Combustion - Chemistry

1. Fundamental Concepts

Definition: A high-temperature exothermic redox reaction where a fuel reacts with an oxidant (usually oxygen in the air).

General Reaction: Fuel + Oxygen $\rightarrow$ Oxides + Energy (Heat/Light).

Complete vs. Incomplete:

Complete: Abundant $O_2$ produces only $CO_2(g)$ and $H_2O(g)$.

Incomplete: Limited $O_2$ produces $CO(g)$ (carbon monoxide) or $C(s)$ (soot/elemental carbon) and $H_2O(g)$.

2. Key Concepts

Hydrocarbon Combustion: The standard reaction involves a compound containing only Carbon and Hydrogen reacting with Oxygen.

General Equation: $$C_xH_y + O_2(g) \rightarrow CO_2(g) + H_2O(g)$$

3. Examples

Easy (Simple Balancing)

Question: Write and balance the complete combustion of Methane ($CH_4$), the primary component of natural gas.

Answer: $$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$$

Medium (Balancing & Isomers)

Question: Write and balance the complete combustion of Propane ($C_3H_8$), commonly used in gas grills.

Answer: $$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g)$$

Hard (Complex Molecules & Incomplete)

Question 1 (Complete - Complex): Write and balance the complete combustion of Glucose ($C_6H_{12}O_6$), which occurs during cellular respiration.

Answer: $$C_6H_{12}O_6(s) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l)$$

Question 2 (Incomplete): Write the incomplete combustion of Octane ($C_8H_{18}$) producing Carbon Monoxide ($CO$).

Answer: $$2C_8H_{18}(l) + 17O_2(g) \rightarrow 16CO(g) + 18H_2O(g)$$

4. Problem-Solving Techniques

The "CHOS" Rule:

1. Carbon: Balance the $CO_2$ to match the number of Carbon atoms in the fuel.

2. Hydrogen: Balance the $H_2O$ to match the number of Hydrogen atoms in the fuel.

3. Oxygen: Count the total Oxygen atoms on the product side and balance the $O_2$ on the reactant side.

4. Simplify: If you get a fraction for $O_2$, multiply the entire equation by the denominator to make it whole numbers.

Check State Symbols: Always double-check if the question specifies water as vapor $(g)$ or liquid $(l)$, and if the fuel is a liquid (like gasoline) or a gas (like methane).