Combustion of Alkenes

  1. Like any other hydrocarbons, alkenes burn in air or oxygen.
  2. Like alkanes,
    1. alkene burns completely in the sufficient oxygen to produce carbon dioxide and water.
    2. alkene burns incompletely in limited supply of oxygen to produce carbon monoxide, carbon (in the form of soot) and water.

Examples
Complete combustion of ethene

C2H4 + 3O2 → 2CO2 + 2H2O

Incomplete combustion ethene

C2H4 + 2O2 → 2CO + 2H2O
C2H4 + O2 → 2C + 2H2O

The combustion of alkenes becomes more difficult and produces more soot when the size of the alkenes molecules increases.

Percentage of Carbon

Example
Find and compare the percentage of carbon of ethene, propene and butene. [Relative atomic mass: C = 12, H = 1]
Answer:
Percentage of carbon of ethene (C2H4)
Percentage of carbon = 2(12) 2(12)+4(1) ×100% =85.7% Percentage of carbon of propene (C3H6)
Percentage of carbon = 3(12) 3(12)+6(1) ×100% =85.7% Percentage of carbon of butene (C4H8)
Percentage of carbon = 4(12) 4(12)+8(1) ×100% =85.7%
Note::
All alkenes have the same percentage of carbon.


Example
Find and compare the percentage of carbon of hexane and hexene.
[Relative atomic mass: C = 12, H = 1]
Answer:
Percentage of carbon of hexane (C6H14)
Percentage of carbon = 6(12) 6(12)+14(1) ×100% =83.7%
 Percentage of carbon of hexene (C6H12) = 85.7% (Refer to example above)

Note:

  1. Percentage of carbon of alkene is higher compare to the alkane that has same number of carbon per molecule.
  2. Therefore, combustion of alkene will produce more sooty flame compared to their corresponding alkane.
Alkenes has higher percentage of carbon compared to their corresponding alkane.

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