Combustion Reaction Calculator
ChemistryCalculate the balanced combustion reaction for any CₓHᵧ or CₓHᵧOᵤ hydrocarbon. Outputs O₂ required, CO₂ and H₂O produced, and air-to-fuel mass ratio.
8
18
0
100
O₂ Coefficient
12.5
CO₂ Coefficient
8
H₂O Coefficient
9
O₂ Needed (g)
350.1
Air Needed (g)
1,509.2
What is a Combustion?
The Combustion Reaction Calculator balances the complete combustion reaction for any organic fuel of formula CₓHᵧOᵤ and computes the stoichiometric quantities: the O₂ coefficient and the masses of O₂, CO₂, H₂O, and air for a given fuel mass.
For a hydrocarbon or oxygenated fuel CₓHᵧOᵤ burning completely: CₓHᵧOᵤ + (x + y/4 − z/2) O₂ → x CO₂ + (y/2) H₂O. The O₂ coefficient (x + y/4 − z/2) determines the stoichiometric air requirement — the minimum air needed for complete combustion. This is the basis of the air-to-fuel ratio (AFR) used in engine calibration. The default example (octane C₈H₁₈, representing petrol) requires 12.5 mol O₂ per mol fuel and approximately 15.1 g air per gram of fuel.
For the reverse direction — determining the molecular formula from combustion products — use the Combustion Analysis Calculator. For air-fuel ratio calculations specific to engines, the AFR Calculator provides stoichiometric, rich, and lean mixture calculations for standard fuels. For the energy released, the Heat of Combustion Calculator computes ΔH_combustion from bond energies or Hess's law.
How to use this Combustion calculator
- Enter Carbon Atoms (x) — the subscript of C in the molecular formula. For C₈H₁₈: x = 8.
- Enter Hydrogen Atoms (y) — subscript of H. For C₈H₁₈: y = 18.
- Enter Oxygen Atoms (z) — subscript of O in the fuel itself (0 for pure hydrocarbons; 1 for ethanol C₂H₆O, 2 for acetic acid C₂H₄O₂).
- Enter Fuel Mass (g) — the amount of fuel to burn.
- Read O₂ Coefficient for the balanced equation and O₂/Air Needed for the given mass.
Formula & Methodology
Balanced combustion for CₓHᵧOᵤ:CₓHᵧOᵤ + (x + y/4 − z/2) O₂ → x CO₂ + (y/2) H₂O O₂ coefficient = x + y/4 − z/2 CO₂ coefficient = x H₂O coefficient = y/2For given fuel mass:M_fuel = 12.011x + 1.008y + 15.999z [g/mol] moles fuel = fuel_mass / M_fuel O₂ mass (g) = moles_fuel × (x + y/4 − z/2) × 31.998 Air mass (g) = O₂_mass / 0.232 (air is 23.2% O₂ by mass)Worked example — ethanol blend E20 comparison: E20 fuel is 20% ethanol (C₂H₆O) + 80% petrol (C₈H₁₈) by volume. Ethanol: O₂ coefficient = 2 + 6/4 − 1/2 = 3.0. M = 46.068 g/mol. O₂ per gram = 3.0 × 31.998/46.068 = 2.083 g O₂/g ethanol. Octane: O₂ per gram = 12.5 × 31.998/114.23 = 3.508 g O₂/g octane. E20 blend (by mass, ≈18.8% ethanol/81.2% octane): O₂ per gram ≈ 0.188 × 2.083 + 0.812 × 3.508 = 3.238 g O₂/g blend → AFR ≈ 3.238/0.232 ≈ 13.96 g air/g blend. Slightly lower than pure octane's 15.12, meaning E20 engines need recalibration — India's E20 rollout under the National Biofuel Policy 2022 requires all new vehicles sold after 2023 to be E20-compatible.
Frequently Asked Questions
A combustion reaction is a chemical reaction between a fuel and oxygen (O₂) that produces heat (and light) as the primary products. For organic fuels containing C, H, and O: CₓHᵧOᵤ + (x + y/4 − z/2) O₂ → x CO₂ + (y/2) H₂O + heat. Complete combustion produces only CO₂ and H₂O (no CO or soot). Incomplete combustion (insufficient O₂) produces CO, soot (C), and partially oxidised products. Industrial combustion in thermal power plants (India's NTPC stations), engines, and furnaces aims for complete combustion for efficiency and emissions compliance.
For CₓHᵧOᵤ + a O₂ → b CO₂ + c H₂O: (1) Balance C: b = x. (2) Balance H: c = y/2. (3) Balance O: a = x + y/4 − z/2 (total O in products minus O already in fuel). For octane C₈H₁₈: a = 8 + 18/4 = 8 + 4.5 = 12.5. Multiply entire equation by 2 to get whole number coefficients: 2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O. The calculator keeps fractional coefficients (12.5 O₂ per mole C₈H₁₈) for mass calculations — these are valid as stoichiometric ratios.
