Methanol, ethanol and propanol are all members of the alcohol homologous series. A chemist is investigating the combustion of these three alcohols and notices that as the carbon chain length increases, the flame produced becomes increasingly smoky and luminous. Explain why alcohols burn in air and describe how the products of complete combustion of ethanol differ from those of incomplete combustion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alcohols are widely used as fuels. Ethanol has the molecular formula C₂H₅OH. Complete combustion requires a plentiful supply of oxygen, whereas incomplete combustion occurs when oxygen is limited.
Model answer (5 marks)
Alcohols contain carbon and hydrogen atoms, so they can react with oxygen in the air to form new bonds.
During complete combustion the reaction is
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Thus the products are carbon dioxide and water.
When oxygen is limited the reaction is incomplete. Carbon atoms are not fully oxidised to CO₂; instead they form CO and elemental carbon (soot). The soot particles give the flame a smoky, luminous appearance.
Therefore, as the carbon chain length increases the amount of carbon available for soot formation rises, making the flame increasingly smoky and luminous.
During complete combustion the reaction is
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Thus the products are carbon dioxide and water.
When oxygen is limited the reaction is incomplete. Carbon atoms are not fully oxidised to CO₂; instead they form CO and elemental carbon (soot). The soot particles give the flame a smoky, luminous appearance.
Therefore, as the carbon chain length increases the amount of carbon available for soot formation rises, making the flame increasingly smoky and luminous.
Examiner tips
- Use the word ‘complete’ to show you know the balanced equation; include CO₂ and H₂O. Mention CO and soot for incomplete combustion. Show the reaction for ethanol explicitly to demonstrate understanding.
- common_mistakes
- :
- Writing the combustion equation for methanol or propanol instead of ethanol. Forgetting to mention CO or soot in the incomplete combustion description. Using ‘carbon monoxide’ but not linking it to the smoky flame.
Mark scheme (5 marks)
- Alcohols contain carbon and hydrogen (and oxygen), so they can burn / react with oxygen
- Complete combustion of ethanol produces carbon dioxide
- Complete combustion of ethanol produces water
- Incomplete combustion produces carbon monoxide instead of (or as well as) carbon dioxide
- Incomplete combustion also produces carbon (soot / particles) / the flame is smokier because carbon particles are released
Key terms in this question
complete combustion · incomplete combustion
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Alcohols, haloalkanes and analysis questions
- A student is trying to identify two unknown liquids. One liquid is a haloalkane …
- Propan-1-ol can be converted into 1-chloropropane, a haloalkane, in a substituti…
- A forensic scientist is analysing an unknown organic liquid found at a crime sce…
- Ethanol is a member of the alcohol family of compounds. Explain what happens whe…