2.5b: relate the physical properties of materials to the types of intermolecular force present, eg: (i) the trends in boiling and melting temperatures of alkanes with increasing chain length (ii) the effect of branching in the carbon chain on the boiling and melting temperatures of alkanes (iii) the relatively low volatility (higher boiling temperatures) of alcohols compared to alkanes with a similar number of electrons (iv) the trends in boiling temperatures of the hydrogen halides HF to HI

The trends in boiling and melting temperatures of alkanes with increasing chain length

• as chain length increases, boiling and melting temperatures increase
• van der Waals forces become larger with more points of contact

The effect of branching in the carbon chain on the boiling and melting temperature of alkanes

• as branching increases, boiling and melting temperature decreases
• side chains interfere with points of contact between atoms, so van der Waals forces are smaller

The relatively low volatility (high boiling point) of alcohols compared to alkanes with a similar number of electrons

• alcohols can form hydrogen bonds as well as van der Waals forces

The trends in boiling temperatures of the hydrogen halides HF to HI

• a gradual increase in boiling temperature from HF to HI would be expected, as the increasing atomic size means more van der Waals forces
• this is true for HBr to HI, but HCl and HF are different
• HCl has a more polar bond, so the permanent dipole-dipole interactions are stronger
• HF can form hydrogen bonds