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:

1. Identify the reactants and products:

   • Reactants: Decane (C₁₀H₂₂) and Oxygen (O₂)
   • Products: Carbon Dioxide (CO₂) and Water (H₂O)

2. Write the unbalanced equation: $2C_{10}H_{22} + O_2 \rightarrow CO_2 + H_2O$

3. Calculate the number of each type of atom on both sides:

   • On the reactant side: C=20, H=44, O varies
   • On the product side: C from CO₂ = 1, H from H₂O = 2

4. Balance Carbon atoms first: There are 20 Carbon atoms from 2 decane molecules, so we need 20 CO₂: $2C_{10}H_{22} + O_2 \rightarrow 20CO_2 + H_2O$

5. Balance Hydrogen atoms next: There are 44 Hydrogen atoms from decane, so we need 22 H₂O: $2C_{10}H_{22} + O_2 \rightarrow 20CO_2 + 22H_2O$

6. Balance Oxygen atoms last: Count the Oxygen:

   • From 20 CO₂: 20 * 2 = 40
   • From 22 H₂O: 22 * 1 = 22
   • Total O needed = 40 + 22 = 62 O atoms
   • On the reactant side, each O₂ has 2 O atoms, so: $O_2 = \frac{62}{2} = 31$

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