How do you balance a combustion reaction?

For complete combustion of a hydrocarbon CxHy (or CxHyOz with oxygen already in the molecule), the balanced equation follows the formula: CxHyOz + (x + y/4 - z/2) O2 → x CO2 + (y/2) H2O. If the coefficient for O2 is not a whole number, multiply all coefficients by 2. For example, propane (C3H8): C3H8 + 5 O2 → 3 CO2 + 4 H2O. The number of moles of O2 needed is x + y/4 - z/2 per mole of fuel. Complete combustion assumes sufficient oxygen so that all carbon becomes CO2 and all hydrogen becomes H2O.

What is the heat released in combustion?

The heat released per mole of fuel burned is given by the standard enthalpy of combustion (delta H combustion), which is always negative (exothermic). It can be estimated from Hess's law using standard enthalpies of formation: delta H combustion = [x times delta Hf(CO2) + (y/2) times delta Hf(H2O)] - delta Hf(fuel). For common fuels: methane releases about 890 kJ/mol, propane about 2,220 kJ/mol, and octane about 5,471 kJ/mol. The heat released per gram of fuel is found by dividing the molar enthalpy by the molar mass of the fuel.

What is the difference between complete and incomplete combustion?

Complete combustion occurs when there is sufficient oxygen for all carbon atoms to form CO2 and all hydrogen atoms to form H2O. Incomplete combustion happens when oxygen is limited, producing carbon monoxide (CO), soot (unburned carbon), or other partially oxidised products alongside CO2 and H2O. Incomplete combustion releases less energy per mole of fuel and produces toxic CO. This calculator models complete combustion only. The stoichiometric air-fuel ratio represents the minimum oxygen needed for complete combustion.

Combustion Reaction Calculator

This calculator balances the complete combustion equation for any hydrocarbon or oxygenated fuel with the formula C_xH_yO_z, then works out how much oxygen it needs and what it produces. Complete combustion stoichiometry matters for chemistry coursework, lab work, and understanding how fuels like methane, petrol or ethanol burn. You can pick a common fuel such as methane, propane, butane, octane, ethanol, methanol or glucose from the dropdown, or enter a custom formula by specifying the number of carbon, hydrogen and oxygen atoms in the molecule. Then enter either the moles of fuel burned or its mass in grams; the calculator converts automatically between the two using the fuel's molar mass. It returns the fully balanced equation, the moles of oxygen required, the moles of carbon dioxide and water produced, the fuel's molar mass, and the total heat released in kilojoules, along with the energy released per gram of fuel. A plain-language summary explains what these numbers mean for the quantity you entered. For the preset fuels, the heat of combustion uses published standard enthalpy values; for any custom formula it is estimated using the Dulong-Petit approximation, and the calculator flags this clearly so you know when a figure is measured rather than approximate. Treat all results as indicative for complete combustion under standard conditions, since real burning is affected by temperature, pressure and how much oxygen is available.

Updated Standard enthalpies of combustion from NIST Chemistry WebBook and IUPAC data.

1. Fuel Formula

Select a common fuel or enter a custom formula

Carbon atoms (x) in CxHyOz

Hydrogen atoms (y)

Oxygen atoms in fuel (z) 0 for pure hydrocarbons

Please enter valid whole numbers: carbon atoms must be at least 1, hydrogen atoms at least 0, and oxygen atoms at least 0.

2. Quantity

Moles of fuel burned

mol

Or enter mass of fuel

Enter either moles or mass. If you enter a mass, moles will be calculated automatically using the molar mass of the fuel.

Balanced Complete Combustion Equation

CH₄ + 2 O₂ → CO₂ + 2 H₂O

Results

O2 Required
2.00
moles of oxygen

CO2 Produced

1.00

moles of carbon dioxide

H2O Produced

2.00

moles of water

Heat Released

890

kJ (for this quantity)

Stoichiometry (per mole of fuel)

Fuel formulaCH4

Molar mass of fuel16.04 g/mol

O2 needed (stoichiometric)2.000 mol/mol fuel

CO2 produced1.000 mol/mol fuel

H2O produced2.000 mol/mol fuel

Enthalpy of combustion-890.4 kJ/mol

For Your Quantity

Moles of fuel1.000 mol

Mass of fuel16.04 g

O2 consumed2.000 mol (64.00 g)

CO2 produced1.000 mol (44.01 g)

H2O produced2.000 mol (36.03 g)

Total heat released890.4 kJ

Heat per gram of fuel55.5 kJ/g

Summary: Burning 1.000 mol (16.04 g) of CH4 requires 2.000 mol of O2 and releases 1.000 mol of CO2, 2.000 mol of H2O, and 890.4 kJ of heat (55.5 kJ/g of fuel).

