Comparison of Combustion Heat of Difference Fuel

The table below shows the heat of combustion of a few fuel. It shows that when the number of carbon in a molecule increase, the heat of combustion will increase.
Alcohol MolecularFormula
DH (kJ mol-1)
Methanol
CH3OH
‑715
Ethanol
C2H5OH
‑1376
Propan-1-ol
C3H7OH
‑2017
Butan-1-ol
C4H9OH
‑2675

If a graph is plotted.

We will find that the heat of combustion is directly proportional to the number of carbon in the fuel.

Example 1
Alcohol Combustion heat (kJ mol-1)
Methanol
Ethanol
Propanol
Pentanol
-710
-1370
-2015
-3300

Table above shows the heat of combustion for different alcohol. Estimate the heat of combustion of butanol. By referring to the table above, find the heat release if 6g propanol is combusted. (Relative molecular mass: Carbon = 12; Hydrogen = 1, Oxygen = 16)

Answer
Heat of combustion of butanol
= -(2015+3300)/2 ≈ 2660 kJ mol-1

Relative molecular mass of propanol (C3H7OH)
= 3(12)+7(1)+16+1
= 60

Number of mol of propanol,
n = 6g/50gmol-1
n = 0.1 mol

Amount of heat released
= 0.1 x 2015kJ
= 201.5kJ

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