The Effect of Carbon Chain Length on Combustion Enthalpy of Alcohols

Introduction

Alcohols are considered as a viable alternative fuel for internal combustion engines due to their reduced environmental impact compared to petroleum-based fuels. They produce fewer greenhouse gases and toxic emissions while enhancing overall energy efficiency. Additionally, their high-octane rating, burning velocities, and wider flammability limits contribute to better environmental sustainability.

Combustion is a chemical process in which a substance reacts rapidly with oxygen and releases heat. In this study, we investigate how the length of the carbon chain in alcohols affects their combustion enthalpy.

The alcohols selected, including methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, all belong to the same homologous series with a common functional group.

The enthalpy of combustion for ethanol, for example, can be represented by the following reaction:

_{C2H5OH (l) + 3O2 (g) -> 2CO2 (g) + 3H2O (g)}

Hypothesis

We hypothesize that the longer the carbon chain in the alcohol molecule, the greater its enthalpy of combustion.

Variables

Independent Variable: The length of the alcohol's carbon chain.

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Dependent Variable: Combustion enthalpy (in kJmol^-1).

Controlled Variables

Chemicals and Apparatus

Procedure

1. Use a measuring cylinder to measure 50g of water and pour it into the calorimeter clamped to a stand.
2. Measure the initial mass of the fuel in the spirit burner.
3. Dip a cotton string in the fuel, leaving a small portion outside the spirit burner for combustion.
4. Place a thermometer in the water and note the initial temperature.
5. Light the burner and position it under the calorimeter at an appropriate distance.
6. Stir the water frequently to ensure even heat distribution.
7. Close the lid to stop combustion when the temperature increases by 30°C.
8. Measure the mass of the burner again and record the final mass.
9. Repeat steps 1 to 8 for all alcohols, conducting at least 3 trials for each alcohol.

Safety, Ethical, and Environmental Issues

Ethical Considerations: This investigation does not involve the use of animal-based products or plant extracts, eliminating ethical concerns.

Analysis

Qualitative Observation

1. The flame of the fuels is consistently yellow at the top.
2. As the carbon chain of alcohol increases, more black residue accumulates at the bottom of the calorimeter.
3. Droplets form on the inner surface of the calorimeter.
4. The time required for combustion decreases as the carbon chain length increases.

Results

Raw Data

Data Processing

Uncertainty Calculation

Percentage Uncertainty = (Absolute Uncertainty / Measured Value) x 100%

Conclusion

The hypothesis that the enthalpy of combustion of alcohols increases as the carbon chain length increases is supported by the results. The negative values of enthalpy of combustion indicate an exothermic process, and the greater the negative value, the greater the combustion enthalpy.

Percentage uncertainty and absolute uncertainty varied for each alcohol, with a trend indicating that longer carbon chains had greater uncertainty. The observed trend can be attributed to the increase in molecular size and intermolecular bond strength with longer carbon chains.

Comparing theoretical and experimental values revealed that experimental values were consistently lower than theoretical values, resulting in percentage errors. The experiment's reliability is supported by the goodness of fit (R² value) of the trend lines.

Overall, the experiment demonstrated that longer carbon chains in alcohols lead to higher combustion enthalpy, highlighting the potential of longer-chain alcohols as alternative fuels.

Evaluation

The study has several advantages, including its simplicity and ease of replication. However, there are areas for improvement, particularly in minimizing random errors. To enhance the precision and accuracy of the experiment, the following improvements are suggested:

By addressing these sources of random errors, future experiments can achieve both precision and accuracy in measuring combustion enthalpy.

Limitations: The study may have been affected by systematic errors, and certain assumptions may have influenced accuracy, particularly in the case of ethanol. Further investigations into these systematic errors are warranted.

Future Research: Future research could explore the effects of other factors, such as pressure and oxygen concentration, on combustion enthalpy to gain a more comprehensive understanding of alcohol fuels' behavior.

Overall, this study contributes to the understanding of the relationship between carbon chain length and combustion enthalpy in alcohols, which has implications for the development of more sustainable fuel alternatives.