How Complete Combustion Is Balanced

Complete combustion of a fuel with formula C_xH_yO_z in excess oxygen always produces carbon dioxide (CO2) and water (H2O). The general balanced equation is:

C_xH_yO_z + (x + y/4 - z/2) O₂ → x CO₂ + (y/2) H₂O

The coefficient for O₂ follows directly from conservation of atoms: each carbon requires one O₂ (contributing 2 oxygen atoms to CO2), each pair of hydrogen atoms requires half an O₂ (contributing one oxygen atom to H2O), and any oxygen already present in the fuel reduces the O₂ needed. When the O₂ coefficient is not a whole number (which happens when y is odd), all coefficients are multiplied by 2 to produce whole-number integers.

Worked Example: Propane

Propane has the formula C₃H₈ (x = 3, y = 8, z = 0). Applying the formula:

O₂ coefficient = 3 + 8/4 - 0/2 = 3 + 2 = 5
CO₂ produced = 3 moles per mole of propane
H₂O produced = 8/2 = 4 moles per mole of propane

Balanced equation: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O. Propane has a standard enthalpy of combustion of -2,220 kJ/mol.

Enthalpies of Combustion for Common Fuels

Fuel	Formula	Molar Mass (g/mol)	Delta H combustion (kJ/mol)	Energy density (kJ/g)
Methane (natural gas)	CH4	16.04	-890	55.5
Ethane	C2H6	30.07	-1,560	51.9
Propane (LPG)	C3H8	44.10	-2,220	50.3
Butane	C4H10	58.12	-2,878	49.5
Octane (petrol proxy)	C8H18	114.23	-5,471	47.9
Ethanol	C2H6O	46.07	-1,368	29.7
Methanol	CH4O	32.04	-726	22.7
Glucose	C6H12O6	180.16	-2,803	15.6
Acetylene	C2H2	26.04	-1,300	49.9
Benzene	C6H6	78.11	-3,268	41.8

For fuels not in the preset list, this calculator estimates enthalpy using the Dulong-Petit approximation: approximately 33.3 kJ/g of carbon, 144 kJ/g of hydrogen, and -9.3 kJ/g of oxygen in the fuel. This is a simplified estimate and actual values should be sourced from NIST or measured calorimetrically for precise work.

Molar Mass Calculation

The molar mass of the fuel C_xH_yO_z is calculated as: M = 12.011x + 1.008y + 15.999z g/mol. This uses IUPAC 2021 standard atomic weights for carbon (12.011), hydrogen (1.008), and oxygen (15.999).

Complete vs Incomplete Combustion

This calculator assumes complete combustion with excess oxygen. In real engines and burners, incomplete combustion is common due to insufficient oxygen, short reaction times, or low temperatures. Incomplete combustion produces carbon monoxide (CO) and unburned carbon (soot) alongside CO2 and H2O, releasing less energy and creating more pollution. The stoichiometric air-fuel ratio (AFR) for complete combustion can be calculated from the O2 coefficient: AFR by mass = (moles O2 x 32 x 100/23.2) / molar mass of fuel, where 23.2% is the mass fraction of oxygen in air.

Related Calculators

Science and Engineering Calculators: all chemistry and physics tools.
Moles Calculator (mass to moles): convert between mass and moles for any element or compound.
Equilibrium Constant (Keq) Calculator: Calculate Keq from concentrations or partial pressures.
Dilution Calculator (C1V1 = C2V2): calculate solution concentrations after dilution.
Avogadro Number Calculator: convert between moles and numbers of particles.
Resistor Power Rating Calculator NZ: Power Dissipation & Rating.
Restraint of Trade Checker NZ 2026: Is Your Clause Likely Enforceable.

Sources and method: NIST Chemistry WebBook (webbook.nist.gov) standard enthalpies of combustion at 25 degC and 1 atm. IUPAC 2021 standard atomic weights (iupac.org). Dulong-Petit approximation used for fuels not in the preset list. Balanced equation derived from conservation of carbon, hydrogen, and oxygen atoms with the formula O2 coefficient = x + y/4 - z/2.

This calculator models ideal complete combustion under standard conditions (25 degC, 1 atm). Actual energy release varies with temperature, pressure, and combustion efficiency. Enthalpy values for preset fuels use published standard values; the Dulong-Petit estimate for custom formulas is approximate. For precise work, consult NIST or conduct calorimetric measurements.