.

[andrewloppol]

.

[superflya]

the black smoke is the carbon, indicating an incomplete reaction. i think

[kenhung123]

I think the explanation would be just that hexene has a double bond and is more reactive that hexane

[fady_22]

Cyclohexene: C6H10:
2C6H10 + 11O2 ------>12CO + 10H20
or 2C6H10 + 5O2 -------> 12C + 10H20

In your explanation, you can comment on the relative number of oxygen molecules required for both reactions to occur. As the reactions of cyclohexene requires more oxygen, there will be more carbon (smoke) being produced than there is in the combustion of cyclohexane. I think that this is valid.... although I could be wrong.

[fady_22]

Cyclohexane
C6H12 + 9O2 --> 6CO2 + 6H2O
C6H12 + 6O2 --> 6CO + 6H2O
C6H12 + 3O2 --> 6C + 6H2O

Cyclohexene
2C6H10 + 17O2 --> 12CO2 + 10H2O
2C6H10 + 11O2 --> 12CO + 10H2O
2C6H10 + 5O2 --> 12C + 10H2O


My reactions going on ?

:
+ this
Complete combustion occurs when the hydrocarbon burns in an excess of oxygen. An excess of air means that there is more than enough oxygen to cause all of the carbon to turn into carbon dioxide. When it burns with an insufficient supply of oxygen the products given may be carbon monoxide and water (CO + H20) instead of carbon dioxide and water (CO2 + H20). The black smoke is due to pure Carbon being released from the combustion reaction.

The cyclohexene reacts faster since its double bond, when broken, releases more heat energy causing the reaction to occur quick. The cyclohexene also, as seen in the equations, needs more oxygen to occur at once, thus due to the faster rate and that cyclohexene has an insufficient supply of oxygen to create carbon dioxide as fast as hexane, the cyclohexene has more black smoke due to incomplete combustion when compared to cyclohexane.

Sound good ?

I don't really know about the double bond breaking and releasing more energy... I think that it would require more energy, instead of releasing more energy.

[fady_22]

Bond Energy
"For any particular chemical bond, say the covalent bond between hydrogen and oxygen, the amount of energy it takes to break that bond is exactly the same as the amount of energy released when the bond is formed. This value is called the bond energy."

Maybe it requires more and releases more too?

"When the bond is formed". We are not forming a bond, but destroying one. The energy required to break that bond is different to the energy released when the bond is broken.

If you have a look at the heat of combustion of an alkane and its corresponding alkene, you'll see that they are very similar values. So I don't really think that the combustion of an alkene would release more energy for the reaction to go forward.

[fady_22]

Just:
"
Complete combustion occurs when the hydrocarbon burns in an excess of oxygen. An excess of air means that there is more than enough oxygen to cause all of the carbon to turn into carbon dioxide. When it burns with an insufficient supply of oxygen the products given may be carbon monoxide and water (CO + H20) instead of carbon dioxide and water (CO2 + H20). The black smoke is due to pure Carbon being released from the combustion reaction.
The cyclohexene has more black smoke due to incomplete combustion when compared to cyclohexane.
"
then ?

Yep, I think that should be enough.