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Category Physics & Science

Substance

Chemical Formula Supports C, H, O, N, S, Cl, parentheses. E.g. Ca(OH)2

Amount

Unit

Excess Air (%)

H₂O Product Phase

Balanced Equation

Integer coefficients

⚗️ Fuel

🔥 O₂ Required

💨 CO₂ Produced

💧 H₂O Produced

ΔH°_rxn

Energy per gram

Theoretical Air (STP)

Actual Air (with excess)

Fuel Molar Mass

Mass Balance

Species	Coefficient	Moles	Mass (g)

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About

Incorrect stoichiometry in combustion analysis leads to miscalculated fuel requirements, flawed emission estimates, and potentially hazardous air-fuel ratios in industrial burners. This calculator parses a chemical formula, balances the complete combustion reaction with O₂, and computes product masses, molar quantities, enthalpy change ΔH°_rxn via Hess's Law, and theoretical air demand at STP. It handles hydrocarbons, alcohols, carboxylic acids, amines, thiols, and simple inorganic fuels containing C, H, O, N, S, and Cl. The balancing algorithm counts atoms across reactants and products, solving for the minimal integer coefficient set. Enthalpy values use NIST-referenced standard enthalpies of formation at 298.15 K and 1 atm.

Limitations apply: the tool assumes complete combustion (no CO or soot formation), standard-state conditions, and ideal gas behavior for air volume. For fuels with metallic elements or halogens beyond Cl, results require manual verification. Pro tip: real combustion systems operate with 10 - 50% excess air to ensure completeness. Use the excess air input to estimate actual air demand.

Formulas

Complete combustion of a generic fuel C_aH_bO_cN_dS_e follows this general scheme:

C_aH_bO_cN_dS_e + n_O₂ O₂ → a CO₂ + b2 H₂O + d2 N₂ + e SO₂

The oxygen coefficient is determined by atom balance:

n_O₂ = a + b4 + e − c2

Enthalpy of reaction via Hess's Law:

ΔH°_rxn = ∑ ΔH°_f(products) − ∑ ΔH°_f(reactants)

Theoretical air volume at STP (0 °C, 1 atm):

V_air = n_O₂ × 22.4140.2095 × (1 + excess%100)

Where a, b, c, d, e are atom counts of C, H, O, N, S in the fuel. n_O₂ is the stoichiometric moles of oxygen. 22.414 L/mol is the molar volume at STP. 0.2095 is the mole fraction of O₂ in dry air.

