AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Alkenes react with oxygen in combustion reactions in the same way as other…
Full specification wording
Reactions of alkenes: Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion.
Like all hydrocarbons, alkenes burn in oxygen, releasing carbon dioxide and water when combustion is complete.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 5
Study notes
From the checked teaching notes for this specification point.
The idea
Like all hydrocarbons, alkenes burn in oxygen, releasing carbon dioxide and water when combustion is complete. The general equation for complete combustion of an alkene is CₙH₂ₙ + 1.5n O₂ → n CO₂ + n H₂O, so ethene gives C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. In a plentiful supply of air, combustion is complete. In a limited supply of air, combustion is incomplete: some carbon is not fully oxidised, producing carbon monoxide, a toxic gas, and solid carbon particles, which appear as soot. These particles glow and give a smoky, yellow flame. Alkenes have a higher carbon-to-hydrogen ratio than the corresponding alkanes, so they tend to burn with a smokier flame.
What earns the marks
- Alkenes are hydrocarbons and burn in oxygen to release energy, forming carbon dioxide and water when combustion is complete.
- The balanced equation for complete combustion of ethene is C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.
- Incomplete combustion occurs when oxygen supply is limited and produces carbon monoxide and/or carbon (soot) instead of carbon dioxide.
- Carbon monoxide is toxic, and carbon particles cause the smoky, yellow flame seen when alkenes burn in air.
- The high carbon-to-hydrogen ratio of alkenes compared with alkanes helps explain why their flames are smokier.
Common mistakes and how to put them right
Writing that incomplete combustion produces only carbon dioxide and water.
Put it right: It also produces carbon monoxide and/or carbon.
Balancing ethene combustion as C₂H₄ + 2O₂ → 2CO₂ + 2H₂O.
Put it right: The correct balanced equation is C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.
Stating that carbon monoxide is harmless.
Put it right: Carbon monoxide is toxic because it reduces the oxygen-carrying capacity of the blood.
Exam tips
- Balance combustion equations by counting carbon, then hydrogen, then oxygen atoms.
- Link a smoky flame to incomplete combustion and name the products carbon monoxide and carbon.
- When comparing alkenes with alkanes, refer to the higher proportion of carbon in alkene molecules.
Try the lesson
The start of the Reactions of alkenes: Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons, but they tend to burn in air with smoky flames because of incomplete combustion. lesson, made from checked teaching notes for this specification point.
Have a go first
Ethene burns in a plentiful supply of air. Which two products are formed when ethene undergoes complete combustion?
Listen
Like all hydrocarbons, alkenes burn in oxygen, releasing carbon dioxide and water when combustion is complete. The general equation for complete combustion of an alkene is CₙH₂ₙ + 1.5n O₂ → n CO₂ + n H₂O, so ethene gives C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. In a plentiful supply of air, combustion is complete. In a limited supply of air, combustion is incomplete: some carbon is not fully oxidised, producing carbon monoxide, a toxic gas, and solid carbon particles, which appear as soot. These particles glow and give a smoky, yellow flame. Alkenes have a higher carbon-to-hydrogen ratio than the corresponding alkanes, so they tend to burn with a smokier flame. A strong answer earns marks for these points. Alkenes are hydrocarbons and burn in oxygen to release energy, forming carbon dioxide and water when combustion is complete. The balanced equation for complete combustion of ethene is C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. Incomplete combustion occurs when oxygen supply is limited and produces carbon monoxide and/or carbon (soot) instead of carbon dioxide. Carbon monoxide is toxic, and carbon particles cause the smoky, yellow flame seen when alkenes burn in air. A common slip: Writing that incomplete combustion produces only carbon dioxide and water. Instead, it also produces carbon monoxide and/or carbon. Exam tip: Balance combustion equations by counting carbon, then hydrogen, then oxygen atoms.
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