Enter Carbon Atoms (x), Hydrogen Atoms (y), and Oxygen Atoms in the fuel (z, for oxygenated fuels like alcohols or esters; enter 0 for pure hydrocarbons). Enter Fuel Mass (g). The calculator outputs O₂, CO₂, and H₂O stoichiometric coefficients, plus the actual O₂ mass needed and air mass needed for complete combustion of the given fuel mass. Default: octane C₈H₁₈ (petrol), 100 g.
CH₄ + 2 O₂ → CO₂ + 2 H₂O. Coefficients: O₂=2, CO₂=1, H₂O=2. Per 100 g CH₄ (M=16.043 g/mol = 6.233 moles): O₂ needed = 6.233 × 2 × 31.998 = 399.0 g; air needed = 399.0/0.232 = 1720 g. Methane is the primary component of natural gas (CNG), widely used in India for cooking (Piped Natural Gas in Mumbai, Delhi), power generation (NTPC gas turbines), and transportation (BEST and DTC CNG buses). India imports ~50% of its gas needs; domestic production from KG Basin, Rajasthan, and Mumbai High fields.
Ethanol C₂H₆O (x=2, y=6, z=1): O₂ coefficient = 2 + 6/4 − 1/2 = 2 + 1.5 − 0.5 = 3.0. Balanced: C₂H₆O + 3 O₂ → 2 CO₂ + 3 H₂O. Ethanol's oxygen content (z=1) reduces the O₂ required compared to a pure hydrocarbon with the same C,H count — this is why ethanol-blended fuels (E10: 10% ethanol, E20: 20% ethanol) have lower O₂ requirements from air, affecting engine air-to-fuel ratio calibration. India's E20 blending mandate (2025 target) affects millions of vehicles; fuel system calibration requires combustion stoichiometry calculations.
For C₈H₁₈ (isooctane, reference fuel): O₂ coefficient = 8 + 18/4 = 12.5. Molar mass of octane = 114.23 g/mol. O₂ per gram of fuel = 12.5 × 31.998 / 114.23 = 3.508 g O₂/g fuel. Air mass = 3.508/0.232 = 15.12 g air/g fuel — this is the stoichiometric air-fuel ratio (AFR) ≈ 15.1:1 for petrol. The [AFR Calculator](/afr-calculator/) computes this for various fuels. Engines run rich (AFR < 15.1) for maximum power or lean (AFR > 15.1) for fuel economy; the λ sensor (oxygen sensor) monitors this ratio in real time.
For 1 kg octane C₈H₁₈: moles octane = 1000/114.23 = 8.753 mol; CO₂ produced = 8.753 × 8 = 70.02 mol CO₂ = 70.02 × 44.009 = 3082 g = 3.08 kg CO₂. Every kilogram of petrol produces ~3.1 kg of CO₂. A typical Indian car uses ~7 L/100 km (density ≈ 0.73 kg/L = 5.11 kg/100 km) → emits ~15.8 kg CO₂/100 km = 158 g/km CO₂. CAFÉ (Corporate Average Fuel Efficiency) standards in India target 113 g/km CO₂ for passenger cars — driving the shift to CNG and EV in Indian urban transport.
Incomplete combustion occurs when insufficient O₂ is available (equivalence ratio > 1, rich mixture). Instead of CO₂, carbon monoxide (CO) is produced: 2C + O₂ → 2CO. CO is colourless, odourless, and extremely toxic — it binds to haemoglobin 200 times more strongly than O₂, causing oxygen deprivation. In India, CO poisoning from poorly ventilated generators, cooking stoves, and car exhaust is a significant public health concern. Building codes (NBC 2016) and BIS standards for generators and stoves specify minimum ventilation to prevent CO accumulation.
Combustion reaction: a chemical equation describing what happens when a fuel burns — the stoichiometry of reactants and products. Combustion analysis: an analytical technique using combustion to determine the elemental composition of an unknown organic compound by measuring the masses of CO₂ and H₂O produced from a known sample mass. The [Combustion Analysis Calculator](/combustion-analysis-calculator/) is for the analytical technique (determining %C, %H from measured products). This calculator is for reaction stoichiometry (given the molecular formula, how much O₂ and air are needed, and how much CO₂ and H₂O are produced).
The heat of combustion (ΔH_c) is the enthalpy change when one mole of fuel burns completely in O₂ at standard conditions (25°C, 1 atm). Octane: ΔH_c = −5471 kJ/mol = −47.9 MJ/kg (lower heating value). Methane: ΔH_c = −890 kJ/mol = −55.5 MJ/kg. Hydrogen: ΔH_c = −286 kJ/mol = −142.0 MJ/kg (highest of any fuel by mass). The [Heat of Combustion Calculator](/heat-of-combustion-calculator/) computes ΔH from bond energies or from Hess's law using formation enthalpies. Combustion reactions are always highly exothermic — this energy release drives engines, power plants, and industrial furnaces.