11.4 Alkanes

1) Combustion
Fuels (organic compounds) react with excess oxygen to produce carbon dioxide and water.
Combustion can be classified into complete and incomplete combustions depends on the oxygen supply.
Complete combustion: enough oxygen supply
CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion: oxygen supply is limited
2CH₄ + 3O₂ → 2CO + 4H₂O
CH₄ + O₂ → C + 2H₂O

Environmental or safety consideration of combustion
CO₂ is a greenhouse gas, and can cause global warming
CO is a toxic gas which can combine with haemoglobin to prevent the carriage of oxygen gas in blood
C is a small particulate (soot) and can cause global dimming

Tips for balancing combustion reaction:
The coefficient before CO₂ (CO or C) depends on the number of C in organic compound
The coefficient before H₂O depends on the number of H in organic compound
The coefficient before O₂ depends on the total number of O in CO₂ (CO or C) and H₂O which have already balanced in the previous two steps.

2) Substitution
Alkanes can react with halogens (Cl₂, Br₂) to produce monohalogenoalkane and hydrogen halide.
CH₄ + Cl₂ → CH₃Cl + HCl

Conditions: UV light (or sunlight)
So, this reaction can be called photochemical reaction.

Actually, all hydrogen atoms in alkane can be displaced by halogen when excess halogens are supplied in this process. So products should be varied, such CH₂Cl₂, CHCl₃ and CCl₄ evenly. But in IG level, you are just required to know monosubstotution as the only product.