1.3.1—Combustion reactions

In complete combustion with excess oxygen, carbon forms CO₂ and hydrogen forms H₂O. The fuel's other elements must also appear in the products.

Write the correct products first, then balance C, H and any other atoms before balancing O₂ last. Check every atom and state symbol.

For ethanol, write C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O by balancing C, then H, then oxygen. Complete combustion specifies products, not automatically conditions or state symbols; use the question's conditions to decide whether water is liquid or vapour.

Representative question

[Maximum number: 1]

Write an equation for the complete combustion of methane.

$\mathrm{CH}_{4}(\mathrm{~g})+2 \mathrm{O}_{2}(\mathrm{~g}) \rightarrow \mathrm{CO}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

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.

1.3.1—Combustion reactions

Complete Combustion

Writing Complete-Combustion Equations

Assessment in practice

Combustion and Thermodynamics Summary

Question 1