Question

3\. If a hydrocarbon is burned with less than the theoretical amount of air, can the combustion be complete?

Short answer

Burning a hydrocarbon with less than the theoretical amount of air results in incomplete combustion.

Step-by-step solution

- Understand Incomplete Combustion

In combustion, a hydrocarbon reacts with oxygen to form carbon dioxide and water. If there isn't enough oxygen, the hydrocarbon cannot fully convert to carbon dioxide and water.

- Theoretical Air Requirement

The theoretical amount of air is the exact quantity needed to provide sufficient oxygen for complete combustion of the hydrocarbon. This ensures that all carbon atoms are converted to carbon dioxide and all hydrogen atoms are converted to water.

- Consequences of Insufficient Air

When less than the theoretical amount of air is available, there is not enough oxygen to fully oxidize the hydrocarbons. This leads to incomplete combustion, producing carbon monoxide (CO), soot (carbon particles), and other hydrocarbons.

- Conclusion

Since the combustion cannot be complete without sufficient oxygen, if the hydrocarbon is burned with less than the theoretical amount of air, the combustion will be incomplete.

Key concepts

hydrocarbon combustion

Hydrocarbon combustion is a chemical process where a hydrocarbon reacts with oxygen to produce carbon dioxide (CO2) and water (H2O). This process releases energy, typically in the form of heat and light, hence why it is commonly seen in fireplaces, car engines, and even power plants. Hydrocarbons are compounds made up of hydrogen (H) and carbon (C) atoms. During combustion, these atoms combine with oxygen (O2).
For example, when methane (CH4), a common hydrocarbon, burns in the presence of oxygen, the reaction can be written as:
\[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \]
In ideal conditions, if there's enough oxygen present (as dictated by the theoretical air requirement), all the carbon in methane turns into CO2, and all the hydrogen turns into H2O. This complete combustion releases the maximum amount of energy possible from the hydrocarbon.
However, if the air supply is limited, not all hydrocarbons will be fully combusted. This leads to incomplete combustion, which produces less energy and results in harmful emissions.

theoretical air requirement

The theoretical air requirement is the exact quantity of air (specifically, the oxygen in the air) necessary for the complete combustion of a given amount of hydrocarbon. In other words, it's the amount needed for all the carbons to form carbon dioxide and all the hydrogens to form water.
This is calculated based on the stoichiometry of the combustion reaction. For instance, returning to the example of methane (CH4):
\[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \]
To completely burn one molecule of methane, we need exactly two molecules of oxygen. The amount of air required comes from knowing that air is approximately 21% oxygen by volume.
Ensuring that this exact amount of air is provided allows for complete combustion, reducing wastage and minimizing harmful by-products. Running an engine or any combustion system at the theoretical air requirement is ideal for efficiency.

oxygen deficiency effects

When there is less than the theoretical amount of air for combustion, there won't be enough oxygen to combine with all the hydrocarbon molecules. This leads to incomplete combustion with several negative consequences.
Some of these are:

• Production of carbon monoxide (CO), a toxic gas
• Formation of soot (carbon particles), which can contribute to air pollution and health problems
• Release of other hydrocarbons that didn't fully combust, impacting air quality.

In the case of methane, incomplete combustion happens as:
\[ 2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O} \]
Notice that instead of producing CO2, incomplete combustion produces CO (carbon monoxide) due to oxygen deficiency. Carbon monoxide is particularly dangerous as it can bind with hemoglobin in our blood, preventing oxygen from reaching cells and tissues.
Oxygen deficiency can also result in lower energy efficiency since incomplete combustion doesn't release as much energy as complete combustion. Thus, ensuring an adequate air supply is crucial for efficient and safe combustion processes.