Reference Data

Substance	Formula	Molar Mass g/mol	ΔH°_f kJ/mol	ΔH°_comb kJ/mol	Products
Methane	CH₄	16.04	−74.8	−890.4	CO₂ + H₂O
Ethane	C₂H₆	30.07	−84.0	−1560.7	CO₂ + H₂O
Propane	C₃H₈	44.10	−103.8	−2219.2	CO₂ + H₂O
Butane	C₄H₁₀	58.12	−125.6	−2877.6	CO₂ + H₂O
Pentane	C₅H₁₂	72.15	−146.4	−3535.8	CO₂ + H₂O
Hexane	C₆H₁₄	86.18	−167.2	−4163.0	CO₂ + H₂O
Octane	C₈H₁₈	114.23	−208.4	−5470.5	CO₂ + H₂O
Ethylene	C₂H₄	28.05	52.4	−1411.2	CO₂ + H₂O
Acetylene	C₂H₂	26.04	226.7	−1299.6	CO₂ + H₂O
Benzene	C₆H₆	78.11	49.1	−3267.6	CO₂ + H₂O
Toluene	C₇H₈	92.14	12.0	−3910.3	CO₂ + H₂O
Methanol	CH₄O	32.04	−239.2	−726.0	CO₂ + H₂O
Ethanol	C₂H₆O	46.07	−277.6	−1366.8	CO₂ + H₂O
1-Propanol	C₃H₈O	60.10	−302.6	−2021.3	CO₂ + H₂O
Acetic acid	C₂H₄O₂	60.05	−484.3	−874.2	CO₂ + H₂O
Formaldehyde	CH₂O	30.03	−108.6	−570.8	CO₂ + H₂O
Acetone	C₃H₆O	58.08	−248.4	−1789.9	CO₂ + H₂O
Glucose	C₆H₁₂O₆	180.16	−1271.0	−2803.0	CO₂ + H₂O
Sucrose	C₁₂H₂₂O₁₁	342.30	−2226.1	−5643.8	CO₂ + H₂O
Diethyl ether	C₄H₁₀O	74.12	−271.2	−2723.7	CO₂ + H₂O
Hydrogen	H₂	2.016	0	−285.8	H₂O
Carbon (graphite)	C	12.01	0	−393.5	CO₂
Carbon monoxide	CO	28.01	−110.5	−283.0	CO₂
Sulfur	S	32.07	0	−296.8	SO₂
Hydrogen sulfide	H₂S	34.08	−20.6	−562.0	SO₂ + H₂O
Dimethyl sulfide	C₂H₆S	62.13	−65.4	−1904.2	CO₂ + H₂O + SO₂
Naphthalene	C₁₀H₈	128.17	78.5	−5156.8	CO₂ + H₂O
Glycerol	C₃H₈O₃	92.09	−669.6	−1654.3	CO₂ + H₂O
Phenol	C₆H₆O	94.11	−165.1	−3053.5	CO₂ + H₂O
Cyclohexane	C₆H₁₂	84.16	−156.4	−3919.6	CO₂ + H₂O

Frequently Asked Questions

Nitrogen in the fuel is assumed to form molecular N₂ (the thermodynamically favored product under standard combustion). Sulfur atoms produce SO₂. Both are included in the oxygen balance: sulfur consumes one mole of O₂ per atom, while nitrogen releases as the diatomic element and does not consume oxygen. The enthalpy calculation uses standard formation enthalpies for SO₂ (−296.8 kJ/mol) and N₂ (0 kJ/mol).

Three common sources of discrepancy: (1) phase state - this calculator uses gas-phase H₂O product by default; if your reference uses liquid water, the difference is approximately 44 kJ/mol per mole of water (the enthalpy of vaporization). (2) Temperature reference - values here are at 298.15 K; other sources may use 25 °C with different conventions for dissolved species. (3) The ΔH°_f database values are rounded to one decimal place.

Stoichiometric (theoretical) air provides exactly the oxygen needed for complete combustion. In practice, furnaces and engines supply 10 - 50% excess air to compensate for imperfect mixing. Too little excess air produces carbon monoxide and soot. Too much excess air wastes energy by heating inert nitrogen. The calculator multiplies theoretical air volume by (1 + excess%/100) to give practical air demand. Typical values: gas burners 10 - 15%, coal boilers 20 - 30%, rotary kilns up to 50%.

Yes. The formula parser handles nested parentheses and subscripts. Enter Ca(OH)2 and the parser expands it to Ca×1, O×2, H×2. However, combustion of inorganic salts like Ca(OH)₂ is not chemically meaningful - the calculator will warn if no combustible elements (C, H, S) are present. The tool is designed for fuels: organic compounds, hydrogen, carbon, sulfur, and their derivatives.

Atomic weights follow IUPAC 2021 standard values rounded to two decimal places (e.g., C = 12.01, H = 1.008, O = 16.00). For most engineering and educational purposes, this gives accuracy within ±0.05%. Isotope-specific calculations (e.g., deuterated compounds) are not supported.

The balancer first computes fractional coefficients from atom balance, then multiplies all coefficients by the least common multiple of their denominators to produce the smallest integer set. For example, combustion of C₂H₆ yields fractional O₂ coefficient of 3.5, which is scaled to 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. You can toggle between per-mole (fractional) and integer representations.