12-oz. aluminium beverage can with cut out top
Bench mat
Digital balance (to 0.001g)
Stand, boss, clamp
Spirit burners
Thermometer
100 cm3 measuring cylinder
III. Data
Combustion of Ethanol:
Combustion of Propan-1-ol:
Combustion of Butan-1-ol:
Calculation:
Experimental enthalpy change of combustion of Ethanol:
– = –1120 kJmol-1
Experimental enthalpy change of combustion of Propan-1-ol:
– = –1510 kJmol-1
Experimental enthalpy change of combustion of Butan-1-ol:
– = –2090 kJmol-1
% error of enthalpy change of combustion of Ethanol:
x 100% = x 100% = 81.9%
As the % error of enthalpy change of combustion of Ethanol = 81.9%,
△[C3H7OH] = –1510 ÷ 81.9% = –1840 kJmol-1
△[C4H9OH] = –2090 ÷ 81.9% = –2550 kJmol-1
Result:
The standard enthalpy of formation of combustion of Propan-1-ol:
–1840 kJmol-1
The standard enthalpy of formation of combustion of Butan-1-ol:
–2550 kJmol-1
IV.Discussion
The standard enthalpy of combustion of alcohols were found.
In general, the standard enthalpy of combustion of alcohols found were lower than the literal value.
Precaution
The mass of water should not be too large or too small. If the mass of the water is too small, the heat absorbed by the apparatus will become greater proportion. If the mass of water is too large, the heat evolved by combustion of alcohols raise the temperature slowly and the heat loss to the surroundings will become large. Thus the mass of water used in this experiment is 300cm3.
Initial temperature must be steady. i.e. room temperature.
Temperature rise employed is around 15℃. As there is heat loss to surroundings, large jumps in temperature would not show up as high as they should. The temperature rise cannot be too high. However, if the temperature is too low, it leads to great inaccuracy as there is measuring error in weighing the alcohol used, measuring the mass of water used and the temperature increase of water (reading the thermometer).
Compare the enthalpy of combustion of the alcohol with the literal value to check out the reliability of the result. i.e. Check the % error
(% error = x 100%) of each alcohol are similar.
If the variation is too large, redo the experiment carefully.
Error
The enthalpy of combustion of alcohols found is slightly smaller than the theoretical value.
There are several reasons to explain the fact:
-
There is heat loss to surroundings. Thus the heat evolved by combustion of the alcohol is underestimated.
-
Heat evolved by the spirit burner is carried away by air current. Thus the heat evolved by combustion of the alcohol is underestimated.
- There is incomplete combustion of the alcohols.
-
The heat capacity of the apparatus are omitted. Some heat are used to increase the temperature of the aluminium can. Thus the heat evolved by combustion of the alcohol is underestimated.
-
The sample of alcohol may not be pure. As there exist other substances, the masses of the alcohols were overestimated. Thus the enthalpy of combustion of the alcohols are smaller.
The difference between the experimental and literal value of enthalpy of combustion of alcohols increases with the relative molecular mass
because:
-
There is incomplete combustion of the alcohols. Larger the relative molecular mass of the alcohol, more difficult for it to undergo complete combustion.
-
There is heat loss to surroundings, larger jumps in temperature would not show up as high as they should. As the relative molecular mass increases, the heat evolved is larger and the temperature jumps less significantly.
The difference between the experimental and literal value of enthalpy of combustion of Propan-1-ol is the largest
because:
-
The sample of alcohol may not be pure. As there exist other substances, the masses of the alcohols were overestimated. The purity of Propan-1-ol may be the lowest among the three samples. Thus, the difference between the experimental and literal value of the enthalpy of combustion is the largest.
Improvements
- Minimize heat loss to the surroundings by placing a lid and insulating materials on the aluminium can.
- Reduce heat carried away by air current from the spirit burner by placing shields around the apparatus
- Find the heat capacity of the apparatus by electrical heating method.
- Use Beckmann thermometer for higher accuracy in reading temperature.
- Record the temperature at regular interval and plot graphs of temperature against time. Find the change in temperature by extrapolation to minimize the error in heat loss to the surroundings.
- Use flame calorimeter instead.
- Ensure the purity of the alcohols before carrying out the experiment.
- The experiment should be carried out immediately as alcohols are volatile.
V. Conclusion
The standard enthalpy of formation of combustion of Ethanol was found to be –1120 kJmol-1
The standard enthalpy of formation of combustion of Propan-1-ol was found to be –1840 kJmol-1
The standard enthalpy of formation of combustion of Butan-1-ol was found to be –2550 kJmol-1