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4.3.2Additional lessons

Incomplete Combustion — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 80 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5When Combustion Is Incomplete

The type of product depends on how much oxygen is present compared with the amount of fuel.

Cause
  • Cause Too little oxygen is available, so some carbon cannot be fully oxidised to carbon dioxide.
Carbon monoxide
  • Carbon monoxide CO forms when oxidation stops part-way, as in 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(l).
Soot
  • Soot Carbon particles form when even less oxygen is present, as in CH₄(g) + O₂(g) → C(s) + 2H₂O(l).
Visible clue
  • Visible clue A yellow, smoky flame indicates incomplete combustion, while a blue flame indicates a plentiful air supply.

Part 2 of 5Hazards and Consequences

1
Carbon monoxide is colourless and odourless, so people cannot detect it without an alarm.
2
CO binds strongly to haemoglobin in red blood cells and prevents oxygen being carried around the body.
3
Poisoning causes headaches, dizziness and unconsciousness, and may lead to death in poorly ventilated rooms.
4
Soot is made of carbon particles that blacken surfaces and can damage lungs and block engines and flues.
5
Incomplete combustion also releases less energy per mole of fuel than complete combustion.

Part 3 of 5Prevention

Prevention
Ventilation
  • Ventilation Fires, boilers and cookers need a free flow of fresh air so oxygen is always in excess.
Maintenance
  • Maintenance Regular servicing of heaters and flues prevents blockages that restrict air.
Detectors
  • Detectors Carbon monoxide alarms warn occupants before concentrations reach dangerous levels.
Engine design
  • Engine design Catalytic converters convert CO to CO₂ in vehicle exhaust gases.
ConditionCarbon-containing products
Excess oxygenCO₂ and H₂O only
Limited oxygenCO and H₂O
Very limited oxygenC and H₂O (and some CO)

Part 4 of 5Worked Example

Question Balance the equation for the incomplete combustion of octane to CO and water, and state how many moles of O₂ react with 1 mole of octane.

  1. Write C₈H₁₈(l) + O₂(g) → CO(g) + H₂O(l).
  2. Carbon8 CO; hydrogen: 9 H₂O.
  3. Oxygen atoms on the right = 8 + 9 = 17, so O₂ = 17/2 = 8.5.
  4. Double to remove the fraction2C₈H₁₈(l) + 17O₂(g) → 16CO(g) + 18H₂O(l).

Answer 8.5 mol O₂ react with each mole of octane

Part 5 of 5Exam Checklist & Tips

4.3.2 – Key Points for the Exam

  • Incomplete combustion Burning in a limited supply of oxygen, forming CO or carbon instead of only CO₂.
  • Carbon monoxide A toxic gas formed by partial oxidation of carbon.
  • Soot Finely divided carbon formed in very oxygen-poor flames.
  • Haemoglobin The protein in red blood cells that carries oxygen.

Common Mistakes

  • Writing carbon dioxide as the product of incomplete combustion.
  • Saying CO is poisonous because it is acidic, when it actually blocks oxygen transport in blood.
  • Forgetting that two equations are needed for carbon and carbon monoxide.
  • Leaving fractional coefficients without stating whether they are acceptable per mole.

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