3.4 Combustion Reactions
Combustion burns a fuel in oxygen, releasing heat and light. It is always exothermic, and balancing follows a reliable order: carbon first, hydrogen second, oxygen last.
Complete Combustion of Hydrocarbons
With plentiful oxygen, a hydrocarbon $\text{C}_x\text{H}_y$ burns fully to carbon dioxide and water:
$$\text{C}_x\text{H}_y + \left(x + \frac{y}{4}\right)\text{O}_2 \rightarrow x\,\text{CO}_2 + \frac{y}{2}\,\text{H}_2\text{O}$$
Example: Balance the combustion of ethane, $\text{C}_2\text{H}_6$. Balance C (2 $\text{CO}_2$), then H (3 $\text{H}_2\text{O}$), then O ($\frac{7}{2}$ $\text{O}_2$), and clear the fraction:
$$2\,\text{C}_2\text{H}_6 + 7\,\text{O}_2 \rightarrow 4\,\text{CO}_2 + 6\,\text{H}_2\text{O}$$
Incomplete Combustion
With limited oxygen, carbon monoxide — or soot (C) — forms instead of $\text{CO}_2$:
$$2\,\text{CH}_4 + 3\,\text{O}_2 \rightarrow 2\,\text{CO} + 4\,\text{H}_2\text{O}$$Combustion of Other Fuels
Oxygen-containing fuels burn the same way. Ethanol: $$\text{C}_2\text{H}_5\text{OH} + 3\,\text{O}_2 \rightarrow 2\,\text{CO}_2 + 3\,\text{H}_2\text{O}$$
Practice Problems
- Balance the complete combustion of propane, C3H8.
- Write the balanced equation for the incomplete combustion of methane producing CO.
- Balance: CH3OH + O2 → CO2 + H2O
- Why is soot a sign of incomplete combustion?
Answers: 1. C3H8 + 5 O2 → 3 CO2 + 4 H2O 2. 2 CH4 + 3 O2 → 2 CO + 4 H2O 3. 2 CH3OH + 3 O2 → 2 CO2 + 4 H2O 4. Soot is unburned carbon — oxygen ran out before every C atom could reach CO2.