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1.3.2—Incomplete combustion

Syllabus
First assessment 2025
Objective
1.3.2
Level
HL

Incomplete Combustion

When oxygen is limited, carbon may form CO and/or elemental carbon instead of only CO₂. The available oxygen and the fuel's elements determine the product set.

Include the products specified by the question, then balance all atoms. Do not replace a required CO or C product with CO₂ simply because the fuel contains carbon.

One valid carbon-monoxide equation is 2CH₄ + 3O₂ → 2CO + 4H₂O; with still less oxygen, elemental carbon can also form. Use the product set named by the question and rebalance from scratch—there is no single universal incomplete-combustion equation.

The product change has chemical consequences: carbon monoxide binds strongly to haemoglobin and reduces oxygen transport, while fine carbon particulates damage respiratory health and can alter atmospheric heating. Incomplete combustion also releases less usable energy per mole of fuel than complete conversion to CO₂ and H₂O; distinguish these separate health, environmental and energy claims.

Deducing Incomplete-Combustion Products

Assessment in practice

Representative question

Question 1

[Maximum number: 1]

Which products may form when propane undergoes incomplete combustion?

A

CO2\mathrm{CO}_{2} and H2\mathrm{H}_{2} only

B

CO, C and H2\mathrm{H}_{2}

C

CO2,H2O\mathrm{CO}_{2}, \mathrm{H}_{2} \mathrm{O} and H2\mathrm{H}_{2}

D

CO2,CO\mathrm{CO}_{2}, \mathrm{CO} and C

Combustion and Thermodynamics Summary

Retrieve the route: identify complete or incomplete combustion products, compare fuels and biofuels, balance fuel-cell half-equations, calculate ΔS° and ΔG°, then use ΔG, Q and K to reason about spontaneity and equilibrium.

Check products before balancing, evidence before evaluation, oxidation versus reduction, kelvin and unit consistency, the sign of ΔG, and whether Q is below, equal to, or above K.