Methanol ---> Normal BP: 64.6°C --> dHvap: 35.3kJ/mol

iso-Propanol ---> Normal BP: 82.3°C --> dHvap: 45.4kJ/mol

Cyclohexane ---> Normal BP: 80.7°C --> dHvap: 33.1kJ/mol

Question: Ethanol has a normal BP of 78.3°C. Why is the normal BP higher than methanol but lower than iso-propanol listed above? Would you expect the heat of vaporization of ethanol to follow the same trend? Explain.

molar mass CH3OH = 32

molar mass ethanol = 46
molar mass isopropanol = 60
longer chains, more complicated molecules, high intermolecular forces. I would expect enthalpy vap to follow the same trend.

Thank you so much :)

To understand why the normal boiling point (BP) of ethanol is higher than methanol and lower than iso-propanol, we need to consider the molecular structure and intermolecular forces of these compounds.

The normal boiling point of a substance is determined by the strength of the intermolecular forces between its molecules. Stronger intermolecular forces result in higher boiling points.

In the case of methanol, it has only one carbon atom, which is smaller than the carbon chains present in iso-propanol and ethanol. Methanol molecules can form hydrogen bonds, but these bonds are weaker compared to iso-propanol due to the smaller carbon chain. As a result, stronger intermolecular forces develop between iso-propanol molecules, leading to a higher boiling point compared to methanol.

Ethanol, with a slightly larger carbon chain than methanol, has a stronger capacity to form hydrogen bonds. The presence of an additional -CH2- group in ethanol increases the molecular size and hence the London dispersion forces. These stronger intermolecular forces in ethanol compared to methanol result in a higher boiling point for ethanol.

However, iso-propanol has a branched carbon chain, which leads to a more compact molecular structure compared to ethanol. This reduces the surface area available for intermolecular interactions or contact between molecules, thereby decreasing the overall strength of intermolecular forces. Hence, iso-propanol has a lower boiling point compared to ethanol, despite having a larger carbon chain.

Regarding the heat of vaporization (dHvap), it generally follows a similar trend to the boiling point. Higher boiling points are often associated with higher values of dHvap. This is because substances with stronger intermolecular forces require more energy to overcome those forces and convert from a liquid to a gaseous state. Therefore, we would expect the heat of vaporization of ethanol to be higher than that of methanol but lower than that of iso-propanol.