Alkanes
Introduced| English | Meaning |
|---|---|
| alkane/ˈælkeɪn/ | A saturated hydrocarbon containing only single bonds between carbon atoms. |
| saturated/ˈsætʃəreɪtɪd/ | Describing a solution unable to dissolve more solute under stated conditions, or an organic molecule with no carbon-carbon double or triple bonds. |
| hydrogenation/haɪˈdrɒdʒəneɪʃn/ | The addition of hydrogen to a substance, often across a carbon-carbon double bond using a catalyst. |
| cracking/ˈkrækɪŋ/ | Breaking long-chain alkanes into shorter alkanes and alkenes using heat and a catalyst. |
| combustion/kəmˈbʌstʃn/ | An exothermic reaction of a fuel with an oxidising agent, usually oxygen. |
| carbon monoxide/ˈkɑːbən mʌˈnɒksaɪd/ | A compound of carbon and oxygen with formula CO, formed for example during incomplete combustion. |
The unreactive backbone
- Alkanes 烷烃 are saturated 饱和 hydrocarbons ($\text{C}_n\text{H}_{2n+2}$).
- They are useful fuels but generally unreactive.
- We make them and burn them.
Alkanes are:
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Alkanes have only single C–C bonds (saturated) and follow CₙH₂ₙ₊₂.
Alkanes are saturated hydrocarbons containing only single bonds.
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General formula CₙH₂ₙ₊₂.
Making alkanes
- hydrogenation 氢化: add hydrogen to an alkene (Pt or Ni catalyst, heat).
- cracking 裂化: break a long-chain alkane into shorter ones (heat with $\text{Al}_2\text{O}_3$).
An alkene decolourises bromine water; an alkane does not
The tetrahedral carbon
Each carbon in an alkane has four bonding pairs and no lone pairs — they spread as far apart as possible into a tetrahedron (109.5°).
Cracking is used to:
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Cracking turns heavy crude-oil fractions into smaller, more useful alkanes and alkenes.
Match each alkane fact.
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Each item links the term to its correct meaning.
Combustion 燃烧
- complete (plenty of oxygen) → carbon dioxide + water.
- incomplete (not enough oxygen) → water + toxic carbon monoxide 一氧化碳 (CO) + soot.
Natural gas (mainly methane, an alkane) burns on a stove
Incomplete combustion of an alkane (not enough oxygen) produces:
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With too little oxygen you get CO (toxic) and carbon (soot) as well as water.
Unreactivity and environment
- Alkanes are unreactive towards polar reagents: the C–H and C–C bonds are strong and have little polarity, so there's no charge to attract attack.
- Burning them in engines gives CO, NOₓ and unburnt hydrocarbons — a catalytic converter removes these.
Alkanes are unreactive towards polar reagents because their bonds are:
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Non-polar, strong C–H and C–C bonds give no charged site for an attacking species.
You've got it
- alkanes = saturated hydrocarbons ($\text{C}_n\text{H}_{2n+2}$); made by hydrogenation or cracking
- complete combustion → CO₂ + water; incomplete → toxic CO + soot
- unreactive (strong, non-polar bonds); engine pollutants removed by a catalytic converter