Determining the effect of carbon chain length of an alcohol on its fuel efficiency

Data Collection and Processing:

 n=m/M (This formula is used to determine the moles of alcohol that was burnt)

Then Q=mc∆T to determine the heat energy given out by each alcohol.

Finally enthalpy of combustion is determined by using the formula ∆H=mc∆T

Quantitative Data:

Calculation for Methanol:

Initial Mass of the Spirit Burner = 207.89g ± 0.01g

Final Mass of the Spirit Burner = 207.59g ± 0.01g

Mass of ethanol burned =0.30g ± 0.02g

Molar Mass of Methanol (CH3OH) = (12.01) + (4*1.01) + 16.00 = 32.05g mol-1

Amount of Ethanol Burned = 0.30g/32.05g mol-1 = 0.009 moles

Heat Energy required to raise the temperature of 20g of water by 34.7oC = (20 *4.18 *34.7) = 2900.92J

It can then be implied that the heat energy evolved from 0.009moles of ethanol = 2900.92J

Heat Energy evolved by one mole of ethanol = 2900.92/0.009 = 322257.778J = -322kJ

Uncertainty of the mass of the water = (0.01/20)*100 = 0.05%

Uncertainty of the Temperature = (0.2/34.7)*100 = 0.58%

Uncertainty of the mass of the ethanol = (0.01/0.30)*100 = 3.33%

Total percentage uncertainties = (0.05+0.58+3.33)= 3.96%

• 3.96% of 322kJ = 12.7 kJ mol-1

Thus CH3OH(l) + O2(g)  CO2(g) + 2H2O(l) ∆H = -322.2 ± 12.7 kJ mol-1

Calculation for Ethanol:

Initial Mass of the Spirit Burner = 210.76g ± 0.01g

Final Mass of the Spirit Burner = 210.36g ± 0.01g

Mass of ethanol burned = 0.40g ± 0.02g

Molar Mass of Ethanol (C2H5OH) = (2*12.01) + (6*1.01) + 16.00 = 46.1g mol-1

Amount of Ethanol Burned = 0.40g/46.1g mol-1 = 0.00868 moles

Heat Energy required to raise the temperature of 20g of water by 37.5oC = (20 *4.18 *37.5) = 3135J

It can then be implied that the heat energy evolved from 0.00868moles of ethanol = 3135J

Heat Energy evolved by one mole of ethanol = 3135/0.00868 = 361175.1152J = -361kJ

Uncertainty of the mass of the water = (0.01/20)*100 = 0.05%

Uncertainty of the Temperature = (0.2/37.5)*100 = 0.53%

Uncertainty of the mass of the ethanol = (0.01/0.40)*100 = 2.5%

Total percentage uncertainties = (0.05+0.53+2.5)= 3.08%

• 3.08% of -361kJ = 11.1 kJ mol-1

Thus C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l) ∆H = -361.0 ± 11.1 kJ mol-1

Calculation for Propanol:

Initial Mass of the Spirit Burner = 217.25g ± 0.01g

Final Mass of the Spirit Burner = 216.69g ± 0.01g

Mass of ethanol burned = 0.56g ± 0.02g

Molar Mass of Propanol (C3H7OH) = (3*12.01) + (8*1.01) + 16.00 = 60.11g mol-1

Amount of Propanol  Burned = 0.56g/60.11g mol-1 = 0.00932 moles

Heat Energy required to raise the temperature of 20g of water by 40.4oC = (20 *4.18 *40.4) = 3377.44J

It can then be implied that the heat energy evolved from 0.00932moles of ethanol = 3377.44J

Heat Energy evolved by one mole of ethanol = 3377.44/0.00932 = 362386.2661J = -362kJ

Uncertainty of the mass of the water = (0.01/20)*100 = 0.05%

Uncertainty of the Temperature = (0.2/40.4)*100 = 0.50%

Uncertainty of the mass of the ethanol = (0.01/0.56)*100 = 1.9%

Total percentage uncertainties = (0.05+0.50+1.9)= 2.45%

• 2.45% of 362kJ = 8.869 kJ mol-1

Thus C3H7OH(l) + 4.5O2(g)  3CO2(g) + 4H2O(l) ∆H = -362.386 ± 8.869 kJ mol-1

Conclusion and Evaluation

The enthalpy values for methanol, ethanol and propanol were -322 kJ mol-1, -361 kJ mol-1 and -362 kJ mol-1 respectively. These values are relatively close to each other and support my hypothesis which was that as the carbon chain length increased the enthalpy of combustion would increase. I encountered several sources of errors during this lab such as heat energy from the flames of the spirit burner which did not fully heat up the water due to the draught of air caused by the air conditioner. This source of error could be overcome by blocking this source of air off or carrying out this experiment at a different location. Another source of error encountered was heat loss from the beaker in which the water was boiled and this was because the beaker is not a perfect heat insulator. To combat this error, aluminum foil could have been used to wrap around the beaker and the experiment, thus reducing the amount of heat lost and ensuring most of the heat from the spirit lamp went to heating up the water.  A major source of error in my lab also was the incomplete combustion of the alcohols due to a limited supply of oxygen thus the incomplete combustion of the alcohol formed carbon as soot and carbon monoxide. Preventing incomplete combustion would be to perform this experiment in an even more oxygen richer environment such that the alcohol could burn completely to form carbon dioxide and water.