Question

Why does the antifreeze ethylene glycol, HO-CH2-CH2-OH, molar mass 62.07 g/mol, have a much higher higher boiling point (197.6°C) than propanol (97.4°C), CH3-CH3-CH2-OH, molar mass 60.09 g/mol, even though they have similar molar masses?

Answer 1

1) Boiling Point : It's the temperature at which vapour pressure of liquid matches the atmospheric pressure.

2) From above definition of B.P. it follows that lower the vapour pressure of liquid higher is the B.P. and Higher vapour pressure lower is the B.P.

3) Vapor pressure of liquid in turns depend on extent of molecular association.

Higher the intermolecular association lower will be the escape of the molecule i.e. lower is the vapour pressure and hence higherwilll be the B.P.

4) Ethylene glycol have two -O-H groups wherase Propanol has 1 -O-H group.

Hence Extent of H-Bonding i.e. intermolecular association in _{Ethylene glycole is more than in propanol.}

In Ethylene glycol as intermolecular association through H-bonding is higher, it has lower degree of vapor pressure and hence higher B.P. than an alcohol Propanol of comparision molar mass.

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