# DIscuss the difference between intermolecular force, and intramolecular force with reference to ethylene and hydrazine. How are these assessed?

May 25, 2017

Do you mean $\text{intermolecular}$ or $\text{intramolecular}$ forces.......?

#### Explanation:

$\text{Intermolecular forces}$ are the forces between molecules, and the normal boiling point and melting point of a molecule are useful metrics to interrogate this interaction.

On the hand, $\text{intramolecular forces}$ are the forces WITHIN molecules; i.e. the forces between $N - H$ and $N - N$ versus $C - H$ and $C - C$ bonds. Most syllabuses make this distinction very clearly.

As regards the $\text{intermolecular force}$, perhaps the best metric we can use for comparison is the $\text{normal boiling point}$, i.e. the temperature at which the vapour pressure of the material is $1 \cdot a t m$.

The normal boiling point of ethylene is $- 103.7$ ""^@C.

The normal boiling point of hydrazine is $114$ ""^@C.

These materials have more or less equivalent molecular mass, yet the difference in volatility is striking. So what has hydrazine (and water) got that ethylene ain't got? The answer is $\text{hydrogen bonding}$, where hydrogen is bound to a strongly electronegative element such as fluorine, or oxygen, or nitrogen. Hydrogen bonding constitutes a potent intermolecular force that elevates the boiling point.