Combustion Chemistry

Every carbon atom in a gallon of gasoline ends up as exactly one molecule of CO2. Not approximately. Exactly. The chemistry is fixed: one carbon in, one CO2 out. No engine redesign, no additive, no catalytic converter changes this. The carbon has to go somewhere, and combustion sends it to the atmosphere.

Combustion is rapid exothermic oxidation: a hydrocarbon reacts with oxygen to produce carbon dioxide, water, and heat. For methane: CH4 + 2O2 -> CO2 + 2H2O + 890 kJ. The reaction breaks weaker C-H and O=O bonds and forms stronger C=O and O-H bonds, releasing the difference as thermal energy. The stoichiometry is exact: every carbon atom produces one CO2 molecule, every two hydrogen atoms produce one H2O molecule.

When oxygen supply is insufficient, combustion is incomplete. Instead of CO2, you get carbon monoxide (CO), unburned hydrocarbons, and soot (particulate matter). Incomplete combustion wastes fuel energy and produces toxic pollutants. This is why furnaces, engines, and power plants are engineered to ensure excess air supply.

Worked Example

Burning one mole of octane (C8H18), the main component of gasoline:

• Write the balanced equation. 2C8H18 + 25O2 -> 16CO2 + 18H2O.
• Per mole of octane: 8 CO2 molecules produced.

If a car burns 3 moles of octane per mile, how many moles of CO2 does it emit per mile?

3 x 8 = 24 moles of CO2 per mile. At 44 g/mol, that is about 1,056 grams, roughly 1 kg of CO2 per mile. The chemistry is the same regardless of engine efficiency; efficiency only determines how many moles of fuel are burned per mile, not the ratio of CO2 per mole.

This fixed stoichiometry is why carbon emissions can be calculated directly from fuel consumption data, without measuring exhaust.

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