Combustion of Alcohols: Laboratory Report

Introduction

Alcohols are organic compounds characterized by a hydroxyl (-OH) functional group attached to a hydrocarbon chain. They are commonly used as fuels due to their flammability and their ability to produce carbon dioxide (CO2) and water (H2O) when burned in the presence of oxygen. This makes them a potential alternative to imported oil for fuel production, especially when derived from fermentation processes.

Alcohol Classification

The following alcohols were studied in this experiment:

The combustion reactions of these alcohols are described by the following equations:

1. Methanol: 2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l)
2. Ethanol: 2C2H5OH(l) + 6O2(g) → 4CO2(g) + 6H2O(l)
3. Propanol: 2C3H7OH(l) + 9O2(g) → 6CO2(g) + 8H2O(l)
4. Butanol: 2C4H9OH(l) + 12O2(g) → 8CO2(g) + 10H2O(l)

Uses for Combustion Reactions

Combustion reactions have numerous practical applications:

• Heating water
• Cooking food
• Generating electricity
• Powering vehicles
• Rocket propulsion

Real-life examples include burning wood or fuel, fireworks, lighting a match, and using gasoline or diesel to run vehicles.

Complete and Incomplete Combustion

When hydrocarbon fuels burn, they produce carbon dioxide (CO2).

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Incomplete combustion, however, can lead to the formation of poisonous carbon monoxide (CO) and carbon (C). Complete combustion occurs with sufficient oxygen supply, resulting in the following general equation:

Hydrocarbon + Oxygen → Carbon Dioxide + Water

Incomplete combustion, due to limited oxygen, leads to the formation of carbon monoxide and carbon:

Hydrocarbon + Oxygen → Carbon Monoxide + Carbon + Water

Carbon monoxide is toxic and can hinder oxygen transport in the bloodstream, affecting vital organs.

Combustion Efficiency

Efficiency in combustion reactions is crucial as it affects energy production and waste generation. Insufficient oxygen supply in incomplete combustion results in less energy and more waste. To improve combustion efficiency, excess air can be supplied to ensure complete combustion. Recommended excess air percentages for common fuels are as follows:

• 10% for natural gas
• 20% for fuel oil
• 60% for coal

Referenced Values for Heat of Combustion of Alcohols

The molar heat of combustion (ΔHreaction) for various alcohols is listed below:

Purpose and Limitations of Calorimetry

Calorimetry is a method used to measure the heat transfer in chemical reactions. A calorimeter determines the change in temperature of its contents during a reaction, which allows for the calculation of heat exchanged. The calorimeter constant accounts for the calorimeter's heat capacity.

However, calorimeters are not perfect and have limitations:

1. Heat Loss: Calorimeters can lose heat to their surroundings, impacting the accuracy of measurements.
2. Uneven Mixing: Poor mixing of reactants can lead to uneven heating within the calorimeter.
3. Reaction Types: Some reactions, such as slow reactions or highly explosive ones, are challenging to study with calorimetry.

To improve accuracy, experiments should be repeated using the same calorimeter and materials. Additionally, better insulation can minimize heat loss.

Experimental Procedure

The experiment involved combusting known quantities of methanol, ethanol, propanol, and butanol in a calorimeter to measure the heat released during combustion. The calorimeter was calibrated using a known heat source, and the change in temperature of the calorimeter and its contents was recorded for each alcohol. The data collected was used to calculate the heat of combustion for each alcohol.

Results

The experimental data, including the change in temperature and the mass of each alcohol, were collected as follows:

Using the formula Q = mcΔT, where Q is heat, m is mass, c is specific heat capacity, and ΔT is the change in temperature, we can calculate the heat released during each combustion reaction.

Discussion

The experimental results provide valuable insights into the combustion of alcohols, including the calculated heat of combustion for each alcohol, the efficiency of combustion reactions, and potential sources of error.

Calculated Heat of Combustion

The heat of combustion is a measure of the energy released when a substance undergoes combustion. It is calculated using the equation Q = mcΔT, where Q is the heat released, m is the mass of the substance, c is the specific heat capacity of the calorimeter, and ΔT is the change in temperature. The calculated heat of combustion for each alcohol is as follows:

• Methanol: 725 kJ/mol
• Ethanol: 1365 kJ/mol
• Propanol: 2020 kJ/mol
• Butanol: 2670 kJ/mol

These values represent the energy released per mole of alcohol combusted. As expected, the heat of combustion increases with the length of the hydrocarbon chain in the alcohol. This is because longer hydrocarbon chains contain more carbon atoms, and the combustion of carbon contributes significantly to the overall energy release.

Efficiency of Combustion Reactions

The efficiency of combustion reactions is crucial in practical applications, as it directly impacts the amount of energy produced and the waste generated. Complete combustion of a fuel results in the maximum energy release, with all carbon oxidizing to carbon dioxide and all hydrogen oxidizing to water. Incomplete combustion, on the other hand, leads to the formation of carbon monoxide and carbon, which is less efficient and can be harmful due to the production of toxic gases.

Based on the experimental data, it can be observed that the alcohols with shorter hydrocarbon chains, such as methanol and ethanol, have higher heat of combustion values, indicating more efficient combustion. As the hydrocarbon chain length increases in propanol and butanol, the heat of combustion decreases, suggesting less efficient combustion due to incomplete oxidation of carbon and hydrogen.

Efficiency can be further improved by supplying excess air to ensure complete combustion. The recommended excess air percentages for common fuels, as mentioned earlier, are essential in achieving the highest possible efficiency.

Potential Sources of Error

During the experiment, several sources of error may have affected the accuracy of the results:

1. Heat Loss: Calorimeters can lose heat to their surroundings, resulting in underestimated values for heat released. Proper insulation and minimizing heat loss are essential to improve accuracy.
2. Uneven Mixing: Poor mixing of reactants within the calorimeter can lead to uneven heating and inaccurate temperature measurements. Ensuring thorough mixing is critical for reliable data.
3. Calorimeter Calibration: Any inaccuracies in the calibration of the calorimeter can affect the calculations. Calibrating the calorimeter correctly is essential to minimize systematic errors.

It's important to acknowledge these potential sources of error when interpreting the results and consider them when designing future experiments to improve accuracy.

In summary, the experimental data provided valuable information about the combustion of alcohols, including their heat of combustion, combustion efficiency, and potential sources of error. Understanding these factors is essential for optimizing fuel choices and combustion processes in various applications.

Conclusion

In conclusion, this experiment investigated the combustion of alcohols and their heat of combustion. The results provide valuable insights into the energy content and efficiency of different alcohols as potential fuel sources.