General Chemistry
Combustion
Rapid oxidation with oxygen — releases heat and light
Combustion is rapid oxidation reaction with oxygen, typically producing CO₂, H₂O, and heat (often light). Hydrocarbon combustion: CnHm + O₂ → CO₂ + H₂O. Complete combustion: full oxidation; CO₂ and H₂O. Incomplete: insufficient O₂; produces CO (toxic), C (soot). Fuels: hydrocarbons (methane, propane, gasoline), wood (cellulose), coal, biofuels. Powers: heating, electricity (power plants), transportation. Chemistry: chain reactions involving free radicals. Activation energy provided by ignition (heat or spark). Critical for energy economy; major CO₂ source.
- Reaction (general)Fuel + O₂ → CO₂ + H₂O + heat
- Methane combustionCH₄ + 2O₂ → CO₂ + 2H₂O; ΔH = -890 kJ/mol
- Complete vs incompleteComplete = CO₂; incomplete = CO, C (soot)
- ActivationSpark or flame for ignition
- Heat of combustion (gasoline)~47 MJ/kg
- MechanismFree radical chain reaction
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Why combustion matters
- Energy. Most electricity, heating, transport.
- Heating. Home, industrial.
- Transportation. Internal combustion engines.
- Power plants. Coal, gas, oil.
- Cooking. Stoves, fires.
- Climate change. Major source of CO₂.
- Safety. Fire prevention/suppression.
Common misconceptions
- Combustion always complete. Often incomplete; produces CO, soot.
- Fire is solid. Hot ionized gases (plasma in some flames).
- Combustion exothermic only. Yes — exothermic; activation energy first.
- Higher temperature better. NO_x increases at high T.
- All fuels burn similarly. Different chemistry, products.
- Heat alone causes combustion. Need fuel + O₂ + heat (fire triangle).
Frequently asked questions
What is combustion?
Rapid oxidation reaction with oxygen, releasing energy as heat (and often light). Fuel + O₂ → CO₂ + H₂O for hydrocarbons. Other fuels: H₂ + O₂ → H₂O. Different fuels: different products. Always exothermic. Often produces flame (rapid light emission from hot gases).
How is methane combusted?
CH₄ + 2O₂ → CO₂ + 2H₂O. ΔH = -890 kJ/mol. Bonds broken: 4 C-H (1648 kJ) + 2 O=O (996 kJ) = 2644 kJ. Bonds formed: 2 C=O (1598) + 4 O-H (1852) = 3450 kJ. Net: -806 kJ (close to actual). Cleanest hydrocarbon fuel — least CO₂ per unit energy.
What's incomplete combustion?
Insufficient O₂. Produces CO (carbon monoxide) and/or C (soot). 2C + O₂ → 2CO (incomplete). C₂H₆ + 5/2 O₂ → 2CO + 3H₂O. CO is toxic (binds hemoglobin). Soot: black smoke; particulates. Common: poor ventilation, badly tuned engines. Solutions: more O₂, complete burning, catalytic converters.
How is energy released?
Bond formation in products (stronger bonds) > bond breaking in reactants. Specifically: O=O (498 kJ/mol, weak) → C=O (799), O-H (463). Net energy release. From thermochemistry standpoint: ΔH < 0 (exothermic). Energy: kinetic energy of products; emitted as heat; flame visible from hot CO₂ + H₂O + soot at ~2000-3000 K.
How does combustion start?
Activation energy needed. Ignition source (spark, flame, hot surface) raises some molecules above Ea. Reaction begins; energy released ignites neighboring molecules; chain reaction. Self-sustaining once started. Quench: remove fuel, oxygen, or heat. Fire triangle: fuel + O₂ + heat (any one removed → fire stops).
What's a free radical chain?
Combustion is chain reaction. Initiation: high T breaks bonds → free radicals (e.g., CH₃·, OH·). Propagation: radicals react with O₂, other molecules; new radicals continue chain. Termination: two radicals combine. Each step: many cycles before termination. Chain length: ~10⁶ for typical combustion.
How does combustion compare to metabolism?
Same overall chemistry: fuel + O₂ → CO₂ + H₂O. Different mechanisms. Combustion: rapid, uncontrolled, mostly heat. Metabolism: slow, enzyme-catalyzed, multi-step, energy captured as ATP (~40% efficiency). Both: net oxidation of organic matter. Body temperature stable because: metabolism rate matches heat loss.