The combustion of 0.25 mol of an unknown organic compound results in the release of 320 kj of energy. which of the compounds in the table could be the unknown compound?
a. methane
b. ethylene
c. ethanol

C. ethanol

The enthalpy of combustion is the amount of heat produced when one mole of ethanol undergoes complete combustion at 25 ° C and 1 atmosphere pressure, yielding products also at 25 ° C and 1 atm.

The enthalpy of combustion of the unknown compound is

ΔH = - 320 kJ / 0.25 mol = - 1280 kJ / mol

To choose a probable compound according to this combustion enthalpy, we must evaluate the deviation in relation to the values reported in the literature for the three probable compounds (methane, ethylene and ethanol). The deviation (e%) will be calculated according to the following equation,

e% =  ( | ΔHx - ΔH | / ΔHx ) x 100%

where ΔHx is the enthalpy of combustion of the probable compound.

The following table shows the combustion enthalpies of the probable compounds and their deviation in relation to the enthalpy of ΔH = - 1280 kJ / mol

CompoundEnthalpy of combustion (kJ/mol)   Deviation

Methane                        - 890.7                                 43.8%

Ehylene                         -1411.2                                   9.3%

Ethanol                        -1368.6                                    6.5%

According to the previous table, we can say that the most probable compound is ethanol, since it has the smallest deviation in relation to the experimental enthalpy value of combustion.