Combustion of Alkanes

  1. All alkanes undergo combustion to produce water and carbon dioxide (or carbon monoxide/carbon in incomplete combustion).
  2. There are 2 types of combustion:
    1. complete combustion
    2. incomplete combustion

Complete Combustion

Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.

Example:
Combustion of methane
CH4 + 2O2 → CO2 + 2H2O

Combustion of ethane

C2H6 + 7/2 O2 → 2CO2 + 3H2O

Combustion of propane

C3H8 + 5O2 → 3CO2 + 4H2O

Incomplete Combustion

  1. Incomplete combustion occurs when there isn't enough oxygen present.
  2. In an incomplete combustion, carbon or carbon monoxide will be produced as the product of the reaction.
  3. Carbon monoxide is a colourless poisonous gas.

Example:
Incomplete combustion of ethane

C2H6 + 5/2 O2 → 2CO + 3H2O

C2H6 + 3/2 O2 → 2C + 3H2O
Incomplete combustion of propane

C3H8 + 7/2 O2 → 3CO + 4H2O

C3H8 + 2 O2 → 3C + 4H2O

MUST Know!
Enough oxygen → Complete combustion
Not enough oxygen → Incomplete combustion

Trends

  1. The hydrocarbons become harder to ignite as the molecules get bigger.
  2. Combustion of alkanes with bigger molecule will produce more soot as the percentage of carbon of the molecule is higher.

Percentage of Carbon

  1. The amount of soot produced depends on the percentage of carbon in a hydrocarbon molecule.
  2. The percentage of carbon in a molecule can be calculated by using the following formula:
  3. Percentage of carbon = Relative mass of carbons in 1 molecule Relative mass of the molecule ×100%
  4. Lets calculate the percentage of carbon in a methane and a hexane.
    1. Percentage of carbon in methane (CH4) [Relative Atomic Mass: Carbon: 12; Hydrogen: 1]
    2. Percentage of carbon = 12 12+4(1) ×100% =75%
    3. Percentage of carbon in hexane (C6H14)
    4. Percentage of carbon = 6(12) 6(12)+14(1) ×100% =83.7%
  5. Percentage of carbon in hexane is higher than in methane, therefore combustion of propane is sootier than combustion of methane
MUST Know!
The bigger the molecule of an alkane, the higher the percentage of carbon, and hence produces more soot during combustion.


1 comment:

  1. Combustion of hexane is sootier than methane not propane

    ReplyDelete