Compare complete and incomplete alkane combustion
Use oxygen availability to predict combustion products and balance equations for an alkane.
Section 14 · Hydrocarbons
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What carbon-containing product indicates complete combustion?
What you will learn
- State products of complete combustion
- Recognise carbon monoxide or carbon as incomplete-combustion products
- Balance alkane combustion equations
Complete combustion has sufficient oxygen
When an alkane burns with sufficient oxygen, its carbon forms carbon dioxide and its hydrogen forms water. For ethane: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Balance carbon and hydrogen first, then oxygen. Fractional oxygen coefficients can be avoided by multiplying the whole equation by a common factor.
Limited oxygen gives incomplete combustion
If oxygen is insufficient, carbon monoxide and/or solid carbon (soot) may form with water. One balanced carbon-monoxide example is 2C₂H₆ + 5O₂ → 4CO + 6H₂O.
The exact products depend on conditions. Do not state that incomplete combustion always makes only CO or only carbon, and do not confuse it with oxidation of an alkene by manganate(VII).
Pause and explain
Which condition favors incomplete combustion?
Worked example
Write balanced equations for complete and one incomplete combustion of ethane.
Show the worked solution
- Complete: form CO₂ and H₂O; balance C and H, then O.
- Incomplete example: form CO and H₂O.
- Check atoms in both equations.
Answer Complete: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Incomplete example: 2C₂H₆ + 5O₂ → 4CO + 6H₂O.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Balance complete combustion of propane: C₃H₈ + O₂ → CO₂ + H₂O.
Give me a hint
Balance C, then H, then O.
Compare my reasoning
- Three carbon atoms require 3CO₂.
- Eight hydrogen atoms require 4H₂O.
- The products contain ten oxygen atoms, so use 5O₂.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
Look for these in your work
- Carbon balanced
- Hydrogen balanced
- Oxygen balanced
Give one balanced incomplete-combustion equation for methane forming carbon monoxide and water.
Give me a hint
Choose CO as the carbon product and balance atoms.
Compare my reasoning
- Start with CH₄ + O₂ → CO + H₂O.
- One carbon gives one CO; four hydrogen gives two H₂O.
- Three product oxygen atoms require 3/2 O₂; double all coefficients.
2CH₄ + 3O₂ → 2CO + 4H₂O.
Look for these in your work
- CO product chosen
- Hydrogen balanced
- Whole-number equation balanced
A burner makes soot when its air inlet is partly blocked. Explain the observation using the oxygen supply.
Give me a hint
Connect limited oxygen to incomplete oxidation of carbon.
Compare my reasoning
- The blocked inlet reduces oxygen mixing with the fuel.
- Some carbon is not fully oxidised to CO₂.
- Fine solid carbon can form as soot.
The reduced oxygen supply can cause incomplete combustion, leaving some carbon as soot.
Look for these in your work
- Oxygen limitation identified
- Incomplete combustion named
- Soot linked to carbon
Common mistakes
- Writing CO₂ as the only carbon product when oxygen is limited
- Balancing oxygen before fixing carbon and hydrogen
- Calling incomplete combustion a complete reaction with less product
What can you explain now?
What carbon-containing product indicates complete combustion?
Compare with the explanation
CO₂
Complete combustion oxidises carbon to carbon dioxide.
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Make it stick
How do oxygen supply and the carbon-containing products distinguish complete from incomplete combustion?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.