Enthalpy of Combustion: Experiment Report

Introduction

The enthalpy of combustion is defined as the heat released when one mole of a substance is completely burned in oxygen under standard conditions (Clark, 2013). In this exothermic reaction, substances react with excess oxygen, ensuring complete combustion, resulting in the production of carbon dioxide and water along with the release of heat. The heat released during this process varies among different substances and is measured to calculate the enthalpy of combustion. As the reaction is exothermic, the enthalpy change is negative.

The aim of this experiment is to determine the relative enthalpies of combustion for five different alcohols: Butan-1-ol, Ethanol, Methanol, Propan-1-ol, and Propan-2-ol. Alcohols are organic compounds that contain a hydroxy functional group (OH) attached to the carbon chain, making them efficient fuels due to their high heat production. Although the structures of these alcohols vary, they all share the common hydroxy functional group. Butan-1-ol has a four-carbon chain, ethanol has two carbons, methanol has one, and both propan-1-ol and propan-2-ol have three-carbon chains.

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Notably, all the alcohols, except propan-2-ol, are primary alcohols, while propan-2-ol is a secondary alcohol.

Experimental

The experimental setup involved an apparatus comprising an alcohol spirit burner, chimney, aluminum beaker, meter rule, digital thermometer, measuring cylinder, and an elastic band. The only change made to the apparatus during the experiment was the use of different spirit burners. Care was taken to ensure that the elastic band on the copper beaker remained in a fixed position, serving as a reference point for clamping it in the same position each time.

Here is the step-by-step procedure:

1. Measure 200 cm³ of tap water using a measuring cylinder and transfer it into the beaker securely clamped to the apparatus.
2. Stir the water using the digital thermometer and record the initial temperature.
3. Weigh the spirit burner containing Butan-1-ol with a calibrated balance, ensuring it starts from 0, and record the initial mass to four decimal places.
4. Light the spirit burner using matches and position it directly underneath the chimney, ensuring it is placed centrally.
5. Observe the temperature increase over time, periodically stirring the water with the thermometer.
6. When the water temperature has risen 150 degrees Celsius greater than the initial temperature, remove the spirit burner from under the chimney and extinguish the flame.
7. Continue to observe the temperature and stir the water until it reaches its maximum value after the spirit burner has been removed. Record this final temperature.
8. Weigh the spirit burner again, with the cap on, and record the final mass.

Repeat the above procedure for the remaining four alcohols, using fresh water in the beaker for each new alcohol. Record all results and observations in a table.

During the experiment, various observations may be made, including:

• Large, bright, sooty flames from Propan-2-ol, accompanied by a slow increase in temperature.
• Dim flames from Ethanol, Methanol, and Propan-1-ol.
• Large, bright flames without soot from Butan-1-ol.

Results

Raw Data:

Calculated Data:

Calibration Calculations

Experiment 1:

Change in mass = 1.2173 g

Moles = mass / Mr

n = 1.2173 g / 74 g/mol = 0.0203 moles

Energy released = moles x enthalpy of combustion

Q = 0.0203 moles x 2021 kJ/mol = 41.0263 kJ

Heat capacity = Q / temperature rise

Heat capacity = 41.0263 kJ / 15.4 K = 2.6640 kJ/K^-1

Experiment 2:

Change in mass = 1.2566 g

Moles = mass / Mr

n = 1.2566 g / 74 g/mol = 0.0209 moles

Energy released = moles x enthalpy of combustion

Q = 0.0209 moles x 2021 kJ/mol = 42.2389 kJ

Heat capacity = Q / temperature rise

Heat capacity = 42.2389 kJ / 15.7 K = 2.6904 kJ/K^-1

Enthalpy of Combustion Values

Calculating energy released: Q (kJ) = heat capacity of apparatus (kJ/K^-1) x temperature rise (K)

Calculating moles of each alcohol: n = Mass (g) / Mr

Calculating Enthalpy of Combustion using: ΔHc = -Q / n (kJ mol^-1)

Discussion

The overall correlation between the number of carbon atoms in the alcohols and the enthalpy of combustion was observed to be negative. Specifically, an increase in the number of carbon atoms led to a greater exothermic enthalpy value. This trend is in line with the principles of combustion reactions. When alcohols and compounds react with oxygen, they initially require energy to break the bonds (endothermic process). Subsequently, the formation of new products during combustion releases energy into the surroundings (exothermic process). An exothermic reaction occurs when the energy released from the formation of new bonds exceeds the energy absorbed to break the initial bonds (Schmidt-Rohr, 2015).

In the context of a combustion reaction, the double bond in O2 is relatively weak and does not require significant energy to break. However, the formation of CO2 and H2O as products involves the creation of stronger bonds, releasing more energy than was initially absorbed to break the O2 molecules. This results in an overall exothermic reaction. The observed trend in the experiment demonstrates that as the number of carbon atoms in the alcohol molecule increases, the enthalpy change becomes more negative. This is because with longer carbon chains, there are more CO2 and H2O bonds formed, which are stronger and release more energy, leading to a more exothermic reaction.

The Y-intercept for Experiment 1 is -275.78, while the Y-intercept for Experiment 2 is -55.38.

Conclusion

In conclusion, the results of the experiment exhibit a consistent negative trend where an increase in the carbon chain length of the alcohols corresponds to a decrease in the enthalpy of combustion. Methanol and ethanol achieved values of approximately -834.6230 and -798.5635 kJ/mol, respectively, which were close to their respective literature values. However, an anomaly was observed with propan-2-ol, which yielded a significantly lower enthalpy value of -780.7579 kJ/mol. This deviation could be attributed to possible heat loss to the surroundings due to minor changes in apparatus setup or variations in wick length, affecting the heat output of the flame.

Comparatively, the enthalpy values for butan-1-ol exhibited a broader range, ranging from -2258.3147 to -3001.5370 kJ/mol. This wide variation may also result from energy losses to the surroundings or potential inaccuracies in the measurement of the spirit burner's mass. To address these issues and improve accuracy, it is essential to maintain a consistent apparatus setup throughout the experiment and ensure that the mass balances are properly calibrated and set to zero before each measurement.

References

• Clark, J. (2010). Various enthalpy change definitions. Chemguide.co.uk. Available at: [Accessed 10 March 2022].
• Schmidt-Rohr, K. (2015). Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2. Journal of Chemical Education, 92(12), 2094-2099.