# How to tell what intermolecular force is involved based on the boiling point and compound?

## E.g. Compound: CH_3F Boiling Point: -78.62

##### 1 Answer
Jun 22, 2017

Well, boiling point is a pretty good indicator of the extent of intermolecular force..........

#### Explanation:

When something boils we interrogate the equilibrium........

$\text{Stuff(l) "rightleftharpoons" Stuff(g)}$

In the liquid state, the stuff exerts a $\text{vapour pressure}$, which is a function of temperature. When the liquid exerts a vapour pressure that is EQUAL to the ambient pressure, boiling occurs, and bubbles of vapour form directly in the liquid.

The $\text{normal boiling point}$ is specified when the vapour pressure of the liquid is equal to $\text{ONE ATMOSPHERE}$.

To draw your attention to a practical examples, we know that the $\text{normal boiling point}$ of water is $100$ ""^@C; i.e. at $100$ ""^@C, water has a vapour pressure of $1 \cdot a t m$. At lower temperatures water exerts an equilibrium vapour pressure, which are extensively tabulated, and which often we must take into account when we (say) collect a volume of gas by displacement of water.

In you example, you quote ${H}_{3} C F$ with a boiling point of $- 78.6$ ""^@C, versus, say, $C {H}_{4}$ with a boiling point of $- 164.0$ ""^@C, versus, say, $C {H}_{2} {F}_{2}$ with a boiling point of $- 52.0$ ""^@C. In this series, dipole-dipole interaction is probably the dominant intermolecular force.