Combustion of Alkanes

Full Notes

Combustion of alkanes and fuels have been outlined more here.
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Heat energy is released when alkanes undergo combustion as the process is exothermic. This released heat energy can be harnessed and used for other things - such as generating electricity. This makes alkanes useful as fuels.

Combustion describes the reaction that occurs when an alkane rapidly reacts with oxygen, at a high temperature.

Alkanes can undergo either complete or incomplete combustion.

Complete Combustion

Complete combustion occurs when there is enough oxygen present and carbon can be fully oxidised, forming carbon dioxide as a product (and water).

Example Equation:
Complete Combustion of Methane (CH₄):
CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete Combustion

Incomplete combustion occurs when there is limited oxygen present and carbon can’t be fully oxidised, meaning carbon monoxide (CO) or carbon (soot) gets formed as a product (along with water).

Example Equation:
Incomplete combustion of methane – carbon monoxide (CO) production:
CH₄ + 1.5O₂ → CO + 2H₂O

Example Equation:
Incomplete combustion of methane – carbon (C, soot) production:
CH₄ + O₂ → C + 2H₂O

Pollutants from Internal Combustion Engines

Internal combustion engines can produce several pollutants, released in exhaust gases:

• Carbon monoxide (CO) – toxic gas that binds to hemoglobin in blood.
• Nitrogen oxides (NO_x) – formed at high temperatures when N₂ and O₂ react.
• NO₂ contributes to acid rain and photochemical smog.
• Carbon (C, soot) – causes respiratory issues and global dimming.
• Unburned hydrocarbons – contribute to smog formation.

Removal of Pollutants Using Catalytic Converters

Catalytic converters in car exhausts convert harmful gases to safer ones.

For example helping convert carbon monoxide (CO) and nitrogen monoxide (NO) into carbon dioxide (CO₂) and nitrogen (N₂)

Sulfur Dioxide and Its Removal from Flue Gases

Sulfur dioxide is formed when fuels containing sulfur (such as alkane-based fossil fuels) combust. Sulfur impurities in the fuel react with oxygen and form sulfur dioxide (SO₂).

Once released into the atmosphere, sulfur dioxide molecules react with water in clouds and forms sulfurous (H₂SO₃) and sulfuric (H₂SO₄) acid.

This causes the rain that falls from the clouds to have a low pH (around 5.5) - acid rain.

SO₂ can be removed from flue gases using calcium oxide (CaO) or calcium carbonate (CaCO₃).

Reactions to remove SO₂:
Using calcium oxide:
CaO + SO₂ → CaSO₃ (calcium sulfite)

Using calcium carbonate:
CaCO₃ + SO₂ → CaSO₃ + CO₂

Summary