Complete vs Incomplete Combustion
| Complete | Incomplete | |
|---|---|---|
| Oxygen supply | Excess | Limited |
| Products | CO₂ + H₂O | CO and/or C (soot) + H₂O |
| Energy released | Maximum | Less. Bonds not fully oxidised |
| Flame | Blue | Yellow / smoky |
Pollutants from Combustion
| Pollutant | Source | Environmental Effect |
|---|---|---|
| CO | Incomplete combustion | Toxic. Binds to haemoglobin, prevents O₂ transport |
| CO₂ | All combustion | Greenhouse gas → global warming |
| SO₂ | Sulfur impurities in fuel | Acid rain (H₂SO₄). Damages buildings, lakes |
| NOx | N₂ + O₂ at high temps in engines | Acid rain (HNO₃), photochemical smog |
| Particulates (soot) | Incomplete combustion | Respiratory disease, reduced visibility |
Origin of Atmospheric Nitrogen Oxides (NOₓ)
NOx is formed from atmospheric N₂ and O₂ reacting at the extreme temperatures inside internal combustion engines. It does not originate from nitrogen impurities in the fuel itself.
Standard Enthalpy of Combustion (ΔHc⦵)
The enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions (298 K, 100 kPa) with all reactants and products in standard states.
Because combustion is always exothermic, ΔHc⦵ is always strictly negative.
Combustion Equations with State Symbols
Complete combustion of methane:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Incomplete combustion (limited oxygen):
2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(l)
Mandatory Liquid State Symbol for Water: H₂O(l)
In standard combustion equations, water must always be written as H₂O(l), not H₂O(g). Standard enthalpy of combustion is defined at 298 K, where water is a liquid. Writing (g) misses the latent heat of condensation, giving a less negative value and losing marks.
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