy mixing of oxygen thus insufficient oxygen supply cause incomplete combution"

Can anyone explain the second reason in more detail?

I can't understand how holes help in mixing oxygen thus providing sufficient oxygen supply for the gas to combust? Why can't a candle thread provide sufficient oxygen?

One more thing , where does the reaction between oxygen and gas take place in the burner? Inside it or outside the burner once the gas comes out of the holes?

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1 Answer 1

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Is has very little to do with saturated/unsaturated nature of hydrocarbons.

The gas burner has gas and air premixed, similar as Bunsen and other lab gas burners. If you did not premix gas with air, there would be incomplete combustion with yellow shining flame, like when you close the air input of a Bunsen burner. The holes help to prevent premature ignition of the gas/air mixture inside the burner.

Candles evaporate the paraffinic wax hydrocarbons. These vapors are not premixed with air and burning happens in transition region where is the hydrocarbon/air ratio range supporting burning. There is no burning in outer regions that have too little of vapors not in the inner regions that have too little of air. If you evaporated wax by electric heater, mixed it with air in ideal ratio and high enough mixture temperature, there would be complete combustion.

Additionally, the size of molecules and carbon chain of long n-alkanes play its role in the burning rate.

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