The combustion of organic compounds produces large quantities of energy. These compounds range from
that of alkanes, to alkenes to even alcohols. Ethanol is a commonly used fuel in motor-cars and its usage
is increasing because it is a form of renewable energy. However, what makes a good fuel?
A good fuel is any substance which gives out large amounts of energy when it is burnt. In most cases,
fuels are burnt in oxygen (air) i.e. they are oxidized. Nonetheless, is there any relationship between the
energy released by one alcohol and another alcohol?
Therefore, the aim of this experiment is to investigate the relationship between the number of carbon
atoms in an alcohol chain and its standard enthalpy change of combustion.
To investigate the relationship between the numbers of carbon atoms in an alcohol chain; methanol,
ethanol, propanol, butanol and pentanol and their respective standard enthalpy change of combustions.
It can be hypothesized that as the number of Carbon atoms in an alcohol increases; the enthalpy of
combustion will also become more negative.
This is because as the number of Carbon atoms increase, the molecule’s shape differs. The hydrocarbon
chain becomes longer. As we move down the homologous group, a spate CH3 molecule is being added
into the alcohol chain. Thus, the extra energy is required to break apart this new molecule. It is much
harder to break up a longer hydrocarbon chain than a shorter one as there are more bonds to break.
Therefore, more energy is required to break a longer hydrocarbon. This can be explained by each
successive member of the series contains one more methylene group (-CH2-) than the previous one. When
it is burnt, there will be one extra C-C bond and two extra C-H bonds to be broken. Therefore, much more
energy is required for combustion.
Moreover, this prediction is made more valid if one talks in terms of the Van der Waal’s forces of each
molecule. As the carbon atoms are added into the alcohol chain, its mass increases. This will increase the
Van der Waal’s forces amidst the alcohols, resulting in stronger intermolecular forces of attraction.
Therefore, this leads to an increase in the enthalpy of combustion of the alcohols.