EN4 Determination of Enthalpy Change of Combustion

4. Calculation/interpretation:

The calibration of the calorimeter, using propan-1-ol:

Heat energy released during the experiment (E) = △HC [propan-1-ol (l)] * n

= -2017 * 0.01833

= 36.978kJ/mol

Calibration factor of the calorimeter (C) = E / △T

= 36.978kJ / 20

= 1.849kJ/mol

Enthalpy Change of combustion of:

Ethanol = (C × △T) / n

= 1.849 * 20.2 / 0.0272

= -1374.4 kJ/mol

Butan-1-ol = 1.849 * 21.4 / 0.0235

= -1682.7 kJ/mol

Pentan-1-ol = 1.849 * 19.8 / 0.0107

= -3427.2 kJ/mol

Hexan-1-ol = 1.849 * 22.2 / 0.00901

= -4600.8 kJ/mol

From data book, the value of enthalpy change of combustion of:

Ethanol: -1367 kJ/mol.

Propan-1-ol: -2017kJ/mol

Butan-1-ol: -2675 kJ/mol.

Pentan-1-ol: 3323kJ/mol.

Hexan-1-ol: -3976kJ/mol.

Difference between the values of enthalpy changes of combustion for each adjacent pair of alcohols in the homologous series:

5. Discussion:

The literature differences between the values of enthalpy change of combustion for each adjacent pair of alcohols are quite even, about 650kJ/mol. However, the experimental differences are rather deviated. The first difference between ethan-1-ol and propan-1-ol is more or less the same as the literature value. But for the others, the difference is either a negative value or a very large value, up to the double or triple of the literature one. Such great error should be due to the errors listed below, due to the alternation of experimental factors in different cases.

It was essential for the volumes of water in the aluminum can to be the same when performing the experiments of different alcohols. 200ml of tap water was used in each case. However, if the volume of water used was not 200ml, the enthalpy change calculated would be slightly different. For instance, if the volume of water was larger, maybe because some water was remained in the can, a greater amount of heat would be required to heat up the water to a certain amount. As a result, more alcohol in the spirit lamp would be used, giving a smaller enthalpy change of combustion of that alcohol. In the tests after the first one, as some water was left in the can, this can explain why the enthalpy changes calculated are so high.

During the experiment, none precautions were taken to prevent heat loss to the surroundings, because it was assumed that the heat loss to the surroundings from the calorimeter was constant when burning the different alcohols. Only can the results obtained from the calorimeter work when the heat loss was assumed to be the same. Nevertheless, the truth would not be like this. For example, the environmental conditions would vary. After performing the first test using propan-1-ol, the aluminum can was heated and not cooled well. So less energy from the spirit lamp was required to heat the water. According to △HC = (C × △T)/n, the calculated enthalpy change of combustion would be greater. The longer the aluminum can was cooled, the smaller the calculated enthalpy change of that alcohol, which should be more accurate.

On the other hand, the aluminum can used might cause errors in the experiment. This is because when the flame of the spirit lamp was too strong, the bottom of the aluminum can would turn black, meaning that the aluminum had changed to aluminum oxide or other compounds. Since the specific heat capacities of aluminum oxide and aluminum are different, it could not keep a constant amount of heat absorbed by the can. The aluminum oxide layer would act as a barrier to make heat transfer from the flame to the water slower. Consequently, more heat would be required from the flame to warm the water inside to a certain extent. More alcohol from the lamp would be needed, giving a smaller enthalpy change of combustion of that alcohol.

Furthermore, it was assumed that the alcohol was burnt completely in the experiment, which can never come true under room conditions, especially for the alcohols with high carbon number. The reaction goes like this:

Alcohol + oxygen → carbon dioxide + carbon monoxide + carbon + water.

It suggests that not all of the alcohols were burnt completely. As a result, the calculated mass of alcohol used was larger than the actual one and so the enthalpy change calculated was smaller than the literature value.

Most importantly, the standard enthalpy change of combustion of propan-1-ol was calculated in an experiment of standard conditions. However, for the first experiment, using propan-1-ol to calculate the calibration factor, the conditions were not standard. This could give a reason for the difference.

Seeing these errors, it was normal to find the calculated values of the enthalpy changes to be different from the literature values.

The difference of the enthalpy changes of adjacent pair of alcohols was found to be roughly constant, for the case of the literature value. This is because the covalent bond of alcohols with higher carbon number is greater and so more energy is required to break the bond. Tracing such difference, the enthalpy change of combustion of other alcohols can be predicted, for example, adding about 650kJ/mol for each next alcohol.

6. Conclusion:

The enthalpy changes of combustion of the alcohols were found to be -1374kJ/mol, -1683kJ/mol, -3427kJ/mol, -4601kJ/mol for ethanol, butan-1-ol, pentan-1-ol and hexan-1-ol, under room conditions of 774mmHg and 19.0℃.