Combustion (Burning) Reactions

A combustion reaction can be identified because the reactants are a hydrocarbon and oxygen.  Hydrocarbons contain carbon and hydrogen (and sometimes oxygen):  examples include CH₄, C₄H₁₀, C₈H₁₈, C₁₂H₂₂O₁₁, and C₂₅H₅₂.  Oxygen gas is always O₂.

No matter what hydrocarbon is being burned, the products of complete combustion are carbon dioxide (CO₂) and water (H₂O).

Hydrocarbon (C_xH_y) + O₂  ---->  CO₂ + H₂O

Examples: 

1.  CH_{4 (g)}  +  O_2(g)  ---> ???

• First, identify the reaction as a combustion reaction by the reactants: hydrocarbon plus oxygen gas.   (CH₄ is called methane.)

• The products of a combustion are always the same so you may immediately write out the word equation:

Methane + Oxygen --->  Carbon dioxide + Water

• Now write out the equation in chemical formulas:

  CH_{4 (g)}  +  O_2(g)  ---> CO_2(g) +  H₂O_(g)

• _{Note that the products are gases because all combustion reactions release heat.}

• _{Finally, balance the equation by adding coefficients:}

  CH_{4 (g)}  + 2O_2(g)  ---> CO_2(g) +  2H₂O_(g)

2.  C₆H₁₂O_6(s) +  O_2(g)  ---> ???

• First, identify the reaction as a combustion reaction by the reactants: hydrocarbon plus oxygen gas.   (C₆H₁₂O₆  is called glucose.)

• The products of a combustion are always the same so you may immediately write out the word equation:

Glucose + Oxygen --->  Carbon dioxide + Water

• Now write out the equation in chemical formulas:

  C₆H₁₂O_{6 (s)}  +  O_2(g)  ---> CO_2(g) +  H₂O_(g)

• _{Note that the products are gases because all combustion reactions release heat.}

• _{Finally, balance the equation by adding coefficients:}

  C₆H₁₂O_{6 (s)}  +  6O_2(g)  ---> 6CO_2(g) +  6H₂O_(g)