Enthalpy Of Combustion Of Methanol Lab Report

From the experimental data and through its analysis, the enthalpy of combustion for the five alcohols were determined; methanol, ethanol, propanol, butanol, and pentanol. As the line of best fit in the graph suggests, the enthalpy of combustion increased as the sizes of the molecules increased. This was predicted in the hypothesis and proves it to be correct. As seen on the graph, the enthalpy of combustion increases from 140kJ/mol for methanol, which has the smallest molecular mass, to 530kJ/mol for propanol, which has the largest molecular mass. The enthalpy of combustion increases as molecular size increases. This is because larger molecular sizes have more bonds, thus bond energy, and requires more energy to combust the substance.

These are the bond enthalpies that are used in this experiment
The chemical reaction to find the enthalpy of combustion of methanol is:
CH2OH + 1.5O2 -> CO2 + 2H2O

The energy needed to break the bonds in the LHS of the equation is:
{3 (C-H): 3 x 412}+{(C-O): 336}+{(O-H): 463}+{1.5(O=O): 1.5 x 498}=2740kJ/mol

The energy needed to make the bonds in the RHS of the equation is:
{2(C=O): 2 x 805}+4{(O-H): 4 x 463}=3466

Therefore, the enthalpy of combustion for methanol= The energy needed to break the bonds - The energy needed to make the bonds=-726kJ/mol

By doing these calculations for the four other types of alcohols, we can obtain the data given in the table below (this corresponds with the values listed in the IB Chemistry textbook).

Enthalpy of Combustion for the Alcohols
Type of Alcohol Accepted Value of Enthalpy of Combustion (kJ/mol)
Methanol -726
Ethanol -1367
Propanol -2021
Butanol -2676
Pentanol