When decane undergoes complete combustion, a mixture of carbon dioxide and water is formed - Edexcel - GCSE Chemistry Combined Science - Question 4 - 2016 - Paper 1

To balance the combustion reaction of decane ( $2C_{10}H_{22} + O_2 \rightarrow CO_2 + H_2O$), we need to find the appropriate coefficients for carbon dioxide and water.

1. Identify the number of carbon atoms in decane: There are 10 carbon atoms in decane, thus we need 10 molecules of carbon dioxide.

$C: 2C_{10}H_{22} \rightarrow 10CO_2$

2. Identify the number of hydrogen atoms in decane: There are 22 hydrogen atoms, which means we need 11 molecules of water to balance the hydrogen.

$H: 2C_{10}H_{22} \rightarrow 11H_2O$

3. Balance the oxygen atoms: Count the total oxygen needed on the product side:

   • From 10 CO₂: 10 \times 2 = 20 O atoms

   • From 11 H₂O: 11 \times 1 = 11 O atoms

   • Total = 20 + 11 = 31 O atoms

   • Each O₂ molecule provides 2 O atoms, hence (\frac{31}{2} = 15.5) O₂ molecules are needed.

   To avoid fractions, we can multiply all coefficients by 2:

   $\text{Final balanced equation: } 2C_{10}H_{22} + 31O_2 \rightarrow 20CO_2 + 22H_2O$