Combustion Reaction Calculator

What Is a Combustion Reaction?

A combustion reaction is a chemical reaction in which a fuel reacts with oxygen to produce carbon dioxide and water vapor. For organic compounds containing only carbon (C), hydrogen (H), and oxygen (O), complete combustion follows a predictable pattern that can be balanced using simple atom-counting rules.

The general combustion reaction for a fuel with formula $C_{\alpha}H_{\beta}O_{\gamma}$ is:

$$C_{\alpha}H_{\beta}O_{\gamma} + aO_2 \rightarrow bCO_2 + cH_2O$$

where $\alpha$, $\beta$, and $\gamma$ are the numbers of carbon, hydrogen, and oxygen atoms in one molecule of fuel.

How to Balance a Combustion Reaction

Balancing a combustion reaction follows a fixed order. Start with carbon, then hydrogen, then oxygen:

1. Balance carbon: $b = \alpha$ (every carbon atom in the fuel becomes one $CO_2$ molecule).
2. Balance hydrogen: $c = \beta/2$ (every two hydrogen atoms produce one $H_2O$ molecule).
3. Balance oxygen: $a = \alpha + \beta/4 - \gamma/2$ (oxygen from $O_2$ must supply both product molecules).
4. Whole numbers: if any coefficient is fractional, multiply all coefficients by the factor needed to clear denominators.

Worked Example: Hexane Combustion

For hexane ($C_6H_{14}$), we have $\alpha = 6$, $\beta = 14$, and $\gamma = 0$:

• $b = 6$ (six $CO_2$ molecules)
• $c = 14/2 = 7$ (seven $H_2O$ molecules)
• $a = 6 + 14/4 = 9.5$ (nine and a half $O_2$ molecules)

Multiplying all coefficients by 2 gives the balanced equation:

$$2C_6H_{14} + 19O_2 \rightarrow 12CO_2 + 14H_2O$$

Common Fuel Combustion Reactions

Methane ($CH_4$) is the simplest hydrocarbon fuel. Its balanced combustion reaction is $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$. Propane ($C_3H_8$), used in camping stoves and heating, balances as $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$. For oxygenated fuels like ethanol ($C_2H_6O$), the oxygen in the fuel reduces external oxygen demand: $C_2H_6O + 3O_2 \rightarrow 2CO_2 + 3H_2O$.

Once you know the balanced reaction, you can calculate the air-fuel ratio (AFR) for stoichiometric combustion, or use the Combustion Analysis Calculator to work backward from combustion products to find an unknown compound's formula.