Pentane burns in plenty of air: CO₂ and H₂O is produced.
C₅H₁₂ + 8 O₂ → 5 CO₂ + 6 H₂O
Pentane burns in limited amount of air: CO or even C is produced along with H₂O.
2 C₅H₁₂ + 11 O₂ → 10 CO + 12 H₂O
Explanation
Pentane is a hydrocarbon. There are five carbon atoms in each of its molecule. Its molecular formula will be C₅H₁₂.
Hydrocarbon fuels burn to produce CO₂ when there's plenty of air.
? C₅H₁₂ + ? O₂ → ? CO₂ + ? H₂O
- Among all species in this reaction, C₅H₁₂ has the largest number of atoms per molecule. Assume that the coefficient of C₅H₁₂ is one.
1 C₅H₁₂ + ? O₂ → ? CO₂ + ? H₂O
- C₅H₁₂ is the only reactant that contains C atoms. There are 5 C atoms in a C₅H₁₂ molecule. There should be the same number of C atoms in the products.
- CO₂ is the only product that contains C atoms. There are one C atom in each CO₂ molecule. 5 C atoms correspond to 5 CO₂ molecules.
1 C₅H₁₂ + ? O₂ → 5 CO₂ + ? H₂O
- Similarly, C₅H₁₂ is the only reactant that contains H atoms. H₂O is the only product that contains H atoms. There are 12 H atoms in one C₅H₁₂ molecule, which corresponds to 6 H₂O molecules.
1 C₅H₁₂ + ? O₂ → 5 CO₂ + 6 H₂O
- Both CO₂ and H₂O are products that contains O atoms. There are 5 × 2 + 6 × 1 = 16 O atoms in total in 5 CO₂ molecules and 6 H₂O molecules. The 16 O atoms on the product side corresponds to 8 O₂ molecules on the reactant side.
1 C₅H₁₂ + 8 O₂ → 5 CO₂ + 6 H₂O
1 C₅H₁₂ + 8 O₂ → 5 CO₂ + 6 H₂O
- All coefficients shall be whole numbers. If there's any fraction in this equation, multiply both sides by the least common multiple of their denominators.
Hydrocarbon fuels burn to produce H₂O and CO when there's a limited supply of air. C (soot) might also be produced. Assuming that only CO is produced. Try to balance the equation using the same method.
1 C₅H₁₂ + 11/2 O₂ → 5 CO + 6 H₂O
2 C₅H₁₂ + 11 O₂ → 10 CO + 12 H₂O
Less O₂ is consumed for each mole of C₅H₁₂.
Consider: What would be the balanced equation when only C is produced?
Reference
"Products and effects of combustion", GCSE Chemistry (Single Science), BBC Bitesize.
