Combustion Reaction Calculator

Related Calculators

Complete combustion: Burning with enough oxygen so fuel carbon becomes CO2 and fuel hydrogen becomes H2O.

Fuel formula: The chemical formula of the fuel, limited here to compounds made of C, H, and optional O.

Stoichiometric oxygen: The exact amount of O2 needed to burn the fuel completely, with no fuel left and no extra O2 left.

Theoretical dry air: The exact dry air needed to supply the stoichiometric oxygen, using 21% O2 in air.

Extra air above the exact need (%): How much more air you supply than the exact theoretical amount.

Fuel molar mass: The mass of 1 mole of the fuel, in g/mol.

Reaction heat: The enthalpy change for the reaction, in kJ. A negative value means the combustion releases heat.

Equation style: Whether the balanced equation is shown with whole-number coefficients or per 1 mole of fuel.

Mistake: Typing a formula with elements other than C, H, and O into Fuel formula , such as sulfur, nitrogen, or chlorine.
Fix: Use this page only for CHO fuels. Enter a valid Fuel formula like CH4, C3H8, or C2H5OH.

Mistake: Entering grams in Fuel amount while Fuel amount entered as is set to Moles.
Fix: Match the number to the basis you picked. If your amount is in grams, change Fuel amount entered as to Mass.

Mistake: Expecting Theoretical dry air needed or Nitrogen carried with air to appear while Air setting is set to Use pure oxygen only.
Fix: Switch Air setting to Use dry air if your homework asks for air instead of pure O2.

Mistake: Using a negative value in Extra air above the exact need (%) .
Fix: Enter 0 or a positive percent. This calculator handles exact air or extra air, not oxygen shortage cases.

Mistake: Reading Oxygen needed as the oxygen actually supplied when you also entered extra air.
Fix: Treat Oxygen needed as the exact stoichiometric need. If you use dry air with extra air, check Extra oxygen left after reaction for the unused part.

Mistake: Expecting Reaction heat for every typed formula in Fuel formula .
Fix: Use a built-in Common fuel if you need Reaction heat . Typed formulas may still show the balanced equation and mole amounts without heat.

• This calculator solves complete combustion only, so it assumes enough oxygen is available to form CO2 and H2O.
• It accepts fuels made only of C, H, and optional O. Formulas with other elements are outside scope.
• The fuel must contain at least one carbon atom and one hydrogen atom under this page's model.
• If Air setting is pure oxygen, air-based results such as Theoretical dry air needed, Nitrogen carried with air, and Extra oxygen left after reaction are not used.
• Dry air is approximated as 21% O2 and 79% N2 by mole, so real air composition details are ignored.
• Reaction heat is available only for supported built-in fuels and uses standard enthalpy values, so real burner conditions can differ.
• The heat result depends on the chosen Water state for heat result because liquid water and water vapor have different formation enthalpies.

How the calculator balances the reaction

For a fuel written as CxHyOz, complete combustion follows the standard pattern CxHyOz + O2 -> CO2 + H2O [1].

CxHyOz + (x + y/4 - z/2) O2 -> x CO2 + (y/2) H2O

The calculator reads the number of C, H, and O atoms in the fuel, then uses that pattern to get the oxygen, carbon dioxide, and water coefficients per 1 mole of fuel. If you pick whole-number style, it multiplies by the smallest factor that clears fractions.

Amount calculations

If you enter mass, the calculator first converts fuel grams to moles with molar mass.

fuel moles = fuel mass (g) / fuel molar mass (g/mol)

It then scales every stoichiometric amount by the fuel moles entered.

species moles = coefficient per mole of fuel x fuel moles

O2 needed = (x + y/4 - z/2) x fuel moles

CO2 formed = x x fuel moles

H2O formed = (y/2) x fuel moles

Air and excess-air calculations

When Air setting is dry air, the calculator converts oxygen need to theoretical air using 0.21 mol O2 per mol dry air.

theoretical dry air = O2 needed / 0.21

If extra air is added, oxygen supplied is increased by the chosen percent, and leftover oxygen is the supplied oxygen minus the exact oxygen need.

O2 supplied = O2 needed x (1 + excess air % / 100)

O2 leftover = O2 supplied - O2 needed

Nitrogen carried with the air is estimated from the dry-air mole ratio 79:

21, so N2 = O2 supplied x 79/21.

N2 in products = O2 supplied x (79/21)

Reaction heat

For supported built-in fuels, the calculator uses standard enthalpies of formation and the reaction-enthalpy relation [2][3]. O2(g) has formation enthalpy 0 [3].

DeltaHrxn = sum(products) - sum(reactants)

DeltaHrxn = sum(nu x DeltaHf products) - sum(nu x DeltaHf reactants)

The sign matters: a negative Reaction heat means the combustion releases heat.

Mini example

For 1 mol of CH4, x = 1, y = 4, and z = 0. So the balanced reaction is CH4 + 2 O2 -> CO2 + 2 H2O. That gives 2 mol O2 needed, 1 mol CO2 formed, and 2 mol H2O formed. In dry air, theoretical air is 2 / 0.21 = 9.5238 mol air. If water is counted as liquid, the heat is about -890.31 kJ for 1 mol CH4 using the stored formation values.

Assumptions behind the math

This method assumes complete combustion, CHO-only fuels, dry air as 21% O2 and 79% N2, and standard-state enthalpy data for the heat result. Real flames can differ if combustion is incomplete, air is humid, or conditions are not standard.