Reactivity 1.3.2 — Incomplete Combustion of Organic Compounds

What Is Incomplete Combustion?

Incomplete combustion occurs when there is an insufficient supply of oxygen for a substance—typically a hydrocarbon—to undergo complete combustion. Instead of forming only carbon dioxide and water, incomplete combustion produces other products such as carbon monoxide (\( \text{CO} \)) and carbon (soot, \( \text{C (s)} \)).

This type of combustion is less efficient and is a major source of air pollution and health hazards.

General Equation:

For a hydrocarbon, complete combustion is:

\[ \text{Hydrocarbon} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} \]

Incomplete combustion can follow either of these pathways:

• Carbon monoxide formation: \[ \text{Hydrocarbon} + \text{O}_2 \rightarrow \text{CO} + \text{H}_2\text{O} \]
• Carbon (soot) formation: \[ \text{Hydrocarbon} + \text{O}_2 \rightarrow \text{C (s)} + \text{H}_2\text{O} \]

Why Incomplete Combustion Occurs

• Limited oxygen supply (e.g., closed or confined spaces).
• Large hydrocarbons are harder to combust completely.
• Poor ventilation or faulty fuel-burning appliances (e.g., stoves, heaters).

Consequences of Incomplete Combustion

1. Carbon Monoxide (CO) – Toxic Gas

• Carbon monoxide is a colorless, odorless, and tasteless gas that is extremely dangerous.
• It forms when carbon in the fuel is only partially oxidized due to a limited oxygen supply.
• CO binds to hemoglobin in the blood with much greater affinity than oxygen, forming carboxyhemoglobin.
• This drastically reduces the blood’s capacity to transport oxygen, causing symptoms such as dizziness, nausea, fatigue, and in high doses, unconsciousness or death.
• CO poisoning is particularly dangerous in enclosed spaces with poor ventilation, such as rooms with faulty gas heaters or running car engines.

2. Carbon (Soot) – Black Particulate Matter

• In very limited oxygen, unburnt carbon is released as soot—fine black particles seen in smoke or deposited on surfaces.
• Soot contributes to respiratory issues, especially in individuals with asthma or bronchitis. It can penetrate deep into the lungs and cause irritation and inflammation.
• Environmentally, soot contributes to urban air pollution and smog formation. It also darkens surfaces and machinery, requiring frequent cleaning and maintenance.
• When deposited on ice or snow, soot absorbs more sunlight and accelerates melting, contributing to climate change through decreased albedo (reflectivity).

3. Lower Energy Output – Reduced Efficiency

• Incomplete combustion is less energy-efficient than complete combustion because not all the fuel is fully oxidized.
• As a result, less heat is released, and more fuel is needed to achieve the same amount of energy, leading to increased fuel consumption and higher costs.
• This inefficiency is particularly critical in engines, furnaces, and power plants, where performance depends on maximizing energy output.
• In the long term, incomplete combustion leads to greater environmental pollution and unnecessary waste of fuel resources.

Example : Incomplete combustion of methane (CH₄)

• • Complete: \[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \]
  • Incomplete (carbon monoxide): \[ 2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O} \]
  • Incomplete (carbon): \[ \text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O} \]

Key Differences from Complete Combustion

Deducing Equations for the Incomplete Combustion of Hydrocarbons and Alcohols

Understanding how to deduce these equations is essential in combustion analysis and environmental chemistry. The products of incomplete combustion often contribute to pollution, smog, and health risks, especially in poorly ventilated spaces or malfunctioning combustion engines.

You can deduce the balanced equations by following a systematic approach:

1. Write the molecular formula of the hydrocarbon or alcohol.
2. List the products: Incomplete combustion forms \( \text{CO} \), \( \text{C} \), and \( \text{H}_2\text{O} \).
3. Balance carbon atoms first, then hydrogen, and finally oxygen.
4. Use fractional coefficients for \( \text{O}_2 \) if necessary, then multiply the entire equation to eliminate fractions.