Enthalpy of Combustion of Alchohols

To find the energy transferred to the water, we can use this equation:

Energy = 4.18 x 100 x 13 = 5354 Joules

Mass of ethanol ÷ Molar mass of ethanol  3.125 g ÷ 46.06844 g/mol  0.0678 moles

Assuming that all the heat is transferred to water, the calculated enthalpy of combustion of ethanol is:

 = 80,147.49263 Joules/mol  80 kJ mol-1

Error Propagations

Uncertainty in mass:

Uncertainty in measuring cylinder:

Uncertainty in temperature:

Total uncertainty: 0.064 + 7.69 + 0.5 = 8.254%

Uncertainty in enthalpy value: 8.254% of 80 kJ/mol = ± 6.6032 kJ/mol

The percentage error compared to literature value:

 

The standard enthalpy change of combustion of ethanol (with uncertainty) is: -80 ± 6.6032 kJ mol-1

Combustion of Methanol

Reaction: 2CH3OH (l) + 3O2 (g) --> 2CO2 (g) + 4H2O (g)        

To find the energy transferred to the water, we can use this equation:

Energy = 4.18 x 100 x 20 = 8360 Joules

Mass of ethanol ÷ Molar mass of ethanol  5.17 g ÷ 46.06844 g/mol  0.11222 moles

Assuming that all the heat is transferred to water, the calculated enthalpy of combustion of ethanol is:

 = 74,496.52468 Joules/mol   74.5 kJ mol-1

Error Propagations

Uncertainty in mass:

Uncertainty in measuring cylinder:

Uncertainty in temperature:

Total uncertainty: 0.0387 + 5 + 0.5 = 5.5387%

Uncertainty in enthalpy value: 5.5387% of 74.5 kJ/mol = ± 4.13 kJ/mol

The percentage error compared to literature value:

 

The standard enthalpy change of combustion of methanol (with uncertainty) is: -74.5 ± 7.413 kJ mol-1